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2005 UW5 (Small NEO observed inside the orbit of the Moon)

LINEAR discovered 2005 UW5 at 6am UT on 2005 Oct 27 on 5 images at about mag +19.5V and moving at about 6"/min to the NW. It was posted on the Minor Planet Center's NEO Confirmation Page with temporary designation AV50318 at 21:42 UT the same day. The initial prediction indicated that the object was rapidly accelerating against the plane of the sky, nearly doubling in speed in 24 hours, a good indication that it was thought to be already very near to the Earth and closing fast. Uncertainties in the prediction were also growing rapidly, so the area of sky it might be found in was getting bigger and bigger as the hours went by.

The night of 2005 Oct 27 was rather poor at Great Shefford, with a lot of drifting cloud and poor transparency at low altitudes. A run was attempted at the predicted place for AV50318 10 minutes after the MPC posted the details on the NEOCP, but nothing was found and the conditions so poor that no further searching was attempted. It turned out later that the object was actually 21' east of that first prediction and 12' outside of the 18'x18' field of view.

LINEAR confirmed their discovery with exposures taken between 04:33 and 05:53 UT on 2005 Oct 28. The MPCs prediction on the NEOCP was updated at 22:29 UT and MPEC 2005-U84 announcing the discovery followed about an hour later. The object was already about twice as far away as the Moon and a very close approach in 24 hours time was indicated coming to within about half the distance of the Moon, or about 17 diameters of the Earth, and at high northern declinations. This would make the time of closest approach particularly favourable to European observers.

Still with a day to go before closest approach, skies partially cleared at Great Shefford just after 1am local time on Oct 29th, with 2005 UW5 moving at about 35"/min and at mag +17.
258 images were taken, 166 were spoiled by clouds but the remaining images were used to produce 9 positions and these were sent off to the Minor Planet Center, helping to refine the ephemeris for the close approach due later that day. The following animation shows 2005 UW5 moving at 38"/min between 01:52 -01:59 UT on 2005 Oct 29:

2005 UW5 2005 Oct 29 01:52-01:59UT
Field 8.5'x8.5', North up. 0.40-m Schmidt-Cassegrain & CCD
Each frame a stack of 20 x 4 second exposures

After a day of blue skies at Great Shefford, by the time darkness fell on the evening of 2005 Oct 29th clouds rolled in and conditions were not looking good for picking up 2005 UW5 at its closest approach, minimum distance due at 00:16 UT, at a distance of 0.0013 AU (the Moons distance is 0.00257 AU). However there were occasional breaks in the cloud and imaging was started at 18:44 UT in the hope that some would show the NEO. A number of runs were successful with the best conditions around 21h UT, just before the sky finally completely clouded over. All of the images taken for astrometry were 1 second exposures with the CCD camera binned 3x3, but the final image taken was a single 60 second exposure binned 2x2 showing 2005 UW5 moving 525" during the exposure (see image above).

Click on the image below to see an 11 frame animation of 30 seconds of motion between 21:12:00 -21:12:30 UT. Each frame is a 1 second exposure and the animation plays back in real time, exactly as the images were taken, with a 2 second gap between frames. The images were binned 3x3 and are 341x341 pixels wide, the field of view is 18.3'x18.3'.

(Please note: clicking on the image below will load a 1Mb animated .gif)

Although at the time of the close approach by 2005 UW5 it was only the 16th closest fly-by of a Minor Planet with a reasonably determined orbit, in fact only 6 of these Minor Planets had ever actually been observed while closer than the Moon. 2005 UW5 was only inside the Moons orbit for about 20 hours and only observable for about 13 of those.


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