Tuesday 10 April 2012

A rush of NEOs near Earth

Rather unusually, two newly discovered near-earth asteroids were observed as bright as 13th magnitude during March. The first to be discovered was 2012 EG5, by the 1.8-m Pan-STARRS telescope on March 13th. It was only 21st magnitude at that time and still 19 days away from closest approach. Many observers reported it brightening steadily on its way in and several posted on the Minor Planet Mailing List (MPML) forum that they had measured it to have a 17.5 minute rotation period, with an amplitude of about 1/3 mag. My last observation, at 23:05 UT on March 31, found it at 13th mag and rushing south at 155"/minute, already at a declination of -25° and altitude of only 11°. It was just 5,000 miles outside of 1 Lunar Distance (LD) from Earth and would reach a minimum distance of 0.6 LD about 10 hours later but at a declination of about -55°. On the MPML, Dr Lance Benner mentioned some days later that although they had expected to be successful, his team using the Goldstone RADAR had failed to record echoes from 2012 EG5, indicating that it might be significantly smaller and/or less reflective than they had thought, based on an assumed diameter of ~50 metres, derived from its absolute magnitude.

The second 13th mag NEO was 2012 FP35. It was discovered by the Catalina Sky Survey with their 0.68-m Schmidt on March 24 at 06:12 UT and in contrast to 2012 EG5 it was just 48 hours from closest approach. By the evening of March 24th in the UK it was still listed as unconfirmed on the NEO Confirmation Page, with an indicated uncertainty in position of about 1/4°. I searched for it but didn't find it in the uncertainty area, finally picking it up about 1/2° away from its predicted place. It was at about 4 LD by then, 18th mag and moving at about 11"/min against the sky. Just 21 hours later the following night, when first picked up at 20:12 UT it was moving about ten times faster at 103"/min and was about 2.5 magnitudes brighter and passed inside 1 LD at about 23:00 UT. I followed it into morning twilight and last registered it at 04:53 UT on March 26 by which time it had approached to 0.44 LD and was screaming along at just over 1,000"/min. At that speed I was limiting my exposures to just 0.2 seconds to reduce image trailing and could only get a batch of 5 images exposed before 2012 FP35 had travelled from one side of my field of view to the other, necessitating frequent telescope repositioning.

NEO 2012 FP35 on 2012 March 26 04:49:08 - 04:49:38 UT, 5 exposures of 0.2 second duration, each separated by 7 seconds. Motion 1,000"/min, mag +14. Distance from Earth 0.44 Lunar Distances.
Galaxy NGC 4026 is to centre-left, magnitude +11.5.

Even though 2012 FP35 was at 13th magnitude and could be easily seen on the individual images, most of the fields taken could not be measured because of a lack of comparison stars bright enough to register due to the very short exposures. 2012 FP35 was intrinsically about 3.5 magnitudes fainter than 2012 EG5, indicating a diameter of only 7-10 metres.

Coincidentally, on the same night another recently discovered NEO, 2012 FS35 was also under observation and also within 1 LD at the same time as 2012 FP35. I last observed 2012 FS35 on March 26 at 02:21 UT as a 17th mag object moving at 90"/min. It was at a distance of 0.57 LD by then but would approach to just 0.17 LD or about 4.5 Earth diameters from the Earth's surface about 15 hours later. However, 2012 FS35 is much smaller than even 2012 FP35, with an estimated diameter of only 2-3 metres so would not have posed any appreciable risk even if it had come closer and impacted the Earth.