Re: Natural Satellites

Date: Thu Jun 08 2000 - 20:15:52 PDT

  • Next message: Ted Molczan: "USA 116 and 129 elements and observation"

    In a message dated 6/8/00 6:49:14 PM Central Daylight Time, 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
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    This archive was generated by hypermail 2b29 : Thu Jun 08 2000 - 20:17:33 PDT