In 1950 Jan Oort inferred the existence
of a huge comet cloud surrounding the solar system. Today the 'Oort Cloud'
is an accepted part of the Solar System, containing perhaps 1012
comets up to 50000 astronomical units (au) from the Sun, though none have
actually been detected. Gerard Kuiper hypothesised that there might be
a second comet cloud much nearer than the Oort Cloud. At distances greater
than 40 - 50 au the space density of proto-planetary material would be
too low to support continued accretion of bodies to sizes greater than
a few hundred kilometres across, but it seemed unlikely that accumulation
would stop sharply at 30 - 40 au (the Neptune-Pluto distance). Naturally
these comets would be extremely faint due to their large distance and assumed
low visual albedo. Several searches took place between 1989 and 1992 using
both photographic and CCD techniques, but all were unsuccessful until the
first such object, 1992 QB1, was finally discovered by researchers using
deep CCD images. They discovered a second, 1993FW, six months later. The
INT prime focus camera with the large format Ford-Loral CCD is ideal for
the search for trans-Neptunian objects since the pixel size of 0.37 arcseconds
allows adequate sampling even during sub-arcsecond seeing, while the large
area of 12.6 arcminutes square enhances the probability of detection.
In September 1993 researchers
obtained 30 minute R band images in clear weather and good seeing, achieving
a limiting magnitude of R=23. Objects at distances of more than 30 au slowly
retrograde along the ecliptic at velocities of less than 4 arcseconds per
hour, and so images of fields were obtained at least 2 hours apart, and
were compared to reveal any moving objects. The search revealed two trans-Neptunian
objects, named 1993 SB and 1993 SC. 1993 SB was barely visible at R=22.7,
while 1993 SC was much easier to detect at R=21.7. Analysis of the positions
by Marsden of the International Astronomical Union gave initial circular
orbits of radii 33.1 au and 34.5 au respectively. Both are located approximately
60 degrees away from Neptune in heliocentric longitude, and therefore there
is a small possibility that they could be Neptune Trojans librating around
the L4 Lagrangian point in the Sun-Neptune system.
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ING facilities involved:
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Isaac Newton Telescope,
with prime focus CCD
Some references:
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Williams, I. et al,
1993, "1993 SB and 1993 SC", IAU Circular, 5869
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Williams, I. et al,
1995, "The slow moving objects 1993 SB and 1993 SC", Icarus, 116,
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