Is This Moon Three?

tetrad writes "The BBC reports that a new object has been discovered orbiting Earth. It's possible that it's just a piece of space junk, but more likely it is a rock that has been recently (in the last year) captured by our planet's gravitational field. If the object is confirmed to be natural, this would be Earth's third moon. (Did you know there were two already?)" Here's our earlier mention of Earth's alleged second moon. Update: 09/12 04:52 GMT by T : Reader cscx adds a link to an article running on space.com which says this newfound object may be some trash from the Apollo missions.

Comments

Re:What's in a moon?

[Bonker]

IANAA, but a professor once told me that a body that orbits another non-stellar body is a moon while a body that orbits a star is a planet or an astroid (astro- from star) depending on its size and regularity of its orbit.

Thus, the Pluto-Charon system is probably much more accurately labeled as either a pair of asteroids due to size considerations, or a dual-planetary system because their orbits are highly regular, albeit at a significant pitch compared to the other 8 planetary systems.

I've also heard that the Earth/Luna system should be considered a dual-planetary system because Luna has a much higher percentage of it's parent planet's mass than other moons... This jives with the 'Planetary Collision' theory of moon formataion, in which the moon is actualy a significant chunk of Earth, torn off early during our planet's formation.

The 'second moon', Cruithne, fits in with a large category of non-moon, non-planetary, non-asteroid bodies in the solar system. If you ever study the 'Trojans', you know that there are huge bodies of apparent moonlets that sit on a sixty-degree angle from Jupiter's, directly along Jupiter's orbit from the sun. (They are apparently held in such a strange place by the gravity of Jupiter vs. the gravity of Sol.) Rather than calling Cruithne a moon, we're probably better off adding a new 'common' cetegory to our solar classification to include it and the Trojans. AFAIC, there's no reason not to call these all Trojans and be done with it.

Re:What's in a moon?

[kzinti]

Just out of curiosity, what is the scientific criterion for a moon?

Isaac Asimov, in one of his popular-science articles, once presented a well-reasoned argument that the Earth and Moon should not be considered a planet and satellite, but a double planet. He formed his argument by comparing the masses of all the other moons in the Solar system to the masses of their primaries, and showed that the Moon:Earth mass ratio was far greater than that of any other planet/satellite pair. He suggested that we could account for this "outlier" by considering the Earth and Moon to be a double planet.

Whether this argument would stand up to real scientific scrutiny, I don't know. It sounded pretty good to me, but I was just 10 at the time. Maybe it was just gee-whiz stuff made up to impress 10-year-olds, but that doesn't really seem like Isaac's style.

Does anybody else remember this essay?

--Jim