----- Original Message ----- From: Rosely Gregio <gregio@dglnet.com.br> To: <FrankEReed@aol.com> Sent: Friday, June 09, 2000 3:00 AM Subject: Re: Natural Satellites > Hi all! > But... and on the Asteroid companion of the Earth with orbit type > horseshoe - 3753 Cruithne (1986), as that is explained? For the that I read, > it was a captured Asteroid! > > Hugs > Rosely Gregio > > ----- Original Message ----- > From: <FrankEReed@aol.com> > To: <patrickfwd@yahoo.com>; <SeeSat-L@blackadder.lmsal.com> > Sent: Friday, June 09, 2000 12:15 AM > Subject: Re: Natural Satellites > > > > 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 > > > ----------------------------------------------------------------- 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 - 22:59:19 PDT