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Quasi-Satellites and Other Co-Orbital Bodies

By upsilon in Science
Tue Oct 22, 2002 at 06:49:12 PM EST
Tags: Science (all tags)
Science

Recently, scientists announced the discovery of an asteroid co-orbital with the Earth, given the temporary label of 2002 AA29. Other asteroids with co-orbital motions have been discovered, and various news stories have given them the dubious labels of "Earth's second (or third or Nth) moon!".

But what exactly is a moon? Can a co-orbital body such as 2002 AA29 truly be said to be a moon of another body?


The definition of a moon or satellite is a rather fuzzy one, so let us first look at what a co-orbital body is. Loosely speaking, co-orbital means that the two bodies orbit the Sun in roughly the same time period, and are gravitationally linked. Even here, though, the term is used for three distinct purposes.

First, there are asteroids such as 3753 Cruithne which is gravitationally bound to the Earth despite not being in the same orbit, or even the same orbital plane. It goes around the Sun roughly once a year (give or take a few hours), but describes a rather elaborate dance when viewed from the Earth. If one were to take up a position above the Sun's north pole and turn once a year (so that the Earth always stayed in the same apparent position), Cruithne would describe a kidney-shaped "orbit".

Over time, though, the location of the "kidney" with respect to the Earth changes. It moves slowly around the Sun, coming close (well, 15 million kilometers) to the Earth from one side then moving around the sun to approach from the other side, and then back, over roughly 770 years.

At various points over the year, Cruithne is on all sides of the Earth, and so in a very loose sense can be considered to "orbit" the earth. But the distances involved are typically vast, compared to the size of the Earth's orbit, and it is only in a very loose sense that one would consider Cruithne to be a satellite; the average layperson would hardly think of it in these terms at all.

Much more recently, 2002 AA29 was discovered. While Cruithne's dance with the Earth is far more complex than 2002 AA29's relationship, 2002 AA29 has a closer relationship. For starters, 2002 AA29 is (roughly) in the Earth's orbital plane. Cruithne's orbit is inclined more than 15 degrees, while 2002 AA29 has no appreciable inclination.

Instead, 2002 AA29's orbit can be more or less described as a corkscrew around the Earth's orbit. Like Cruithne, it starts close to the Earth and gradually makes its way around the Earth's orbit to approach from the other side some 95 years later.

However, it occasionally comes even closer and gets stuck in a pseudo-orbit around the Earth with a period of one year, for as long as fifty years at a stretch. The next time this will happen will be in roughly 2600 AD. Here, too, the orbit maintains its corkscrew appearance, but now the Earth is in the center. Each loop of the corkscrew takes one year.

2002 AA29 also comes much closer to the Earth than Cruithne ever does. On January 8, 2003, it will be only 12 times as far away as the Moon; that's only 4.6 million kilometers away.

A third sort of co-orbital motion has been known for a long time. Here the asteroid follows the same orbit as the planet (or other larger body) but never comes close, such as the Trojan asteroids of Jupiter. There are two places on Jupiter's orbit, 60 degrees ahead of and 60 degrees behind the planet where groups of asteroids have been discovered. These two locations are known as the L4 and L5 points, where L stands for Lagrange. (The other three Lagrange points do not lie on the orbit of the planet, and indeed are unstable, so a co-orbital body found there will not be co-orbital for very long.)

Scientists started naming the asteroids in the forward group after Greek heroes of the Trojan war, and the rear group was named after the Trojans. The name "Trojan asteroids" soon referred to both groups, and indeed to other instances of asteroids found in Lagrange points, such as the six Martian Trojan asteroids known to lie in the L4 and L5 points on the orbit of Mars.

Clearly, though, Trojan asteroids do not meet the definition of satellite in any sense of the term. They are gravitationally bound to the planet, and are interesting to study in their own right, but are conclusively not satellites.

So, having looked at three different examples of co-orbital motions, have we reached any definite conclusions as to what a satellite is? Well, no. There does not seem to be a hard and fast rule as to what a satellite is. Clearly the Moon is a satellite and a Trojan asteroid is not, but what about Cruithne or 2002 AA29? The best solution is perhaps one favored by some scientists, who have started to favor the term "quasi-satellite" to describe these elaborate (yet relatively stable) co-orbital motions. While somewhat awkward and thus unlikely to gather any momentum in the popular press, this term does concisely describe the somewhat fuzzy line between satellite and Trojan, and is likely the best one can do, given the wide range of orbital motions one can find.

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Poll
Which term best describes 2002 AA29?
o Satellite 0%
o Moon 0%
o Quasi-satellite 18%
o Co-orbital body 51%
o Asteroid 11%
o Celestial corkscrew 18%

Votes: 43
Results | Other Polls

Related Links
o an asteroid co-orbital with the Earth
o Other asteroids with co-orbital motions
o 3753 Cruithne
o a rather elaborate dance
o 2002 AA29
o corkscrew around the Earth's orbit
o pseudo-orb it around the Earth
o six Martian Trojan asteroids
o quasi-sate llite
o Also by upsilon


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Quasi-Satellites and Other Co-Orbital Bodies | 19 comments (12 topical, 7 editorial, 0 hidden)
Nice. +1 FP. (4.00 / 3) (#1)
by graal on Tue Oct 22, 2002 at 03:26:51 PM EST

Fits the science section perfectly. Now if you could only work in a recipe or something...

--
For Thou hast commanded, and so it is, that every
inordinate affection should be its own punishment.
-- St. Augustine (Confessions, i)

near-earth objects (5.00 / 1) (#8)
by arthurpsmith on Wed Oct 23, 2002 at 12:28:45 AM EST

Since asteroids can have high concentrations of carbon and other elements that may be hard to find on the Moon, space development in the near term seems to require both lunar and near-Earth asteroid resources. This 100-meter size example (and there may be dozens more like it) would not be much harder to get to than the Moon itself - easier as far as propellant is concerned, although the trip would take a bit longer. Finding more like this will make a big difference to the practicality of space development plans.

There are currently over 2000 known "Near Earth Objects", according to the minor planet center. Many of these have been discovered very recently through the Near-Earth Asteroid Tracking project. But what we really need is a space-based system to detect these things - unfortunately very little money is being spent on the search right now, even though these objects represent both great opportunity, and great danger.

Energy - our most critical problem; the solution may be in space.


Alas, (5.00 / 1) (#18)
by JChen on Fri Oct 25, 2002 at 11:54:22 AM EST

there are more nuclear warheads at the ends of rockets today than there are command modules.

Let us do as we say.
[ Parent ]
standards (5.00 / 1) (#9)
by dirvish on Wed Oct 23, 2002 at 01:27:20 AM EST

I think we need a Universal Big Rocks Standards Association to take care of classifying this sort of thing. That would help avoid confusion.

Technical Certification Blog, Anti Spam Blog
Binary planet (4.50 / 2) (#10)
by bloat on Wed Oct 23, 2002 at 05:57:07 AM EST

Excuse me for being vague but I remember reading something about how some astronomers did not consider the Moon to be a true moon, but rather that the Moon and the Earth form a binary planet system.

The reason behind this was that firstly the Moon is extremely large relative to the Earth when you compare typical moons to their planets, and secondly that the Moon's orbit is never curves away from the Sun like other moon's orbits do. Instead the Moon's orbit always curves towards the Sun, like the Earth's does.

CheersAndrewC.
--
There are no PanAsian supermarkets down in Hell, so you can't buy Golden Boy peanuts there.
Part of that doesn't make sense (5.00 / 1) (#11)
by mcherm on Wed Oct 23, 2002 at 09:44:05 AM EST

The reason behind this was that firstly the Moon is extremely large relative to the Earth when you compare typical moons to their planets,

Sure, that's true.

...and secondly that the Moon's orbit is never curves away from the Sun like other moon's orbits do. Instead the Moon's orbit always curves towards the Sun, like the Earth's does.

Huh? The moon orbits the earth in an ellipse (with SLIGHT perturbation from the Sun) that is quite close to the plane of their joint orbit around the sun. So the moon spends about 50% of it's time going "toward" the sun, and about 50% going "away".

-- Michael Chermside
[ Parent ]

Concave orbits (4.66 / 3) (#12)
by bloat on Wed Oct 23, 2002 at 10:14:49 AM EST

Huh? The moon orbits the earth in an ellipse (with SLIGHT perturbation from the Sun) that is quite close to the plane of their joint orbit around the sun. So the moon spends about 50% of it's time going "toward" the sun, and about 50% going "away".

Yes, it looks like an ellipse from the Earth, but if you are at the Sun looking at the Moon and ignoring the Earth, the Moon's path is always concave, it never curves away.

A brief Google reveals that the vague source I mentioned is actually an Isaac Asimov article. Here's a reference to a page that talks about it in lots of detail.

Just Mooning Around

CheersAndrewC.
--
There are no PanAsian supermarkets down in Hell, so you can't buy Golden Boy peanuts there.
[ Parent ]
"Concave orbits" (5.00 / 1) (#14)
by p3d0 on Wed Oct 23, 2002 at 11:50:25 AM EST

As I understand it, there's no such thing as "concave orbits" and "convex orbits". Any smooth curve, no matter how strange, can always be considered either concave or convex depending on your point of view, and what piece of the curve you're looking at. (If I'm wrong on this, please let me know.)

Rather, the interesting thing is whether a particular orbit is "always concave toward the sun/earth/whatever". A body X can be considered to be orbiting another body Y if X's orbit is always concave toward Y. In the case of the moon, I suppose you could say it orbits both the sun and the earth, while you could not say the same for (I think) any other satellite in the solar system.
--
Patrick Doyle
My comments do not reflect the opinions of my employer.
[ Parent ]

Ya learn something new every day. (5.00 / 1) (#15)
by mcherm on Wed Oct 23, 2002 at 02:58:27 PM EST

Fascinating link... I particularly liked figure a which sums it up in a visual diagram.

-- Michael Chermside
[ Parent ]
You're both right (4.66 / 3) (#13)
by upsilon on Wed Oct 23, 2002 at 10:57:51 AM EST

You are right, in that of course the Sun-Moon distance is increasing about 50% of the time and that same distance is decreasing about 50% of the time. If the Moon were always heading towards the Sun, well, it would fall in eventually!

What bloat was referring to (and seemingly just got a little confused in the terminology) is the fact that if you watch the Moon going around the Sun, it always follows a concave track.

If you imagine yourself in a race car going around a track, a concave turn means that you are always going straight or turning left, never turning right. Another way to think of it is that there is no point in space outside the Sun-Moon orbit where the Moon's orbit surrounds you on more than 180 degrees.

Nearly every other planetary satellite in the solar system has an orbit (relative to the Sun) that is sometimes convex (the other notable exception being Pluto's moon Charon).

So perhaps I chose a poor example in pointing out the Moon as a satellite, since there is some doubt of this in certain circles... uh, no pun intended.
--
Once, I was the King of Spain.
[ Parent ]

try to use hexadecimal (1.57 / 7) (#17)
by Fen on Thu Oct 24, 2002 at 10:22:32 AM EST

It's easy, just use metric. Give meters in hexadecimal (no kilometers). Do it now.
--Self.
If it ain't made of cheese then it ain't no moon (none / 0) (#19)
by ThreadSafe on Sun Nov 03, 2002 at 01:58:38 PM EST


Make a clone of me. And fucking listen to it! - Faik

Quasi-Satellites and Other Co-Orbital Bodies | 19 comments (12 topical, 7 editorial, 0 hidden)
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