Image of the Month - November 2004

Triple Stack

Minor planet 2004 TD18 had been listed with potential impact solutions on the JPL risk page from 15th October 2004, 2 days after discovery with the Spacewatch 0.9m telescope at Kitt Peak. The Spaceguard Central Node had posted an observing campaign for 2004 TD18 on October 18, asking for more astrometry to help refine it's orbit before the object faded from view. Listed as magnitude +21.1 by the Minor planet Center, Astrometrica's track & stack feature enabled Peter Birtwhistle to recorded the NEO in the night of October 21/22 with a S/N ratio of 9.0 in this stack of 120 exposures of 30 seconds each. Birtwhistle used a CCD camera on the 0.3m f/6.3 Schmidt-Cassegrain telescope of his garden observatory located in Great Shefford, England. With the astrometry from these images, and another set from the Spacewatch II telescope on October 23, JPL was able to remove the last predicted impact solutions.

2004 TD18
Credit & Copyright: Peter Birtwhistle, Great Shefford Observatory

On the left panel in the picture above, the images have been stacked to compensate the motion of the NEO, moving 2.1" per minute in PA 230°. Most of the trails in the image were caused by field stars, but to the lower right of 2004 TD18 there is a trail in a somewhat different direction: It was caused by another minor planet, (42454) 4134 T-3. The trail of that 16mag main belter easily shows up in the middle panel, where the images have been stacked without compensating for any motion - but the faint NEO, of course, is lost now. In the right panels, the frames were stacked to compensate for the motion of (42454) 4134 T-3 (0.5"/minute in PA 259°), which is now visible as a bright dot among shorter star trails. "The two minor planets passed each other by about 18" about 1/2 hour after the last image was taken, at ~01:14 UT on 2004 October 22", notes Birtwhistle. The field shown in these frames covers 10' x 10', and id enlarged by 50% from the original image scale.

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