Earth's Little Brother Found

loconet writes "The BBC is reporting that astronomers have discovered the first object ever that is in a companion orbit to the Earth. Asteroid 2002 AA29 is only about 100 metres wide and never comes closer than 3.6 million miles to our planet."

Comments

See the orbital motion for yourself

[StupendousMan]

JPL has a very nice tool for looking at the orbits of asteroids. Go to

http://neo.jpl.nasa.gov/orbits/

for the general case. For 2002AA29 in particular, you can use

http://neo.jpl.nasa.gov/cgi-bin/db?name=2002AA29&g roup=all&search=Search

Keep in mind that the orbital solution is based on only a short arc: only 28 days, about one twelfth of a complete revolution. Our estimates of the orbital parameters -- and behavior -- could change quite a bit over the next few months.

Re:Not quite a planet, eh?

[ocie]

Interesting physics, but Kepler's third law says:

The squares of the periods of the planets are proportional to the cubes of their semimajor axes
(http://home.cvc.org/science/kepler.htm).

So the mass of a planet has nothing to do with its orbital period (well, assuming it is small enough that it doesn't make the sun orbit it). So anything placed at Earth's distance from the sun and moving at the same speed would orbit the sun in the same path the Earth does regaurdless of its mass.

Re:Not quite a planet, eh?

[targo]

roughly the same orbit around the sun, a much smaller mass has to travel MUCH slower than the Earth to maintain that orbit.

Wtf? Orbital velocity is a constant that depends only on the mass of the parent body, as long as the orbiting body is significantly lighter.
After all, geosynchronous satellites are all at approximately same height, although they have the same speed (to maintain synch), but different mass.

The formula for calculating orbits is:
T=2*pi*(a+h)/v
where T = period, a = radius of the parent body, h = orbit height, and v = satellite velocity, which can be calculated from:
v = sqrt(g/(a+h)),
where g is gravitational acceleration of the parent body.
You don't see the mass of the satellite anywhere here.

Re:Not quite a planet, eh?

[GreenPhreak]

The reason this discovery is useful and more than 'whoop-de-doo' is because of what was mentioned in the end of the article: it is an extra-terrestrial body that is very close to the Earth. It would not be outside our reach to visit this object with current technology and learn more about the composition of asteroids and other minor planets in the solar system.

It is also intriguing since no 'trojans' have been discovered for the Earth and this could signal that we do in fact have some. Trojans are asteroids that occupy the 4th and 5th Lagrangian points about a larger body (Jupiter has the most, due to its large mass). Because of the physics involved in a 2 body system where any additional bodies have negligible mass compared to the original 2, there are a few 'stable' points where the gravitational forces cancel out...these are known as Lagrangian points. L4 and L5 are co-orbital to the less-massive object (Jupiter, Earth, whatever).

Although this object is not a trojan, since it has a horseshoe orbit and temporarily gets caught up in Earth's orbit, it suggests that there are bodies out there that could be trojans. Perhaps as our detection abilities progress, we will discover some Earth-trojans.

Re:Horseshoe orbit?

[Link310]

http://www.paias.com/paias/home/Science/Newton/New t8Fig5Orbits.htm explains it. From what I understood, it's actually orbiting the L4 and L5 Lagrange points of earth.

This picture illustrates it pretty well.

Re:Not quite a planet, eh?

[geoswan]

Of course, the part I don't get, *why* can't it hit the Earth? In roughly the same orbit around the sun, a much smaller mass has to travel MUCH slower than the Earth to maintain that orbit...

I don't have the equation for the gravitational attraction between two bodies. But I know it is a function of the SUM of the masses of the two objects. So, how much do you think the sum of the masses of the sun and the Earth differs from the sum of the masses of the sun and 2002 AA29?

There are lots of explanations of horseshoe orbits on the web. Basically, if two objects share the same, or very similar, orbits, they are attracted to one another. That gravitational attraction drains kinetic energy from the leading object, and slightly adds kinetic energy to the trailing object.

The leading object, having lost energy, moves closer to the primary. Its year gets slightly shorter, and its actual velocity relative to the primary speeds up. Similarly, the trailing object moves farther away, and its year grows slightly longer.

So the leading objects closer orbit has it revolve around the Primary more quickly, and it will slowly move away from the trailing object. Eventually the leading object is exactly opposite from the trailing object. According to the BBC article, this takes 95 years.

Once the object that was leading is more than 180 degrees ahead in it orbit from the object that was trailing, their mutual attraction starts to add energy to its orbit, and raise it to a higher orbit. Similarly, the mutual attraction drains energy from the other object.

What we have just seen is the two objects trade places. The object that was the trailing object is now the trailing object.

It seems paradoxical that mutual attraction should tear the two object apart. Until you remember that the Sun's influence on the object's trajectories is much more important than their attraction to one another.

At least that is my understanding of the BBC's article.

How does this mechanism allow 2002 AA29 to be briefly captured by the Earth? I'd welcome an explanation of this.

uh... 'scuse me?

[TWX_the_Linux_Zealot]

Here is Paul Wiegert's information on Cruithne, which has much of the same characteristic as this current space body, but his explanation actually makes sense for what appears to be a horseshoe orbit, when in reality it's only a horseshoe orbit from Earth's perspective, and is relatively sane looking when viewed off of the solar system plane.