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I'm reading Jim Lovell (Apollo 8 and 13) and Jeffrey Kluger's book Apollo 13, which is a fantastic read about a long past era I only have kindergarten memories of. On page 54 there is a paragraph that reads (emphasis mine):

Also, unlike the LOI burn, during TEI there would be no free-return slingshot to send the ship home in the event that the engine failed to light. If the hydrazine, the dimethylhydrazine, and nitrogen tetroxide did not mix and burn and discharge just so, Frank Borman, Jim Lovell, and Bill Anders would become permanent satellites of Earth's lunar satellite, expiring from suffocation in about a week and then continue to circle the moon, once every two hours, for hundreds — no thousands; no, millions — of years.

Apollo 8 was in an elliptical orbit of 169.1 × 60.6 miles. I am skeptic about how long such an orbit would be stable. My skills in orbital mechanics are rudimentary at best and I know there is no atmosphere to cause orbital decay due to atmospheric drag. But I have heard about other effects causing orbits to decay over time, such as variations in the moon's gravitational field (density anomalies, mascons). This may cause lunar satellites to eventually crash into the moon. Can someone estimate how long this would be for the case of the Apollo 8 lunar orbit?

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  • $\begingroup$ Where have you heard that orbits about the moon could decay? I'm doubtful that variation in the gravity field could cause that. I can't think of any kind of tidal drag that could do it either. $\endgroup$
    – Mike Dunlavey
    May 13, 2012 at 2:18
  • $\begingroup$ @MikeDunlavey It is known that two of the Lunar Module's ascent stages, which were left in a lunar orbit, eventually crashed on the moon. See en.wikipedia.org/wiki/Apollo_Lunar_Module (LM-5 and LM-11). Other LMs were deliberately crashed on the moon, these two were left to orbital decay. $\endgroup$
    – Jens
    May 13, 2012 at 11:39
  • $\begingroup$ OK, I learned something. Good question. $\endgroup$
    – Mike Dunlavey
    May 14, 2012 at 0:56

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Your skepticism is well founded in this case. For a low orbit such as this, the leading perturbation will be from the mass concentrations (colloquially "mascons") in the Moon, which produce large variations in the gravitational field. The relevant timescale is probably weeks at best.

The orbit of the Apollo 16 subsatellite PFS-2 decayed so rapidly that the instrument impacted the Moon after just 34 days. The GRAIL mission (at the time of writing this answer in mid-May 2012, in the science phase) planned to take advantage of this behavior in order to smoothly vary the orbits and study the gravitational field of the Moon. The spacecrafts were planned for decommission on May 29, 2012 and were expected to impact within one month.

It is worth noting that at some inclinations, low orbits may be much more stable (7º, 50º, 76º, and 86º), but Apollo 8 was inserted at 12º. At this inclination the orbit was actually observed to be perturbated by several kilometers over the course of the 20 hour mission.

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    $\begingroup$ Grammar grump here :) I would say "perturbed". $\endgroup$
    – Mike Dunlavey
    May 14, 2012 at 15:26
  • $\begingroup$ @MikeDunlavey I agree, that's how I originally wrote it, but I deferred to one of my sources on this usage. If I have time to add some more physics to the answer I'll change it. If it really bugs you, feel free =) $\endgroup$
    – tmac
    May 14, 2012 at 17:03
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    $\begingroup$ Well (sheepishly) I tried, but it won't take small edits. But it's a great answer. It never dawned on me that the raisins in the muffin would have that much effect on orbits, but on low orbits, of course they would. I suppose the perturbations would affect both high and low apsides. $\endgroup$
    – Mike Dunlavey
    May 14, 2012 at 20:16

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