NASA, the US Department of Defense, and other agencies are considering near-future applications of this technology. Perhaps the earliest application will be orbit transfer.

Assume that you've paid the launch cost to put a satellite into low Earth orbit a few hundred kilometers above Earth's surface and you wish to raise its orbital height to a geosynchronous altitude, about 36,000 kilometers above Earth's surface.

You could of course do this with a conventional upper stage using chemical propulsion, but a hydrogen-expelling solar-thermal rocket has about twice the efficiency of the best chemical rocket. So there is a significant economy in developing a solar-thermal tug to loft payloads between low Earth and geosynchronous orbits. This economy will be of interest to developers of communication, navigation, and Earth-viewing satellites.

But, as generally happens in the space business, there are trade-offs. A chemical rocket is a high-thrust device and the orbit transfer can be accomplished within hours or, at most, a few days. Although a solar-thermal rocket has a higher thrust than a solar-electric rocket, the orbit-

transfer time between low Earth orbit and geosynchronous orbit is considerably longer than for a chemical rocket.

Orbital-transfer tugs based upon the solar-thermal rocket (or the solar-electric rocket) are certainly feasible. But payloads on board these tugs will require more shielding during their transfer through Earth's Van Allen radiation belts than payloads on board chemical rocket orbital-transfer tugs due to the fact that they will be spending more time in the radiation belts, increasing the total radiation exposure and potentially causing more radiation-induced damage.

At least two flight tests of the technology were proposed in the 1990s. The Shooting Star Experiment was to have launched from the space shuttle orbiter and demonstrate the fundamentals of the technology. An artist concept for the Shooting Star is shown in Figure 12.2. Boeing, working with the US Air Force, was tasked to develop a space tug using solar-thermal propulsion called the "Solar Orbital Transfer Vehicle (SOTV).'' Neither the Shooting Star Experiment nor the SOTV flew.

Thermal Propulsion

FIGURE 12.2 Artist's concept of NASA's shooting star experiment. (Courtesy NASA)

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