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Occasionally Artificial Satellites are picked up by chance from Great
Shefford as they pass through the telescope/CCD field of view and
noticed in near real-time as the images are being transferred from CCD
to computer.
Low
altitude satellites move very fast and traverse the entire field
of view in a fraction of the length of a normal exposure and it is
therefore impossible to determine the direction they are headed or at
what speed, so any attempt at trying to follow them is hopeless.
However, some satellites move much slower, at speeds similar to Near
Earth Asteroids when making very close approaches to Earth (e.g.
anywhere from 100"/min up to 2,000"/min+). Although very
unlikely, it cannot be ruled out immediately that the observed object
may indeed be a natural Near Earth object (though rapid flashing tends
to indicate an artificial object), so further positions are obtained to
try and determine an orbit good enough to prove whether it is in a
geocentric orbit and therefore almost certainly artificial.
These slower objects will be in the same field of view for several
consecutive exposures, allowing an initial estimate of speed and
direction of motion to be made. To have any chance of orbit
determination the object needs to be followed immediately to allow
further positions to be obtained. As these exposures are being taken
Astrometrica is used to measure the exposures already made and these
early positions then used to make a linear extrapolation for subsequent
telescope positioning.
However, any object very close to the Earth will
be either accelerating or decelerating fast as seen against the sky and a
linear interpolation will often only suffice to keep up with the object
for the first 10-20 minutes. This period can be extended significantly
by fitting the observed RA & Declination co-ordinates to a quadratic
rather than a linear fit, but sooner or later FindOrb is needed to
calculate an initial orbit and ephemerides to predict further positions.
So far these exercises at Great Shefford have led to the
determination of a number of geocentric orbits (see some examples below) but maybe one
day a real NEO making a very close approach will be identified?
List of orbital elements and ephemerides:
Bill Gray's artificial satellite "Pseudo-MPEC"
index page
GS733A = 1987-105D = NORAD 18704 (4th
stage of rocket that launched COSMOS 1903 into a Molniya orbit on 21 Dec
1987)
GS697A = 1990-71D = NORAD 20745 (4th
stage of rocket that launched Molniya 1-78 on 10 Aug 1990)
GS695A = 1994-48D = NORAD 23197 (4th
stage of rocket that launched COSMOS 2286 into a Molniya orbit on 05 Aug
1994)
GS61LA = (unidentified, geocentric
orbit)
GS436A = (unidentified, geocentric
orbit)
GS428A
(link to Bill Grays' site) = (Unidentified
Molniya)
IMP8 (including J95-centric 'dark hours
only' ephemeris covering 2008-03-07 to 2009-03-01) from observations
2007-03-21 to 2007-11-05.
1983-020A (including J95-centric 'dark hours
only' ephemeris covering 2008-03-03 to 2009-03-02)
1983-020D (including J95-centric 'dark hours
only' ephemeris covering 2008-03-03 to 2009-03-02)
GEOTAIL (including J95-centric 'dark hours
only' ephemeris covering 2008-03-03 to 2009-03-02)
WMAP (including J95-centric 'dark hours
only' ephemeris covering 2008-03-03 to 2009-03-02)
2007-004B (THEMIS B) (including J95-centric 'dark hours
only' ephemeris covering 2008-03-03 to 2008-03-12). See
here for more information on this unusual object. Orbit corrections
appear to be made frequently, possibly every few weeks, making
predictions difficult.
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