The Case for Mars (57 page)

Perigee:
The lowest point in an orbit around a planet.

Pyrolyze:
The use of heat to split a compound into its elemental constituents.

Regolith:
What most commonly refer to as dirt.

rem:
The measure of radiation dose most commonly used in the United States. One hundred rems equals one Sievert, the European unit. It is estimated that radiation doses of about 60 to 80 rem are sufficient to increase a person’s probability of fatal cancer at some time later in life by 1 percent. Typical background radiation on Earth is about 0.2 rem/year.

RWGS:
Reverse water-gas shift reaction.

RIG:
Radioisotope thermoelectric generator.

Sabatier reaction:
A reaction in which hydrogen and carbon dioxide are combined to produce methane and water. The Sabatier reaction is exothermic, with aypicalquilibrium constant (see above).

Saturn V:
The heavy-lift launch vehicle used to send the Apollo astronauts to the Moon. The Saturn V could lift about 140 tonnes to LEO.

SEI:
Space Exploration Initiative.

SNC meteorites:
Named for the locations where the first three were found (Shergotty, Nakhla, and Chassigny), SNC meteorites are believed on the basis of very strong chemical, geologic, and isotopic evidence to be debris thrown off of Mars by impacting meteorites.

Sol:
One Martian day; 24.6 hours long.

Solar flare:
A sudden eruption on the surface of the Sun that can deliver immense amounts of radiation across vast stretches of space.

SPE:
Solid polymer electrolyte.

Specific impulse:
The specific impulse of a rocket engine is the number of seconds it can make a pound of propellant deliver a pound of thrust. If you multiply the specific impulse of a rocket engine, given in seconds, by 9.8, you will obtain the engine’s exhaust velocity in units of meters/second. Specific impulse is generally viewed as the most important factor in judging a rocket engine’s performance. Frequently abbreviated “Isp.”

SRB:
Solid rocket booster.

SSME:
Space Shuttle main engine.

SSTO:
Single-stage-to-orbit.

Stable equilibrium:
An equilibrium condition, which, if displaced by some external force, will return on its own to its original state. A ball on top of a hill is in unstable equilibrium, because if pushed in either direction it will roll away, accelerating itself from its original position. A ball on a flat surface in the bottom of a bowl is in stable equilibrium, because if pushed, it will roll back to its starting point.

STR:
Solar thermal rocket.

Telerobotic operation:
Remote control of some device, such as a small Mars rover equipped with TV cameras, by human operators at a significant distance away.

Thrust:
The amount of force a rocket engine can exert to accelerate a spacecraft.

Titan IV:
An expendable launch vehicle manufactured by the Lockheed Martin Corporation capable of delivering 20,000 kg to LEO or 5,000 kg to a minimum energy trans-Mars trajectory.

TMI:
Trans-Mars injection, a maneuver which places a payload or spacecraft on a trajectory to Mars.

TW:
Terrawatt, one terrawatt equals 1,000,000 megawatts. Human civilization today uses about 13 TW

TW-year:
The total amount of energy associated with the use of one terrawatt for one year.

Unstable equilibnum: See
stable equilibrium, above.

Vapor pressure:
The pressure exerted by the gas emitted by a substance at a certain temperature. At 100°C, the vapor pressure of water is greater than the Earth’s atmospheric pressure and so it will boil.

W/kg:
watts per kilogram.

NOTES

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4.
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23.
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24.
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25.
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26.
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27.
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31.
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32.
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36.
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37.
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38.
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