IMP-8
Put on the NEOCP with temporary designation NK0154 in Feb 2003 this
'NEO' turned out to be an interesting Artificial Satellite called IMP-8
(Interplanetary Monitoring Platform) in a 12 day orbit around Earth,
launched in 1973.
A very interesting account of the follow-up recovery
by Reiner Stoss can be read
here and more about the
satellite itself can be found from the IMP-8
NASA web site.
For NEO astrometrists IMP-8 serves a good target to practice very
fast moving object (VFMO) work, it can generally be imaged for 1-2 days
in each 12 day orbit, travelling at 70 - 90 arcsec per minute at about
mag. +18 to +19. Bill Gray has provided astrometric results for IMP-8
on his ProjectPluto site and current ephemerides can be generated from
the MPC's Distant
Artificial Satellites Observation Page. A review of the history of
IMP-8's orbital status from 1973 right up to 2003 was available
here by
Heather Franz, a member of the IMP-8 team (though by Jan 2004 this had
been removed). |
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With four months of astrometric observations available to help predict
the satellite's position, Bill Gray determined that the ephemeris was only
30" arc off by the
time it was imaged on 17 Nov 2003. In a fairly crowded starfield and
moving at 66"/minute the satellite was revealed by stacking images
in a slightly different way to that done for more 'normal' targets. A
series of 45 short (6 second) exposures was taken, with 20 seconds
between the start of each image. Normally these would
be stacked in consecutive groups (e.g. with 45 exposures stacking images 1-15 as one
image, then stacking images 16-30 as the next and 31-45 as the next).
However, with a very fast moving object, trying to find a single
faint dot on three frames in widely different places on the images is
quite difficult. So instead the images were stacked in interlaced
sets (e.g. images 1,4,7,10 etc were stacked as one image, images
2,5,8,11 etc as the next and 3,6,9,12 etc as the third image). The
advantage is that this places the target object in a similar place on
all three images (where the target actually was at the times of images 1, 2 and
3). See the interlaced stacking
page for more details.
Below is the full field of view from 17 Nov 2003 showing the
rich starfield and the track of IMP-8. A close-up animation of the
stacked images then reveals IMP-8 clearly by its motion. As with the
original single frames there is an interval of just 20 seconds between
the start of each stack, though each stack actually has a total exposure
of 36 seconds, (each being a stack of 6 images). Only the images in the
first half of the run of 45 were used, as the target ran close to some
field stars later as can be seen on the indicated track below.
Animation using interlaced
stacking: 
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