Sunday 5 July 2009

2009 June notes: Observing statistics, 2009 KL8, 2009 MU, 2001 FE90

2009 has provided some exceptional statistics for Great Shefford so far, more usable nights (103) & more hours used (455) in the first 6 months of the year since the observatory was commissioned in 2002, beating the previous best Jan-June (2005) by 10%. Hopefully the last half of the year will be equally good.

June started with the close fly-by of Apollo 2009 KL8, passing Earth at 5 Lunar Distances (LD) on June 2nd. That night it was moving at 60"/min and fading fast, from 19th to 20th mag. during the course of the night. Two nights before, even though about 30% further away it was about 1.5 magnitudes brighter because of its smaller phase angle and I followed it that night for 1h 45 minutes to try and determine its rotation period. That turned out to be just 2 mins 41 seconds, varying by 0.75 magnitudes and it had managed to complete 32 rotations while I was observing it!

Another close approach occurred on June 24 at 16:50 UT when 2009 MU passed at 2.3 LD. The night before it had been well placed throughout the few dark hours available from the UK and was followed at mag. +16, its speed accelerating beyond 100"/min by dawn. Initial indications are that its lightcurve is irregular, suggesting that it may be tumbling, but further analysis needs to be completed.

The Apollo 2001 FE90 was also observed towards the end of the month. Discovered in 2001 when it reached 18th magnitude, this year it was predicted to brighten to mag. +13.3 as it sped from north to south in the evening sky and was scheduled to be a RADAR target at Goldstone and Arecibo at the end of June. I first observed it briefly following a thunderstorm just after midnight on 26th June and in the 22 minutes before it was obscured behind trees it showed a dramatic variation of over two magnitudes in brightness! I observed it on the three following nights and watched the amplitude reduce to 1 magnitude as the phase angle reduced from 79° to 34°.

The lightcurve derived from about 30 minutes of images taken on the evening of 27 June 2009 is shown, including a mosaic of the images from the same sessions, showing the rise and fall in brightness of the Minor Planet as it trailed (from left to right). As can be seen, the observations cover just a couple of minutes less than one full rotation period.
2001 FE90 - light variations on 27 June 2009