Asteroids in LDROs as tether anchors

[Hop_David]

Early versions of the Asteroid Redirect Mission suggested parking a 500 tonne rock in a Distant Retrograde Orbit (DRO). Retrograde lunar orbits are stable up to about 40,000 km altitude. They can remain there centuries without station keeping.

A rock in retrograde lunar orbit could be an anchor for a vertical tether. It would also be a momentum bank.

In the Keck proposal, the rock is retrieved and parked with ion engines.

A tether with ion engines is a way to have your ion ISP and eat your Oberth cake too. An ion engine can gradually build up orbital momentum. Then a tether can do the equivalent of an impulsive burn via catching or throwing a payload.

Here's a pic of one possible tether:



When released from the foot, a payload will drop to the lunar surface and impact at 1.04 km/s. A soft landing would be somewhat easier than from a circular low lunar orbit (about 1.6 km/s). Rendezvous with the tether foot could be accomplished by launching at 1.04 km/s for a suborbital hop. Sadly, this is good for only for the lower latitudes. An orbit passing over the poles wouldn't be retrograde.

When released from the top, a payload will fly off in a hyperbolic orbit with regard to the moon. Vinf of this hyperbola is 1.62 km/s.

Many paths are possible depending on when the payload is released. The tether circles the moon each 7.16 hours. So over 7.16 hours, the tether goes through all the barycentric longitudes.

Here is a release when barycentric longitude is 129º:



From tether release to perigee is 51 hours. At perigee it's traveling about 10.9 km/s with regard to earth. This is a hyperbolic orbit with regard to earth with a Vinf of about .3 km/s. A healthy Oberth benefit could be enjoyed by doing a burn at this perigee.

Here the payload is released when the tether is at 0 degrees:



The 1.6 Vinf vector is nearly parallel with the moon's velocity vector so the 1.6 is added to the moon's 1 km/s. So velocity vector is about 2.6 km/s with regard to earth. Vinf is about 2.2 km/s with regard to earth.

Shortly after release, a spacecraft would be moving 1.96 km/s. A burn at this speed would enjoy some Oberth benefit and boost Vinf wrt moon.

Assuming Kevlar with 3600 Mpa tensile strength and density 1.44 g/cm³, this tether would have a taper ratio of 2.12.