Sunday, 10 April 2011

2011 March notes: 4 close approach NEOs

Fortunately March broke the 4-month run of very poor observing conditions at Great Shefford, with 16 usable nights and additionally plenty of Near-Earth objects to observe. March and Oct/Nov are the months that the surveys are generally most successful at discovering very close-approaching minor planets and this month they again had a bumper crop.

2011 EY11 discovered on March 5th with the Mt. Bigelow Schmidt by the Catalina Sky Survey team made a very close pass to just 0.34 Lunar distances (LD) from Earth at 03:36 UT on March 7th. It was picked up from Great Shefford just before 8pm on March 6th when it was already 16th mag and moving at 170"/min. It had come inside the orbit of the Moon about an hour before, but when last recorded, at 01:55 UT on March 7th was 0.38 LD away and travelling at over 800"/min. Heading almost due South its declination decreased from +16° to -25° in the 6 hours it was under observation and was likely to be only about 6 meters in diameter. It showed large and very rapid variations in brightness of 1 mag or more, but a lightcuve has not yet been reduced.

Another discovery from March 5th with the Mt. Bigelow telescope was 2011 EC12, which was to make an approach to 3.3 LD during the early evening of March 8th. It was observed at mag +16.7 and moving at 100"/min on the night before closest approach but when observed at the point of closest approach on March 8th was up to 0.7 mags fainter due to the rapidly increasing phase angle and it had accelerated to 150"/min too, both factors making it a more challenging object that second night.

2011 EU20 was first picked up by the Mt. Lemmon 1.5-m telescope of the Catalina Sky Survey on March 8th, 3 days before making a pass to within 1.62 LD of Earth. Observed on the night of March 9th at mag +17.2 and again the next night at mag +16.1. When last detected at 03:36 UT on March 11, 7 hours before closest approach it was at 1.7 LD and moving at 160"/min. Again, relatively small, with an estimated diameter of about 11 meters.

2011 EW74 discovered on March 15 from Mt. Bigelow was a larger object with an estimated diameter of 65 meters and made an approach to 10 LD on March 21. Even though more distant than the other objects mentioned, because of its larger size it would still reach mag +16.1 for a few days either side of closest approach. Unfortunately this coincided with the full Moon and for the three days when 2011 EW74 was at its brightest it was always less than 40° from the Moon, reducing the signal/noise ratio of the images and making photometry more difficult. No obvious brightness variations were noted but a full reduction of the images obtained has yet to be completed.