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Friday, November 9


Technology at NASA
Mars Exploration Technology
Mars Exploration Technology Program

Mars Technology Program (MTP)

The Mars Technology Program (MTP) is responsible for technology-development plans that are consistent with NASA's Mars Exploration vision, and implementing and infusing those technologies into future missions.


Technologies That Enable Mars Exploration
Technology development makes missions possible. Each Mars mission is part of a continuing chain of innovation: each relies on past missions for new technologies and contributes its own innovations to future missions. This chain allows NASA to continue to push the boundaries of what is currently possible, while relying on proven technologies as well.

Technologies of Broad Benefit
Propulsion:for providing the energy to get to Mars and conduct long-term studies
Power:for providing more efficient and increased electricity to the spacecraft and its subsystems
Telecommunications:for sending commands and receiving data faster and in greater amounts
Avionics:electronics for operating the spacecraft and its subsystems
Software Engineering:for providing the computing and commands necessary to operate the spacecraft and its subsystems
In-situ Exploration and Sample Return
Entry, Descent, and Landing:for ensuring precise and safe landings
Autonomous Planetary Mobility:for enabling roversairplanes, and balloons to make decisions and avoid hazards on their own
Technologies for Severe Environments:for making systems robust enough to handle extreme conditions in space and on Mars
Sample Return Technologies:for collecting and returning rock, soil, and atmospheric samples back to Earth for further laboratory analysis
Planetary Protection Technologies:for cleaning and sterilizing spacecraft and handling soil, rock, and atmospheric samples
Science Instruments
Remote Science Instrumentation:for collecting Mars data from orbit
In-Situ Instrumentation:for collecting Mars data from the surface


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(34) Orbits in Space

    Index

 28.Spaceflight
--------------------------

 29. Spacecraft   (and 5 more)
-------------------

 30.To Space by Cannon?

 30a.Project HARP

 31.Nuclear Spaceflight?

 32. Solar Sails

 32a. Early Warning of
        Solar Shocks

 33. Ion Rockets

 34.Orbits in Space

 34a. L1 Lagrangian pt.

 34b. L4/L5 Points (1)

 34c.L4/L5 Points (2)

 35. Gravity Assist

 36. Pelton Turbine

         Afterword 

Synchronous Orbits

All space orbits obey the laws of Kepler and Newton. As already noted, for circular orbits Kepler's third law may be written
T = 5063 seconds R3/2 = 5063 seconds R * SQRT(R)
where T is the orbital period, * marks multiplication, R is the orbit's radius in units of Earth radii (= 6371 km) and SQRT(R) is the square root of R.
 NASA's communication
 satellite TDRRS, used to relay
  data from orbiting spacecraft
    From this one finds that for T = 86400 sec = 24 hours, R = 6.6 Earth radii. An equatorial satellite at this distance has a period of 24 hours and therefore, as the Earth rotates, it stays above the same point on the Earth's equator
    (more accurately, the orbital period is 235.9 seconds short of 24 hours, equal to the Earth's true rotation period).
    Such an orbit is ideal for a communication satellite, for then a "satellite dish" linked to it need not track it across the sky, but can stay pointed in a fixed direction.
    It was the British science fiction writer Arthur Clarke who first proposed the use of this "synchronous" orbit, long before the first artificial satellites. Clarke later wrote the book "Fountains of Paradise" (set in Sri Lanka, to which he had moved) in which thin cables linked synchronous satellites to the ground. A material strong enough and light enough for such cables does not exist, and is so far beyond anything known that it is probably impossible; but it makes a good story. About 200 satellites now inhabit synchronous orbit, some owned by governments for their own use, many operated by telecommunication companies.

Atmospheric Re-entry

The Kepler formula also applies to elliptical motion, provided R is replaced by the semi-major axis a of the orbit. Over time however orbits stray from exact Keplerian ellipses because to additional forces, such as the attraction of the Moon and the Sun. For elongated ellipses, this causes the lowest point in the orbit ("perigee") to move up and down, ultimately reaching the atmosphere and causing satellite to be lost.
    Atmospheric friction also causes low-altitude satellites to re-enter, sooner or later: all these, as they lose energy, descend deeper and deeper into the atmosphere, and ultimately reach denser regions, where they burn up. That was the fate of the Skylab space station in July 1979: NASA had hoped to use the Space Shuttle to boost it into a higher orbit, but the shuttle was not ready on time.
    Meanwhile the peak of the 11-year sunspot cycle arrived, a more active peak than NASA had hoped for, bringing a greater intensity of solar x-rays and extreme ultra-violet radiation. These radiations are absorbed in the uppermost fringes of the atmosphere, heat them up and make them expand outwards, more at "solar maximum" than at other times. Their expansion increased the air resistance ("drag") to the motion of Skylab and caused its early demise.

The Bulge of the Earth

    If the Earth were a perfect sphere, orbit calculations could assume that all its mass was concentrated at its center: the force, at least outside the Earth, would have been exactly the same. However, the centrifugal force associated with the Earth's rotation makes it slightly non-spherical, wider across the equator by a few kilometers than from pole to pole.
    That modifies the orbits of satellites and must be taken into account. When the orbital plane is inclined to the equator, the equatorial bulge slowly rotates it around the Earth: a line perpendicular to the orbit plane gradually traces a cone. Interestingly, there exists a situation where one can take advantage of this rotation.
Ordinarily, a satellite's orbit is fixed in space, and as the Earth goes around the Sun, its orientation relative to the Sun constantly changes. Take for example the case of a low altitude satellite whose orbit plane contains the axis of the Earth (i.e. it passes right above the north and south poles). If in June that plane happens to be lined up with the dawn-dusk direction, i. e. the division between the sunny side of Earth from the shaded one, then in September it matches the noon-midnight direction, a rotation of 90 degrees. Note that the June orbit enjoys 24-hour sunlight, but the September orbit does not.
    However certain orbits exist, passing just a few degrees from the poles, whose planes are rotated by the bulge of the Earth by exactly one rotation per year. Such "sun synchronous" orbits, can be made to always face the Sun, or always go through midnight. The DMSP satellites have such orbits(the picture here, of the aurora above the Great Lakes, was taken by one of these satellites; note Florida at bottom right), and so did Magsat.
Earth observation satellites such as Landsat  andSPOT  (Satellite Pour l'Observation de la Terre) also prefer sun-synchronous orbits, which ensure that images from different dates are always taken at the same time of the day. Without this, the difference in the shadows may confuse their interpretation.

Lagrangian Points

    By Kepler's 3rd law, a spacecraft going around the Sun in a circle smaller than the Earth's orbit will always have a shorter period and will move faster, and if launched from Earth its distance will grow until it and the Earth are well separated. Yet a way exists for keeping the two together.If the spacecraft is placed between the Earth and the Sun, the opposing pull of the Earth reduces the effective pull of the Sun, allowing the spacecraft to orbit the Sun more slowly. If the distance is properly chosen, the orbital motion will match that of Earth, allowing the two to stay together throughout the Earth's annual journey around the Sun.
    The point where this happens is the L1 Lagrangian point (after Joseph Lagrange, the Italian-French mathematician who pointed it out). It is about 4 times more distant than the Moon, at about 1/100 of the distance of the Sun. The next section gives a calculation of the L1 distance, using results from sections (20),(21) and (M-5).
The L1 point provides a very useful position for monitoring the solar wind before it reaches Earth and for other purposes. Currently two spacecraft are stationed near L1-- ACE , studying "anomalous cosmic rays" and also observing the solar wind, and SOHO , which observes the sun. WIND  which also occupied this region has moved to a new orbit.
        SOHO, a joint effort by NASA and the European space agency ESA, was nearly lost in June 1998 when by accident its high-gain antenna was shifted to where it no longer pointed at Earth. It also lost power because its solar cells no longer squarely faced the Sun, and the fuel of its on-board rocket got badly chilled. Some anxious months  followed, during which the controllers tried various ways of re-asserting control, even using the giant Arecibo radio telescope as a radar dish to pin-point the location of the spacecraft. Control was regained in the second half of September 1998 and the spacecraft resumed operation; unfortunately, its last gyro failed in December 1998. As of April 1999, its attitude is successfully controlled by inertia wheels, adapted for a job for which they were not originally intended.
    Four more Lagrangian points exist in the Earth-Sun system, including L2, symmetric to L1 on the nightside of the Earth. On June 30, 2001 NASA launched its Microwave Anisotropy Probe (MAP)  towards the L2 point. MAP is a follow-up on the successful Cosmic Background Explorer (COBE)  which in 1992 carefully measured the brightness and wavelength distribution of the "cosmic background" microwave radiation left over from the "big bang", when the existence of the universe began (read Stephen Hawking's "A Brief History of Time" or Steve Weinberg "The First Three Minutes").
  The radiation COBE observed was almost equally bright in all directions ("isotropic"), but not completely so: a small unevenness ("anisotropy") remained, and MAP is designed to observe it better. That unevenness may be a clue to uneven distribution of matter soon after the big bang, when the volume of the universe was still small and those microwave started out as high-energy gamma rays. Their uneven distribution may be related to the observed fact that matter in today's universe seems to be clumped in distinct galaxies, with big voids between them.
  NASA also plans to place its "Next Generation Space Telescope" (NGST), the successor to the orbiting Hubble telescope, at or near L2. For more details about these points and on related space missions, click here .
    The L3 point is on the far side of the Sun, invisible from Earth, unstable and probably unimportant. Of greater interest are the L4 and L5 points, on the Earth's orbit but 60° off the Sun-Earth line (viewed from above the north pole, L5 is 60° clockwise from that line, L4 60° counterclockwise). The mathematical proof that objects at these locations can orbit the Sun with the same period as the Earth while maintaining a fixed angle to the Sun-Earth line is quite long. If you are familiar with algebra and trig and have the patience for a long calculation, you may find the derivation in section #34b of this web site.
    If other planets did not interfere, these would be stable positions for small third bodies keeping a fixed station relative to Earth, and bodies close to such orbits would remain close to them. Claims exist of small asteroids observed near the L4 and/or L5 points of Earth, Mars and Venus, but the effect is most pronounced at the Lagrangian points of Jupiter, the heavyweight among planets. Several hundred of so-called Trojan Asteroids  are located in the vicinity of Jupiter's L4 and L5 points, orbiting the Sun with the same period as Jupiter. They are known as "Trojan" because their names come from characters in Homer's Iliad.
    The Earth-Moon system also has its Lagrangian points, and the L4 and L5 points on the Moon's orbit have been proposed as sites for self-contained "space colonies."
Note added June 2003
        MAP is now in position and has yielded some interesting results . It was renamed WMAP, the Wilkinson Microwave Anisotropy Probe, after David Wilkinson of Princeton University, a key member of the MAP team who died in 2002. NGST was also renamed by NASA to become the "James Webb Telescope," after one of the agency's late administrators.
                While the Lagrangian point orbits were historically the first 3-body orbits studied in detail, other such orbits are now known, in particular a figure-8 orbit which is stable. See here .

Cruising through the Solar System

To escape Earth a spacecraft needs a high velocity: 8 km/s to enter low Earth orbit, 11.2 km/s to escape Earth altogether. The only known way of getting such velocities are rockets.
    But even when a spacecraft escapes from Earth, it is still held by the Sun's gravity. While no longer bound to Earth, it will still orbit the Sun, at about the same distance as before. To move around the solar system, additional velocity is needed.
    The hardest object to reach would be the Sun itself. Our imaginary spacecraft, freed from the Earth, would be moving like the Earth around the Sun at about 30 km/sec. The only way for it to reach the Sun is to somehow kill that velocity--for instance, by a rocket imparting 30 km/s in the opposite direction; if that were done, the spacecraft would be pulled in by the Sun. The people who propose sending nuclear waste by rocket into the Sun do not seem to know much about orbits!
Considering the great difficulty in giving a spacecraft even the 8 km/s required for a low Earth orbit, the great rocket power required for reaching distant planets is a serious obstacle. Luckily, one can often take advantage of planetary gravity-assist maneuvers, as discussed in the next regular section.

Questions from Users:   Why is it so hard to reach the Sun?
                Also asked:  Why are satellites launched eastwards?;
        also answers:  What is a sun-synchronous orbit?
                        Why are satellites launched from near the equator?        
***      Point of gravity equilibrium

Calculation:       #34a The Distance to the L1 Point
Next Regular Stop: #35 To the Planets, to the Stars
            Timeline                     Glossary                     Back to the Master List

Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   stargaze("at" symbol)phy6.org .

Last updated: 9-24-2004
Re-formatted 27 March 2006

SPACE EXPLORATION MERIT BADGE

Galileo Launch from Orbit How Orbits Work

What an Orbit Really Is

Orbit DiagramThe drawings at the right simplify the physics of orbiting Earth. We see Earth with a huge, tall mountain rising from it. The mountain, as Isaac Newton first envisioned, has a cannon at the top. When the cannon is fired, the cannonball follows its ballistic arc, falling as a result of Earth's gravity, and it hits Earth some distance away from the mountain. If we put more gunpowder in the cannon, the next time it's fired, the cannonball goes halfway around the planet before it hits the ground. With still more gunpowder, the cannonball goes so far that it never touches down at all. It falls completely around Earth. It has achieved orbit.
If you were riding along with the cannonball, you would feel as if you were falling. The condition is called free fall. You'd find yourself falling at the same rate as the cannonball, which would appear to be floating there (falling) beside you. You'd never hit the ground. Notice that the cannonball has not escaped Earth's gravity, which is very much present -- it is causing the mass to fall. It just happens to be balanced out by the speed provided by the cannon.

Getting Into Orbit

The cannonball provides us with a pretty good analogy. It makes it clear that to get a spacecraft into orbit you need to
  • Raise It Up (the mountain) to a high enough altitude so that Earth's atmosphere isn't going to slow it down too much. In practical terms you don't generally want to be less than about 100 miles above the surface of the Earth. At that altitude, the atmosphere is so thin that it doesn't present much frictional drag to slow you down.
  • Accelerate It until it is going so fast that as it falls, it just falls completely around the planet.The required speed for a particular altitude A can be found from the formula
    v = 1,113,263/sqrt(3963 + A)where A is in miles and v comes out in miles per hour. So for example the shuttle, orbiting at 200 miles up travels at
    v = 1,113,263/sqrt(3963 + 200) = 17,254 mphAt that speed, it takes about 90 minutes to complete one orbit (an hour and a half to go all the way around the Earth!).

    If we place a satellite way up - at an altitude of 22,284 miles, then to stay in orbit, the satellite should travel at
    v = 1,113,263/sqrt(3963 + 22,284) = 6872 mphAt that speed, you can show that it takes 24 hours to orbit the Earth. But since the Earth is rotating once every 24 hours, the satellite is going around the Earth at the same exact rate that the Earth is turning. The satellite stays above the same point on the Earth, or looking at it from the Earth's surface, the satellite stays in the same place in the sky. This is called a "geostationary" orbit, since the satellite seems to be stationary - it looks like it doesn't move! This is great if you have to point your satellite dish to pick up a signal from this satellite. Point it once and you're done.
    Geostationary Orbit

Apogee Kick

How does a satellite get from low earth orbit (where the shuttle lets go of it) to geosynchronous orbit?
  • Elliptical Orbits: most orbits are not perfectly circular. All orbits are ellipses (flattened circles) with a high point (apogee) and a low point (perigee).
    • At apogee, when the satellite is farthest from the earth, it is going the slowest - it's ready to fall back toward the earth.
    • As the satellite falls it gains speed, and "overshoots" the earth, swinging quickly through perigee, then gaining altitude back toward apogee.
    • The satellite doesn't stay in orbit at the apogee distance because it isn't going fast enough when it reaches that point. It doesn't stay in orbit at the perigee distance because it's picked up so much speed by that point that it starts climbing again.
Elliptical Orbit
  • Transfer Orbit:
    • If we speed the satellite up while it's in low circular earth orbit it will go into elliptical orbit, heading up to apogee.
    • If we do nothing else, it will stay in this elliptical orbit, going from apogee to perigee and back again.
    • BUT, if we fire a rocket motor when the satellite's at apogee, and speed it up to the required circular orbit speed, it will stay at that altitude in circular orbit. Firing a rocket motor at apogee is called "apogee kick", and the motor is called the "apogee kick motor".
Apogee Kick

 Gimme More! Orbital Mechanics Web Page  an outstanding reference!
 Back to Space Exploration Home Page

Questions

Your questions and comments regarding this page are welcome. You can e-mail Randy Culp for inquiries, suggestions, new ideas or just to chat.
Updated 20 March 2004

Space Environment

How do objects in space travel?


What is an orbit?

An orbit is a regular, repeating path that an object in space takes around another one. An object in an orbit is called a satellite. A satellite can be natural, like the moon, or human (or extraterrestrial?) -made.

(This drawing is not a scale drawing)

In our solar system, the Earth orbits the Sun, as do the other eight planets. They all travel on or near the orbital plane, an imaginary disk-shaped surface in space. All of the orbits are circular or elliptical in their shape. In addition to the planets' orbits, many planets have moons which are in orbit around them.

 What causes an orbit to happen?
What is a satellite?
 What travels in an orbit?
 How do we put a spacecraft into orbit?
 Once a ship is in orbit, do we have to do anything to keep it there?


 How is an ellipse different from a circle?
 Are there orbits within orbits?
 What is the orbital plane?
 What are some kinds of orbits?

 What are the orbital lengths and distances of objects in our solar system?
 What could cause an orbit to fail?
 How do spacecraft use an orbit to move from planet to planet?