Thursday, 5 July 2007

2007 June notes: 6R10DB9's fourth and final perigee

As anyone in the UK will already know, the poor weather in May just got worse through June. I suffered the least number of hours observing since December 2002.

The unusual object with provisional designation 6R10DB9 made it's fourth and final perigee in June. Back during the March perigee it was observed with the 10-m SALT telescope and found to have a rotation period of only 2.75 minutes and an amplitude at that time of 1.2 magnitudes. This time it approached from the sunward side of the Earth, passed over the North pole and was picked up again a day after perigee on June 15th in Draco, about 1 degree from Theta Cep while only 0.80 Lunar Distances from Earth, magnitude +19 - +20 and moving at 47"/min. Never 'bright', it was recorded at mag +18.6 on June 18th and at about +19.5 on the 20th, fading again as it receded from both Sun and Earth. The only other observations reported at the time of writing are radar detections from Goldstone on June 12 & 14 and optical astrometry from the 1.5-m Mt Lemmon telescope on June 22. By the end of June it was below 20th mag and too far south to attempt from the UK.

The main indicator that this object is likely to be natural rather than a piece of artificial space junk come from determinations of solar radiation pressure (SRP) acting on it. Bill Gray's FindOrb freeware program includes routines to calculate SRP, given as the area-to-mass ratio or AMRAT in m^2/kg. With astrometry extending for 10 months now, it gives a value of 0.0011, with an RMS residual from 132 observations of 2.5". Not including SRP increases the RMS residual to an unsatisfactory 9.1". The JPL Horizons orbit solution which also includes the Goldstone Radar observations, gives an even smaller value of AMRAT of 0.0007. In comparison, various distant artificial satellites, such as IMP8, Geotail, J002E3 (the piece of old Apollo hardware discovered by Bill Yeung in 2003), WMAP etc have values between 10 and 50 times larger, indicating that 6R10DB9 is much more massive and much less prone to the perturbing effects of SRP than artificial satellites and is therefore likely to be a rocky body, probably about 4 metres in diameter. There is still a chance that time might be spent at some of the large observatories to get spectra and other measurements during July before it goes out of range completely.