V244 A

where D — diameter of the beaming mirror, d — diameter of the receiving mirror on the spacecraft, Str — Strehl ratio « 0.1, and A — laser wavelength [Eckel and Schall, 2008].

For instance, a CO2 laser (A — 10.6 mm), beamed by a 5m diameter mirror could be received by a 1 m diameter mirror at about 140 km, assuming a Strehl factor 0.5.

In space this range can be higher, since the Strehl ratio is close to 1. Chemical oxygen-iodine lasers (COIL), with their 1.3-mm wavelength, offer a range almost an order of magnitude longer. Free electron lasers (FEL) may have a range of wavelengths, but operate in the pulsed mode only: the continous wave (CW) or pulsed mode operation is an important issue, since it directly affects thrust.

Once received, the power can be used in a variety of propulsion strategies. A semi-empirical quantity, the ''coupling coefficient CT", expresses how much of the incident power is converted into thrust. CT depends on the particular strategy chosen to produce thrust from the power transmitted by the laser beam, and permits analysis of Moon-launching without bothering with the specifics of propulsion. If sufficiently large, or lasting, or both, laser power becomes thrust capable of lifting payload from the Moon and injecting it into orbit. Notice that small thrust lasers are still capable of accelerating (of course, thrust must be at least equal to the lunar weight), but the Rayleigh equation sets a crude distance and time limit on how long acceleration may last. In fact, if a0 is the acceleration (assumed constant) imparted to the craft, T is the thrust, V the lunar escape speed, t the escape time, neglecting gravity work for simplicity, it must be

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