2005 WN3 (Extremely small NEO caught inside the orbit of the Moon)
(Negative of original images shown to aid the visibility
of the faint object)
This object was discovered with the 1.5-m reflector of the Mt. Lemmon
Survey (part of the Catalina
Sky Survey) on four exposures made between 07:24 and 08:00 UT on 25
November 2005 and followed up again at 09:02 and 09:45UT. In those 141
minutes the object's apparent motion against the sky background
accelerated from 19"/min to 24"/min and it was obvious that
the object was already very close by and heading in towards Earth.
The first prediction by the Minor Planet Center was posted on the NEO
Confirmation Page (NEOCP) at 19:14 UT, by chance just minutes after the
equipment at Great Shefford Observatory had been turned on ready for
another night of observing. By 19:38UT the CCD camera had cooled down
ready to start taking images. Broken cloud would occasionally interrupt
the evening.
Even though it was only 10 hours after having been last seen at Mt
Lemmon, the uncertainty area indicated by the Minor Planet Center was
already about 4 degrees long and expanding very rapidly. The area
extended from the nominal position (expected to be the most likely place
to find the object) off to the north east. The points on the uncertainty
map were coloured red, indicating that the object was thought to be
within 0.01 AU of Earth.
A search for the object was started at 19:40UT and by that time the
prediction indicated that it would be moving at about 170"/min and
be about mag +17.7. An initial run of 21 short 4 second exposures was
taken at the nominal position, then further overlapping fields were
taken, working along the uncertainty area to the north east. By 20:01UT,
after covering about 1 degree the search had to be abandoned.
At 21:50UT the search was restarted and by now the MPC had completely
reworked the uncertainty map and had the range of possible positions
extending equally either side of nominal and the revised calculations
were actually showing a smaller range of positions than on the initial
map. The dots on the map were now coloured black, signifying that the
object was very close, within the orbit of the Moon. The entire
uncertainty area was covered but by now the prediction indicated the
speed had increased to about 500"/min and detecting it would be
very difficult. Nothing was detected on the images and so one last
attempt at detecting the object was made. The uncertainty area was
aligned along the direction of motion and so the telescope was
positioned slightly ahead of the start of the area and images taken
continuously as the uncertainty area passed through the field of view.
Nothing was detected using this method and by the time this was
completed at 22:20UT the predicted speed of the object had risen to
nearly 700"/min. The sky clouded up completely soon after.
The next day all the images were re-examined by using the technique
of interlaced
stacking, to help reveal fast moving objects. Nothing was found in
any of the images apart from the very first 5 images taken at the start
of the evening. Faintly visible on all five the object could be seen
moving swiftly out of the field of view to the bottom right (see
animation above). Stacking all five images, compensating for motion of
175"/min strengthened the image (see the background image with the
object indicated at bottom right. The animation is made up of
independent pairs of images stacked together, so images 1 & 2 were
stacked, then 3 & 4 and then 5 & 6. Image 6 was used to balance
the animation, the object is actually cut in half by the edge of the
frame in image 5. Subsequently, two telescope positioning images taken
immediately prior to the 21 exposure run (at low resolution) were found
to contain faint images of the object and these were also stacked
together. The four stacks were sent off to the Minor Planet Center at
13;28UT on 26 Nov 2005 and circular MPEC
2005-W65 was issued 5 hours later, announcing the object as 2005 WN3.
The object had been first picked up by the Mt. Lemmon survey when
about three times further away than the Moon. It was last seen from
there when it was about 2.6 times further away than the Moon. When
picked up at Great Shefford it was just inside the Moons orbit at about
0.9 Lunar Distances and passed Earth 4h 50min later, five times closer
to the Earth than the Moon, or about 6.5 Earth diameters away! At that
time it was in PiscisAustrinus, near the first magnitude star Fomalhaut,
magnitude +17 and moving at about 2,700"/min or about the diameter
of the full Moon every 40 seconds. Immediately after closest approach,
with the phase angle increasing very rapidly it will have faded very
rapidly at about 3 magnitudes per hour.
The size of the object has been estimated very roughly from its
brightness as being between 3 and 6 metres in diameter, making it one of
the smallest objects to have had a good orbit determined for it. At the time of discovery
only 2003
SQ222 was listed as smaller in the Minor Planet
Center's list of Closest
Approaches by Minor Planets and was the 4th
closest approach ever observed.
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