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.

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.