2011 April notes: 2011 GP59 and NEOCP changes

April continued the good spell of weather that set in during March and ended up being the warmest April since records began for the British Isles. Plenty of Near-Earth asteroids were picked up by the surveys but probably the most interesting object was 2011 GP59 which was discovered by the amateur run La Sagra survey in southern Spain just before midnight on April 8th. It was described as mag +17 by the discoverers and was heavily observed from Europe in the next few hours, with 45 positions being reported by the time I sent my first position in, just 2 hours after discovery. It was immediately obvious that it was varying greatly in brightness in the space of just a few minutes and it could be seen to brighten up and then fade completely from view in real-time as sequential images were captured and displayed on the pc. Also evident early on that first night was that it would brighten over the coming days and make a close approach about a week later. It passed Earth at 1.4 Lunar Distances on the evening of April 15 but at a southerly declination and running into evening twilight so difficult to observe from the UK. Nick James posted a very good animation on YouTube showing the rapid brightness changes from the night of April 11th.

I last observed it in the early hours of April 12th when it was 16th mag and still moving relatively slowly at 9"/min., obtaining 376 images over a three hour period to try and determine a lightcurve. The initial reduction of the data showed a dramatic curve with a 2 mag amplitude and a period of just over 7 minutes. However, there was quite a noticeable scatter in the brighter segments of the curve, just where the errors would normally be expected to be at their smallest.



Initial lightcurve reduction, showing large scatter at the brightest part of the curve, where scatter would normally expected to be least



So Canopus was used to try and determine whether the object was tumbling and if a secondary period was contributing to the scatter of the main curve. Canopus has functionality to determine an initial lightcurve and can then subtract that modelled variation from the original data points. The adjusted data points can then be used to try and solve for a secondary period. If a secondary period is apparent, then this can in turn be subtracted from the original data points and a fresh determination of the primary period  made. A few of those iterations for 2011 GP59 resulted in the two lightcurves shown here, the main one with a 7.352 +/- 0.002 minute period and 1.8 mag amplitude, while the secondary period was found to be 10.25 +/- 0.02 minutes with an amplitude of approximately 0.4 magnitudes. There is a great deal of scatter throughout the secondary lightcurve and it is best viewed from several feet away(!) when the sinusoidal lightcurve becomes much more apparent. The overall noise is mainly due to the secondary period being superimposed on the large 1.8 magnitude variation of the primary period and therefore both the maximum and minimum of the secondary curve have data points contributed from the faintest parts (as well as the brightest parts) of the overall variation, so low signal/noise ratio measures, with large scatter are unfortunately present throughout the secondary curve.



Primary lightcurve with secondary subtracted



Secondary lightcurve with primary subtracted

Using measurements obtained by the Lowell Observatory from the previous night, tumbling asteroid expert Dr Petr Pravec reported similar results in the Minor Planet Mailing list here http://tech.groups.yahoo.com/group/mpml/message/25234 with periods of 7.3501 +/- 0.0004 minutes and 10.258 +/- 0.003 minutes.

Users of the NEO Confirmation web page will have noticed during late April that the number of objects on the page has exploded from what in previous months had been at most around 30 objects at any one time to much higher numbers, for a while in the first few days of May there were over 160 objects listed. The surveys are not necessarily picking up any more NEOs than they had done a few days earlier, rather the Minor Planet Center (MPC) has changed the threshold "NEO probability" that a newly discovered object has to score to get onto the page. Following a workshop in March that included representatives from all the NASA funded surveys, JPL and the MPC, the attendees overwhelmingly voted for more objects to appear on the page, so objects that only have a small chance of actually being a NEO (such as objects with Mars crossing orbits, Hungarias, Phocaeas etc.) are now appearing on the NEOCP along with more definite NEOs.

As an observer, the sheer number of objects on the page makes target choice very difficult and to help, the MPC are making some modifications. Initially they have started displaying the NEO probability as a percentage against each object, so the observer can choose objects with high scores, say 50%+ if they want to have a good chance of observing a new NEO. In beta test now and hopefully soon to be introduced fully is a means of filtering the page by magnitude, declination and NEO probability which should make planning an observing session much easier.

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.

Visit to the Catalina Sky Survey

During the first week of February my job took me to Phoenix, AZ and before flying home, I took the opportunity to visit the Catalina Sky Survey (CSS) team in Tucson. I met up with Principal Investigator Ed Beshore and co-P.I. Steve Larson at the Lunar and Planetary Laboratory within the University of Arizona campus for lunch and we were joined soon after by Richard Kowalski and Rik Hill from the CSS and by Carl Hergenrother from the Minor Planet Center (MPC), also based in Tucson.

Later in the afternoon Ed drove me up the winding road to Mt. Lemmon, a 1.5 hour journey, initially crossing the Catalina foothills covered with magnificent Saguaro Cacti. Half way up the mountain, stunning views unfolded across Tucson, to Kitt Peak 55 miles to the southwest and to Mt. Hopkins with the MMT 50 miles south, not surprising the area is dubbed "Optical Valley". Some miles before reaching our destination we passed the snow line and saw the devastation caused by the forest fires that came close to destroying the CSS observatories back in the summer of 2003. Once we arrived at the summit of Mt. Lemmon (9200ft above sea level), the Large Binocular Telescope on the summit of Mt. Graham 50 miles to the northwest was plainly visible, with the lengthening shadow of Mt. Lemmon pointing almost directly at it.

The shadow of Mt. Lemmon pointing almost directly at the Large Binocular Telescope on Mt. Graham 50 miles away (top right in insert)

As the Sun was setting, Ed opened up the dome of the 1.5-m reflector (observatory code G96) and initialised the equipment before the start of the nights NEO surveying. The 4K x 4K CCD is normally operated at -100C but that temperature has recently been difficult to get down to, an air dryer unit used to extract water from the cooling system suspected of being full of water and needing replacing soon.

Ed Beshore opens up the lower shutter of the Mt. Lemmon 1.5-m telescope

The observer for the night, Alex Gibbs arrived and continued preparing the telescope, running through focussing sequences and also choosing fields in the early evening sky ready for the survey work to start. Two computer flat screen monitors arranged side-by-side displayed the sky divided up into the fixed 1.2 x 1.2 field centres that are used night after night for all the survey work. Settings allowed the areas of sky that CSS and the other surveys such as LINEAR and Spacewatch had covered in recent nights to be colour-coded so that fresh unsurveyed sky could be targeted that evening.

Field centres for the 1.5-m telescope, colour-coded showing where NEO surveying had been done in recent nights by other surveys

Alex clicked on sets of 12 adjacent fields with the mouse, each set of 12 fields would be automatically imaged one after another, going around the set a total of four times. All the images would then be fed into the processing pipeline for automatic moving object detection. Lines on the computer screen marking out 60 elongation from the Sun indicate the westernmost boundary and surveying then proceeds to the east as the night progresses, with the observer manually choosing the areas of sky to be targeted. Earlier in the week Pan-STARRS had been given a complete night dedicated to NEO detection, resulting in 27 Pan-STARRS objects being on the NEO Confirmation Page, most very faint at mag. 22-23 and out of the reach of amateurs. Alex selected some of the sets of survey fields to cover the uncertainty areas where some of the Pan-STARRS objects were located to try and help recover them. As astronomical twilight ended the exposures started and soon the first set of 12 x 4 images had been processed and was ready for examination.



Alex Gibbs selecting fields at the start of a night of NEO surveying by the 1.5-m Mt. Lemmon telescope

The CSS software detects objects that have consistent movement on all four frames of each field, down to a signal-to-noise threshold of about 1.2 (i.e. deep into the random noise in the images) and then matches those detections against the latest orbits in the MPC's MPCORB Minor Planet database. Each set of four images is then blinked, with known objects marked in green. All the remaining detections are then visually checked by the observer and those that look convincing are manually selected, the rest discarded. A score is then determined for each new object depending on its rate and direction of motion, together with where it is located in the sky to indicate its chance of being a NEO. Astrometry is measured for all the objects and sent off to the MPC, any objects that look like they are particularly interesting have further exposures scheduled for later in the night to help with the follow-up effort.

With the 1.5-m telescope in full operation, Ed and I left to drive the 7 miles down to Mt. Bigalow where the CSS 0.68-m Schmidt is located (observatory code 703), 1000ft below the Mt. Lemmon telescope. That night Andrea Boattini was the observer (he was the only member of the CSS team I had met before, at the MACE meeting in Mallorca in 2003) and as we arrived he was busy blinking a set of images with 100+ moving objects, expertly selecting or rejecting the automatic detections at a remarkably fast rate!

Andrea Boattini blinking newly detected moving objects at the controls of the CSS 0.68-m Schmidt

Ed and Andrea discussed recent adjustments to their procedures for the Schmidt, increasing exposure length to 60 seconds to try and detect fainter objects, at the expense of reducing the amount of sky they would be able to cover per night. We left Andrea blinking the latest set of images processed through the pipeline and drove back down the mountain with both telescopes continuing to work flat out.  What a night...



2010 XM56

December and November were very disappointing months, the smallest amount of clear skies here for seven years, indeed my counts of nights used (169) and hours spent imaging (702) for the year were also the lowest since 2003. I recall writing this time last year how 2009 had produced the best observing figures since my observatory was commissioned in May 2002... I should have kept my mouth shut!

On one of the rare decent nights in the month, I followed Apollo 2010 XM56 for 5.9 hours on December 16th. It had been  discovered a week earlier by LINEAR and with an estimated diameter of only about 30 meters it was predicted to reach mag. +15 as it passed by Earth at slightly less than 2 Lunar distances. During the night its apparent speed accelerated from about 160"/min up to 250"/min and only stayed in the same field of view for 4 minutes at the start of the night and 3 minutes by the end. It was obviously varying substantially in brightness with a period of about 2 hours. I stacked all the usable images from each of the separate fields of view taken during the night and ended up with 96 photometric measures. The raw lightcurve shows a plot of relative  magnitude against fractional Julian Day (0.5 = December 17.0 UT) and shows the amplitude of the brightness variation increasing from an already substantial 2.5 magnitudes at the start to 4.4 magnitudes by the end of the night! By the end, the object was changing from being very well recorded on individual 2 second exposures at maximum, then fading within 1/2 an hour to being completely invisible on individual images at the deep minimum, though the multiple image stacks made at minimum still recorded it well. Some of the increase in amplitude will be due to the rapidly increasing phase angle (47° increasing to 65°) as shadows lengthened on the surface of the object although there may also be some variation due to tumbling. The period was determined to be 2.35 +/- 0.02 h.

Raw lightcurve of 2010 XM56 from 2010 Dec 16/17th

Chang'e 2 booster

Here is a lightcurve derived for 2010-050B which appears to be the booster that helped launch the Chinese Chang'e lunar mission. It was on the NEOCP for a while and looks like it will be visible for a while at least as a NEO-like distant artificial satellite. The lightcurve is from 246 exposures taken on 25 Nov. which shows a 26.89 sec rotation period with an amplitude of about 3.0 mags and I interpret it as representing a generally regular, elongated object, but as the "dark" ends point towards us, one end produces a bright (2+ mag) flash, lasting just 2.6 seconds centered at about phase=0.56 and the other end produces a much fainter flash (<1 mag.) for about the same duration at phase=0.06.

Lightcurve of 2010-050B (the booster for the Chinese Chang'e mission)