On Feb 20, 5:38 pm, will...@cfa.harvard.edu (Steve Willner) wrote: > In article <3246ca96-d581-45bc-b8b5-25d4ebd07...@j9g2000vbz.googlegroups.com>, > Robert Clark <rgregorycl...@yahoo.com> writes: > > > First though, note there are many ways a fragment could be separated > > from the main asteroid. > > As I understand it, about 1/3 of near-Earth asteroids have moons. > > > depending on the direction the fragment is sent, once the asteroid > > comes around to the Earth or Moon or other planet on a close approach, > > that fragment could be much closer to that large gravitating body than > > the asteroid and therefore be sent on a different orbit. > > In which case it's an independent object following its own orbit, no > longer a companion of the first object. > > > Then on subsequent orbits it could impact the gravitating body. > > Yes, it would on average have just as much or as little chance of > impacting as any other NEO. For a specific object, one would have to > know the orbit. > > > Indeed it could even be captured by the gravitating body, such as the > > Earth, > > Only if it comes close enough to lose energy in the atmosphere. > That's a tiny probability but not zero. > > > For instance asteroid 2012 DA14 was traveling at 18,641 mph, > > about 8.3 km/s on closest approach. At the distance it passed the > > Earth at 17,000 miles this is greater than escape velocity. But it's > > less than escape velocity at the Earth's surface. > > If it had come closer to Earth, it would have been going faster > because Earth's gravity would have accelerated it more. > > --
Thanks for the info. On this page it says the orbits "cross" at two locations, or more precisely, which body is on the outside orbit changes twice per orbit:
La Sagra Observatory discovers very near-Earth asteroid 2012 DA14. Posted By Jaime Nomen 2012/03/27 05:20 CDT "The preliminary orbit shows that 2012 DA14 has a very Earth-Like orbit with a period of 366.24 days, just one more day than our terrestrial year. The orbit is nearly circular but just elliptical enough to jump inside and outside of the path of Earth two times per year. Because objects move faster when they are closer to the Sun, the relative motion is similar to some sports races: when the Earth is on the outer track, it is overtaken by 2012 DA14, but when the asteroid crosses Earth's orbit, Earth overtakes it and passes by. It is during the orbit crossings when the closest encounters occur, and when there is potential for a future impact." http://www.planetary.org/blogs/guest-blogs/3418.html
Does this mean there are two close approaches per orbit? It doesn't necessarily have to be since where the two orbits "cross" does not mean the two bodies have to be there at the same time. On the other hand because they have similar orbits and therefor similar speeds, if they are close at one "crossing" point, they should be relatively close at the other.