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

Maybe an old Apollo booster?

[ishmalius]

Here is a link I saw just before your posting:

Space.com

Re:What's in a moon?

[NetFu]

New Page 1

Well, here's what Merriam-Webster's Collegiate Dictionary says:

Moon:
-- 1a : a natural satellite of a planet

Satellite:
-- 2a : a celestial body orbiting another of larger size

I think based on these common definitions that these objects, assuming they are found be natural and that they can be proved to orbit Earth, should be called moons. Maybe they don't fit our cultural, unwritten definition of a Moon, but that doesn't change the facts. So, maybe we need to change what most people think of as the Moon...

Ephemeris information for the object

[xlation]

The JPL has an ephemeris generator that now calculates the position of the object.
To see the data:

1. Click the "Target Body" Button
2. Choose "Spacecraft" from the "Select Major Body" dropdown.
3. Select "J002E3 Spacecraft (UNCONFIRMED)"

Re:Captured How?

[FuzzyDaddy]

No, this wouldn't work. The slingshot effect changes the energy of a small object near a large one in a frame of reference that is not comoving the large object - for example, a rubber ball bouncing off a brick wall does not change it's energy in the frame of reference relative to the wall, but if you bounce it off of a moving truck, the energy in ground frame of reference will change - but not in the truck frame of reference.

Because in SOME frame of reference, the energy of the object doesn't change, this type of elastic collision CANNOT move an object from a bound to an unbound orbit.

The 60-degree angle, moons, and whatnot

[devphil]

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.)

Anytime you have something (Foo) orbiting something else (Bar), i.e., once the requirements of "orbit" are met, there are five points of gravitational equilibrium set up amongst the two bodies. They're called LaGrange points. The last two, L4 and L5, are on Foo's orbit around Bar, sixty degrees ahead of Foo (L4) and sixty degrees behind (L5).

L4 and L5 by themselves, ignoring L1-L3, are often called Trojan points, named for this particular group of satellites.

As for the defintion of moon versus just another satellite in general, I believe it has to do with respective mass ratios, and where the fulcrum point of rotation is between the two bodies. Right now our own moon isn't in a true rotation around us, we're in a sort of dumbbell tumble, and the center of the dumbbell is a bit below the ground.

(Actual astronomers please correct me, I'm on a number of narcotic-containing painkillers right now and could have gotten some words tumbled.)

Re:What's in a moon?

[Autonomous Crowhard]

The best explanation I've ever heard of if a two body system is a planet-moon or two-planet combination is this:

If the center of gravity of two bodies lies inside one of the bodies then that is the planet and the other is the moon. If the center of gravity is between the two objects then it is a two planet system.

Re:How is Cruithne a moon?

[kalidasa]

Cruithne isn't really a moon - it's a coorbital companion. And if you look at the Cruithne FAQ fm6 linked to, you'll see that it's not the only one. So if these coorbital companions counted as moons, this new object (if it isn't just a spent rocket booster) really would be the fifth moon.

Does the Earth have any other companions or moons?

Yes, the Moon. But apart from that, there are no natural objects known to be in close dynamical relationships with the Earth. NEW! Asteroid 1998 UP1 and 2000 PH5 have been found to be in similar relationships with our planet. We are currently (18 Sept 2001) working on publishing these results. Look for more info here soon.

In that case....

[Anonymous Coward]

...then, since the barycenter is inside the Earth, the Moon is a moon.

As for Cruithne

[KewlPC]

As for our second alleged moon: it is not a moon!

It orbits the sun in a horseshoe-shaped orbit that goes quite a distance above and below the elliptical plane. The horseshoe orbits overlap, but don't take 770 years. However, because these horseshoe-shaped orbits overlap, Cruithne eventually goes all the way around the sun. It is this that takes 770 years.

It is affected by Earth's gravity (indeed, that is why its orbit is shaped like a horseshoe), but that doesn't make it a satellite of Earth.

Therefore, this "third" moon is actually the second. :b

I'm pretty sure you're correct

[GePS]

I'm quite sure that Luna is the techincal name for the first moon that humans noticed orbiting Earth. Why do I think this? It fits the pattern of the true name of the sun, the other major body that is readibly visible to us earthlings.

"the sun" actually = "sol" (latin in origin, i believe)

"the moon" actually = "luna" (latin in origin, i believe).

It makes sense to me, but then again, I dunno what validity that holds. :)