Why isn't the moon headed to the earth?

August 21, 2006 7:48 AM

AstroPhysics: Why doesn't the moon move towards the earth, and other satellite related questions.

From time to time there is a story about how a satellite that one country or another launched several years ago is going to reenter the earth's atomosphere and crash into the ocean/land/burn up/whatever. This makes me think that satellites are in orbits that move continuously closer to the earth (googling around shows this to be, possibly, "orbit decay"). I assume this is becuase the satellite does not have enough velocity to continue in its orbit, and gravity pulls it down. And that makes sense to my tiny brain.

But what I can't figure out is why other orbits don't decay. The moon doesn't get closer to the earth, and the earth doesn't get closer to the sun (and neither to do the other planets), unless someone forgot to tell me something in grade school. Why? Googling gets me lots of stories about satellites and orbits, but no explanation for the differnce between planets and man-made objects. (And some man-made objects don't have orbit decay, maybe? Like the space station? Becuase that isn't going to crash back to the earth, is it?)

posted by dpx.mfx to science & nature (22 comments total)

Eventually, the moon will crash into the earth. This is nothing you need to worry about. That is to say, other orbits decay too, but at very very slow rates.
posted by sonofsamiam at 7:54 AM on August 21, 2006

The moon "steals" some of Earth's rotational spin as it orbits the planet, gradually moving further and further away. Earth days get longer (albeit slowly) as its spin is slowed. Man-made satellites are not rotating enough to grab sufficient momentum from the Earth.
posted by Blazecock Pileon at 7:56 AM on August 21, 2006

Satellites close to the earth experience orbital decay because they are actually still (just) in the earth's atmosphere, and so there is a little drag affecting them. The space-station stays up because we use rockets to keep it in position.

The moon and our geo-syncrounous satellites (and interplanetary missions, etc) are sufficiently far away that they are not experiencing any drag, and so move according to Newton's laws of motion.

Except that the moon is receding and will eventually depart the earth-luna system. Not sure on the reason, but it is probably just that the moon has some extra energy left over from its creation. This energy translates to an ever-larger orbit.
posted by clord at 7:59 AM on August 21, 2006

Man-made satellites decay because they are all close enough to be affected by the Earth's atmosphere. While the atmosphere is extremely thin at satellite heights, there's still enough air to gradually slow down satellites, which causes them to eventually fall to Earth.

There's a wide range in orbital heights. Mir, the international space station, and a variety of satellites are in low earth orbit, which decays rapidly (more atmosphere). Many communications satellites are in geosynchronous orbit, which is much much farther away from the Earth, and these orbits decay rather slowly (which is causing a space litter problem).

The moon is a good long way away, and isn't affected by Earth's atmosphere. The Moon is actually getting further and further away from Earth, at about 1.5 inches/year. The Moon is stealing rotational velocity from the Earth, slowing the Earth's rotation and speeding the Moon up (which makes it orbit further away). Eventually the Earth and Moon will match rotations, and the Moon will cease getting further away.
posted by jellicle at 8:11 AM on August 21, 2006

clord's got it. The reason for the very slight changes in the Moon's orbit are tidal forces between the two bodies. Wikipedia explains this well. This will happen between certain other astronomical bodies as well, but on a different scale depending on the planets/stars/whatever involved.

There are even tinier effects due to general relativity, that result in orbiting objects moving closer together rather than further away, but for anything other than closely orbiting pairs of black holes or neutron stars it's totally unmeasurable. It'd take like hundreds of lifetimes of the universe before a system like the Moon and the Earth moved much at all (I can't recall the exact figures, but they are ludicrously tiny ones).

I don't think that what Blazecock said is quite right - it's the fact that the satellites are so tiny and have negligible gravitational pull that means they'll not get much angular momentum transfer, I think.
posted by edd at 8:15 AM on August 21, 2006

Wow. I feel silly for not knowing that the Moon is getting farther away. I keep trying to think if maybe I learned that and forgot, but I really don't think I did. Probably when I was learning about the moon I had a teacher who thought it would be too confusing to explain such a thing. Sigh.

I suppose this also means that we're also going to crash into the sun someday, assuming the sun doesn't die first.
posted by dpx.mfx at 8:17 AM on August 21, 2006

The sun will expand out to reach us first I think. It's got a lifetime on the order of billions of years, and the general relativity decay will be on the order of gazillions of years, to use the very precise technical term.

Mind you, the ever detailed Wiki says: While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out, preventing it from being engulfed.
posted by edd at 8:21 AM on August 21, 2006

A couple nights ago I saw a documentary on the science channel that spoke about this topic. They did say that the moon is moving away, however did not mention anything about the moon reaching a time where it will stop moving further! According to them the moon will eventually be so far that it will cause the earth to wiggle around it's axis. And of course that full eclipses will become a thing of the past.
posted by convex at 8:25 AM on August 21, 2006

There are a lot of interactions going on between celestial bodies, and the relationships have puzzled many of the greatest minds in Western civilization for millenia. Don't feel too stupid.

The distances between bodies like the sun, earth and its moon DO change. The orbits of the planets aren't even circular, and while periodic, they are constantly perturbed by changing proximity to other planets which pull and tug on them. Even distant stars exert SOME pull on us...

Those pulls and tugs are clues as to where new objects can be found, too. Many of the outer planets were identified first by their periodic impact on the expected positions of others. It's really pretty cool to know this, and should give you a real appreciation of the minds of folks like Galileo.

Your presumptions regarding celestial stability sound rooted in a really short time frame... even 1000 human lifetimes is barely enough to tell that 'average' distances have changed.

As clord said, atmospheric drag accounts for the decay of low earth orbit objects (shuttle and space station, etc.) and their orbits are corrected / boosted periodically to maintain them. The same is true of the geostationary satellites in the Clarke orbit, which get perturbed, too. They have no substantial drag component, but must be orbit adjusted using a limited supply of station keeping fuel. When it is exhausted, they are retired, powered down, and parked in place. Lots of junk up there!
posted by FauxScot at 8:34 AM on August 21, 2006

So, will the moon eventually move far enough away from our planet to effect...what, exactly? Will humans be kissing existence goodbye as a result? How will this effect our planet?
posted by NoMich at 8:35 AM on August 21, 2006

This is an example of people providing more information than necessary.

For the purposes of your question, the Moon is in a stable orbit around the Earth because it circles the Earth exactly fast enough to balance the mutual gravitational attraction between the Earth and Moon.

Manmade satellites can be in a variety of orbits, some geosynchronous (above the same point on the Earth) and some not. Low orbit satellites that are not in geosynchronous orbit will see the greatest amount of atmospheric drag which will degrade the orbit over relatively short periods of time. Even high orbit satellites that are in geosynchronous orbit may have orbits that are eventually disturbed by complex gravitational interactions between the Earth and the Moon within the span of a human life. (Note that "geosynchronous" doesn't mean "a stable orbit around the Earth" because, for example, when you're standing on the Earth you're necessarily in a geosynchronous orbit, though you'd fall if you had nothing to stand upon.)

As mentioned above, orbits which would otherwise be completely stable "forever" can be unstable over large amounts of time because of gravitational interactions with other relatively nearby bodies (thus the solar system, as FauxScot says, is involved in a complex interelationship which causes the orbits of all bodies to change over time), but more relevant to this discussion orbits can change due to tidal interactions.

Tidal interactions can be most easily understood if you realize that in the real world, these objects are not points in space, but massive bodies where necessarily some parts are farther from another gravitational source than others. What results from this are tidal interactions. In the case of the Earth/Moon system, these tidal interactions have resulted in the Moon being "tidally locked" in its relationship to its orbit around the Earth—this is why the same side of the Moon is always facing the Earth. But it probably occurs to you that the same side of the Earth is not always facing the Moon! This is because the Earth is not yet tidally locked in its relationship to the Moon. Because of this, the Moon is effectively acting as a "drag" on the Earth's rotation and eventually the Earth's rotation will slow down enough so that the two bodies will be completely tidally locked, always in the same relative position to each other, the same sides facing each other. At this point, for these purposes excluding the even longer-range interaction I mention above, the Earth/Moon system will be stable.
posted by Ethereal Bligh at 9:19 AM on August 21, 2006

Yeah, don't feel stupid. Because the short answer to your question is: it's complicated. The reason satellites are easy to understand is because they're artificially simple. We launched them, and we determined their orbits. Nature, though far more beautiful, also is far more complex.

NoMich: The above-cited Wikipedia article has a good explanation: "This effect is expected to continue until the spin of the Earth has slowed to match the orbital period of the Moon. At that point the tidal effect of the Sun will dominate, further slowing the Earth and thereafter causing the orbit of the Moon to steadily shrink. However these effects would not be expected to occur until long after the Sun has become a red giant." So basically, there's nothing to worry about. The only immediate changes will be a slight lengthening of the day and a slight diminishing in the size of the tides. Nothing that will be noticeable in day-to-day life.
posted by markcholden at 9:22 AM on August 21, 2006

Ivars Peterson, in his very entertaining Newton's Clock, reports work which asserts that our solar system as a whole is not stable, and will eventually come apart. As I recall, this was deduced from a claim that Pluto's orbit is chaotic.

Also as I recall, ignoring the changes in the Sun mentioned above, once the Moon begins moving back toward the Earth, it won't stop before it reaches Roche's Limit, at which point it will be torn apart by tidal forces, producing something like the rings of Saturn.
posted by jamjam at 9:59 AM on August 21, 2006

(I'm not a scientician, so this may be bullshit, but I'm surprised no one else has said it yet)

The moon is in orbit because when it was captured by Earth it had roughly the right amount of momentum (and in exactly the right direction) to balance Earth's gravity. If it had had significantly more, it would have gained atittude and eventually disappeared into space. If it had significantly less, it would have fallen to Earth long ago. But no, it had near enough to the right amount for neither to have happened yet.

(all the objects that didn't have the right amount have disappeared or crashed, obviously)
posted by cillit bang at 10:20 AM on August 21, 2006

As others pointed out, the transfer of energy from the earth to the moon means that the moon gets farther away, the lunar month gets longer, and the earth's day gets longer. These tiny effects have been directly measured with atomic clocks and lasers.

But the effects have also been verified on a larger scale. By looking at and counting sedimentary layers from millions of years ago, geologists can determine the number of days between monthly high and low tides. They have also counted growth rings on buried corals to determine the number of days in a year.

One study shows that 900 million years ago, the day was only 18 hours long and that there were 481 eighteen-hour days in a year. (The length of a year has not changed, just the number of days in the year.)
posted by JackFlash at 10:56 AM on August 21, 2006 [1 favorite has favorites]

The moon is in orbit because when it was captured by Earth

... or was formed in a collision with a Mars-sized object (the giant impact hypothesis).
posted by hangashore at 11:06 AM on August 21, 2006

The length of a year has not changed

Actually, it has. It has lengthened. The same kind of tidal forces that do work within the earth-moon system also do work within the sun-earth+moon system. The orbit of the earth+moon will have enlarged to compensate for the reduced energy in the system. Therefore the length of the solar year will have increased.
posted by meehawl at 11:47 AM on August 21, 2006

The length of a year has not changed

Actually, it has. It has lengthened.

But not to a measureable extent over a 4 billion year history. That is because the sun is approximately a million times more massive than the earth.
posted by JackFlash at 12:02 PM on August 21, 2006

Thank you, JackFlash. Some years ago when I heard that birds have a free-running circadian cycle duration such that ~440(?) would fit into a solar orbital year, I tried to determine whether that corresponded to the length of a day at some critical point in the evolution birds as a whole, but I couldn't find any good evidence or a reasonable calculation about the historical chronology of day length, so I just forgot about it until you reminded me today.
posted by jamjam at 12:16 PM on August 21, 2006

and speeding the Moon up (which makes it orbit further away).

Perhaps I misread Heinlein, but speeding up *tightens* your orbit, no?
posted by baylink at 1:37 PM on August 21, 2006

baylink: if you're talking about Niven's Integral Trees, the orbital mechanics adage is "West takes you In, In takes you East, East takes you Out, Out takes you West, North and South bring you back again." (where East is prograde, West is retrograde).

So, East takes you Out, speeding up extends your orbit.
posted by nervestaple at 2:20 PM on August 21, 2006

not to a measureable extent

I make it about 5 billionths of a year longer since the sun began shining.

Wikipedia has a simple explanation of tidal locking.
posted by meehawl at 4:04 PM on August 21, 2006