The Case for Mars (56 page)

Ballistic NIMF. (Artwork Robert Murray, courtesy Lockheed Martin)

NIMF
Rocketplane. (Artwork Robert Murray, courtesy Lockheed Martin)

NIMFs—Nuclear Rockets using Indigenous Martian Fuels—as rocketplanes or as ballistic spacecraft would afford Mars explorers and later colonists unlimited mobility on a planetary scale.

Exploration team on a partially terraformed Mars. (Artwork by Michael Carroll)

The New Created World. (Artwork by Michael Carroll)

Liquid water once coursed over the face of Mars and, given the technological capabilities of the twenty-first century, it may once again. Several decades of terraforming could transform Mars into a relatively warm and slightly moist planet suitable some day for explorers without space-suits, although breathing gear would still be required. Returning oceans to Mars is actually a possibility for the distant future.

GLOSSARY

Aerobraking:
A spacecraft maneuver using friction with a planetary atmosphere to decelerate from an interplanetary orbit to one about a planet.

Aeroshell:
A heat shield used to protect a spacecraft from atmospheric heating during aerobraking.

Apogee:
The highest point in an orbit about a planet.

Atmospheric pressure:
The pressure an atmosphere exerts. On Earth at sea level, the atmospheric pressure is 14.7 pounds per square inch. This amount of pressure is therefore known as one “atmosphere” or one “bar.”

BEIR:
Biological effects of ionizing radiation.

Bipropellant:
A rocket propellant combination including both a fuel and oxidizer. Examples include methane/oxygen, hydrogen/oxygen, kerosene/hydrogen-peroxide, etc.

Buffer gas:
An effectively inert gas that is used to dilute the oxygen required to support breathing or combustion. On Earth, the 80 percent nitrogen found in air serves as a buffer gas.

Conjunctionem> The position of a planet behind the Sun as seen from another planet. When Earth and Mars are in conjunction, they are on opposite sides of the Sun.

Conjunction mission:
A mission that flies about half of the way around the Sun to travel from one planet to another. Conjunction missions have the lowest propulsion requirements.

Cosmic ray:
A particle, such as an atomic nucleus, traveling through space at very high velocity. Cosmic rays originate outside of our solar system. They typically have energies of billions of volts and require meters of solid shielding to stop.

Cryogenic:
Ultra-cold. Liquid oxygen and hydrogen are both cryogenic fluids as they require temperatures of-180° and -250°C, respectively, for storage.

Delta 2:
An expendable launch vehicle manufactured by McDonell Douglas, capable of throwing 1,000 kg on a direct trajectory from Earth to Mars.

Delta-V
(also written ΔV): The velocity change required to move a spacecraft from one orbit to another. A typical delta-V required to go from low Earth orbit to a trans-Mars trajectory would be about 4 km/s.

Departure velocity:
The velocity of a spacecraft relative to a planet after effectively leaving the planet’s gravitational field. Also known as hyperbolic velocity.

Direct entry:
A maneuver in which a spacecraft enters a planet’s atmosphere and uses it to decelerate and land without going into orbit.

Direct launch:
A maneuver in which a spacecraft is launched directly from one planet to another without being assembled in orbit.

Electrolysis:
The use of electricity to split a chemical compound into its elemental components. Electrolysis of water splits it into hydrogen and oxygen.

Electron density:
The number of electrons per cubic centimeter. The higher the electron density of an ionosphere, the better it reflects radio waves.

Endothermic:
A chemical reaction requiring the addition of energy to occur.

Epicycle:
A small circle whose center travels along the path of a larger circle. Ancient and medieval astronomers described the motion of the planets by envisioning that each planet traveled in a circle—the epicycle—whose center moved along a larger circle centered on the Earth.

Equilibnum constant:
A number that characterizes the degree to which a chemical reaction will proceed to completion. A very high equilibrium constant implies near complete reaction.

ERV:
Earth return vehicle.

ET:
External tank.

EVA:
Extravehicular activity.

Exhaust velocity:
The speed of the gases emitted from a rocket nozzle.

Exothermic:
A chemical reaction that releases energy when it occurs.

Fairing:
The protective streamlined shell containing a payload that sits on top of a launch vehicle.

Fast conjunction mission:
A conjunction-type mission (see above) inwhich some extra propellant is used to shorten the flight time.

Free-return trajectory:
A trajectory which, after departing Earth, will eventually return to the Earth without any additional propulsive maneuvers.

GCMS:
Gas chromatograph mass spectrometer.

Geothermal energy:
Energy produced by using naturally hot underground materials to heat a fluid, which can then be expanded in a turbine-generator to produce electricity.

Gravity assist:
A maneuver in which a spacecraft flying by a planet uses that planet’s gravity to create a slingshot effect which adds to the spacecraft’s velocity without any requirement for the use of rocket propellant.

Heliocentric:
Centered about the Sun. A heliocentric orbit is one that trans-verses interplanetary space and is not bound to the Earth or any other planet.

Hohmann transfer orbit:
An elliptical orbit, one of whose ends is tangent to the orbit of the planet of departure, and whose other end is tangent to the orbit of the planet of destination. The Hohmann transfer orbit is the purest incarnation of the conjunction-class orbit, and as such is the lowest-energy path from one planet to another.

Hydrazine:
A rocket propellant whose formula is N
₂
H
₄
. Hydrazine is a mono-propellant, which means that it can release energy by decomposing, without any additional oxidizer required for combustion.

Hyperbolic velocity:
The velocity of a spacecraft relative to a planet before entering, or after effectively leaving, the planet’s gravitational field. Also known as approach or departure velocity.

Hypersonic:
A speed many times the speed of sound; in common usage Mach 5 or greater.

Ionosphere:
The upper layer of a planet’s atmosphere in which a significant fraction of the gas atoms have split into free positively charged ions and negatively charged electrons. Because of the presence of freely moving charged particles, an ionosphere can reflect radio waves.

Isp:
A commonly used abbreviation for specific impulse (see below).

ISPP:
In-situ propellant production.

JSC:
Johnson Space Center, Houston, Texas.

kb/s:
kilobits per second.

Kelvin degrees:
The Kelvin or “absolute” scale is a method of measuring temperature which starts with its zero point set at “absolute zero,” the temperature at which a body in fact possesses no heat. Thus, 273 degrees Kelvin is the same temperature as 0° centigrade, the freezing point of water. Each additional degree Kelvin corresponds to one additional degree centigrade.

kHz:
kilohertz, a measure of frequency used in radio. One kHz equals 1,000 cycles per second.

km/s:
kilometers per second.

kW:
kilowatts.

kWe:
kilowatts of electricity.

kWe-hr:
The total amount of energy associated with the use of one kilowatt of electriciy for one hour.

kWh:
The total amount of energy associated with the use of one kilowatt for one hour.

LEO:
Low Earth orbit.

LOR:
Lunar orbit rendezvous.

LOX:
Liquid oxygen.

MAV:
Mars ascent vehicle.

Methanation reaction:
A chemical reaction forming methane. In the Mars Direct mission, the methanation reaction is the Sabatier reaction in which hydrogen is combined with carbon dioxide to produce methane and water.

MHz:
megahertz, a measure of frequency used in radio. One MHz equals 1,000,000 cycles per second.

millirem:
1/1,000th of a rem (see below).

Minimum energy trajectory:
The trajectory between two planets requiring the least amount of rocket propellant to attain (see Hohmann transfer).

m/s:
meters per second.

MOR:
Mars orbit rendezvous.

MSR:
Mars sample return.

MSR-ISPP:
Mars sample return employing in-situ propellant production.

MWe:
Megawatts of electricity.

MWt:
Megawatts of heat. One megawatt equals 1,000 kilowatts.

NEP:
Nuclear electric propulsion.

NIMF:
Nuclear rocket using indigenous Martian fuel.

NTR:
Nuclear thermal rocket.

Opposition:
The position of a planet in the opposite direction from the Sun as seen from another planet. When Earth and Mars are in opposition, they are on the same side of the Sun, and thus closest to each other.

Opposition mission:
A mission that flies most or all of the way around the Sun (~360 degrees) to travel from one planet to another, swinging into the inner solar system in the process in order to increase speeds. Opposition missions have the highest propulsion requirements.