PDA

View Full Version : stray asteroids orbiting earth



dohbot
2009-Jul-12, 11:27 AM
how come the satellites of other planets like neptune or jupiter that look like rocks are considered moons, but the asteroids that were caught by earth and orbiting it are not considered moons of earth?

Hornblower
2009-Jul-12, 11:45 AM
how come the satellites of other planets like neptune or jupiter that look like rocks are considered moons, but the asteroids that were caught by earth and orbiting it are not considered moons of earth?

Are there any such objects known to be orbiting Earth? If so, it is news to me.

thoth II
2009-Jul-12, 12:06 PM
I think the dynamics basically rule out any terrestrial planet from capturing asteroids.

Rhaedas
2009-Jul-12, 12:16 PM
Closest one I've heard of is Cruithne (http://en.wikipedia.org/wiki/3753_Cruithne), sometimes referred to as Earth's second moon, but it isn't really because it doesn't actually orbit the Earth.

dohbot
2009-Jul-12, 01:44 PM
thanks for sending the link, that was what i was hoping to find.

R.A.F.
2009-Jul-12, 02:02 PM
how come the satellites of other planets like neptune or jupiter that look like rocks are considered moons, but the asteroids that were caught by earth and orbiting it are not considered moons of earth?

Actually, to be "nitpick-ily" correct...there is only one Moon (notice the capital). It is the natural satellite of the Earth. (sometimes referred to as Luna). The so called "moons" of other planets should rightfully be referred to as the satellites of those respective planets.

At least that's how I learned it...

TonyE
2009-Jul-12, 05:39 PM
3753 Cruithne is an Asteroid and is sometimes called "Earth's Companion" even though it always remains in orbit round the Sun, not Earth.

This is because it has an orbital period almost exactly one year. The orbit is said to be in 1:1 resonance with Earth. However, its orbit is quite inclined to Earth's orbit and somewhat more eccentric. From time to time Cruithne comes quite close to Earth and when that happens Earth causes Cruithne to change its orbit a bit. The resulting relative motions are sometimes called a "horseshoe orbit".

There is a simulation of Cruithne's orbit here (http://www.myastrostuff.com/integration/webpages/cruithnea.htm):

mugaliens
2009-Jul-13, 02:33 AM
More near-Earth objects (NEOs) have since been discovered. These include 54509 YORP, (85770) 1998 UP1, 2002 AA29, and 2009BD which exist in resonant orbits similar to Cruithne's.

Again, like Cruithne, they orbit the Sun, not Earth.

Cruithne's oribit is horseshoe only when viewed from Earth. It's merely eccentric when viewed from above the solar system's ecliptic plane.

tony873004
2009-Jul-13, 02:52 AM
Earth can capture asteroids, and sometimes it does. 2006 RH120 (aka 6R10DB9) is an asteroid that was captured by Earth in 2006. Unlike Cruithne, it directly orbited Earth. It was a true moon. It completed a few orbits before escaping back to interplanetary space. One day it may be captured again. Here's a link to a simulation I made, describing the capture.

http://www.orbitsimulator.com/cgi-bin/yabb/YaBB.pl?num=1182030550

Jens
2009-Jul-13, 02:56 AM
Actually, to be "nitpick-ily" correct...there is only one Moon (notice the capital). It is the natural satellite of the Earth. (sometimes referred to as Luna). The so called "moons" of other planets should rightfully be referred to as the satellites of those respective planets.


In general usage, though, people often say "the moons of Jupiter" and I don't have any problem with it at all. It expresses the idea quite adequately, and somehow "satellites" sounds scientifically formal or something.

AndreasJ
2009-Jul-13, 10:07 AM
Also, to most people "satellite" means "artificial satellite", so refering to the moons of Jupiter is less likely to confuse people than to speak of the satellites of same.