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Have your ion ISP and eat Oberth cake too.

21 May 2015, 00:37 UTC
Have your ion ISP and eat Oberth cake too.
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Early versions of the Asteroid Redirect Mission suggested retrieving an asteroid in heliocentric orbit and parking it in a lunar Distant Retrograde Orbit (DRO).DROs can be stable for centuries without station keeping. Once we park an asteroid in such an orbit, it's not going anywhere. DROs can go about as high as 40,000 km. Once we get higher than about 3/5 of a Hill Sphere radius, the orbits are no longer stable.The proposed asteroid retrieval vehicle would be powered by solar arrays and propelled by Hall Thrusters using xenon as reaction mass.A 500 tonne rock in a DRO along with solar panels and Hall Thrusters would make a great anchor for a tether. It would also serve as a momentum bank.Hall Thrusters could build the tether's momentum gradually. But catching or releasing a payload is a sudden event equivalent to an impulsive burn from a high thrust chemical rocket.Here is one possible vertical tether whose foot is 300 km above the lunar surface:This ~6000 km tether is a lot shorter than a full blown Clarke tower. The moon's gravity well is also shallower than earth's. So this tether endures much less stress. Kevlar would suffice for the tether material and ...

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