Why do artificial satellites need orbit correction, but natural ones don't?

Question

No matter how much you fine-tune a satellite's orbit up front, it needs periodic adjustments. It's usually done through tiny rocket bursts. The system and fuel to do that costs money and weight, and, when it runs out of fuel, the satellite's life is near its end.

See Can an artificial satellite stay in orbit forever?.

By contrast, the Moon has been around forever, with no adjustments. It's getting further from Earth at about 4cm per year, which is negligible in the span of decades for most practical purposes. See http://en.wikipedia.org/wiki/Moon.

My question is, why is the orbit stability seemingly inconsistent between artificial and natural satellites? Is it just an effect of the Moon's much larger mass, or is there something else going on? If it's just mass, how about Saturn's rings, made up from countless small rocks that have also been around forever?

Answer 1

The instability in orbits of our artificial satellites come from a few basic causes:

1. Atmospheric drag and solar wind effects
2. The Earth isn't a perfect uniform sphere but is slightly lumpy, which means its gravitational field isn't uniform
3. Other massive objects in the solar system perturb their orbits with their gravity

So let's consider them one by one.

First, the atmospheric drag effect is substantial for low earth orbit satellites, but the moon is about 1000 times further than LEO, and the relative effect of atmospheric drag is less for larger objects, so for the moon this is completely negligible. Similarly, solar wind affects large objects less than small ones.

Second, the gravitational irregularity becomes less significant with distance as well; at the moon's distance, the Earth's gravity behaves very much like a perfect "point mass".

Third, perturbation from other sources is a factor, but the planets and their natural satellites have had billions of years to settle into positions which are mutually stable; the planets are far enough from each other to interfere only slightly, and the moons are far enough away from their primaries and each other to accept a little bit of perturbation over astronomical timescales.

So, fundamentally, artificial satellites in low Earth orbit need correction because they are easily "tossed around" and have so little margin for error -- if their altitude decreases by 50km, they're lost, whereas 50km of variation in the Moon's orbit wouldn't have any significant effect on the Earth or the Moon.

Artificial satellites in geosynch orbit need correction because we want them to be in very specific orbits (i.e. above particular positions on the Earth), which again isn't a concern for natural satellites -- it doesn't matter to anyone exactly where they go (with the possible exception of interplanetary mission planners).

Most of this also applies to the other natural satellites, even though our Moon is a bit of an outlier -- it's a lot larger than most, and it's the only massive body orbiting the Earth.

The individual bodies making up the rings of the outer planets are constantly changing their orbits due to collision and perturbation by the moons of those planets. The ring systems as a whole appear to be stabilized by the positions of the moons.

Answer 2

Essentially, this is a result of observational bias. A natural satellite will only orbit a parent for extended time periods precisely because the orbit it is in is stable ^†.

The plain truth of the matter is that we are simply injecting satellites into unstable orbits. If you were to move natural satellites into the same orbits, they'd be unstable too.

Take for example, the moon:

Gravitational anomalies slightly distorting the orbits of some Lunar Orbiters led to the discovery of mass concentrations (dubbed mascons), beneath the lunar surface caused by large impacting bodies at some remote time in the past. These anomalies are significant enough to cause a lunar orbit to change significantly over the course of several days.

From Lunar Orbit.

What this is saying is that there's no (or more correctly, very few) stable orbits around the moon due to its lumpy gravitational field. Why do we not see natural satellites orbiting the moon? Because they would have decayed due to the orbit being unstable!

Another example, asteroid 3753 Cruithne.

3753 Cruithne is an Aten asteroid in orbit around the Sun in 1:1 orbital resonance with Earth, making it a co-orbital object. It is a minor planet in solar orbit that, relative to Earth, orbits in a bean-shaped orbit that ultimately effectively describes a horseshoe, and which can transition into a quasi-satellite orbit.

Its orbit, is too, unstable. On the timescale of millions of years, it will transition out of its current arrangement too:

After many years, the Earth will have fallen so far behind that Cruithne will then actually be "catching up" on the Earth from "behind". When it eventually does catch up, Cruithne will make a series of annual close approaches to the Earth and gravitationally exchange orbital energy with Earth; this will alter Cruithne's orbit by a little over half a million kilometres—while Earth's orbit is altered by about 1.3 centimetres (0.51 in)—so that its period of revolution around the Sun will then become slightly more than a year.

But there's even natural examples of unstable orbits on human timescales. Look at 2006 RH₁₂₀

2006 RH120 is a tiny near-Earth asteroid with a diameter of about 2–3 meters that ordinarily orbits the Sun but makes close approaches to the Earth–Moon system every twenty years or so, when it can temporarily enter Earth orbit through temporary satellite capture (TSC). Most recently it was in Earth orbit from September 2006 to June 2007.

But, we can also inject artifical satellites into stable orbits, as well. Now that Dawn is orbiting Ceres, it will stay there for many hundreds of years. You could easily consider that stable on human timescales.

To summarize, orbits don't care whether the bodies involved are artificial or natural. You're only likely to find natural satellites in stable orbits, because the chaotic nature of orbits takes place over geologic timescales.

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^{†From a physical perspective, no orbit is ever stable. Tidal effects and gravitational influences mean that most of the orbits we consider stable, on human timescale, are unstable in geologic timescales. Additionally, gravitational radiation results in orbital decay on a timespan longer than the scale of the universe. All orbits are merely a human approximation of an unstable, chaotic system.}

Answer 3

The most general answer is this: The Solar System is 4.6 billion years old, anything that happens "quickly" has happened a very long time ago. For example, it is thought that in the early solar system the orbits of Neptune and Uranus where switched, with Neptune being further in than Uranus, but the gentle tug of Jupiter and Saturn eventually pushed them into their current orbits.

I would give a lot to know how close they must have gotten to colliding at one point.

As Andrew Thompson pointed out, man made objects are on orbits where no natural body is likely to be, because of the large orbital interference in lower orbits. We put them there, because that is were we get the most use out of them.

An earth surveillance satellite has to be as close to earth as possible to take usable pictures. We can't put it somewhere else, just because it would stay up indefinitely.

Also, nature doesn't care for any particular orbital arrangement, whereas, for example, TV satellites work only on a narrowly defined geostationary orbit. So we need to make sure they stay there. Putting an object on just any orbit is not as hard as putting it in a certain orbit and making sure it stays there. There are disused man-made objects like old probes and Apollo upper stages drifting around the solar system that may keep flying around for billions of years.

Answer 4

The short answer is that they DON'T need a correction to stay in orbit. But, as Russell, Rikki, Ross, and others pointed out, they need correction for the "stationary" part of a "stationary geo-synchronous orbit" because they need to stay where our terrestrial dishes are pointing. If they correct small errors they only require small amounts of energy (fuel). If they are allowed to drift, then the correction takes more fuel, possibly an acceleration to get back to the correct location, and a deceleration to stay there and not over-shoot. Hence the need for tiny, but frequent, corrections for geo-stationary satellites.

Answer 5

There is somewhat of a Darwinian nuance here, even though this is not a question of biology. If something has "survived" potentially billions of years of stable orbit, it is not one of a large number of things that either fell to earth or went away. If the question is, "Why do artificial and natural satellites stay at orbit," the answer is that artificial satellites stay in motion because they are "propped up" to a very slight degree, and existing natural satellites are a tiny minority of satellites that have persistently remained in orbits (as opposed to a silent majority, meaning dead, collection of immeasurably more particles that have either fallen to earth or escaped earth's orbit).

Answer 6

The important thing is that artificial satellites have a function that often requires the satellite to be in a specific orbit. Keeping it in that orbit requires correction. Many artificial satellites, once they fail, run out of fuel, or are no longer useful, have the stationkeeping stopped. If they are high enough, they stay in orbit just fine, but not any specific one.

Answer 7

Russell has given a very good response on sources of orbit perturbation, but I am with Rikki on this one!

The main reason that "heavenly bodies" do not need orbit correction is that we do not have any means of correcting their orbits!

Moon has been around the earth for a long time and will be there for a while!

I know this is not the scientific response you were looking for or this answer may even receive some negative points, but believe me, if we were able to move Jupiter moons or even our moon, we would have found a scientific need to do so!