Did you hear that, Neil?
June 19, 2019 11:29 AM   Subscribe

On July 20, 1969, approx. 20:00 UTC, How many spacecraft/ satellites were orbiting earth's moon?
posted by clavdivs to Science & Nature (12 answers total) 4 users marked this as a favorite
 
Two?
posted by Melismata at 11:35 AM on June 19, 2019 [1 favorite]


I'm going to say 2: Apollo 11 and Luna 15. 1969 was a busy year for lunar visits!
posted by jquinby at 11:36 AM on June 19, 2019 [1 favorite]


Explorer 35?
posted by JoeZydeco at 11:47 AM on June 19, 2019 [3 favorites]


I was gonna be nitpicky and say there were actually 2 Apollo 11 craft in orbit at 20:00, but the Apollo 11 Command Module was orbiting the moon and the Lunar Module was out of orbit and descending to the surface. So it's still one.
posted by JoeZydeco at 1:12 PM on June 19, 2019 [1 favorite]


'obtain improved communications.
102:27:32
19:59:32
20 Jul 1969
LM altitude 50,000 feet.
102:32:55
20:04:55
20 Jul 1969
LM propellant settling firing started.
102:32:58
20:04:58
20 Jul 1969'

The LM began its landing sequence with a Descent Orbit Insertion (DOI) burn to lower their periapsis to about 50,000 feet (15 km; 8.2 nmi), chosen to avoid hitting lunar mountains reaching heights of 20,000 feet (6.1 km; 3.3 nmi).

Call it draw at two... three... Four...so far?
posted by clavdivs at 3:48 PM on June 19, 2019 [1 favorite]


The Saturn V third stage, called an S-IVB, from both Apollo 10 and Apollo 11 are both currently in heliocentric (that is, solar) orbit out in space. At the time 11 landed on the moon, its S-IVB would've still been in Lunar Orbit, but it was a hyperbolic orbit which would carry it out of the moon's influence. It's up to you to decide whether the latter mission's S-IVB counts as a lunar satellite.

Apollo 12's S-IVB also did this, but turns out to have been recaptured by Earth, and 13-17's rocket stages were all sent to collide with the moon in order to give some data to the seismometers placed there. (The moon rings like a bell, it turns out.)
posted by Sunburnt at 3:51 PM on June 19, 2019 [4 favorites]


I don't think the USSR confirmed that their previous successful Luna orbiters crash landed. From what I have read, unlike the US they collected data until the batteries ran out. I don't know how long they could maintain their orbit without power. So it seems possible that, say, Luna 14 which was launched in 1968 could have still been in orbit?
posted by muddgirl at 3:59 PM on June 19, 2019 [1 favorite]


We can pretty clearly identify the candidate objects, thanks to this handy Wikipedia listing of Missions to the Moon:
  • Luna 10 - Aug 1966, 2400 km, first man-made object to orbit the moon
  • Luna 11 - Aug 1966, 2415 km
  • Luna 12 - Oct 1966, 2405 km
  • Luna 15 - July 1969, still in orbit at the time you specified, landed while Armstrong & Aldrin were still on the moon
  • Lunar Orbiter 1 - Aug 1966, deorbit Oct 1966
  • Lunar Orbiter 2 - Nov 1966, deorbit Oct 1967
  • Lunar Orbiter 3 - Feb 1967, deorbit Oct 1967
  • Lunar Orbiter 4 - May 1967, deorbit Oct 1967
  • Lunar Orbiter 5 - Aug 1967, deorbit Jan 1968
  • Explorer 35 - July 1967, 7886 km, estimated to have stayed in orbit until the mid-late 1970s
  • Apollo 8 & 10 orbited the moon in December 1968 & May 1969, but successfully returned to Earth and as far as I know, didn't leave anything behind in lunar orbit.
  • Apollo 11 - 3 pieces (Lunar Module, Command Module, Saturn V 3rd stage) potentially in some form of lunar orbit
Some of those were deorbited or returned to Earth. So they're off the list.

Anything in low lunar orbit will decay rather quickly, unless very lucky, due to so-called perturbation effects. But all the Luna orbiters were in rather high lunar orbits and could probably last 5-10 years. I'm assuming they lasted at least three years. I'm assuming the USSR did not deliberately de-orbit their lunar orbiters, as the U.S. did.

Explorer 35 continued an active mission into the early 1970s and so was definitely in orbit on the date you specify.

I'm assuming when you say "orbiting earth's moon" you mean stable lunar orbit, not the kind of hyperbolic lunar orbit Sunburnt identifies above for the Apollo 11 Saturn V 3rd stage at the time you specify. So at the time you specific the Lunar Module has landed, the Command Module is in lunar orbit, and the Saturn V 3rd stage is entering solar orbit. (And if you think the Saturn V 3rd stage qualifies, add one more to the list.)

Putting all that together, here is the list:
  1. Luna 10
  2. Luna 11
  3. Luna 12
  4. Luna 15
  5. Explorer 35
  6. Apollo 11, Command Module
  7. (Apollo 11, Saturn V 3rd Stage)
So six total (or seven, if you count the Saturn V 3rd Stage).
posted by flug at 12:19 AM on June 20, 2019 [6 favorites]


Your list is missing Luna 14, so that makes 7 or 8.

I was using this list of extraterrestrial orbiters which doesn't include Luna 15 for some reason.
posted by muddgirl at 12:02 PM on June 20, 2019 [3 favorites]


Yes, you're right--Luna 14 (April 1968) is definitely in there, orbit ranged from 1894-2607 km so it should have been high enough to survive for a while as well.

Luna 15 was designed as a lander, so that's probably why it's not on the list of orbiters. It "just happened" to be in lunar orbit at the moment Apollo 11 landed.

(Actually, no "just happened" about it--the Soviets were trying to return a lunar soil sample via their robotic probe, before the U.S. could do so via astronauts. And if Luna 15 hadn't malfunctioned just after the lunar de-orbit burn they might well have succeeded.)

I intentionally omitted Luna 13 from the list, because it, too was a lander that landed on the moon some years before Apollo 11.

Also, looking a bit closer at the Luna orbiters, although their average orbital height seems enough to be safe, they all had somewhat eccentric orbits, and the closest points ranged 80-120 miles above the moon's surface. So . . . that could be close enough that the infamous lunar low-orbit perturbation effect disturbed their orbits and caused them to crash-land into the moon sooner than I might have expected.

So it's at least somewhat reasonable to expect they would stay in orbit the 1.5-3 years required.

Giving more weight to that idea is this example where NASA found Chandrayaan-1 about 8 years after it was lost, and in pretty close to the orbit that was expected. Chandrayaan-1 was in a slightly lower orbit than any of the Lunas.

FYI the existence of stable low-altitude lunar orbits was only discovered in 2001! Here is a NASA article--they call them Bizarre Lunar Orbits.

And . . . it turns out that high lunar orbits are also unstable for a different reason (the earth's gravity)

But low-altitude lunar orbits are considered as 60-ish miles altitude (above the lunar surface) and lower, while high-altitude is 750 miles or higher. So all the Lunas, and Chandrayaan, were pretty much in the "Goldilocks Zone" where they were neither too high nor too low. So they probably easily lasted 3 years and in fact, there is some slight chance they are still there today.

TLDR: As muddgirl said, the answer looks to be 7 or 8.
posted by flug at 1:43 PM on June 20, 2019 [1 favorite]


Technical point: Sometimes periselene and aposelene (nearest and furthest points in an object's lunar orbit) are given in reference to the center of mass of the moon, and sometimes to its surface.

(Center of mass is more relevant for orbital calculations, but distance from the surface is more relevant if you are interested in keeping your spacecraft from crashing into the object in question, or its atmosphere.

Just for example, the Space Station is about 254 miles above the earth's surface, but about 4213 miles above the center of the earth. Those are two very different numbers to describe the same orbit!)


Wikipedia articles for the Luna spacecraft seem to be using the center of mass distance. So for example periselene/aposelene for Luna 14 is listed as 1894/2607 km and for Luna 10 2,088/2,738 km, etc.

Lunar radius is about 1737 km so you have to subtract that from the numbers above to get the lowest orbital altitude. So for Luna 14 that would be 147km and Luna 10, 351km.

Then convert to miles if you want to compare with altitude given in NASAs articles about low-altitude lunar orbits. So Luna 14 is 91 miles, Luna 10 is 218 miles at lowest lunar altitude.

Anyway, it looks like the Lunas are pretty much above the low-lunar-orbit danger zone level.
posted by flug at 1:58 PM on June 20, 2019 [3 favorites]


Wow, thanks folks, great work.
posted by clavdivs at 5:17 PM on July 19, 2019


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