Thursday, December 16, 2010
If you put a long boom on a satellite, it will align to the "local vertical". This was first demonstrated on the GOES 3 probe in 1978. It works because the forces of gravity and the "centrifugal force" balance at the center of mass.
Today, long life space tethers are available, using existing materials they are light enough for long lengths to be launched on a cheap launch vehicle like SpaceX's Falcon 9. A length of 500 km, with a lifetime of 10 years, would have mass of just 1275 kg. The tether would stretch from the altitude of the International Space Station (~340 km) up past the altitude of the Hubble Space Telescope (~595 km), out to where the polar orbiting satellites do their job (~700 km+). Within equal masses on each end of the tether, a force of 0.1g would be experienced, with no rotation of the structure required.
Of course, this only works while in orbit around a planetary body. Around the Sun, say for generating artificial gravity for a trip to Mars, a much longer tether would be required and it quickly becomes infeasible. So what good is it? Currently, we have very little data on how the human body responds to partial gravity. Sure, we know lots about how it responds to 1g, and we have some ideas about how it responds to "zero g", but we don't have any data at all about anything in-between. 0.1g is just enough gravity for a human to stand upright and walk around (although Joe Carroll recently suggested that 0.06g was sufficient). If we could put some astronauts on a station with this level of partial gravity for a few months it would give us some vital data for determining whether people can actually live on the Moon or Mars or even asteroids for long periods of time without serious health issues.
Having the low end of the tether at the same orbit as the ISS, and having it not rotating around at high speed would make getting to the experiment very convenient. The high end of the tether could simply be the upper stage of the launch vehicle. SpaceX has already demonstrated that their second stage can be relighted, and flying to a circular orbit of 800km seems well within their capabilities. A 500km tether reel could fit in the trunk of the Dragon, which can remain connected to the second stage after separation.
Considering the availability and low cost of the hardware, the low risk of non-spinning artificial gravity, and the massive scientific payoff, NASA should be pursing this.. now.