2007 September notes: 2007 RJ1, 2007 SU1 and 2007 RS1

Plenty of clear nights in September with quite a lot of activity from the surveys on the NEO Confirmation page. Indeed, around September 17th there were 32 objects listed, which may be a record.

Several close approaches were observed during the month, including 2007 RJ1 which was followed for 12 days and ended up at 2.8 Lunar Distances (LD) on Sep 16.1UT moving at 102"/min. 2007 SU1 was seen at 6 LD, moving at 44"/min on Sep 27.0UT.

However, the headline object for me was 2007 RS1 on Sep 4th. This tiny NEO was discovered by Steve Larson using the 1.5-m Mt. Lemmon Catalina Sky Survey telescope just before 6am UT on the 4th. It was unusually faint at mag. +20.4 considering it was already moving at 13"/min. Steve managed to track it for over 3 1/2 hours, by which time it had accelerated to a speed of 19"/min and brightened by half a magnitude.

By the time the sky was getting dark at Great Shefford later that day 2007 RS1 was already closer than 1 LD and moving at 150"/min but had not been reported since the Mt Lemmon observations. The positional uncertainty on the sky was rapidly getting worse and predicted by the Minor Planet Center to be about 3.5 degrees at 20:00 UT (11 times larger than my field of view) and set to double in size within the next 70 minutes. I started trying to cover the uncertainty area, starting at the nominal position, but after taking just a couple of fields got a phone call that I needed to pick my daughter up from Newbury (a 20 mile round trip). All I could do was to set the telescope some distance ahead of the predicted place and let it take images while I was away, hoping that the object might just pass through my 18'x18' field of view. I set the exposure length to 4 seconds which would cause the object to trail if it did pass through, making it more obvious to identify but at the expense of some accuracy in measurement.

Background stars appear as lines of dots as the fast moving asteroid is kept in the centre of the image and itself appears as a small streak due to its own movement during each individual exposure

By the time I returned from Newbury the sky was clouding over for the night but I had amassed a total of 756 images, 564 taken in one long sequence while I was driving. After spending a number of hours searching the images I eventually managed to locate the tell-tale streaks of the NEO speeding through some of the images taken while I was away from the observatory. Remarkably it passed almost centrally through the field of view, taking just four minutes to pass from one side to the other. With the ephemeris corrected I was then able to find a few more images of it entering the field of view at the very end of one of the early runs I took before I left.

The last images that recorded it were taken at 20:39 UT when it was mag +18, moving at 246"/minute and with it 0.55 LD from Earth. It was to pass just 0.19 LD from Earth, or less than 6 Earth diameters away at 01:18 UT on September 5th, but was it was not reported again.

JPL is listing 2007 RS1 with the faintest absolute magnitude of any Minor Planet, with H = 30.98 +/-0.36, the previous record being 2003 SQ222 at H = 29.99 +/-0.70. It is likely that 2007 RS1 is only about 1 or 2 meters in diameter.