Thursday, 4 September 2008

2008 August notes: Observing statistics, 2004 SB56, 2007 RT12, 2001 QL153, 2008 PG1

The weather was very poor at Great Shefford in July and August. With 25 hours at the telescope, July provided less than half the observing hours logged for that month in any of the last three years. August was officially the cloudiest August in the UK since records began in 1929 and provided only about 50 (generally poor quality) hours, again well below totals from recent years.

However, the night of August 3/4th provided the opportunity to recover three NEOs not seen since their discovery apparitions and a somewhat unusual confirmation of a NEO Confirmation Page (NEOCP) object.

Aten 2004 SB56 had sped northwards after conjunction with the Sun in July and in the 10 days before recovery had brightened 3 magnitudes and increased elongation from 35 to 72°. With an uncertainty area on the sky of 14' it was picked up only 3' from prediction at mag +16.7 and about 6° from the North Pole, a relatively easy recovery.

Apollo 2007 RT12 had an uncertainty of about 1° and at mag +20.0 was a potentially difficult object to recover, just 1° east of Alpha Cep, deep in the Milky Way. Fortunately it was only 8' from prediction and not involved with any field stars during the time it was imaged. With an absolute magnitude of +23.9, equating to an estimated diameter of 30-95 meters, there have only been 10 other NEOs recovered at a second opposition that are smaller than 2007 RT12. Included in this set are the Earth co-orbital companion 2002 AA29 and the tiny 2006 RH120 that was temporarily captured in Earth orbit during 2006 & 2007. My thanks to Sergio Foglia for providing a list of small NEOs seen at more than one apparition.

The third object was 2001 QL153, an Amor that had been observed for about 4 months and had not been seen since January 2002. It too had an uncertainty of about 1° and predicted to be mag +20.5 and could have posed a problem to pick up. It was in a less crowded field than 2007 RT12 and after more than six years since last being observed was recovered only 9' from prediction.

A LINEAR discovery put on the NEOCP on August 3rd turned out to be something of a puzzle. 2008 PG1 was observed from 9-10am UT on August 3 by LINEAR at mag +19, moving at 9"/min from west to east, in the morning sky about 13° north of Mira Ceti. By 2am on August 4th it was predicted to be accelerating and to have an uncertainty area 1.7° long, extending roughly West to East, in the direction of motion. I eventually confirmed it 43' further west than the most westerly point of the uncertainty area (quite unusual for the MPC prediction to be this far out) and moving at about the same speed as when LINEAR discovered it. The orbit appears to be rather indeterminate but it was announced on MPEC 2008-P14 with just the observations from LINEAR and my set of confirmation positions. The orbital eccentricity had been assumed (done sometimes when the orbit is rather indeterminate, to try and help improve the orbit solution). No other observatories had reported positions for the NEO before I got my next chance to observe the area early on August 9th. By then, using FindOrb software from Bill Gray to generate a large number of 'Monte Carlo' orbits to estimate the likely uncertainty area, it looks like it could have been anywhere in a nearly 3° long band. I didn't manage to locate it that morning nor on my next opportunity 4 days later and it has not been reported from anywhere else since. It is now listed as a Virtual Impactor by JPL with 97 potential impacts, the first in only 5 years time, but with such large uncertainties in its orbital elements the chance of collision then is given as just 1 in 200 million!

(Update 10 May 2012: 2008 PG1 was linked to 2009 EV by the Minor Planet Center, read more here)