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Asteroid Gallery

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.

See the diagram here showing where this near miss places 2005 WN3 in the list of only 9 minor planets with well determined orbits that have been observed while closer to the Earth than the Moon.

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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