2006 BV39 (Extremely small NEO caught inside the orbit of the Moon)
On Jan 26th Terry Bressi and Mike Read using the Spacewatch 1.8-m
reflector discovered 2006 BV39, confirmed 15 hours later from
Klet Observatory in the Czech Republic. It was about 4 lunar distances
away from Earth at discovery, mag +19.5, moving at 14"/min and was
closing in fast.
In a conversation with Monty Robson at 932 (John
J. McCarthy Observatory in Connecticut) on the morning of Jan 27th I
mentioned that 2006 BV39 would be an interesting target the
next night (when it would be very close to Earth). Later that day,
nowhere had reported it to the Minor Planet Center after the
confirmation from Klet and by the time images were being taken at
midnight on Jan 28th from Great Shefford the uncertainty of the
ephemeris was quite large and it was not found on the images.
On checking with Monty, he mentioned that he had got it the night
before but had not reduced the astrometry yet. He kindly measured the
positions immediately and sent them through to Great Shefford, allowing
the ephemeris to be updated and that was good enough to locate the NEO
during its very close approach to Earth.
It was 23' W of the MPC's prediction when recovered and would have
been very difficult to find without the ephemeris correction. By this
time it was moving at 210"/min and was just 5,000 miles outside the
Moon's orbit. It was followed at Great Shefford until 05:14UT on the
29th by which time it had accelerated to 277"/min and had crossed
28,000 miles inside the Moon's orbit with closest approach about 2 hours
later at 0.877 Lunar distances. This was the third NEO tracked from
Great Shefford while inside the Moon's orbit in four months! (See 2005
UW5 and 2005 WN3).
All three appear in this diagram
of the handful of NEOs with well defined orbits that have been observed
while closer to the Earth than the Moon.
2006 BV39 is very small and with an absolute magnitude of
+29.0 is estimated to be between just 4 and 9 meters in diameter. With such a
small object, large variations of brightness can result if the object is
irregularly shaped and if spinning fast can result in large variations
of magnitude in short periods of time. This appears to be the case with
2006 BV39, where it can be seen to peak at a magnitude of
+16.8 in frame 2, then fade at least 1.8 magnitudes within 86 seconds so
as to be virtually invisible by the last frame in the animation below.
Each frame keeps the NEO centered in the field of view and is the result
of stacking 9 x 1 second exposures together (with a 2 second gap between
exposures), so averaging the brightness of the NEO over a period of
about 24 seconds. The rapid fading is very evident and on the last frame
there is only the hint that the object has been recorded at all.
2006 BV39 was 0.00227 AU from Earth (or 0.88
lunar distances) when the images for this animation were taken., each
frame has been enlarged x2 and is a stack of 9 x 1 second exposures using the 0.40-m Schmidt-Cassegrain
and CCD. The field of view is 4' x 4'.