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2004 OH9, 2004 OJ9, 2004 PN2 and 2004 PO95 (a relay race of minor planet discoveries made at Great Shefford)

Chart prepared with SkyMap Pro Version 9

Chart showing minor planet movements from 2004 July 12 - Aug 21. 
Red and yellow dots depict positions of minor planets mentioned below on discovery nights.
White squares indicate field of view on discovery images

An object with temporary designation AO60675 was posted on the NEO Confirmation page on 2004 July 17th, listed at 18th magnitude and moving relatively slowly at 0.5"/minute. It was observed at Great Shefford early on July 20th and was given the provisional designation 2004 NN31 by the Minor Planet Center later that same day. It turned out to have a perihelion distance just larger than 1.3 AU (the upper limit for Amor type objects) and so was not categorised as a Near Earth Object.

The chart above shows a ~ 4° x 6° area of the vicinity in western Aquarius, about 1° south west of Uranus where 2004 NN31 was at discovery, together with a number of nearby minor planets observed in the following weeks.

While exposures were still being taken that night, the images already available were being stacked to make sure the new object was visible. It was found in the stacked images, but also another brighter minor planet could also be seen moving at about the same speed, moving north to south, fast approaching a field star. At that time this object did not show up on the Minor Planet Checker (it did by the next day, with designation 2004 NX21) and so, thinking it may be a new, previously unknown minor planet, further exposures were taken.

Eventually, after 2004 NX21 had moved past the field star, all the exposures were stacked and examined. As well as the two minor planets already noted, three fainter ones were found, none of which showed up on the Minor Planet Checker. Each of these was measured and the astrometry sent off to the Minor Planet Center.

Soon afterwards the Minor Planet Center identified one as 2001 TN218. This object had been followed for 10 weeks between October and December 2001 and had not been seen since,  by the time it was picked up in the field of 2004 NN31 it was about 0.5° off track. Two cloudy nights followed before confirmation positions could be obtained for the other two objects and sent off to the Minor Planet Center. Designations came back indicating that they were new objects 2004 OH9 and 2004 OJ9, credited to Great Shefford Observatory. Väisälä orbits were calculated for the two by the Minor planet Center (some simplifying assumptions being made because of insufficient positions being available to allow a general elliptical orbit to be determined).

Bad weather continued, further positions for 2004 OH9 were obtained on July 24.0 in poor conditions (2004 OJ9 could not be recorded) and positions for 2004 OJ9 on July 28.0. The next opportunity on Aug 7.0 was used to get further positions for 2004 OJ9 but these images showed another moving object that could not be identified using the Minor Planet Checker. A second night of positions were obtained the next night on Aug 8.0 and sent off to the Minor Planet Center. This object was eventually designated 2004 PN2 and again credited as a discovery to Great Shefford.

By this time, both 2004 OH9 and 2004 OJ9 had enough observations (including some from the surveys LINEAR, NEAT and LONEOS) for reasonably good orbits to be calculated, so attention was switched to getting at least three nights for 2004 PN2. The next opportunity was Aug 13.0 and positions were obtained. Remarkably, another moving object was found on the stacked images and once again did not match any object on the MPC's Minor Planet Checker. The following night was cloudy, but a second night was made on Aug 15.0 and the measurements sent off to the Minor Planet Center. The designation for the new object was returned as 2004 PO95 and again credited to Great Shefford.

So, in a period of four weeks, two minor planets were discovered in a single field while following up a NEO Confirmation Page object and two further minor planets were found while following up on previous discoveries.

By August 16th the Minor Planet Center had managed to link observations of 2004 OJ9 to a single night of positions reported by Spacewatch in February 1997 and from single nights reported by Spacewatch, LINEAR and NEAT surveys in February & March 2002.

Update March 2006: 2004 OJ9 was recovered at its next opposition from Great Shefford on 05 Nov 2005 and was last observed in Feb 2006. It received permanent number (128465) in March 2006 from the Minor Planet Center and becomes the first minor planet discovery from Great Shefford to be numbered.

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