In a message dated 6/8/00 6:49:14 PM Central Daylight Time, patrickfwd@yahoo.com writes: << Are there any naked eye visible natural satellites in low orbits? I suppose they wouldn't be there for long, but I would think there would be a few big ones, maybe would-be bolides that missed the Earth by a bit and were captured in orbit? >> No. As far as anyone knows, there are no natural satellites of the Earth besides the Moon. It is very difficult for satellites to be captured into orbit around the Earth. Only rare multiple interactions involving atmospheric deceleration and then a precise interaction with the Moon would slow a passing potential moonlet to orbital speed and then place it in an orbit that does not immediately decay. Suppose you have a small asteroid coming in from interplanetary space. It will be approaching the Earth at a speed ranging from 11km/sec (Earth's escape velocity) up to as much as 50km/sec or more. The trajectory will usually be a symmetrical hyperbolic path. Basically the inbound motion will be identical to the outbound motion with a deflection of a few degrees. Gravitation is a "conservative force" which means (roughly) in this case "trajectory in = trajectory out" --the speed leaving the system will equal the speed entering the system. The asteroid cannot be captured into orbit. But what if the asteroid passes very close to the Earth and enters the upper atmosphere? Friction and atmospheric pressure, unlike gravitation, are non-conservative forces. They can irreversibly reduce the energy of the meteor. Usually this ends with the meteor entering the lower atmosphere and burning up. But it's also possible for a meteor to skip out of the atmosphere. In that case, you would have a an object with a speed quite possibly reduced below Earth's escape velocity. This would place the new moonlet on a bound orbit around the Earth. Unfortunately, the perigee of the bound orbit would necessarily be located at the point in the atmosphere where the moonlet experienced its deceleration. This would be a long-ish elliptical orbit with the low end of the ellipse in the upper atmosphere. The moonlet would almost certainly enter on the next orbit and be destroyed in the atmosphere. It could survive as a natural satellite for hours or some days at most. This final fate can be avoided only if some interaction at the far end of the elliptical orbit pulls on the moonlet and lifts its perigee out of the upper atmosphere. A moderatly close pass by the Moon could accomplish this. That could provide a stable orbit for some months. But this second interaction is also a problem. In order to be useful for lifting the perigee, the pass by the Moon has to be relatively close. But this means that the moonlet will pass close to the Moon again within a few months. The second pass will most likely eject the new moonlet from the Earth-Moon system or cause it to impact the Moon. There are stable locations for small orbiting objects 60 degrees ahead and behind the Moon at the same distance from the Earth. These are the Lagrangian points L4 and L5. Artificial satellites and space stations may someday be parked there. Astronomers have searched for natural satellites in these regiond but have found no large objects. There's nothing bigger than a meter across at the Lagrangian points at present. Frank E. Reed Chicago, IL www.clockwk.com/fer ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Thu Jun 08 2000 - 20:17:33 PDT