Thursday, 5 June 2008

2008 May notes: 2008 JL24

After a promising opening week, May turned out to be yet another disappointing month, with just a handful of (cloud interrupted) nights.

The most interesting object observed during the month was 2008 JL24, a mag +17.5 LINEAR discovery added to the NEO Confirmation Page (NEOCP) just before midnight on 11th May and predicted to be moving at about 25"/min. The sky cleared at Great Shefford a couple of hours later and I started searching for it at 00:25 UT. The uncertainty area indicated by the Minor Planet Center was about 80' wide, much bigger than my 18'x18' field of view. I started imaging at the nominal position, at the western end of the area and while the first set of images was being taken I checked with FindOrb whether by chance any of the astrometric positions submitted by LINEAR had poor residuals, in case I could possibly improve the prediction. All the positions looked good, but the ephemeris generated by FindOrb had the object way off to the east, beyond the furthest eastern extent of the NEOCP uncertainty area.

I decided to start searching from the FindOrb prediction and to work my way back to the western side of the NEOCP area with overlapping fields. I picked up the NEO in the corner of the second field attempted, over 1.5 degrees from the nominal position, but in fact only 10 minutes after I had started observing! I re-centred and obtained some further images and sent off the astrometry to the Minor Planet Center.
Initial uncertainty area for 2008 JL24.
White cross is MPC expected position, orange and red dots indicate MPC expected uncertainty area, yellow boxes are telescope fields originally planned to cover MPC uncertainty area.
4 coloured boxes on left show the search fields taken, with the actual position of 2008 JL24 marked with a red cross. Arrow length indicates the motion of the NEO in 1 hour.
Sky chart and field boxes plotted using SkyMap Pro 9.

When LINEAR discovered 2008 JL24 it was just inside the orbit of the Moon, at 0.94 Lunar Distances (LD) and by the time I confirmed it 18 hours later it had already receded to 1.4 LD. Over the years LINEAR has discovered a number of fast moving objects that were probably closer than the Moon at the time, but never picked up again and in fact 2008 JL24 is the first time that a NEO has been discovered while closer than the Moon and then subsequently confirmed. It was well observed over the next five days before fading out of sight just before the full Moon.