Apollo Discussions > The Reality of Apollo

Lunar Orbit

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Not Myself:
How low can one go and still be "safe", where you don't have to worry about the perturbations causing you to form a new impact crater?

ka9q:
I'm not sure that any lunar orbit is indefinitely stable. Everything hits sooner or later. The perturbations change the orbital eccentricity over time until perilune happens to intersect the surface.

At low altitude, the mascons are the major perturbation. If you try to orbit higher to get away from them, the earth and sun become significant.

When you think about it, the only way that a lunar orbit can decay is for the eccentricity to change. Changing the semi-major axis (or mean motion) implies changes in orbital energy and angular momentum, and without an atmosphere to provide drag I can't think of any way to do that. But you can change the eccentricity while keeping the same semi-major axis (and orbital energy).

ka9q:
To illustrate just how significant the perturbation from the earth can be, consider a non-nominal lunar orbit insertion burn.

If the burn doesn't happen at all, you come back home on your free-return trajectory (assuming you were on one).

If you burn too long, you impact on the near side a half orbit later, just as you'd expect. The astronauts watched the clock, ready to push the manual stop button if the computer didn't stop the burn as it should.

If you burn just a little too short, you'll go into an elliptical lunar orbit with apolune on the near side. Again, just as you'd expect.

But as you decrease the burn time further, something interesting happens. Although the apolune continues to increase, the perilune also decreases. There is a range of burn times, all less than the nominal time, that can result in lunar impact within a single orbit. The cause of this decrease in perilune is the perturbation from earth's gravity at apolune on the near side. You're at a higher altitude, increasing the earth/lunar gravity ratio, and the orbital period also increases, so the earth tugs on you for a longer time.

Apollo had emergency "bailout" procedures in case a dangerously non-nominal burn had ever occurred.

Kiwi:

--- Quote from: Coelacanth on May 12, 2012, 05:10:01 PM ---How low can one go and still be "safe", where you don't have to worry about the perturbations causing you to form a new impact crater?
--- End quote ---

I was a bit confused over this post until I read Ka9q's reply, because I thought you were asking our honourable Master of Ceremonies, LunarOrbit, in a rather clever way but in the wrong part of the forum, how low you could stoop as a poster before he went ballistic and dug a hole to bury you in because of "perturbations" from other members.

Got it now.  Case closed!

LunarOrbit:
I've been stable for 36 years. No one has hit me (so far) despite all of my perturbations.