[X] My Assistant

[Bart]

Hi, I’ve been lurking here for a while, but this is my first post. The recent talk on the Uranus & Neptune orbiter forums about Galileo- vs. Cassini-style tours of the moons of the outer planets reminded me of an idea that I had a while ago.

Would it be possible to arrange a Cassini-style trajectory in the Earth-Moon system that would allow periodic visits of NEOs as they passed by? If you limited yourself to visits within 2-3 million km, you would have about one opportunity per year.

Lunar gravity assists should provide enough delta-v for the necessary plane changes, and the flyby orbits would have periods of ~100 days, allowing for a few assists between flybys.

If you could do close flybys of 5 different NEOs over 5 years without going more than 3 million km from Earth, that would seem like an ideal candidate for a Discovery mission.

The potential problem I see with this idea is that Earth’s Hill sphere is only ~1.5 million km, so you’d be spending a good part of each rev orbiting the Sun instead of the Earth. Would it be prohibitive either in terms of time or delta-v to get a spacecraft from a solar orbit 3 million km from Earth to a lunar return trajectory?

[Bob Shaw]

Bart:

Earth-Moon slingshots and peculiar orbits seem to be something of a neglected art - though it has (once) happened. About ten years ago a comsat was left in the wrong transfer orbit and - believe it or not - it's remaining fuel was largely used to perform a Lunar flyby, with the result that it was actually perturbed into a useful orbit! I can't remember the exact satellite name, though it may have been one of the Galaxy series - and yes, when I first heard about it I had some difficulty in believing it at all, as it appeared to be something from the plot of a naive SF TV show.

So, I think the answer to your question is 'probably'. There are a range of funny orbits, libration points and the rest, and you'd think that a bit of creative number crunching might well show up a few good 'uns.

Bob Shaw

[djellison]

iirc Genesis used lunar flybys to line up for re-entry, and Contour used them I believe. Stereo will use them to send the two spacecraft in opposite directions as well.

Doug

[tasp]

ICE used several lunar encounters to reach comet Giacobini-Zinner.

Another class of mission trajectories might go as follows:

Launch from earth in an exact 2 year orbit. At aphelion, you would encounter a convenient asteroid or two, and then fall back towards earth.

At earth encounter, pump up orbit to exactly 4 years (or 3, I don't care) and go further into asteroid belt for another asteroid encounter or two.

Then fall back to earth and pump up the orbit again. Now you are able to fly through the asteroid belt and see a another rock or two, and then continue out to a leading Jupiter Trojan asteroid.

Keep up these encounters till the craft expires, NASA zorches the funding, or all the principle investigators go on Social Security.

You could probably nudge the inclination a little, or pump the period every time you come back 'round earth.

Might even want to catch an outer satellite of Jupiter for comparison to the Trojan asteroid . . . . .

[JTN]

About ten years ago a comsat was left in the wrong transfer orbit and - believe it or not - it's remaining fuel was largely used to perform a Lunar flyby, with the result that it was actually perturbed into a useful  orbit! I can't remember the exact satellite name, though it may have been one of the Galaxy series - and yes, when I first heard about it I had some difficulty in believing it at all, as it appeared to be something from the plot of a naive SF TV show.

It had different names at different times, but one was HGS-1.

[Phil Stooke]

I looked into this lunar gravity assist question. The communication satellite was Asiasat 3, named HGS 1 after it was salvaged by Hughes. Its lunar flybys were on 13 May 1998 and 7 June 1998.

I didn't find anything about Genesis doing a lunar flyby to set up its re-entry - it did an Earth flyby, certainly. Possibly it did a lunar flyby initially to get out to L1?

That idea of using lunar flybys to get to a Lagrange point was used by WMAP on 30 July 2001. It was done in reverse by ISEE-3 (later renamed ICE, the comet probe) on five occasions, the last on 22 December 1983, to transition from an L1 halo orbit to its comet trajectory. Also, Geotail and Wind, two solar wind probes, used repeated lunar flybys to keep their apogees on the desired side of Earth throughout the year (Geotail on the night side, Wind on the day side).

Phil

[Bob Shaw]

More on the Asiasat 3/HGS 1 Lunar flyby:

http://www.spacedaily.com/news/Book_Reveal...at_In_1997.html

Bob Shaw