From the Moon to the Earth

August 17, 2006 12:33 PM

Hypothetical from Moon-orbit to Earth-orbit question. I already know about free return trajectories, but what if something went wrong and your ship (currently orbiting the moon) didn't leave lunar orbit at the point it was supposed to? What might happen?

Let's say for sake or argument that there was a technical problem which kept the spacecraft from exiting Lunar Orbit (to return to earth) at the specific time or location it was supposed to. Can some smart people help me out figure out several ways this mistake might affect the return trip home?

Is it something that could be rectified on the way back to earth, with no real issues? Or would it require landing somewhere different than where you had originally planned? Would it increase the risk of coming into earth orbit at a dangerous angle and/or speed?

Help me think of any and every worst case scenario for this scenario. Thanks!

posted by np312 to technology (8 comments total)

Whoops-- I meant "theoretical," not "hypothetical." Sorry.
posted by np312 at 12:38 PM on August 17, 2006

So by "didn't leave at the point it was supposed to" you mean the ship left at a different point, or just plain didn't leave? If it didn't leave, seems like you'd just wait for the next go-around and try again. If you left at the incorrect orbit, you'd have to do a course correcting burn sometime on your way to earth, and there's probably a point-of-no-return beyond which the capabilities of your burns cannot compensate for your mistake. So, the sooner corrected, the better.
posted by odinsdream at 12:45 PM on August 17, 2006

I already know about free return trajectories

Are you sure? The whole point is that there is no "exit" of "Lunar orbit" because the spacecraft never actually orbits the moon.
posted by vacapinta at 12:46 PM on August 17, 2006

The Moon has almost no atmosphere to slow you down. So if you're headed there from Earth, there are a few choices:

-- you don't hit it and you don't burn fuel to slow down = you slingshot around it and come back
-- you hit it = will have to burn much fuel to return to Earth
--you burn a little fuel to slow down and settle into a Moon orbit = you'd have to burn roughly the same amount of fuel to speed up, leave the Moon's orbit, and return to Earth

If you had unlimited air (by which I mean, "time"), and for some reason things were screwed up and you didn't have as much fuel as planned, you could time your engine burns as you orbited the Moon to get maximum effect, sort of like pumping a playground swing. Each time you go around, you burn just a bit more. Your orbit around the Moon gets more and more elliptical. Eventually you do a bit of a lunar slingshot and head for Earth.

If you have plenty of fuel but you just missed departing lunar orbit on a particular go-round, probably doing it the next time around doesn't make much difference. The Apollo flights, for example - they had a module in lunar orbit and another that went down to the Moon. When they packed up and left, it didn't matter exactly which revolution of the Moon that they actually did the burn to start heading back to Earth.

If you did a burn that was WRONG, ill-timed or whatever, then you're in deep trouble. if you break away from lunar orbit but not headed toward Earth, you could end up anywhere, which pretty much means dead.
posted by jellicle at 1:05 PM on August 17, 2006

This was discussed in layman's terms in Ron Howard's Apollo 13 (and I assume also in Jim Lovell's book upon which the movie was based).

Basically, If the space craft doesn't hit the Earth at the right angle, it either plummets into the atmosphere and burns up or skips right past the planet into deep space where the occupants die a cold, lonesome death. The Apollo astronauts solved this problem when it occured for them by reigniting their maneuvering boosters and manually aiming for the Earth. Obviously, their aim was true, but it certainly doesn't seem like an easy feat. To be honest, I'd expect that today's astronauts (with less piloting experience and a higher dependence on computers) wouldn't pull it off quite as well in the same circumstances.
posted by Terminal Verbosity at 1:05 PM on August 17, 2006

The range of initial lunar orbital accelerations conditions which suffice to produce a small window of acceptable Earth re-entry conditions is vanishingly small. As in tenths of a second of burn from small rocket motors, on the lunar orbit acceleration end. It was so difficult to do this right, that every Apollo mission had planned course corrections on the return flights, in order that they not rely on a single, critical lunar orbit burn (which would have been the most fuel efficient strategy) to return.

Any acceleration from lunar orbit that doesn't place you pretty close to the optimum Earth return path has some finite probability of:

A) Making you a higher orbit satellite of the moon, awaiting orbital decay at some later date, or
B) Breaking you loose from the Earth-Moon 2 body gravitational system, for an infinite tour of cold, dark interplanetary, or even interstellar space, or
C) Making you high Earth orbit piece of space junk, or
D) Making you a pretty meteor for a few tenths of a second, burning up in Earth's atmosphere and/or crashing into Earth's surface, or
E) Sending you into a slowly or quickly decaying orbit of the Sun, where you eventually crash into the Sun itself, or enjoy the vanishingly low probability events of being captured as a satellite of Venus or Mercury, or crashing into them as a final destination.

Actually, the list of possible fates for a lunar return burn gone wrong is probably infinitely God awful, including getting whacked by asteroids, becoming a satellite of outer planets, etc. etc. The point is, there are nearly infinitely more ways for things to go wrong, than go right on the maneuver.
posted by paulsc at 1:16 PM on August 17, 2006

Thanks for answers so far everyone! However, if paulsc or someone else could please explain/expound a little on this interesting quote from above: "The range of initial lunar orbital accelerations conditions which suffice to produce a small window of acceptable Earth re-entry conditions is vanishingly small. As in tenths of a second of burn from small rocket motors, on the lunar orbit acceleration end."

Am I correct in gleaning that correcting your course would be something you'd have to do while still in lunar orbit or in transit to Earth, but that if you did manage to basically reach earth atmosphere somehow, at that point there would be nothing you could do to make sure your angle was okay for re-entry? That all your highly minute course-adjusting and angle/fixing would have had to be done BEFORE you got close to home?

I'm working on a story where a spacecraft in lunar orbit leaves for earth at slightly the wrong time, which means it gets back to earth but then there is a scramble to find the right course trajectory, angle of rentry, etc for their new flight path to keep them from burning up in/bouncing off the atmosphere.
posted by np312 at 1:44 PM on August 17, 2006

A description of the Apollo CSM from Wikipedia could be instructive, as would the Apollo Lunar Mission Tutorial. The Apollo system was fairly complicated, trying at every stage of the mission to dump mass that wouldn't be needed subsequently, to conserve fuel. For standard rocketry on a single vehicle mission, this was a pretty important flight constraint. If you can't gas up along the route, all you've got to get you back is what you had when you left. So, that's something to bear in mind.

If your story technology doesn't have that huge operational limitation, because you've got huge amounts of energy from nuclear rockets on tap at all times, or because you can warp mass arbitrarily by some advanced technology, the importance of when and how you do your burns becomes significantly less. You only need Apollo's efficiencies if you have to pay for every pound of reaction mass you'll need to return, with 100 pounds of take off thrust.

In Apollo's case, they generally tried to make a short return initiating burn "behind" the moon from Earth view, which brought the Command Service Module up past lunar orbit speed, but still below the needed return path optimal speed. This was because they couldn't know the vehicle mass very accurately (how many pounds of lunar rocks were coming back, etc.), and so they just do an approximate burn on the low side of calculations to get started, and then do a precision radar fix of the vehicle from Earth to get speed and position as it left lunar orbit. From that, they could get a really accurate mass of the vehicle, and calculate a/some mid-course correction(s), which they would then do typically at the point in the return path calculated to require minimal fuel for the burn(s). A further requirement was to try to minimize the number of engine starts required for safe transit.

Finally, near the end of the return flight, they made another deceleration burn to inject the vehicle into direct Earth re-entry, which again was a fuel saving strategy, and a pretty gutsy move.

But again, you as a fiction writer can just wave your pen, and put some advanced propulsion on board your spacecraft, and your protagonists won't have to worry about economy, or be so clever. But if you need to do this for dramatic reasons, have 'em fly the typical Apollo profile, for white knuckle goodness.
posted by paulsc at 2:41 PM on August 17, 2006