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Voyager Humankind has been exploring space for four decades, and in that time our reach has extended throughout the solar system with the use of unmanned probes. A spacecraft in the making, "Pluto Express," will end the planetary saga for such probes. But, what about manned missions to Mars, Jupiter, and so forth? What about the mysteries that lay beyond? What about the termination shock of the heliosphere (200 AU; for reference, Pluto is ~40 AU from the Sun), where ionized particles can glean clues to the origins of the galaxy and ultimately the universe? What about the Oort Cloud (10,000 AU), filled with countless comets and other clumps of matter? Finally, what about the exploration of other solar systems?

X-33 These issues are being addressed by the NASA Advanced Space Transportation Program (ASTP), which is currently investigating new ways to propel a unmanned spacecraft to Alpha Centauri in the span of a human lifetime of 50 years. It is the same program that is planning a manned mission to Mars. Both tasks suffer the same dilemma: chemical propellants simply will not work. For the first case, chemical propellants lack the energy needed to boost a space probe up to 10% the speed of light. The overall mass of such a booster would be unthinkable. For the latter case, the spacecraft only needs to obtain the velocity necessary to get to Mars within 3-6 months; however, the mass of a manned payload once again places a burden on the size of the booster engine.

Many concepts have been devised. For years, scientists have suggested nuclear fission as an alternative approach for sending a manned spacecraft to Mars. Although the specific impulse (Isp) is still too low for interstellar missions, it does open new avenues near the vicinity of Earth. Unfortunately, environmental issues have all but "grounded" the use of nuclear fission as a propulsion source. Nuclear fusion is cleaner, and it is a more exciting prospect with its higher energy density and specific impulse. However, scientists are still developing such a device that offers beyond break-even energy (more energy output than input), let alone making the same device small enough to be sent into deep space. Last, electric propulsion, as used for Deep Space I, cannot accelerate a spacecraft fast enough for the tasks mentioned above due to its low thrust-to-weight ratio.

The Sun Laser and solar sails have the attractiveness of providing a virtually engineless craft; photons simply provide the momentum necessary to send the spacecraft to its proper destination. But these, too, have their share of disadvantages: pointing accuracy of laser-driven probes, low intensity of solar light outside the solar system, ultra-low payload mass, etc.

It is here that antimatter addresses attention. Upon annihilation with matter, antimatter offers the highest energy density of any material currently found on Earth. As shown in the table1,2 below, this indicates that antimatter offers the greatest specific impulse of any propellant currently available or in development, and its thrust-to-weight ratio is still comparable with that of chemical propulsion. Simply put, it would take only 100 milligrams of antimatter to equal the propulsive energy of the Space Shuttle.

Propulsion Type Specific Impulse [sec] Thrust-to-Weight Ratio
Chemical Bipropellant 200 - 410 .1 - 10
Electromagnetic 1200 - 5000 10-4 - 10-3
Nuclear Fission 500 - 3000 .01 - 10
Nuclear Fusion 10+4 - 10+5 10-5 - 10-2
Antimatter Annihilation 10+3 - 10+6 10-3 - 1

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This page was last modified on February 27, 2001.