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Sunday, February 10

INTERPLANETARY TRANSPORT NETWORK


                                           UNIT- I
                   INTERPLANETARY TRANSPORT NETWORK
INTRODUCTION 
The interplanetary transport network (IPN) is a collection of gravitationally determined pathways through the Solar System that require very little energy for an object to follow. The ITN makes particular use of Lagrange points as locations where trajectories through space are redirected using little or no energy. These points have the peculiar property of allowing objects to orbit around them, despite the absence of any material object therein. While they use little energy, the transport can take a very long time.
           

A low energy transfer, or low energy trajectory, is a route in space which allows spacecraft to change orbits using very little fuel. [1][2] These routes work in the Earth-Moon system and also in other systems, such as traveling from Earth to Mars or between the satellites of Jupiter. The drawback of such trajectories is that they take longer to complete than higher energy (more fuel) transfers such as Hohmann transfer orbits

1.2 History
The key to the interplanetary transport network was investigating the exact nature of these winding paths near the Lagrange points. They were first investigated by Jules-Henri Poincaré in the 1890s. He noticed that the paths leading to and from any of these points would almost always settle, for a time, on the orbit around it. There are in fact an infinite number of paths taking one to the point and back away from it, and all of them require no energy to reach. When plotted, they form a tube with the orbit around the point at one end, a view which traces back to mathematicians Charles C. Conley and Richard P. McGehee in the 1960s. Theoretical work by Edward Belbruno in 1994 provided the first insight into the nature of the ITN between the Earth and the Moon that was used by Hiten, Japan's first lunar probe. Beginning in 1997 Martin Lo, Shane D. Ross, and others wrote a series of papers identifying the mathematical basis and applying the technique to the Genesis solar wind sample return, along with Lunar and Jovian missions. They referred to an Interplanetary Superhighway (IPS)

As it turns out, it is very easy to transit from a path leading to the point to one leading back out. This makes sense, since the orbit is unstable, which implies one will eventually end up on one of the outbound paths after spending no energy at all. However, with careful calculation, one can pick which outbound path one wants. This turned out to be exciting, because many of these paths lead right by some interesting points in space, like the Earth's Moon or the Galilean moons of Jupiter. That means that for the cost of getting to the Earth–Sun L2 point, which is rather low, one can travel to a huge number of very interesting points for a low additional fuel cost or even for free.

The transfers are so low-energy that they make travel to almost any point in the Solar System possible. On the downside, these transfers are very slow, and only useful for automated probes. Nevertheless, they have already been used to transfer spacecraft out to the Earth–Sun L1 point, a useful point for studying the Sun that was used in a number of recent missions, including the Genesis mission. The Solar and Heliospheric Observatory began operations at L1 in 1996. The network is also relevant to understanding Solar System dynamics; Comet Shoemaker–Levy 9followed such a trajectory to collide with Jupiter.
1.3 Further explanation
In addition to orbits around Lagrange points, the rich dynamics that arise from the gravitational pull of more than one mass yield interesting trajectories, also known as low energy transfers. For example, the gravity environment of the Sun–Earth–Moon system allows spacecraft to travel great distances on very little fuel, albeit on an often circuitous route. Launched in 1978, the ISEE-3 spacecraft was sent on a mission to orbit around one of the Lagrange points. The spacecraft was able to maneuver around the Earth's neighborhood using little fuel by taking advantage of the unique gravity environment. After the primary mission was completed, ISEE-3 went on to accomplish other goals, including a flight through the geomagnetic tail and a comet flyby. The mission was subsequently renamed the International Cometary Explorer (ICE).
The first low energy transfer utilizing this network was the rescue of Japan's Hiten lunar mission in 1991. Another example of the use of the ITN was NASA's 2001–2003 Genesis mission, which orbited the Sun–Earth L1 point for over two years collecting material, before being redirected to the L2 Lagrange point, and finally redirected from there back to Earth. The 2003–2006 SMART-1 of the European Space Agency used another low energy transfer from the ITN.
The ITN is based around a series of orbital paths predicted by chaos theory and the restricted three-body problem leading to and from the unstable orbits around the Lagrange points – points in space where the gravity between various bodies balances with the centrifugal force of an object there. For any two bodies in which one body orbits around the other, such as a star/planet or planet/moon system, there are three such points, denoted L1 through L3. For instance, the Earth–Moon L1 point lies on a line between the two, where gravitational forces between them exactly balance with the centrifugal force of an object placed in orbit there. For two bodies whose ratio of masses exceeds 24.96, there are two additional stable points denoted as L4 and L5. These five points have particularly low delta-v requirements, and appear to be the lowest-energy transfers possible, even lower than the common Hohmann transfer orbit that has dominated orbital navigation in the past.
Although the forces balance at these points, the first three points (the ones on the line between a certain large mass (e.g. a star) and a smaller, orbiting mass (e.g. a planet)) are not stable equilibrium points. If a spacecraft placed at the Earth–Moon L1 point is given even a slight nudge towards the Moon, for instance, the Moon's gravity will now be greater and the spacecraft will be pulled away from the L1 point. The entire system is in motion, so the spacecraft will not actually hit the Moon, but will travel in a winding path, off into space. There is, however, a semi-stable orbit around each of these points. The orbits for two of the points, L4 and L5, are stable, but the orbits for L1 through L3 are stable only on the order of months

1.4 Lagrangian points 
The Lagrangian points  are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be part of a constant-shape pattern with two larger objects (such as a satellite with respect to the Earth and Moon). The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centripetal force required to orbit with them.
Lagrangian points are the constant-pattern solutions of the restricted three-body problem. For example, given two massive bodies in orbits around their common center of mass, there are five positions in space where a third body, of comparatively negligible mass, could be placed so as to maintain its position relative to the two massive bodies. As seen in a rotating reference frame matching the angular velocity of the two co-orbiting bodies, the gravitational fields of two massive bodies combined with the satellite's acceleration are in balance at the Lagrangian points, allowing the third body to be relatively stationary with respect to the first two bodies.
1.4.1 History and concepts
The three collinear Lagrange points (L1, L2, L3 ) were discovered by Leonhard Euler a few years before Lagrange discovered the remaining two.
In 1772, the Italian-French mathematician Joseph Louis Lagrange was working on the famous three-body problem when he discovered an interesting quirk in the results. Originally, he had set out to discover a way to easily calculate the gravitational interaction between arbitrary numbers of bodies in a system, because Newtonian mechanics concludes that such a system results in the bodies orbiting chaotically until there is a collision, or a body is thrown out of the system so that equilibrium can be achieved.
The logic behind this conclusion is that a system with one body is trivial, as it is merely static relative to itself; a system with two bodies is the relatively simple two-body problem, with the bodies orbiting around their common center of mass. However, once more than two bodies are introduced, the mathematical calculations become very complicated. It becomes necessary to calculate the gravitational interaction between every pair of objects at every point along their trajectory.
Lagrange, however, wanted to make this simpler. He did so with a simple hypothesis: The trajectory of an object is determined by finding a path that minimizes the action over time. This is found by subtracting the potential energy from the kinetic energy. With this way of thinking, Lagrange re-formulated the classical Newtonian mechanics to give rise to Lagrangian mechanics.
Common opinion has been that Lagrange himself considered how a third body of negligible mass would orbit around two larger bodies which were already in a near-circular orbit, and found that in a frame of reference that rotates with the larger bodies, there are five specific fixed points where the third body experiences zero net force as it follows the circular orbit of its host bodies (planets). However, that is false.
Actually, Lagrange considered in the first chapter of the Essai  the general three-body problem. From that, in the second chapter, he demonstrated two special constant-pattern solutions, the collinear and the equilateral, for any three masses, with conic section orbits. Thence, if one mass is made negligible, one immediately gets the five positions now known as the Lagrange Points; but Lagrange himself apparently did not note that.
In the more general case of elliptical orbits, there are no longer stationary points in the same sense: it becomes more of a Lagrangian “area”. The Lagrangian points constructed at each point in time, as in the circular case, form stationary elliptical orbits which are similar to the orbits of the massive bodies. This is due to Newton's second law (Force = Mass times Acceleration, or   ), where p = mv (p the momentum, m the mass, and v the velocity) is invariant if force and position are scaled by the same factor. A body at a Lagrangian point orbits with the same period as the two massive bodies in the circular case, implying that it has the same ratio of gravitational force to radial distance as they do. This fact is independent of the circularity of the orbits, and it implies that the elliptical orbits traced by the Lagrangian points are solutions of the equation of motion of the third body.
Early in the 20th century, Trojan asteroids were discovered at the L4 and L5 Lagrange points of the Sun–Jupiter system.
FIVE POINTS
A diagram   showing the five Lagrangian points in a two-body system with one body far more  massive than the other (e.g. the Sun  and the Earth). In such a system, L3–L5 will appear to share the secondary's orbit, although in fact they are situated slightly outside it.

The five Lagrangian points are labeled and defined as follows:
L1
The L1 point lies on the line defined by the two large masses M1 and M2, and between them. It is the most intuitively understood of the Lagrangian points: the one where the gravitational attraction of M2 partially cancels M1 gravitational attraction.
Example: An object which orbits the Sun more closely than the Earth would normally have a shorter orbital period than the Earth, but that ignores the effect of the Earth's own gravitational pull. If the object is directly between the Earth and the Sun, then the Earth's gravity weakens the force pulling the object towards the Sun, and therefore increases the orbital period of the object. The closer to Earth the object is, the greater this effect is. At the L1 point, the orbital period of the object becomes exactly equal to the Earth's orbital period. L1 is about 1.5 million kilometers from the Earth.
The Sun–Earth L1 is suited for making observations of the Sun–Earth system. Objects here are never shadowed by the Earth or the Moon. The first mission of this type was the International Sun Earth Explorer 3 (ISEE3) mission used as an interplanetary early warning storm monitor for solar disturbances. The feasibility of this orbit was the result of a PhD thesis by the astrodynamicist Robert W. Farquhar. Subsequently the Solar and Heliospheric Observatory (SOHO) was stationed in a Halo orbit at L1, and the Advanced Composition Explorer(ACE) in a Lissajous orbit, also at the L1 point. WIND is also at L1.
The Earth–Moon L1 allows comparatively easy access to lunar and earth orbits with minimal change in velocity and has this as an advantage to position a half-way manned space station intended to help transport cargo and personnel to the Moon and back.
In a binary star system, the Roche lobe has its apex located at L1; if a star overflows its Roche lobe then it will lose matter to its companion star.
L2

A diagram showing the Sun–Earth L2 point, which lies well beyond the Moon's orbit around the Earth
The L2 point lies on the line defined by the two large masses, beyond the smaller of the two. Here, the gravitational forces of the two large masses balance the centrifugal effect on a body at L2.
Example: On the side of the Earth away from the Sun, the orbital period of an object would normally be greater than that of the Earth. The extra pull of the Earth's gravity decreases the orbital period of the object, and at the L2 point that orbital period becomes equal to the Earth's.
The Sun–Earth L2 is a good spot for space-based observatories. Because an object around L2 will maintain the same relative position with respect to the Sun and Earth, shielding and calibration are much simpler. It is, however, slightly beyond the reach of Earth's umbra, so solar radiation is not completely blocked. The Herschel Space Observatory, Planck space observatory are already, Chang'e 2 was until April 2012, and the Wilkinson Microwave Anisotropy Probe [10] was until October 2010, in orbit around the Sun–Earth L2. The Gaia probe and James Webb Space Telescope will be placed at the Sun–Earth L2. Earth–Moon L2 would be a good location for a communications satellite covering the Moon's far side. Earth–Moon L2 would be "an ideal location" for a propellant depot as part of the proposed depot-based space transportation architecture.
If the mass of the smaller object (M2) is much smaller than the mass of the larger object (M1) then L1 and L2 are at approximately equal distances r from the smaller object, equal to the radius of the Hill sphere, given by:

where R is the distance between the two bodies.
This distance can be described as being such that the orbital period, corresponding to a circular orbit with this distance as radius around M2 in the absence of M1, is that of M2 around M1, divided by  :

Examples
•           Sun and Earth: 1,500,000 km (930,000 mi) from the Earth
•           Earth and Moon: 60,000 km (37,000 mi) from the Moon
L3
The L3 point lies on the line defined by the two large masses, beyond the larger of the two.
Example: L3 in the Sun–Earth system exists on the opposite side of the Sun, a little outside the Earth's orbit but slightly closer to the Sun than the Earth is. (This apparent contradiction is because the Sun is also affected by the Earth's gravity, and so orbits around the two bodies' barycenter, which is, however, well inside the body of the Sun.) At the L3 point, the combined pull of the Earth and Sun again causes the object to orbit with the same period as the Earth.
The Sun–Earth L3 point was a popular place to put a "Counter-Earth" in pulp science fiction and comic books. Once space-based observation became possible via satellites and probes, it was shown to hold no such object. The Sun–Earth L3 is unstable and could not contain an object, large or small, for very long. This is because the gravitational forces of the other planets are stronger than that of the Earth (Venus, for example, comes within 0.3 AU of this L3 every 20 months). In addition, because Earth's orbit is elliptical and because the barycenter of the Sun–Jupiter system is unbalanced relative to Earth (that is, the Sun orbits the Sun–Jupiter center of mass, which is outside of the Sun itself), such a Counter-Earth would frequently be visible from Earth.
A spacecraft orbiting near Sun–Earth L3 would be able to closely monitor the evolution of active sunspot regions before they rotate into a geoeffective position, so that a 7-day early warning could be issued by the NOAA Space Weather Prediction Center. Moreover, a satellite near Sun–Earth L3 would provide very important observations not only for Earth forecasts, but also for deep space support (Mars predictions and for manned mission to near-Earth asteroids). In 2010, spacecraft transfer trajectories to Sun–Earth L3 were studied and several designs were considered.
One example of asteroids which visit an L3 point is the Hilda family whose orbit brings them to the Sun–Jupiter L3 point.
L4 and L5

Gravitational accelerations at L4

The L4 and L5 points lie at the third corners of the two equilateral triangles in the plane of orbit whose common base is the line between the centers of the two masses, such that the point lies behind (L5) or ahead of (L4) the smaller mass with regard to its orbit around the larger mass.
•           The reason these points are in balance is that, at L4 and L5, the distances to the two masses are equal. Accordingly, the gravitational forces from the two massive bodies are in the same ratio as the masses of the two bodies, and so the resultant force acts through the barycenter of the system; additionally, the geometry of the triangle ensures that the resultant acceleration is to the distance from the barycenter in the same ratio as for the two massive bodies. The barycenter being both the center of mass and center of rotation of the system, this resultant force is exactly that required to keep a body at the Lagrange point in orbital equilibrium with the rest of the system. (Indeed, the third body need not have negligible mass). The general triangular configuration was discovered by Lagrange in work on the 3-body problem.
•           L4 and L5 are sometimes called triangular Lagrange points or Trojan points. The name Trojan points comes from the Trojan asteroids at the Sun–Jupiter L4and L5 points, which themselves are named after characters from Homer's Iliad (the legendary siege of Troy). Asteroids at the L4 point, which leads Jupiter, are referred to as the "Greek camp", while those at the L5 point are referred to as the "Trojan camp". These asteroids are (largely) named after characters from the respective sides of the Trojan War.
Examples
The Sun–Earth L4 and L5 points lie 60° ahead of and 60° behind the Earth as it orbits the Sun. The regions around these points contain interplanetary dust and at least one asteroid, 2010 TK7, detected October 2010 by WISE and announced July 2011. Watch NASA's animated clip..
•           The Earth–Moon L4 and L5 points lie 60° ahead of and 60° behind the Moon as it orbits the Earth. They may contain interplanetary dust in what is called Kordylewski clouds; however, the Hiten spacecraft's Munich Dust Counter (MDC) detected no increase in dust during its passes through these points.
•           The region around the Sun–Jupiter L4 and L5 points are occupied by the Trojan asteroids.
•           The region around the Sun–Neptune L4 and L5 points have trojan objects.
•           Saturn's moon Tethys has two much smaller satellites at its L4 and L5 points named Telesto and Calypso, respectively.
•           Saturn's moon Dione has smaller moons Helene and Polydeuces at its L4 and L5 points, respectively.
•           One version of the giant impact hypothesis suggests that an object named Theia formed at the Sun–Earth L4 or L5 points and crashed into the Earth after its orbit destabilized, forming the Moon.
Stability
•           The first three Lagrangian points are technically stable only in the plane perpendicular to the line between the two bodies. This can be seen most easily by considering the L1 point. A test mass displaced perpendicularly from the central line would feel a force pulling it back towards the equilibrium point. This is because the lateral components of the two masses' gravity would add to produce this force, whereas the components along the axis between them would balance out. However, if an object located at the L1 point drifted closer to one of the masses, the gravitational attraction it felt from that mass would be greater, and it would be pulled closer. (The pattern is very similar to that of tidal forces.)
•           Although the L1, L2, and L3 points are nominally unstable, it turns out that it is possible to find stable periodic orbits around these points, at least in the restricted three-body problem. These perfectly periodic orbits, referred to as "halo" orbits, do not exist in a full n-body dynamical system such as the Solar System. However, quasi-periodic (i.e., bounded but not precisely repeating) orbits following Lissajous-curve trajectories do exist in the n-body system. These quasi-periodic Lissajous orbits are what most of Lagrangian-point missions to date have used. Although they are not perfectly stable, a relatively modest effort at station keeping can allow a spacecraft to stay in a desired Lissajous orbit for an extended period of time. It also turns out that, at least in the case of Sun–Earth-L1 missions, it is actually preferable to place the spacecraft in a large-amplitude  Lissajous orbit, instead of having it sit at the Lagrangian point, because this keeps the spacecraft off the direct Sun–Earth line, thereby reducing the impact of solar interference on Earth–spacecraft communications. Another interesting and useful property of the collinear Lagrangian points and their associated Lissajous orbits is that they serve as "gateways" to control the chaotic trajectories of the Interplanetary Transport Network.
•           In contrast to the collinear Lagrangian points, the triangular points (L4 and L5) are stable equilibria (cf. attractor), provided that the ratio of M1/M2 is greater than 24.96. This is the case for the Sun–Earth system, the Sun–Jupiter system, and, by a smaller margin, the Earth–Moon system. When a body at these points is perturbed, it moves away from the point, but the factor opposite of that which is increased or decreased by the perturbation (either gravity or angular momentum-induced speed) will also increase or decrease, bending the object's path into a stable , kidney-bean-shaped orbit around the point (as seen in the rotating frame of reference). However, in the Earth–Moon case, the problem of stability is greatly complicated by the appreciable solar gravitational influence.
1.5 Trajectory
A  trajectory is the path that a moving the object follows through space as a function of time. The object might be a projectile or a satellite, for example. It thus includes the meaning of orbit—the path of a planet, an asteroid or a comet as it travels around a central mass. A trajectory can be described mathematically either by the geometry of the path, or as the position of the object over time.
In control theory a trajectory is a time-ordered set of states of a dynamical system (see e.g. Poincaré map). In discrete mathematics, a trajectory is a sequence Description: (f^k(x))_{k \in \mathbb{N}} of values calculated by the iterated application of a mapping Description: f to an element Description: x of its source.
Figure Illustration showing the trajectory of a bullet fired at an uphill target
Range and height

Trajectories of projectiles launched at different elevation angles but the same speed of 10 m/s in a vacuum and uniform downward gravity field of 10 m/s2. Points are at 0.05 s intervals and length of their tails is linearly proportional to their speed. t = time from launch, T = time of flight, R = range and H = highest point of trajectory (indicated with arrows).
The range, R, is the greatest distance the object travels along the x-axis in the I sector. The initial velocity, vi, is the speed at which said object is launched from the point of origin. The initial angle, θi, is the angle at which said object is released. The g is the respective gravitational pull on the object within a null-medium.
Description: R={v_i^2\sin2\theta_i\over g}
The height, h, is the greatest parabolic height said object reaches within its trajectory
Description: h={v_i^2\sin^2\theta_i\over 2g}
Hard Stuff
In terms of angle of elevation Description: \theta and initial speed Description: v:
Description: v_h=v \cos \theta,\quad v_v=v \sin \theta \;
giving the range as
Description: R= 2 v^2 \cos(\theta) \sin(\theta) / g = v^2 \sin(2\theta) / g\,.
This equation can be rearranged to find the angle for a required range
Description: { \theta } = \frac 1 2 \sin^{-1} \left( { {g R} \over { v^2 } } \right) (Equation II: angle of projectile launch)
Note that the sine function is such that there are two solutions for Description: \theta for a given range  Description: d_h. The angle Description: \theta giving the maximum range can be found by considering the derivative or Description: R with respect to Description: \thetaand setting it to zero.
Description: {\mathrm{d}R\over \mathrm{d}\theta}={2v^2\over g} \cos(2\theta)=0
which has a nontrivial solution at   Description: 2\theta=\pi/2=90^\circ, or Description: \theta=45^\circ.                                 The maximum range is then   Description: R_{max} = v^2/g\,.                                                                       At this angle
 Description: sin(\pi/2)=1,     so the maximum height obtained is Description: {v^2 \over 4g}.
To find the angle giving the maximum height for a given speed calculate the derivative of the maximum height    Description: H=v^2 sin^2(\theta) /(2g) with respect to  Description: \theta, that is   Description: {\mathrm{d}H\over \mathrm{d}\theta}=v^2 2\cos(\theta)\sin(\theta) /(2g) which is zero when Description: \theta=\pi/2=90^\circ. So the maximum height

Unit –ii
2.1 Gravitational keyhole
A gravitational keyhole is a tiny region of space where a planet's gravity would alter the orbit of a passing asteroid such that the asteroid would collide with that planet on a given future orbital pass. The word "keyhole" contrasts the large uncertainty of trajectory calculations (between the time of the observations of the asteroid and the first encounter with the planet) with the relatively narrow bundle(s) of critical trajectories. The term was coined by P. W. Chodas in 1999. It gained some public interest when it became clear, in January 2005, that the Asteroid (99942 Apophis would miss the earth in 2029 but may go through one or another keyhole leading to impacts in 2036 or 2037. This has been ruled out in 2012.
Keyholes for the nearer or farther future are named by the numbers of orbital periods of the planet and the asteroid, respectively, between the two encounters There are even more but smaller secondary keyholes, with trajectories including a less close intermediate encounter (bank shots). Secondary gravitational keyholes are searched for by importance sampling: Virtual asteroid trajectories (or rather their ‘initial’ values at the time of the first encounter) are sampled according to their likelihood given the


 2.2 elliptical orbit
Due to observational inaccuracies, bias in the frame of reference stars, and largely unknown non-gravitational forces on the asteroid, mainly the Yarkovsky effect, its position at the predicted time of encounter is uncertain in three dimensions. Typically, the region of probable positions is formed like a hair, thin and elongated, because visual observations yield 2-dimensional positions at the sky but no distances. If the region is not too extended, less than about one percent of the orbital radius, it may be represented as a 3-dimensional uncertainty ellipsoid and the orbits (ignoring the interaction) approximated as straight lines.
Now imagine a plane comoving with the planet and perpendicular to the incoming velocity of the asteroid (unperturbed by the interaction). This target plane is named b-plane after the collision parameter b, which is the distance of a point in the plane to the planet at its coordinate origin. Depending on a trajectory's position in the b-plane its post-encounter direction and kinetic energy is affected. The orbital energy is directly related to the length of the semi-major axis and also to the orbital period. If the post-encounter orbital period of the asteroid is a fractional multiple of the orbital period of the planet, there will be a close encounter at the same orbital position after the given numbers of orbits.
According  to  theory of close encounters, the set of points in the b-plane leading to a given resonance ratio forms a circle. Lying on this circle are the planet and two gravitational keyholes, which are images of the planet in the b-plane of the future encounter (or rather of the slighly larger catchment area due to gravitational focusing). The form of the keyholes is a small circle elongated and bent along the circle for the given resonance ratio. The keyhole which is closer to the planet is smaller than the other because the variation of deflection becomes steeper with decreasing collision parameter b
Accuracy may matter
Relevant keyholes are those which are close to the uncertainty ellipsoid projected onto the b-plane, where it becomes an elongated ellipse. The ellipse shrinks and jitters as new observations of the asteroid are added to the evaluation. If the probable path of the asteroid keeps close to a keyhole, the precise position of the keyhole itself would matter. It varies with the incoming direction and velocitiy of the asteroid and with the non-gravitational forces acting on it between the two encounters. Thus, “a miss is as good as a mile,” does not apply to a keyhole of several hundred meter width. However, changing the path of an asteroid by a mile is not a huge task if the first encounter is still years away. Deflecting the asteroid after the fly-by would need a much stronger kick.
For a rapidly rotating planet as the earth, calculation of trajectories passing close to it, less than a dozen radii, shall include the oblateness of the planet—its gravitational field is not spherically symmetric. For even closer trajectories, gravity anomalies may be important.
For a large asteroid (or comet) passing close to the Roche limit, its size, which is inferred from its magnitude, affects not only the Roche limit but also the trajectory because the center of gravitational force on the body deviates from its center of mass resulting in a higher-order tidal force shifting the keyhole.

2.3 Hohmann transfer orbit
In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits of different altitudes, in the same plane.
The orbital maneuver to perform the Hohmann transfer uses two engine impulses, one to move a spacecraft onto the transfer orbit and a second to move off it. This maneuver was named after Walter Hohmann, the German scientist who published a description of it in his 1925 book Die Erreichbarkeit der Himmelskörper (The Accessibility of Celestial Bodies). Hohmann was influenced in part by the German science fiction author Kurd Laßwitz and his 1897 book Two Planets.

stage
Description: http://upload.wikimedia.org/wikipedia/commons/thumb/d/df/Hohmann_transfer_orbit.svg/220px-Hohmann_transfer_orbit.svg.png
The diagram shows a Hohmann transfer orbit to bring a spacecraft from a lower circular orbit into a higher one. It is one half of an elliptic orbit that touches both the lower circular orbit that one wishes to leave (labeled 1 on diagram) and the higher circular orbit that one wishes to reach (3 on diagram). The transfer (2 on diagram) is initiated by firing the spacecraft's engine in order to accelerate it so that it will follow the elliptical orbit; this adds energy to the spacecraft's orbit. When the spacecraft has reached its destination orbit, its orbital speed (and hence its orbital energy) must be increased again in order to change the elliptic orbit to the larger circular one.
Due to the reversibility of orbits, Hohmann transfer orbits also work to bring a spacecraft from a higher orbit into a lower one; in this case, the spacecraft's engine is fired in the opposite direction to its current path, decelerating the spacecraft and causing it to drop into the lower-energy elliptical transfer orbit. The engine is then fired again at the lower distance to decelerate the spacecraft into the lower circular orbit.
The Hohmann transfer orbit is theoretically based on two instantaneous velocity changes. Extra fuel is required to compensate for the fact that in reality the bursts take time; this is minimized by using high thrust engines to minimize the duration of the bursts. Low thrust engines can perform an approximation of a Hohmann transfer orbit, by creating a gradual enlargement of the initial circular orbit through carefully timed engine firings. This requires a change in velocity (delta-v) that is up to 141% greater than the two impulse transfer orbit (see also below), and takes longer to complete.[citation needed]
Calculation
For a small body orbiting another, very much larger body (such as a satellite orbiting the earth), the total energy of the body is the sum of its kinetic energy and potential energy, and this total energy also equals half the potential at the average distance  , (the semi-major axis):

Solving this equation for velocity results in the Vis-viva equation,

Where
 Is the speed of an orbiting body?
•           is the standard gravitational parameter of the primary body, assuming   is not significantly bigger than   (which makes)
•           is the distance of the orbiting body from the primary focus
•           is the semi-major axis of the body's orbit.        
•           Therefore the delta-v required for the Hohmann transfer can be computed as follows, under the assumption of instantaneous impulses:

Where   and   are, respectively, the radii of the departure and arrival circular orbits; the smaller (greater) of   and   corresponds to the periapsis distance (apoapsis distance) of the Hohmann elliptical transfer orbit. The total   is then:
 Whether moving into a higher or lower orbit, by Kepler's third law, the time taken to transfer between the orbits is:
Description: t_H = \begin{matrix}\frac12\end{matrix} \sqrt{\frac{4\pi^2 a^3_H}{\mu}} = \pi \sqrt{\frac {(r_1 + r_2)^3}{8\mu}}
(one half of the orbital period for the whole ellipse), where Description: a_H\,\! is length of semi-major axis of the Hohmann transfer orbit.




              
2.4 Horseshoe orbit 
A horseshoe orbit is a type of co-orbital motion of a small orbiting body relative to a larger orbiting body (such as Earth). The orbital period of the smaller body is very nearly the same as for the larger body, and its path appears to have a horseshoe shape in a rotating reference frame as viewed from the larger object.
The loop is not closed but will drift forward or backward slightly each time, so that the point it circles will appear to move smoothly along Earth's orbit over a long period of time. When the object approaches Earth closely at either end of its trajectory, its apparent direction changes. Over an entire cycle the center traces the outline of a horseshoe, with the Earth between the 'horns'.
Asteroids in horseshoe orbits with respect to Earth include 54509 YORP, 2002 AA29, and 2010 SO16, and possibly 2001 GO2. A broader definition includes 3753 Cruithne, which can be said to be in a compound and/or transition orbit,   or (85770) 1998 UP1 and 2003 YN107.
Saturn's moons Epimetheus and Janus occupy horseshoe orbits with respect to each other (in their case, there is no repeated looping: each one traces a full horseshoe with respect to the other).
Explanation of horseshoe orbit
The following explanation relates to an asteroid which is in such an orbit around the Sun, and is also affected by the Earth.
The asteroid is in almost the same solar orbit as Earth. Both take approximately one year to orbit the Sun.
It is also necessary to grasp two rules of orbit dynamics:
1.     A body closer to the Sun completes an orbit more quickly than a body further away.
2.     If a body accelerates along its orbit, its orbit moves outwards from the Sun. If it decelerates, the orbital radius decreases.
The horseshoe orbit arises because the gravitational attraction of the Earth changes the shape of the elliptical orbit of the asteroid. The shape changes are very small but result in significant changes relative to the Earth.
The horseshoe becomes apparent only when mapping the movement of the asteroid relative to both the Sun and the Earth. The asteroid always orbits the Sun in the same direction. However, it goes through a cycle of catching up with the Earth and falling behind, so that its movement relative to both the Sun and the Earth traces a shape like the outline of a horseshoe.
Stages of the orbit

Figure 1. Plan showing  possible orbits along gravitational contours. In this image, the Earth (and the whole image with it) is rotating counterclockwise around the Sun.
Starting out at point A on the inner ring between L5 and Earth, the satellite is orbiting faster than the Earth. It's on its way toward passing between the Earth and the Sun. But Earth's gravity exerts an outward accelerating force, pulling the satellite into a higher orbit which (per Kepler's third law) decreases its angular speed.
 figure 2 : thin horseshoe orbit
When the satellite gets to point B, it is traveling at the same speed as Earth. Earth's gravity is still accelerating the satellite along the orbital path, and continues to pull the satellite into a higher orbit. Eventually, at C, the satellite reaches a high enough, slow enough orbit and starts to lag behind Earth. It then spends the next century or more appearing to drift 'backwards' around the orbit when viewed relative to the Earth. Its orbit around the Sun still takes only slightly more than one Earth year.
2.5 Gravity assist
In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement and gravity of a planet or other celestial body to alter the path and speed of a spacecraft, typically in order to save propellant, time, and expense. Gravity assistance can be used to accelerate (both positively and negatively) and/or re-direct the path of a spacecraft.
The "assist" is provided by the motion of the gravitating body as it pulls on the spacecraft. The technique was first proposed as a mid-course manoeuvre in 1961,  and used by interplanetary probes from Mariner 10 onwards, including the two Voyager probes' notable fly-bys of Jupiter and Saturn
Elastic collision
                                                 
A gravity assist or slingshot maneuver around a planet changes a spacecraft's velocity relative to the Sun, though the spacecraft's speedrelative to the planet on effectively entering and leaving its gravitational field, will remain the same (as it must according to the law of conservation of energy). To a first approximation, from a large distance, the spacecraft appears to have bounced off the planet. Physicists call this an elastic collision even though no actual contact occurs. A slingshot maneuver can therefore be used to change the spaceship's trajectory and speed relative to the Sun.
A close terrestrial analogy is provided by a tennis ball bouncing off a moving train. In the cartoon at right, a boy throws a ball at 30 mph toward a train approaching at 50 mph. The engineer of the train sees the ball approaching at 80 mph and then departing at 80 mph after the ball bounces elastically off the front of the train. Because of the train's motion, however, that departure is at 130 mph relative to the station.
Description: http://upload.wikimedia.org/wikipedia/commons/thumb/3/3a/Gravitational_slingshot.svg/200px-Gravitational_slingshot.svg.png

Over-simplified example of gravitational slingshot: the spacecraft's velocity changes by up to twice the planet's velocity
Translating this analogy into space, then, a "stationary" observer sees a planet moving left at speed U and a spaceship moving right at speed v. If the spaceship has the proper trajectory, it will pass close to the planet, moving at speed U + v relative to the planet's surface because the planet is moving in the opposite direction at speed U. When the spaceship leaves orbit, it is still moving at U + v relative to the planet's surface but in the opposite direction (to the left). Since the planet is moving left at speed U, the total velocity of the rocket relative to the observer will be the velocity of the moving planet plus the velocity of the rocket with respect to the planet. So the velocity will be U + ( U + v ), that is 2U + v.
This oversimplified example is impossible to refine without additional details regarding the orbit, but if the spaceship travels in a path which forms aparabola, it can leave the planet in the opposite direction without firing its engine, the speed gain at large distance is indeed 2U once it has left the gravity of the planet far behind.
This explanation might seem to violate the conservation of energy and momentum, but the spacecraft's effects on the planet have not been considered. The linear momentum gained by the spaceship is equal in magnitude to that lost by the planet, though the planet's enormous mass compared to the spacecraft makes the resulting change in its speed negligibly small. These effects on the planet are so slight (because planets are so much more massive than spacecraft) that they can be ignored in the calculation.[2]
Realistic portrayals of encounters in space require the consideration of three dimensions. The same principles apply, only adding the planet's velocity to that of the spacecraft requires vector addition, as shown below.
Description: http://upload.wikimedia.org/wikipedia/commons/thumb/d/d0/Grav_slingshot_diagram.png/400px-Grav_slingshot_diagram.png
Description: http://bits.wikimedia.org/static-1.21wmf1/skins/common/images/magnify-clip.png
2 dimensional schematic of gravitational slingshot. The arrows show the direction in which the spacecraft is traveling before and after the encounter. The arrows' length shows the spacecraft's speed.
Due to the reversibility of orbits, gravitational slingshots can also be used to decelerate a spacecraft. Both Mariner 10 and MESSENGER performed this maneuver to reach Mercury.












Unit-iii
3.1 Communication network in space
DEFINITIONS FROM OSI BASIC REFERENCE MODEL
Most of the CCSDS space communications protocols are defined using the style established
by the Open Systems Interconnection (OSI) Basic Reference Model (reference [2]).  This
model provides a common framework for the development of standards in the field of
systems interconnection.  It defines concepts and terms associated with a layered architecture
and introduces seven specific layers.  The concepts and terms defined in this model are
extensively used in the Blue Books that define CCSDS space communications protocols.  If
the reader is not familiar with this model, an excellent introduction can be found in a
textbook on computer networks such as reference
2.2 PROTOCOL LAYERS
2.2.1 SUMMARY       
A communications protocol is usually associated with one of the seven layers defined in the
OSI Basic Reference Model (reference [2]).  Although some space communications protocols
do not fit well with the OSI seven-layer model, this Report uses this model for categorizing
the space communications protocols.
The space communications protocols are defined for the following five layers of the ISO
Model:

·        Physical Layer;
·        Data Link Layer;
·        Network Layer;
·        Transport Layer;
·        Application Layer.

3.1Physical layer
Compression
IPSec
Figure 2-1:  Space Communications Protocols Reference Model
Figure 2-2:  Some Possible Combinations of Space Communications Protocols
In figure 2-1, there are two protocols that do not correspond to a single layer.  CCSDS File
Delivery Protocol (CFDP) has the functionality of the Transport and Application Layers. 












3.2 PHYSICAL LAYER

CCSDS has a standard for the Physical Layer called the Radio Frequency and Modulation
Systems to be used for  space links between spacecraft and ground stations. 
The Proximity-1 Space Link Protocol also contains recommendations for the Physical Layer
of proximity space links.










3.2  DATA LINK LAYER

CCSDS defines two Sub layers in the Data Link Layer of the OSI Model:  Data Link Protocol
Sublayer and Synchronization and Channel Coding  Sublayer.  The  Data Link Protocol
Sublayer specifies methods of transferring data units provided by the higher layer over a
space link using data units known as Transfer Frames.  The Synchronization and Channel
Coding Sublayer specifies methods of synchronization and channel coding for transferring
Transfer Frames over a space link.
CCSDS has developed four protocols for the Data Link Protocol Sublayer of the Data Link Layer:
a) TM Space Data Link Protocol.
b) TC Space Data Link Protocol.
c) AOS Space Data Link Protocol  
d) Proximity-1 Space Link Protocol—Data Link Layer.
The above protocols provide the capability to send data over a single space link.
CCSDS has developed three standards for the Synchronization and Channel Coding Sublayer
 Data Link Layer:
a) TM Synchronization and Channel Coding (reference [8]);
b) TC Synchronization and Channel Coding (reference [9]);
c) Proximity-1 Space Link Protocol—Coding and Synchronization Layer (reference [19]).
TM Synchronization and Channel Coding is used with the TM or AOS Space Data Link
Protocol, TC Synchronization and Channel Coding is used with the TC Space Data Link
Protocol, and the Proximity-1 Space Link Protocol—Coding and Synchronization Layer is
used with the Proximity-1 Space Link Protocol—Data Link Layer.



2.2.4 NETWORK LAYER

Space communications protocols of the Network Layer provide the function of routing
Higher-layer data through the entire data system that includes both onboard and ground
Sub networks.
CCSDS has developed two protocols for the Network Layer:
a) Space Packet Protocol  
b) SCPS Network Protocol (SCPS-NP).
In some cases, Protocol Data Units (PDUs) of the Space Packet Protocol are generated and
consumed by application processes themselves on a spacecraft, instead of being generated
and consumed by a separate protocol entity, and in these cases this protocol is used both as a
Network Layer protocol and as an Application Layer protocol.
PDUs of a Network Layer protocol are transferred with Space Data Link Protocols over a

The following protocols developed by the Internet can also be transferred with Space Data
Link Protocols over a space link, multiplexed or not-multiplexed with the Space Packet
Protocol and/or SCPS-NP:
a) Internet Protocol (IP), Version
b) Internet Protocol (IP), Version




3.4 TRANSPORT LAYER
Space communications protocols of the Transport Layer provide users with end-to-end
Transport services.
CCSDS has developed the SCPS Transport Protocol (SCPS-TP)  
Transport Layer.  The CCSDS File Delivery Protocol (CFDP) (reference [15]) also provides
PDUs of a Transport Layer protocol are usually transferred with a protocol of the Network
Layer over a space link, but they can be transferred directly with a Space Data Link Protocol
If certain conditions are met.
Transport protocols used in the Internet (such as TCP, , and UDP,
 can also be used on top of SCPS-NP, IP Version 4, and IP Version 6 over
space links.
SCPS Security Protocol (SCPS-SP)  and IPSec may be used
with a Transport protocol to provide end-to-end data protection capability.





3.5 APPLICATION LAYER
Space communications protocols of the Application Layer provide users with end-to-end
application services such as file transfer and data compression.
CCSDS has developed three protocols for the Application Layer:
a) SCPS File Protocol (SCPS-FP) (reference  
b) Lossless Data Compression  
c) Image Data Compression
The CCSDS File Delivery Protocol (CFDP)   provides the functionality of the
Application Layer (i.e., functions for file management), but it also provides functions of the
Transport Layer.
Each project (or Agency) may elect to use application-specific protocols not recommended
By CCSDS to fulfill their mission requirements in the Application Layer over CCSDS space
Communications protocols.
PDUs of an Application Layer protocol (excluding CFDP) are usually transferred with a
Protocol of the Transport Layer over a space link, but they can be transferred directly with a
Protocol of the Network Layer if certain conditions are met.
Applications protocols used in the Internet can also be used on
top of SCPS-TP, TCP and UDP over space links.









                                                                 UNIT – IV  
SPACECRAFT PROPULSION

Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the back/rear of the vehicle at very high speed through a supersonic de Laval nozzle. This sort of engine is called a rocket engine.
All current spacecraft use chemical rockets (bipropellant or solid-fuel) for launch, though some (such as the Pegasus rocket and SpaceShipOne) have used air-breathing engines on their first stage. Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resist jet rockets for orbital station-keeping and some use momentum wheels for attitude control. Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north-south station keeping and orbit rising. Interplanetary vehicles mostly use chemical rockets as well, although a few have used ion thrusters and Hall Effect thrusters (two different types of electric propulsion) to great success.                    

Types of propulsion
·        Solid propellant
·        Liquid propellant
·        Semi-liquid propellant
·        Chemical propellant
·        Nuclear propellant






4.1 Solid propellant
A solid rocket or a solid-fuel rocket is a rocket with a motor that uses solid propellants (fuel/oxidizer). The earliest rockets were solid-fuel rockets powered by gunpowder; they were used by the Chinese, Indians, Mongols and Arabs, in warfare as early as the 13th century.[1]
All rockets used some form of solid or powdered propellant up until the 20th century, when liquid rockets and hybrid rockets offered more efficient and controllable alternatives. Solid rockets are still used today in model rockets and on larger applications for their simplicity and reliability.
Since solid-fuel rockets can remain in storage for long periods, and then reliably launch on short notice, they have been frequently used in military applications such as missiles. The lower performance of solid propellants (as compared to liquids) does not favor their use as primary propulsion in modern medium-to-large launch vehicles customarily used to orbit commercial satellites and launch major space probes. Solids are, however, frequently used as strap-on boosters to increase payload capacity or as spin-stabilized add-on upper stages when higher-than-normal velocities are required. Solid rockets are used as light launch vehicles for low Earth orbit (LEO) payloads under 2 tons or escape payloads up to 1100 pounds.
Design
Design begins with the total impulse required, which determines the fuel/oxidizer mass. Grain geometry and chemistry are then chosen to satisfy the required motor characteristics.
The following are chosen or solved simultaneously. The results are exact dimensions for grain, nozzle, and case geometries:
§  The grain burns at a predictable rate, given its surface area and chamber pressure.
§  The chamber pressure is determined by the nozzle orifice diameter and grain burn rate.
§  Allowable chamber pressure is a function of casing design.
§  The length of burn time is determined by the grain 'web thickness'.
The grain may or may not be bonded to the casing. Case-bonded motors are more difficult to design since the deformation of the case and the grain under flight must be compatible.
Common modes of failure in solid rocket motors include fracture of the grain, failure of case bonding, and air pockets in the grain. All of these produce an instantaneous increase in burn surface area and a corresponding increase in exhaust gas and pressure, which may rupture the casing.
Another failure mode is casing seal design. Seals are required in casings that have to be
opened to load the grain. Once a seal fails, hot gas will erode the escape path and result in failure. This was the cause of the Space Shuttle Challenger disaster.



4.2 Liquid propellant
Another failure mode is casing seal design. Seals are required in casings that have to be
opened to load the grain. Once a seal fails, hot gas will erode the escape path and result in failure. This was the cause of the Space Shuttle Challenger disaster.











A liquid-propellant rocket or a liquid rocket is a rocket engine that uses propellants in liquid form. Liquids are desirable because their reasonably high density allows the volume of the propellant tanks to be relatively low, and it is possible to use lightweight pumps to pump the propellant from the tanks into the engines, which means that the propellants can be kept under low pressure. This permits the use of low mass propellant tanks, permitting a high mass ratio for the rocket.
Liquid rockets have been built as monopropellant rockets using a single type of propellant, bipropellant rockets using two types of propellant, or more exotic tripropellant rockets using three types of propellant. Bipropellant liquid rockets generally use one liquid fuel and one liquid oxidizer, such as liquid hydrogen or a hydrocarbon fuel such as RP-1, and liquid oxygen. This example also shows that liquid-propellant rockets sometimes use cryogenic rocket engines, where fuel or oxidizer are gases liquefied at very low temperatures.
Liquid propellant rockets can be throttled in realtime, and have control of mixture ratio; they can also be shut down, and, with a suitable ignition system or self-igniting propellant, restarted.
Liquid propellants are also sometimes used in hybrid rockets, in which they are combined with a solid or gaseous propellant.

Principle of operation
All liquid rocket engines have tankage and pipes to store and transfer propellant, an injector system, a combustion chamber which is very typically cylindrical, and one (sometimes two or more) rocket nozzles. Liquid systems enable higher specific impulse than solids and hybrid rocket engines and can provide very high tankage efficiency.
Unlike gases, a typical liquid propellant has a density similar to water, approximately 0.7-1.4g/cm³ (except liquid hydrogen which has a much lower density), while requiring only relatively modest pressure to prevent vapourisation. This combination of density and low pressure permits very lightweight tankage; approximately 1% of the contents for dense propellants and around 10% for liquid hydrogen (due to its low density and the mass of the required insulation).
For injection into the combustion chamber the propellant pressure at the injectors needs to be greater than the chamber pressure; this can be achieved with a pump. Suitable pumps usually useturbopumps due to their high power and lightweight, although reciprocating pumps have been employed in the past. Turbopumps are usually extremely lightweight and can give excellent performance; with an on-Earth weight well under 1% of the thrust. Indeed, overall rocket engine thrust to weight ratios including a turbo pump have been as high as 133:1 with the Soviet NK-33rocket engine.
Alternatively, instead of a pump, a heavy tank can be used, and the pump forgone; but the delta-v that the stage can achieve is often much lower due to the extra mass of the tankage reducing performance; but for high altitude or vacuum use the tankage mass can be acceptable.
A liquid rocket engine (LRE) can be tested prior to use, whereas for a solid rocket motor a rigorous quality management must be applied during manufacturing to ensure high reliability.[5] A LRE can also usually be reused for several flights, as in the Space Shuttle.
Use of liquid propellants can be associated with a number of issues:
§  Because the propellant is a very large proportion of the mass of the vehicle, the center of mass shifts significantly rearward as the propellant is used; one will typically lose control of the vehicle if its center mass gets too close to the center of drag.
§  When operated within an atmosphere, pressurization of the typically very thin-walled propellant tanks must guarantee positive gauge pressure at all times to avoid catastrophic collapse of the tank.
§  Liquid propellants are subject to slosh, which has frequently led to loss of control of the vehicle. This can be controlled with slosh baffles in the tanks as well as judicious control laws in the guidance system.
§  They can suffer from pogo oscillation where the rocket suffers from uncommanded cycles of acceleration.
§  Liquid propellants often need ullage motors in zero-gravity or during staging to avoid sucking gas into engines at start up. They are also subject to vortexing within the tank, particularly towards the end of the burn, which can also result in gas being sucked into the engine or pump.
§  Liquid propellants can leak, especially hydrogen, possibly leading to the formation of an explosive mixture.
§  Turbopumps to pump liquid propellants are complex to design, and can suffer serious failure modes, such as overspeeding if they run dry or shedding fragments at high speed if metal particles from the manufacturing process enter the pump.
§  Cryogenic propellants, such as liquid oxygen, freeze atmospheric water vapour into very hard crystals. This can damage or block seals and valves and can cause leaks and other failures. Avoiding this problem often requires lengthy chilldown procedures which attempt to remove as much of the vapour from the system as possible. Ice can also form on the outside of the tank, and later fall and damage the vehicle. External foam insulation can cause issues as shown by the Space Shuttle Columbia disaster. Non-cryogenic propellants do not cause such problems.
§  Non-storable liquid rockets require considerable preparation immediately before launch. This makes them less practical than solid rockets for most weapon systems.




4.3 Nuclear propellant
Nuclear propulsion includes a wide variety of propulsion methods that fulfill the promise of the Atomic Age by using some form of nuclear reaction as their primary power source.
Many types of nuclear propulsion have been proposed, and some of them (e.g. NERVA) tested, for spacecraft applications:
Description: http://upload.wikimedia.org/wikipedia/commons/thumb/e/e0/Bimodal_Nuclear_Thermal_Rocket.jpg/300px-Bimodal_Nuclear_Thermal_Rocket.jpg
Bimodal Nuclear Thermal Rockets - conduct nuclear fission reactions similar to those employed at nuclear power plants including submarines. The energy is used to heat the liquid hydrogen propellant. Courtesy of NASA Glenn Research Center
Nuclear pulse propulsion
§  Project Orion, first engineering design study of nuclear pulse (i.e., atomic explosion) propulsion
§  Project Daedalus, 1970s British Interplanetary Society study of a fusion rocket
§  Project Longshot, US Naval Academy-NASA nuclear pulse propulsion design

Nuclear thermal rocket
§  Bimodal Nuclear Thermal Rockets conduct nuclear fission reactions similar to those safely employed at nuclear power plants including submarines. The energy is used to heat the liquid hydrogen propellant. Advocates of nuclear powered spacecraft point out that at the time of launch, there is almost no radiation released from the nuclear reactors. The nuclear-powered rockets are not used to lift off the Earth. Nuclear thermal rockets can provide great performance advantages compared to chemical propulsion systems. Nuclear power sources could also be used to provide the spacecraft with electrical power for operations and scientific instrumentation.
§  NERVA - NASA's Nuclear Energy for Rocket Vehicle Applications, a US nuclear thermal rocket program
§  Project Prometheus, NASA development of nuclear propulsion for long-duration spaceflight, begun in 2003
§  Project Rover - an American project to develop a nuclear thermal rocket. The program ran at the Los Alamos Scientific Laboratory from 1955 through 1972.
Ramjet
§  Bussard ramjet
§  Fission-fragment rocket
§  Fission sail
§  Fusion rocket
§  Gas core reactor rocket
§  Nuclear salt-water rocket
§  Radioisotope rocket
§  Nuclear photonic rocket
Nuclear electric
§  Nuclear electric rocket
§  RKA (Russian Federal Space Agency) NPS Development
Anatolij Perminov, head of Russian Space Agency announced that RKA is going to develop a nuclear powered spacecraft for deep space travel. Design will be done by 2012, and 9 more years for development (in space assembly). The price is set to 17 billion rubles (600 million dollars). The nuclear propulsion would have mega-watt class, provided necessary funding, Roscosmos Head stated.
This system would consist of a space nuclear power and the matrix of ion engines. "...Hot inert gas temperature of 1500 °C from the reactor turns turbines. The turbine turns the generator and compressor, which circulates the working fluid in a closed circuit. The working fluid is cooled in the radiator. The generator produces electricity for the same ion (plasma) engine..." [10]
According to him, the propulsion will be able to support human mission to Mars, with cosmonauts staying on the Red planet for 30 days. This journey to Mars with nuclear propulsion and a steady accelaration would take 6 weeks, instead of 8 months by using chemical propulsion - assuming thrust of 300 times higher than that of chemical propulsion

Analysis the performance of chemical propellant
, we note again the existence of a zone AC
 The propellants go from a series of liquid jets issuing through a multiplicity
Of small injector holes, through breakup of these jets into droplets, impingement
 Of jets or droplet streams on each other, dispersion of the
Droplets into a recirculating mass of combustion products, evaporation of the
Droplets,  interdiffusion of the vapors and kinetically controlled combustion. These
Are obviously complicated processes, and a comprehensive analysis good enough
For first principles design requires large-scale computation In fact,
the largest number of existing liquid rocket combustors, those dating from before
1970, were developed mainly through empirical methods, supplemented by very
Extensive testing. Improved modeling and computational capabilities have more
Recently permitted a more direct approach, with fewer hardware iterations, but
Theory is still far from completely developed in this area, and serves at this point
Mainly to ascertain trends and verify mechanisms. For an in-depth discussion of
Liquid propellant combustion, Here we will only review the
Fundamental concepts which underlie current spray combustion models

COMBUSTION INSTABILITIES IN LIQUID ROCKETS 
rocket engines  is  brief,  but nevertheless  we shall often refer to results achieved
in other systems as well, especially to encourage workers in the field to be aware
of,  if not  conversant  with,  combustion  instabilities  in  all  types  of propulsion
systems.
To  begin  to  understand  the  essential  characteristics  of combustion  instabilities,  it  is  best  first  to  distinguish  linear  and  nonlinear  behavior.  Linear  behavior presents  only  one  general  problem,  linear  instability,  which  received
widespread  attention  during  the  1950s  and  1960s;  see,  e.g.,  the  monograph
by   Crocco  and Cheng
l and  the  comprehensive  compilation  of works  edited by
Harrje  and Reardon.2  Any  disturbance may  be synthesized  as  an  infinite series
of harmonic motions. An  approximate analysis developed over many years (see
3
and  Culick  and  Yang

)  allows  one  to  use  classical  acoustic  modes  as
the  terms  in  the series  and  to  compute the  perturbations  of the  complex  wave
number for  each  mode  due  to  various  contributing  processes  in  a  combustion
chamber.  The real  part  of the  wave  number  gives  the  frequency  shift,  and  the
imaginary part gives the growth (or decay)  constant associated with each mode.
Vanishment of the imaginary part determines the formal condition for linear stability,  whose  dependence  on  the  parameters  characterizing  the  system  is  then
known.
Two  basic  nonlinear problems  arise  when  dealing with  combustion instabilities: determining the conditions for the existence and stability of limit cycles for a
linearly unstable system and finding the conditions under which a linearly stable
system may become unstable to a sufficiently large disturbance. In  the language
of modern dynamical systems theory, these two problems are identified as supercritical  and subcritical  bifurcations, respectively.  The term  bifurcation refers  to
the characteristic that the character of the steady behavior of the system suffers a
qualitative change abruptly  as  a parameter of the system is  varied continuously.
This may at first seem an unnecessarily formal description of the phenomenon. In fact, the framework provided by the approximate analysis and application of some
of the ideas of dynamical systems theory forms  a widely  useful  and  convenient basis for understanding combustion instabilities







Unit -5
5.1 Space travel
According to current reports in the media, travelling to outer space should become possible for everyone by the beginning of the next century. In April 1998, the newspaper Berlin Morgenpost reported that the international Hilton  Hotel corporation is planning the construction of a 5-star hotel, the "Lunar Hilton ," on the Moon. A team of British architects was commissioned to develop plans for the gigantic building project. The luxury hotel with 5000 beds and a height of 325 metres provides its own beach at its private "ocean," and should be equipped with all the amenities one can expect from a first class hotel.
·         
Description: http://www.spacefuture.com/archive/images/the_future_of_space_tourism.2.gif
Fig. 2: The space tourism roadmap with its four sub-scenarios [ESA98, ESA99]
However, the short-term realisation of this utopian project of the HiltonDescription: http://www.spacefuture.com/images/icons/who Group will most likely take a little more time, and for the first half of the 21st century it must be regarded as pure science fiction. Apart from the technical feasibility of some of the construction plans, economic analyses performed by DLR and the Technical University of BerlinDescription: http://www.spacefuture.com/images/icons/who show that the transportation of humans and building materials to the Moon would be extraordinarily expensive, even if very optimistic economic conditions in space operations are assumed [LAssMANN94, REICHERT97a]. With the announcement of its spectacular project, the HiltonDescription: http://www.spacefuture.com/images/icons/whoCorporation competes with three Japanese corporations, ShimizuDescription: http://www.spacefuture.com/images/icons/who, the construction firm Nishimatsu, and Obayashi, all of which have already invested millions into futuristic colonising concepts for the moon.
Life-cycle cost analyses performed in [ESA99, REICHERT97b/c/d] for the planet Mars indicate, that the transportation of humans would require ticket prices in the range of hundreds of millions of dollars, even if very favourable economic conditions in space operations are assumed. In addition, one must consider that after the successful completion of the Apollo program no transportation infrastructure is currently available that would allow humans to be transported beyond the Earth orbital. The creation of such a new infrastructure would require investments of billions of dollars. The Moon and Mars are therefore not likely to play a role in international tourism in the first half of the next century, although Mars represents one of the most promising travel destination because of its many similarities to Earth and its potential of possible former life forms.
Description: http://www.spacefuture.com/archive/images/the_future_of_space_tourism.3.gif
Fig. 3: The first humans explore the Martian surface
Therefore future space tourism will focus for a long time on Earth orbit which can be reached more easily. Figure 2 shows the expected roadmap for space tourism for the next decades with its four sub-scenarios. It is likely, that the space tourism business is initiated by very short suborbital flights, followed by short Earth orbital tourism in advanced, reusable spacecraft which allow several orbits around Earth. Before extended stays in space hotels will become a reality in the far future, touristic trips to existing orbital facilities (e.g. the international space station) might represent an intermediate step.
Already since the mid-sixties, several important analyses of Earth orbital space tourism have been carried out [EHRICKE67]. Various scenarios of transportation and accommodation of tourists in space have been analysed in the frame of industrial research in Japan, increasingly over the last 10 years [COLLINS97, MATSUMOTO97]. In the United States, NASADescription: http://www.spacefuture.com/images/icons/who and the American "Space Transportation AssociationDescription: http://www.spacefuture.com/images/icons/who" have completed a study of the feasibility of space tourism with some promising results [O'NEIL97] in 1998. Furthermore, DASA ( DaimlerChrysler Aerospace) investigates touristic spacecraft and space hotel concepts and supported the first international symposium on space tourism in the spring of 1997, which took place in Bremen (Germany) and attracted more than 100 experts from around the world. The symposium also found a significant echo in the media. Moreover, in 1998/99, the general feasibility and economics of space tourism played also a considerable role in an ESADescription: http://www.spacefuture.com/images/icons/who study under the lead of DLR [ESA98, ESA99, REICHERT98]. All of these activities indicate that space tourism is gaining more and more attention and significance. Today, it is already possible to book atmospheric parabolic flights in aeroplanes for the short-term simulation of zerogravity, and to reserve future passenger flights into suborbital space, even though the technology for the latter has not been developed yet.
5.2 THE INDUSTRIAL SIGNIFICANCE OF FUTURE SPACE TOURISM
According to an estimate of the World Travel Tourism Council, annual global expenditures in the terrestrial tourism sector amount to about 3400 billion US dollars for the year 1995. Tourism thus globally represents one of the largest industries. If it would be possible to shift only a few percent of the world-wide terrestrial tourist expenditures to a future space tourism market, this could double the civil space budgets to 60 billion dollars. This would create up to half a million new jobs also in the high technology space industry, if it is assumed that a sales volume of $50000 to $250000 creates one job.
If a space tourism business can be established in the future, the scientific and operational know-how - globally gained after decades of research and experience in manned space flight (e.g. by astronaut training centers or institutions of aerospace medicine) - could be applied to the space flight preparation and medical supervision of future space tourists. Furthermore, touristic spaceflights could be planned, controlled and monitored by existing space operation centers in Europe, the USA or Russia. As soon as a space tourism market is established, large commercially oriented industrial conglomerates will cooperate in strategic alliances to operate and expand the space tourism business comparable to the terrestrial tourism and telecommunications industries. The main motivation for an industrial investment in the space tourism sector will be the potential of very high achievable profits, which can be expected due to multi-billion dollar market potential.
5.3  THE MARKET FOR SPACE TOURISM 
As soon as the "space ticket" can be purchased for some $10000 - in the best of all cases a ticket price comparable to that of a transatlantic flight with the Concorde - one can expect that a global space tourism market will be established. This has been confirmed by first national and international polls and market analyses (e.g. in the US, Japan, Europe) which indicate that a remarkable segment of clients would be already willing to make considerable financial commitments for short trips into space. For example, 4.3% of all Germans are willing to pay several $10000 - which is roughly comparable to an annual income - for a space trip [ABITZSCH97]. This could be a ticket price which might be achievable with future generations of spacecraft, and it is comparable to the cost of a vacation on a cruise ship which already attracts a steady clientele. Figure 4 shows the expected number of passengers for a space trip as a function of the ticket costs according to international polls performed in the US, Japan, and Europe. With current costs of several ten million dollars to transport a human into space, space tourism is not affordable and finds no acceptance in the public.
However, with declining space travel ticket prices the situation will change. Once a ticket price of $ 1000 is obtainable, a passenger volume of about 20 million is expected, and even with a ticket price of $ 50000 a passenger rate of 1 million per year can be expected. [ABITZSCH97]. Nevertheless, the above mentioned polls have to be critically assessed with respect to their accuracy and credibility. Therefore, more detailed and representative polls and market research in this area have to be carried out.
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Fig. 4: Expected passengers as a function of the cost/price per ticket [ABITZSCH97]
Generally, any average person in good health and with appropriate preparation is able to go on a space trip. NASADescription: http://www.spacefuture.com/images/icons/who, for example, just recently decided to send 77-year old former astronaut and Senator John Glenn into space with the Space Shuttle (Fig. 5). He took part in important medical research experiments from which especially the older generation will profit.
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Fig. 5: A space trip at the age of 77 years: Ex-astronaut and US-Senator John Glenn [NEWSWEEK]
5.4 FUTURE MANNED SPACE TRANSPORTATION SYSTEM
The fact that tourist space travel has not been established yet is mostly due to the high costs of manned space travel. The transportation of a passenger into orbit, for example with the Russian rocket launcher Soyuz still costs several $10 million. Therefore, alternative and less expensive space transportation system concepts have to be identified.
Already in 1979, a manned Space Shuttle was proposed in [DURST79], with a cabin module designed to offer a seat capacity for 74 passengers (fig. 6). The costs for this design were calculated in 1997 to be $3.6 million per passenger, assuming a launch rate of 12 flights per year [KOELLE97].
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Fig. 6: A modified Space Shuttle with a seat capacity of 74 Passengers [DURST79]
A further reduction of the costs is only expected by the development of advanced, reusable single-staged spacecraft. An example of such a spacecraft is the Japanese design for the single staged Kankoh-MaruDescription: http://www.spacefuture.com/images/icons/term launcher (fig. 7) [COLLINS97]. This model with a capacity of 50 passengers is propelled by oxygen/hydrogen engines and is supposed to be operated like regular airplanes on conventional airports.
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Fig. 7: The Japanese Kankoh-MaruDescription: http://www.spacefuture.com/images/icons/term single stage vehicle in comparison with a BoeingDescription: http://www.spacefuture.com/images/icons/who 737 and 747 aircraft [COLLINS97]
The launch costs for this design are estimated in [KOELLE97] to be $300000 per passenger (assuming 10 launches per year) and and in [ESA98] to be about $50000 to $100000 (optimistically assuming 1 launch per day). The illustration shows the Japanese single staged launcher in comparison to a BoeingDescription: http://www.spacefuture.com/images/icons/who 747 and 737.
A further considerable cost reduction - ticket prices ranging from $10000 to $100000 - can therefore be only expected by the development of future generations of launchers, which have extremely high launch rates, are fully reusable and are operated with a minimum maintenance effort, comparable to today's aircraft fleets in the commercial airline business. The X-33Description: http://www.spacefuture.com/images/icons/term Space Shuttle Successor (fig. 8) - a one billion-dollar development program initiated by NASADescription: http://www.spacefuture.com/images/icons/who, can be considered as a first step to lower the transportation cost into space by using a single stage to orbit launching system for the first time.
Description: http://www.spacefuture.com/archive/images/the_future_of_space_tourism.8.gifFig. 8: The American X-33Description: http://www.spacefuture.com/images/icons/termconcept[NASADescription: http://www.spacefuture.com/images/icons/who
According to the American company Zegrahm suborbital flights will play a major role, already in the next decade, as a precursor to initiate touristic space trips. These short space trips either consist of a vertical ascent into space or end after one orbit around Earth with a landing at the departure airport. Although the advanced technology for suborbital flights is not developed yet, it is already possible to make reservations for such space trips for the beginning of the next decade at a ticket price of about $100000. However, the past experience shows, that the new development of high technology launchers generally requires long periods of research and testing and also investments typically in the range of several billion dollars. Taking these circumstances into account, the cost calculations and especially the short time frame for the first suborbital flights already for the beginning of the next decade appears very optimistic. Moreover, the first X-prize (promising $10 million prize for the first private manned rocket, which is launched to an altitude of 100 kilometer) candidates ran out of business.
Another project aims at the same objective: the proposed AscenderDescription: http://www.spacefuture.com/images/icons/term spaceplane (fig. 9) which is designed to perform several flights per day with a crew of 4 members. AscenderDescription: http://www.spacefuture.com/images/icons/term is equipped with two Williams-Rolls FJ44 turbofans and a Pratt & Whitney RL10 rocket engine. After take-off from a conventional runway it performs a subsonic ascent to an altitude of 8 kilometre on jet power by using its two turbofans (fig. 10). Afterwards the rocket engine is ignited which lifts the spaceplane to an altitude of about 100 km. After re-entry into the atmosphere the spaceplane returns to the departure runway. The price per passenger is estimated [ASHFORD97] to be $ 5000 within the next 10 to 20 years. This cost estimate must be critically assessed, and appears too optimistic, especially in comparison with military jets with one flight hour costing up to several $10000. Furthermore, up to now, the RL10 rocket engine is not designed to be operated fully reusable and several times a day.
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Fig. 9: The suborbital AscenderDescription: http://www.spacefuture.com/images/icons/term spaceplane [ASHFORD97]
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Fig. 10: A typical ascent/descent trajectory of the Ascender spaceplane [ASHFORD97]
This might significantly increase the cost per flight and indicates that a costly development program has to be initiated to modify the RL10 engine according to the requirements of the Ascender  spaceplane. Finally, Ascender s maximum take-off weight of only about 4000 kg seems very optimistic and too low to fulfil the mission requirements. Nevertheless, the generalAscender spaceplane approach looks promising and could result in an important technological development program, which leads to a single stage to orbit precursor
5.5 SPACE CRAFT OF EUROPE  
For this reason a first rough return on investment analysis has been carried out for the suborbital scenario in [ESA98, ESA99]. Space tourism is expected to be carried out by commercial companies which expect a profit from their business. Furthermore, in general, initial investments for the development and production have to be refinanced. Figure 11 shows the profit as a function of the operational year including and excluding financing costs, assuming a fare per passenger of $50000. If the financing costs are neglected, a first profit can be achieved in the 9th operational year which increases to about $36.6 billion in the 30th operational year. Considering financing costs, the date of the first profit shiftes to the 11th year and increases to about $34 billion in the 30th operational year. This decreased profit is caused by financing costs, which sum up to about $2.5 billion within the 30 year life cycle. The date of the first profit could be even shifted to earlier years, if the repayment of the development and production costs is spread over a longer period. Because of the relatively early return of i nvestment and the high achievable profits, the suborbital flights scenario generally looks very promising from an economic standpoint. This is also confirmed by the fact, that international polls ind icate, that the assumed ticket price of $50000 could lead to more than one million passengers per year [ABITZSCH97]. Due to the fact, that the fleet of 10 spaceplanes is only capable of transporting 43800 passengers per year, a further extension of the suborbital flights business seems very likely with decreasing ticket costs. However, it should kept in mind that the achieved profit mainly depends on low operation costs per flight (currently not state of the art) and the ability to realise the AscenderDescription: http://www.spacefuture.com/images/icons/term spaceplane with very low mass budgets as designed by [ASHFORD97].
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Fig. 11: Return on investment analysis for the suborbital flights scenario [ESA98]
A completely different design approach is implemented in the concept of the single stage ALLTRA-M1 rocket (fig. 12) of the German FAR research group, which could also represent a promising X-prize candidate [FAR99]. A capsule, which can accommodate up to 3 crew members is mounted on top of the rocket, which is able to reach an altitude of at least 100 kilometres during a vertical ascent. The ALLTRA-M1 rocket has a total mass of only about 10 tons and has almost the dimensions of a garage. The rocket is equipped with a hybrid propulsion system, in which liquid oxygen (in the central tank) is burned up with ordinary solid plastics in the lateral boosters. The hybrid propulsion system represents an interesting propulsion alternative between the classical solid and liquid/liquid propulsion systems. Its simple design, generous production tolerances and environmentally friendly and inexpensive fuels, possibly produced from recycled materials, make them a good candidate for low-cost high power propulsion systems. Thus a complete refuelling of the ALLTRA-M1 rocket costs only about $5000 and a 2-staged rocket concept could even reach a stable orbit around Earth. However, the overall economy, the atmospheric re-entry with acceptable G-loads, the soft landing and the general reusability still have to be demonstrated for the ALLTRA-M1 rocket in further studies.
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Fig. 12: The ALLTRA-M1 hybrid rocket [FAR99]
5.6 TOURISM IN SPACE HOTELS
In order to provide longer touristic stays in Earth orbit, concepts for space hotels have been investigated world-wide. In Japan, several designs for large-scale space hotels were analysed in the context of industrial studies like the ShimizuDescription: http://www.spacefuture.com/images/icons/who space hotel illustrated in fig. 13 [MATSUMOTO97]. It has a total mass of 8000 tons and offers all amenities and entertainment opportunities, one can expect from such a giant hotel complex. It is difficult to imagine from a current standpoint, that this giant hotel complex can be financed, built up and constructed in the near future. However, this sophisticated hotel concept might represent the second generation of space hotels maybe in the second half of the 21st century.
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Fig. 13: The Japanese space hotel concept by ShimizuDescription: http://www.spacefuture.com/images/icons/who [MATSUMOTO97]
A different design philosophy is considered in the Space Hotel Berlin concept (fig. 14), which was evaluated to some extend at DLR in the context of an ESADescription: http://www.spacefuture.com/images/icons/who study [ESA98, ESA99]. For this concept, mainly existing technologies are used by connecting modified habitat modules derived from the International Space StationDescription: http://www.spacefuture.com/images/icons/term (e.g. COF) as "apartments" to a circular ring-structure.
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Fig. 14: The rotating Space Hotel Berlin concept in Earth Orbit [REICHERT98, ESA99]
Rotating the circular structure with different velocities creates a wide variety of artificial gravity levels. With regard to mass and costs, the Space Hotel Berlin is about comparable to the International Space Station . In case of a 100 percent rate of capacity utilization, first rough life cycle cost analyses indicate that the accommodation of tourists seems possible at a price of about $100000 per overnight stay. This depends on the assumed life time of the space hotel complex, ranging from 10 to 30 years [ESA98, ESA99]. However, these costs do not include financing cost (for refinancing the development and production phase), a profit for a commercial company and the expensive Earth to LEO transportation of the tourists. This means, that a space hotel can be realized at the earliest, as soon as future generations of launchers provide extreme cost-efficient tickets, which are decreased by a factor of about hundred. Moreover.
Figure 15 shows the major subsystems of one basic element, of which the Space Hotel Berlin concepts  consists during the first build-up phase. The central subsystem is a cylindrical "apartment"-module with a large panoramic window which is capable to accommodate about 4 tourists. Connected to the module is a solar array which provides in combination with a rechargeable battery pack sufficient electrical energy for the flight phases when the space hotel enters into the Earth's shadow for about 40 minutes. Furthermore, a multifunctional connecting node, which can be entered by humans, is docked at the apartment module. This node provides five further docking ports which can be used, if needed in subsequent build-up phases, to connect additional apartment modules. For safety reasons each node has its own rescue capsule, which can be used in emergencies for the immediate return of the tourists back to Earth. With respect to the main apartment axis, the connecting node is 30o aligned, which causes the circular structure of the Space Hotel Berlin with a capacity of about 50 tourists.
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Fig. 15: The basic element of the Space Hotel Berlin concept [REICHERT98, ESA98]
Figure 16 shows an artist view of the Space Hotel Europe concept, which is derived from the circular structure of the Space Hotel Berlin to simplify the rendezvous/docking man oeuvres and to improve the living conditions.
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Fig. 16: TheSpace Hotel Europe concept [ESA99]
6. THE FASCINATION OF SPACE TRAVEL
The fact, that a significant portion of the public is willing to spend a lot of money on space trips proofs that these are regarded as very promising and fascinating. The confrontation with the high technology of space missions, the heroic myth of astronauts, and the possibility of orbiting planet Earth in just a little more than 80 minutes with a speed of about 30000 kilometers per hour (compared to the 80 days needed a hundred years ago) will represent for each tourist an unforgettable adventure and event. Furthermore, for the first time in his life, the tourist in a space hotel will experience an entirely different environment. Depending on the rotational velocity of the Space Hotel Berlin various levels of artificial gravity can be obtained, ranging from customary terrestrial gravity (1G) to Mars gravity (1/3 G) and even Lunar gravity (1/6 G), i.e. a human being will weigh only one sixth of his or her earthly weight. In the central node of the Space Hotel Berlin there will be nearly zero gravity. Here the tourist can experience weightlessness. How does one behave if one can not get from A to B in a normal way and "above" or "below" are without meaning? Many of the questions that astronauts were asked - how they manage eating, sleeping, and personal hygiene - can be explored by tourists themselves. The zero gravity area also offers fascinating opportunities for entirely new sorts of entertainment, games and sports. For example a ball game in zero-gravity and three-dimensional space. Or one can imagine a swimming pool. The water would not be in a basin, but would float as wobbly water bubbles in space, some of which are several meters in diameter, and one can swim and dive through them. One can imagine that some tourists may want to use the zero-gravity zones for future medical therapies: This could represent a first step towards a future space hospital. Furthermore, the Coriolis force, which only appears within rotating systems, will baffle the tourist, since it will push him, like a magic force, into a certain direction, depending on his direction of movement. Probably, the predominant part of the vacation will be spent with the breathtaking view each tourist will have from the panoramic window of each apartment onto the blue home planet from a distance of 400 kilometres. Even if initially concentrating on his home city and country, soon the tourist will discover earth with its thin and precious atmosphere and a wide variety of picturesque structures on its surface as a totality without national borders. On the other hand, the view of the infinite expanse of outer space will dramatically symbolise that we owe our human existence, history and future a singular, beautiful, tiny "grain of sand" which we call Earth.
After returning to Earth, the consciousness of many tourists may have changed; it will be expanded and globalised with many potentially positive social consequences, which could help, for example, to lower the dangers for environmental pollution, local conflicts, and war. Once space tourism is affordable to the broad public, space activities - especially manned space programs - will develop and increase in a way that is hardly imaginable today. A wide variety of space station concepts have been investigated for decades (fig. 17) and first concepts for gigantic, circular cities in orbit are available. Perhaps many humans increasingly will harbour the wish to visit the more remote planetary worlds of Mars and of the Earth's Moon some day, according to the Russian space pioneer Ziolkowksy who proclaimed around the year 1900: "Earth is the cradle of humanity, but one cannot always remain in the cradle."
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Fig.17: Wernher von BraunDescription: http://www.spacefuture.com/images/icons/who`s ring-shaped space station of 1952
The cover picture and some artist views in the text are provided by ALLTRA. You are invited to visit the complete space gallery of ALLTRA









Glossary
Absolute Brightness (Absolute Magnitude)
A measure of the true brightness of an object. The absolute brightness or magnitude of an object is the apparent brightness or magnitude it would have if it were located exactly 32.6 light-years (10 parsecs) away. For example, the apparent brightness of our Sun is much greater than that of the star Rigel in the constellation Orion because it is so close to us. However, if both objects were placed at the same distance from us, Rigel would appear much brighter than our Sun because its absolute brightness is much larger.

Angular Resolution
The ability of an instrument, such as a telescope, to distinguish objects that are very close to each other. The angular resolution of an instrument is the smallest angular separation at which the instrument can observe two neighboring objects as two separate objects. The angular resolution of the human eye is about a minute of arc. As car headlights approach from a far-off point, they appear as a single light until the separation between the lights increases to a point where they can be resolved as two separate lights.

Angular Size
The apparent size of an object as seen by an observer; expressed in units of degrees (of arc), arc minutes, or arc seconds. The moon, as viewed from the Earth, has an angular diameter of one-half a degree.

Apparent Brightness (Apparent Magnitude)
A measure of the brightness of a celestial object as it appears from Earth. The Sun is the brightest object in Earth's sky and has the greatest apparent magnitude, with the moon second. Apparent brightness does not take into account how far away the object is from Earth.

Arc Minute
One arc minute is 1/60 of a degree of arc. The angular diameter of the full moon or the Sun as seen from Earth is about 30 arc minutes.

Arc Second
One arc second is 1/60 of an arc minute and 1/3600 of an arc degree. The apparent size of a dime about 3.7 kilometers (2.3 miles) away would be an arc second. The angular diameter of Jupiter varies from about 30 to 50 arc seconds, depending on its distance from Earth.

Astronomer
A scientist who studies the universe and the celestial bodies residing in it, including their composition, history, location, and motion. Many of the scientists at the Space Telescope Science Institute are astronomers. Astronomers from all over the world use the Hubble Space Telescope.

Astronomical Unit (AU)
The average distance between the Earth and the Sun, which is about 150 million kilometers (93 million miles). This unit of length is commonly used for measuring the distances between objects within the solar system.

Baseline
The distance between two or more telescopes that are working together as a single instrument to observe celestial objects. The wider the baseline, the greater the resolving power.

Blueshift
The shortening of a light wave from an object moving toward an observer. For example, when a star is traveling toward Earth, its light appears bluer.

Celestial Sphere
An imaginary sphere encompassing the Earth that represents the sky. Astronomers chart the sky using the celestial coordinates of the sphere to locate objects in the cosmos. This sphere is divided into 88 sections called constellations. Objects are sometimes named for the major constellation in which they appear.

Collecting Area
The area of a telescope’s primary light-collecting mirror. A telescope’s light-gathering power rises with an increase in its collecting area.

Constellation
A geometric pattern of bright stars that appears grouped in the sky. Ancient observers named many constellations after gods, heroes, animals, and mythological beings. Leo (the Lion) is one example of the 88 constellations.



Cosmic Abundances
The relative proportions of chemical elements in the Sun, the solar system, and the local region of the Milky Way galaxy. These proportions are determined by studies of the spectral lines in astronomical objects and are averaged for many stars in our cosmic neighborhood. For example, for every million hydrogen atoms in an average star like our Sun, there are 98,000 helium atoms, 360 carbon atoms, 110 nitrogen atoms, 850 oxygen atoms, and so on.

Declination (DEC)
One of two celestial coordinates required to locate an astronomical object, such as a star, on the celestial sphere. Declination is the measure of angular distance of a celestial object above or below the celestial equator and is comparable to latitude. To familiarize yourself with declination, hold out your arm in the direction of the North Star (Polaris). You are now pointing at plus 90 degrees declination. Move your arm downward by 90 degrees. You are now pointing at 0 degrees declination.

Degree of Arc
One degree of arc is 1/360 of a full circle. The apparent sizes of objects as seen from Earth can be measured in degrees of arc. The angular diameter of the full moon or the Sun as seen from Earth is one-half of a degree.

Differentiation
The separation of heavy matter from light matter, thus causing a variation in density and composition. Differentiation occurs in an object like a planet as gravity draws heavier material toward the planet’s center and lighter material rises to the surface.

Diffraction Grating
A device that splits light into its component parts or spectrum. A diffraction grating often consists of a mirror with thousands of closely spaced parallel lines, which spread out the light into parallel bands of colors or distinct fine lines or bars.

Ellipse
A special kind of elongated circle. The orbits of the solar system planets form ellipses.


Field of View (FOV)
A telescope’s viewing area, measured in degrees, arc minutes, or arc seconds. A telescope that can just fit the full moon into its complete viewing area has a field of view of roughly 30 arc minutes.

Geocentric
An adjective meaning “centered on the Earth.” Most early civilizations had a geocentric view of the universe.

Infrared Telescope
An instrument that collects the infrared radiation emitted by celestial objects. There are several Earth- and space-based infrared observatories. The Infrared Telescope Facility, an Earth-bound infrared telescope, is the U.S. national infrared observing facility at the summit of Mauna Kea, Hawaii. A planned space-based infrared observatory is the Space Infrared Telescope Facility (SIRTF).

Interferometer
An instrument that combines the signal from two or more telescopes to produce a sharper image than the telescopes could achieve separately.

Jets
Narrow, high-energy streams of gas and other particles generally ejected in two opposite directions from some central source. Jets appear to originate in the vicinity of an extremely dense object, such as a black hole, pulsar, or protostar, with a surrounding accretion disk. These jets are thought to be perpendicular to the plane of the accretion disk.

Kepler’s Laws
Three laws, derived by 17th century German astronomer Johannes Kepler, that describe planetary motion.

Kepler’s first law: The orbits of planets are ellipses, with the Sun at one focus. Therefore, each planet moves in an elliptical orbit around the Sun.




Light Curve
A plot showing how the light output of a star (or other variable astronomical object) changes with time.

Light-Year
The distance that a particle of light (photon) will travel in a year — about 10 trillion kilometers (6 trillion miles). It is a useful unit for measuring distances between stars.

Luminosity
The amount of energy radiated into space every second by a celestial object, such as a star. It is closely related to the absolute brightness of a celestial object.

Megaparsec (MPC)
Equals one million parsecs (3.26 million light-years) and is the unit of distance commonly used to measure the distance between galaxies.

North Celestial Pole (NCP)
A direction determined by the projection of the Earth’s North Pole onto the celestial sphere. It corresponds to a declination of +90 degrees. The North Star, Polaris, sits roughly at the NCP.

Observable Universe
The portion of the entire universe that can be seen from Earth.

Optical Telescope
A telescope that gathers and magnifies visible light. The two basic types of optical telescopes are refracting (using lenses) and reflecting (using mirrors). The Hubble Space Telescope is an example of a reflecting telescope.



Parallax
The apparent shift of an object’s position when viewed from different locations. Parallax, also called trigonometric parallax, is used to determine the distance to nearby stars. As the Earth’s position changes during its yearly orbit around the Sun, the apparent locations of nearby stars slightly shift. The stars’ distances can be calculated from those slight shifts with basic trigonometric methods.

Parsec (PC)
A useful unit for measuring the distances between astronomical objects, equal to 3.26 light-years and 3.085678 * 1013 kilometers, or approximately 18 trillion miles. A parsec is also equivalent to 103,132 trips to the Sun and back.

Period-Luminosity Law
A relationship that describes how the luminosity or absolute brightness of a Cepheid variable star depends on the period of time over which that brightness varies.

Photometer
An instrument that measures the intensity of light. Astronomers use photometers to measure the brightness of celestial objects.

Photometry
A technique for measuring the brightness of celestial objects.

Proper Motion
The apparent motion of a star across the sky (not including a star’s parallax), arising from the star’s velocity through space with respect to the Sun.

Radial Motion
The component of an object’s velocity (speed and direction) as measured along an observer’s line of sight.

Recessional Velocity
The velocity at which an object moves away from an observer. The recessional velocity of a distant galaxy is proportional to its distance from Earth. Therefore, the greater the recessional velocity, the more distant the object.

Redshift
The lengthening of a light wave from an object that is moving away from an observer. For example, when a galaxy is traveling away from Earth, its light shifts to the red end of the electromagnetic spectrum.

Reflector
A type of telescope, also known as a reflecting telescope, that uses one or more polished, curved mirrors to gather light and reflect it to a focal point.

Refractor
A telescope, also known as a refracting telescope, that uses a transparent lens to gather light and bend it to a focus.

Revolution
The orbital motion of one object around another. The Earth revolves around the Sun in one year. The moon revolves around the Earth in approximately 28 days.

Right Ascension (RA)
A coordinate used by astronomers to locate stars and other celestial objects in the sky. Right ascension is comparable to longitude, but it is measured in hours, minutes, and seconds because the entire sky appears to pass overhead over a period of 24 hours. The zero hour corresponds to the apparent location of the Sun with respect to the stars on the day of the vernal (spring) equinox (approximately March 21).

Roche Limit
The smallest distance at which two celestial bodies can remain in a stable orbit around each other without one of them being torn apart by tidal forces. The distance depends on the densities of the two bodies and their orbit around each other.

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