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2003 NZ6 (A personal account of a brief encounter with an Aten)

The Search

2003 NZ6 was discovered by LINEAR early on 9th July 2003 and posted on the NEOCP with temporary designation AH13022. It was listed as moving at around 14"/minute, quite bright at V=+17 and very well placed during the short summer night from Great Shefford.

The original ephemeris generated from the MPC's NEOCP for Great Shefford that night is shown below:

The NEOCP main page still listed the object as having been added (not updated) and still a 1 Nighter. This, together with the  uncertainty map/offsets column still being present in the ephemeris indicated that the object had not yet been confirmed by any other site since having been posted on the NEOCP.

The motion in the ephemeris showed rapid acceleration in just a few hours, indicating the object was expected to be rapidly closing in on the Earth and the uncertainty map (see below) defining the area the object was likely to be found already extended over 1.5 in RA and 3.5 in Dec, less than 14 hours after LINEAR had last imaged it, though the most likely region it was to be found was still concentrated within +/- 0.5 of the nominal ephemeris position.

If the object wasn't picked up soon the uncertainty region would become so large it would be too big to practically search. Indeed, the NEOCP ephemeris, when extended to the next evening was showing motion of 250"/minute, a VFMO! As it turned out it didn't get anywhere near that fast...

The uncertainty map for AH13022 at 22:00UT on 09 July 2003
adapted from original data available on the MPC's NEO Confirmation Page

Being the middle of summer the ambient temperature didn't allow the CCD to cool down to -40C (the temperature it is normally operated at) and imaging started at 22:00 UT with the CCD at -33.3C.

As there was potentially a large area to search the camera was binned 2x2, cutting the image download time by a factor of four and allowing two 13 second exposures to be taken per minute. With a field of 25x25 arcmin the search was conducted in 20x20 arcmin squares, giving some overlap to help counteract the effects of vignetting. For convenience the grid for the uncertainty map above was set to match the search field size. 

6 images of each search area were taken before moving to the next area (sometimes the magnitude given on the NEOCP can be significantly brighter than an object really is, so 6 images allows detection even if the object turned out to be fainter than the indicated 17th mag).

The first set of 6 images was taken centred on the nominal ephemeris position. The next run was chosen to be centred on the adjacent part of the uncertainty map to the SouthWest, re-centering 10 arcmin South and 10 arcmin West,to align with the 20 arcmin squares on the grid. While the second run of 6 images was exposing Astrometrica was used to blink the images from the first run to manually search for the object. When the second run had finished the camera was centred 20 arcmin south. The third run of 6 exposures was then started and the second set was blinked with Astrometrica.

The third run was stopped half way through its sequence because the object had been located on the edge of the second run of images! The MPC prediction had been good, only 16 arcmin west and 22 arcmin south of the nominal position. I was lucky that just a fraction of the indicated 1.5x3.5 uncertainty area had needed to be searched before locating the object.

 A full frame from the second run showing the object close to the edge of the field and an animation of the movement that revealed it appears below:

AH13022 animation 09 Jul 2003

Main image: 13 second exposure started 22:07:10UT 09 July 2003, field 25'x25'
Animation: six 13 second exposures, start times from 22:06:40 to 22:09:10 09 July 2003, field 2'x2'

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