In article .com,
"David Summers" wrote:

The real disadvantage to using microwaves is the spot size on the
spacecraft. Jordin Kare's analysis showed that once the spacecraft was
most of the way to orbit, an optical system would only be delivering
about half of the energy output onto the vehicle. With microwaves,
that would be much worse.

There are probably ways around this though. Perhaps using some
material with a negative permeability could focus more tightly. Those
materials exist for microwaves, but are challenging for optical
wavelengths.

Another possible approach would be to hand off powering of the craft to
an orbital source. This could probably be located in LEO, so it
wouldn't need to have an enormous antenna. But it would itself need a
pretty massive power source -- either a huge solar array and well-timed
launches, or a smaller array with a large power storage system (perhaps
ultracapacitors). Certainly not cheap infrastructure, but if it's the
sort of thing that enables low-cost, high-rate launches, maybe it'd be
worth it.

- Joe

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