On Sun, 28 Jul 2002, Alex Fraser wrote:
> ERPS will not be able to take a picture of an object passing near the earth.
> Not a close up. You couldn't get the picture back to earth anyway, so what's
> the point?
Perhaps not ERPS itself, but there are people in the small-satellite
community who know how to do such things on less-than-NASA budgets. Not
trivial -- might take more than one try to get it right -- but possible.
> If you got close by some stroke of luck...
Not luck -- onboard propulsion, plus optical navigation.
> then how would you aim the camera?
Attitude control system. Probably reaction wheels, low-precision rate
sensors, and a star tracker.
> Oh radio control, is that what you are thinking? Then how are you
> going to aim the antenna?
Attitude control, plus possibly (depending on design) a movable antenna
mount.
> If you get away with a small antenna on the craft, then you need a good
> bit of gain on the terrestrial side (path loss!).
Correct. Big antenna dishes exist. For example, Stanford has a big
radio-astronomy dish -- it was used in the hunt for signals from Mars
Polar Lander.
> You of course would need to have
> antennas all the way around the planet as it seems to spin on a daily basis.
Continuous coverage is nice, but for a brief asteroid encounter, you can
plan the encounter so it's within view of your main ground station.
> Space based, can you rent airtime?
There are no space-based antennas at this time. However, you can rent
airtime on global ground-based antenna networks.
> just how would you actually
> move the hopefully stable machine into that orientation and keep it there?
> Motive force?
Reaction wheels. Commercially available for small spacecraft. Probably
need a custom design for something this small. A hassle but possible.
Cold-gas jets might be simpler for a brief mission.
> One kilogram? A box filled with voodoo?
No, filled with electronics. See ssdl.stanford.edu/cubesat for some idea
of what's possible with one kilogram these days.
Henry Spencer
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