Hello! I'm trying to get two questions answered quickly: 1. One of our gas giants has a moon that is split in two and when the two half's meet then exchange places. What are the names of the two (half) moons? 2. What's the name of that asteroid that's going to pass very close to us in the near future? Thanks! Rusty Note: Because, if at all possible, I need answers quickly, I've posted this question to 2 different forums... some forums hate that... forgive me this time ok. The other forum doesn't seem to know.
The two moons that "exchange places" are the Saturn moon's Janus and Epimethis. (they swap orbits every four years). However, I haven't seen anything that suggests that it is thought that they were ever once part of the same body. The asteroid that you might be thinking of is 99942 Apophis, which will make a near pass in 2029.
REALLY!!! Aren't they both hemispheres?! When they exchange places don't they almost fit exactly together into a sphere?! I really haven't looked at (studied) our GG moons in 20 years at least so I'm sure you are right. I was sure they were always thought to be 2 halves of the same moon that broke apart. Perhaps 20 years ago... well shit, actually its more like 35 years (how can I be so old!! Don't feel it!) anyway perhaps back then it was believed that they were but now we know better. Got me. :shrug: Thanks so much for the info!!
This is Janus: Please Register or Log in to view the hidden image! And this is Epimetheus: Please Register or Log in to view the hidden image! No, they pass within a few km of each other.
These 2 moons share a very similar orbit (almost exactly the same) and as a result astronomers used to think there was only one moon in that orbit. They knew something was wrong though because photographs of these moons did not match up to their expectations of the orbital behaviour of a single body in that orbit. They eventually realised there were 2 moons.
Apparently the closest they come is about 10,000 km of each other, but that is pretty close in astronomical terms. http://en.wikipedia.org/wiki/Epimetheus_(moon)
