Wednesday, 5 October 2011

2011 September notes

A big improvement over recent months, September ended with a remarkable and record breaking heatwave in the UK and more than half the nights in the month being at least partially usable at Great Shefford, though fog and mist stopped work several hours before dawn on a number of occasions.

After a quiet summer, both PANSTARRS and the Catalina Sky Survey (CSS) telescopes were in full survey mode, looking for new Near Earth Objects and astrometry was contributed for 43 of the objects posted on the NEO Confirmation Page.

PANSTARRS picked up 2011 SP25 on September 20th, relatively faint at mag +20 and slow moving, I observed it 2 nights later and measured it at (red) mag +19.7. It is not a NEO and at the time of writing (Oct. 5th) there are now two weeks of astrometry available and the elements show it to be in a highly eccentric retrograde Halley-type orbit with period 89 years and perihelion at 2.27 AU due this coming November. It looked completely stellar in my images and will probably stay that way, but may be worth keeping an eye on in the next couple of months.

A couple of moderately close NEO passes were well observed, 2011 LJ19, discovered back in June by CSS came closest mid-month, at about 17 Lunar Distances (LD) and maintained 16th mag or brighter for about 2 weeks. A CSS discovery from September, 2011 SR5, passed about 22 LD towards the end of the month and peaked at mag +16 for about 5 days, reaching a speed against the sky slightly faster than 2011 LJ19 at 35"/min.

The closest observed approach though was 2011 SE58, a very faint mag +20.8 discovery made early on September 23 by the Spacewatch team using the 1.8-m reflector on Kitt Peak, when it was moving slowly at 0.8"/min. Spacewatch observed it again the next night by which time it had tripled in speed and was about 0.7 mags brighter, closing in fast at a range of about 14 LD.

I observed it 11 hours later on September 24 at 21:00 UT and found it more than a magnitude brighter at +18.7 and moving at 4"/min. Its distance had approximately halved to 8 LD in that time and a very close approach to 0.6 LD was predicted for September 27.1 UT. The next night was cloudy in Great Shefford but the night of the close approach started out clear. By 23:00 UT 2011 SE58 had risen to an altitude of 14 degrees, just high enough in the east to clear my house roof but it was moving so fast due east and still accelerating that it almost kept pace with the Earth's diurnal motion and stayed in approximately the same place just above the rooftop, edging in azimuth towards the south for the next hour, until mist came down and stopped any further observation. Although registering between mags +14-15 it was moving so fast and at a low altitude that it was unfortunately never recorded strongly enough for any accurate photometry to be attempted.

Here 2011 SE58 is seen passing from west to east through the 18'x18' field of view in less than 2 minutes

When first picked up that night it was moving at 480"/min at 0.80 LD and when last recorded 58 minutes later had accelerated to 600"/min, had approached to 0.72LD and moved a total of 9 degrees against the sky background. The Minor Planet Center Daily Orbit Update published the next morning included the measures I had made that night and also included some pre-discovery PANSTAARS astrometry from 2 days before the Spacewatch discovery observations, when it was 22nd magnitude and had only moved 9" in the hour it was under observation, quite a change in a week!