Finding needles in haystacks, minor planet style

Early in February the Minor Planet Center's (MPC) Bright Recovery Opportunities page was listing NEO 2006 AL8 as a bright, well placed but fast moving evening target, ideal for a recovery attempt.

It had been discovered on 08 Jan 2006 by Gordon Garradd and Rob McNaught from Siding Spring and then followed for 7 weeks. With a period of 6 years, this was its first return to the Sun since discovery and unfortunately the 3-sigma uncertainty in the predicted ephemeris position by 02 Feb 2012 covered a massive 54° of sky, 33° to the east of the nominal position and 21° to the west. However it was also predicted to be 16th mag. and moving at about 27"/min, so if it could be caught in the telescope field of view it ought to be a very noticeable moving object. The Moon was due to be full on Feb. 8th and so many fainter NEO targets were already getting blotted out by the strengthening moonlight, searching for 2006 AL8 seemed like a good way to use an otherwise not particularly useful sky.

First step in the search was to retrieve all the existing astrometry for the NEO via the MPC's MPC Database option, one of the standard options on the left of most of the MPC's web pages. This astrometry was then pasted into Bill Gray's Find_Orb orbit determination program to generate an orbit and have a look at the residuals from the original astrometry. One of the functions of the program allows the filtering out of any astrometry with residuals larger than a set maximum and this was done over a series of steps, starting with a fairly generous limit of 2 arc-seconds, then gradually reducing to a harsh 0.75 arc-seconds, causing a number of the positions to be ignored in the orbit determination. Care has to be taken here, the intention is to only remove positions with large random errors and pushing the limit too small can start to drop good positions and degrade the quality of the solution.

The resulting orbit was then used to generate an ephemeris for the current night and this was compared with the MPC's nominal prediction which was generated from an orbit which only excluded positions with residuals in excess of about 2 arc-seconds. The Find_Orb prediction placed 2006 AL8 to the east of the MPC prediction, but only a couple of degrees from the MPC place. A few other orbits with some different selections of positions were calculated and all placed the NEO to the east of the MPC position, with relatively little scatter.

At 18:28 UT on 02 Feb I started my search at the MPC nominal position and then used the hint from Find_Orb to move east along the line of variation, taking 21 x 4 second exposures of each overlapping field. As each set of images was being exposed they were blinked using Astrometrica, looking for the moving object and in the 6th field 2006 AL8 was glaringly obvious, moving west to east at mag +16.3R, 1.2° from nominal.

NEO 2006 AL8 can be seen to the lower right, moving right to left in this video made from 21 images, each frame a 4 second exposure.

It had only taken 25 minutes from taking the first image to recover it. Another set of images was obtained the next night and the two nights of astrometry were sent off to the MPC at 18:40 UT on 03 Feb. Within the hour the MPC published MPEC 2012-C17 announcing the recovery.

Large uncertainties like the 54° in this case need not be quite as daunting as they might initially seem. If the object is bright enough to be seen in individual images and careful handling of the available astrometry is done, in the majority of cases the correct side of an uncertainty area can be chosen, halving the potential search and often also helping to give a reasonable clue to the whereabouts within the remaining uncertainty area. The same process often helps with newly discovered NEOs on the NEO Confirmation page when uncertainties are large, though often this is made more difficult when there are just a handful of positions for a new object, making it impossible to decide which positions are more in error than others, compared with the recovery of an existing object where there may be dozens or even hundreds of positions available.

2011 July notes: 2011 MD and recovering 2000 CP101

Early in July the remarkable NEO 2011 MD was still under observation following its exceedingly close pass to just 7600 miles above the Earth's surface at the end of June. It was visible at high northerly declinations before and after the near-miss only because of the severe perturbations caused by the Earth's gravity, putting a 130° kink in its trajectory (see the JPL news story here)

The final images I obtained were taken during the early morning of July 3 with 2011 MD moving relatively slowly at 5"/min in Cassiopeia in a very rich Milky Way star field, near the border with Cepheus and Lacerta. Even though more than 5 days after the close approach, 2011 MD was still only 2.5 Lunar Distances from Earth and with the object at nearly 20th magnitude this combination caused a rather unusual problem when trying to measure positions. Plenty of images needed to be taken to register a 20th mag target and so I took 200 x 20-second exposures between 00:37 and 02:07 UT, before twilight started to interfere. However, because of the rich star field, wherever the asteroid passed too close to stars many of the images would have to be discarded. Normally, this process is time consuming but straightforward, involving identifying the stars on a line from the first to the last ephemeris position and estimating which of the corresponding images should be left out before stacking the remaining images together to enhance the feeble light from the faint asteroid.

When I did this and started stacking various combinations of images with Astrometrica I kept on finding the faint image of the NEO was contaminated with star trails, which should not have been happening if I had properly identified the images to discard.

The cause of the problem was that the object was so close to the Earth, but moving so slowly against the sky that the Earth's diurnal rotation was causing the apparent track of the NEO to be significantly curved. The image shows the difference between the more usual straight track and the actual curved track.

Track of 2011 MD on 2011 July 3.07 UT, 9'x9' field of view

Armed with the new track and this time leaving out the correct images I managed to get three sets of images stacked and measured and sent off to the Minor Planet Center. The position measured from the final stack, shown here was the last reported sighting of 2011 MD received by the MPC.

Later in the month I searched for NEO 2000 CP101, discovered by LINEAR on 2000 Feb 10 and observed for the next 30 days but not seen in the 11 years since. By July 24 it was expected to be 18th magnitude but the 3-sigma uncertainty area was a daunting +/- 21 degrees long, stretching east and west from the nominal prediction, far too big to sensibly search the whole area using my 0.3 degree field of view.

In an effort to try and reduce the scale of the search I retrieved all the original astrometry via the Minor Planet Center's MPCOBS facility at http://www.minorplanetcenter.net/db_search and then used FindOrb, brutally filtering out any observations that had residuals bigger than 0.5" and then producing an ephemeris from the resultant orbit. The MPC prediction by contrast was much more lenient and included most of the original observations. My ephemeris placed the object a couple of degrees to the east of the MPC prediction and hoping for the best I started searching from that point, working outwards both to the east and west.

I started at dusk and after 4.5 hours, covering 13 fields of view, I eventually picked up the telltale motion of 2000 CP101 just before dawn. It turned out to be 3.7 degrees east of the MPC prediction. After obtaining confirming images the following night, the recovery was announced in Minor Planet Circular MPEC 2011-O29.



The 13 fields taken during the search for 2000 CP101, Delphinus is at top left and Altair at lower right.
Graphic generated using SkyMap Pro 9

 

2010 August notes: (105) Artemis occultation, NEO discoveries and recoveries

The amount of activity on the Minor Planet Center's NEO Confirmation Page (NEOCP) was as expected, quite low during August, with the telescopes of the Catalina Sky Survey being out of action during the monsoon season in the South-West USA. However, LINEAR picked up a number of new objects, as did the WISE Infrared satellite above the atmosphere.

A LINEAR discovery designated 2010 PJ9 passed by at just under 3 Lunar Distances (LD) late on August 9th but weather only permitted catching it from Great Shefford on the morning of the 8th when it was at 6 LD and 16th mag, moving at 30"/min.

2010 PR66 was another LINEAR discovery made on August 15th and confirmed from Great Shefford later the same day. It was about 18th mag at that time but had reached 14th mag 3 weeks before discovery as it made a 10 LD approach in the evening sky that had gone completely unobserved. LINEAR's discovery was impressive, deep in the rich star fields midway between Sagitta and Delphinus, their search algorithms must be very good. Finding moving objects in rich star fields is something that LINEAR still seems to do better than the other surveys. For me, even knowing where to look, it took some time and many exposures to pick it out from the background stars, just to get a few position measurements.

Another stellar occultation was recorded, this time by (105) Artemis on the evening of August 18th (see details including the drift-scan image here). I recorded an 8 second event and Roger Stapleton in St. Andrews also recorded a positive event. The timings from these, together with three other observers in Europe allowed the 123 x 87km oval shape of the asteroid to be mapped out and the result can be seen on the European Asteroidal Occultation Results page for 2010. Click on the chords link to see the result and the link to the observers list showing all those contributing measurements to the the diagram.
I took some images of (105) Artemis before the occultation and measured astrometry from them, sending the positions of (105) Artemis to the Minor Planet Center. It was only later that I recalled that Artemis had been a target of mine 33 years earlier, from my previous location of Cheam (station 499) where I had submitted a single position from 1977 May 22, taken with a 300mm focal length telephoto lens with a 2x converter on Tri-X film, reduced by hand with the SAO Star Catalogue. That result still sits in the Minor Planet Center database and is now joined by my three (new technology) positions from 18 August 2010!

During a quiet NEOCP period towards the end of the month I set about trying to recover some NEOs that had only been seen at one opposition and were predicted to be having reasonably favourable apparitions now. I managed to locate four objects (2006 FE, 2007 VC138, 2008 TC4 and 2009 PY) on the night of August 30th and the weather held up enough for me to get confirming positions the next evening. I was very lucky with 2006 FE to pick it up by chance in a very rich star field in northwest Aquila in the first set of exposures taken, travelling across a little 1 arcmin void amongst the milky way background. The most challenging recovery though was 2009 PY, 20th magnitude and half a degree from prediction, requiring a number of my 18'x18' fields of view to be searched before finding the tell-tale motion of the faint NEO.

2008 August notes: Observing statistics, 2004 SB56, 2007 RT12, 2001 QL153, 2008 PG1

The weather was very poor at Great Shefford in July and August. With 25 hours at the telescope, July provided less than half the observing hours logged for that month in any of the last three years. August was officially the cloudiest August in the UK since records began in 1929 and provided only about 50 (generally poor quality) hours, again well below totals from recent years.

However, the night of August 3/4th provided the opportunity to recover three NEOs not seen since their discovery apparitions and a somewhat unusual confirmation of a NEO Confirmation Page (NEOCP) object.

Aten 2004 SB56 had sped northwards after conjunction with the Sun in July and in the 10 days before recovery had brightened 3 magnitudes and increased elongation from 35 to 72°. With an uncertainty area on the sky of 14' it was picked up only 3' from prediction at mag +16.7 and about 6° from the North Pole, a relatively easy recovery.

Apollo 2007 RT12 had an uncertainty of about 1° and at mag +20.0 was a potentially difficult object to recover, just 1° east of Alpha Cep, deep in the Milky Way. Fortunately it was only 8' from prediction and not involved with any field stars during the time it was imaged. With an absolute magnitude of +23.9, equating to an estimated diameter of 30-95 meters, there have only been 10 other NEOs recovered at a second opposition that are smaller than 2007 RT12. Included in this set are the Earth co-orbital companion 2002 AA29 and the tiny 2006 RH120 that was temporarily captured in Earth orbit during 2006 & 2007. My thanks to Sergio Foglia for providing a list of small NEOs seen at more than one apparition.

The third object was 2001 QL153, an Amor that had been observed for about 4 months and had not been seen since January 2002. It too had an uncertainty of about 1° and predicted to be mag +20.5 and could have posed a problem to pick up. It was in a less crowded field than 2007 RT12 and after more than six years since last being observed was recovered only 9' from prediction.

A LINEAR discovery put on the NEOCP on August 3rd turned out to be something of a puzzle. 2008 PG1 was observed from 9-10am UT on August 3 by LINEAR at mag +19, moving at 9"/min from west to east, in the morning sky about 13° north of Mira Ceti. By 2am on August 4th it was predicted to be accelerating and to have an uncertainty area 1.7° long, extending roughly West to East, in the direction of motion. I eventually confirmed it 43' further west than the most westerly point of the uncertainty area (quite unusual for the MPC prediction to be this far out) and moving at about the same speed as when LINEAR discovered it. The orbit appears to be rather indeterminate but it was announced on MPEC 2008-P14 with just the observations from LINEAR and my set of confirmation positions. The orbital eccentricity had been assumed (done sometimes when the orbit is rather indeterminate, to try and help improve the orbit solution). No other observatories had reported positions for the NEO before I got my next chance to observe the area early on August 9th. By then, using FindOrb software from Bill Gray to generate a large number of 'Monte Carlo' orbits to estimate the likely uncertainty area, it looks like it could have been anywhere in a nearly 3° long band. I didn't manage to locate it that morning nor on my next opportunity 4 days later and it has not been reported from anywhere else since. It is now listed as a Virtual Impactor by JPL with 97 potential impacts, the first in only 5 years time, but with such large uncertainties in its orbital elements the chance of collision then is given as just 1 in 200 million!

(Update 10 May 2012: 2008 PG1 was linked to 2009 EV by the Minor Planet Center, read more here)

2008 June notes: 2008 LA, 2008 LG2, 2008 LH2, 2008 LB, 2003 OB4

After a rather lack-lustre May, June provided the best stats I've logged for that month since my records started in 2002, with 16 at least partially usable nights and 47 hours at the telescope.

This time of year, activity on the NEO Confirmation Page is tailing off as the main NASA survey sites in the southern USA get closer to the summer monsoon season, even so, there were 22 new NEO discoveries announced and 9 of those were observed, with 2008 LA and 2008 LG2 being followed on 5 nights each and still being listed (July 5th) as 'virtual impactors'. Another NEO 2008 LH2 was confirmed soon after LINEAR discovered it, just after its closest approach to 9 Lunar Distances (LD) and followed for a total 7 nights.

2008 LB was the closest and fastest moving NEO seen during the month, a difficult Apollo object, last recorded 16 hours before its closest approach at about 3.8 LD, mag +17.9 and moving at 83"/min through the very crowded star fields just 5° south-east of the Scutum star cloud.

Another object recorded in a rich star field was 2003 OB4, this Amor type NEO had been discovered by NEAT on 23 July 2003 and, with prediscovery images, had been tracked for three months that year but not seen since. I searched for it on several nights in eastern Ophiuchus and found it over 0.5° off-track on 9th June at mag +19, confirming it the next night, with the recovery being announced in MPEC 2008-L43.

2008 January notes: 2005 BT1, BI10604, 2007 XS16, 2008 AV72 = 2000 JF4 = 2006 PH11

January was rather disappointing with fewer hours logged observing than for the same month in any of the previous 5 years.

A highlight was the recovery of the Apollo 2005 BT1, originally discovered by LINEAR on 16 Jan 2005 and followed for 47 days. Predicted to have a favourable approach in Jan 2008, a search was started from Great Shefford on Jan 24th, the expected magnitude given by the Minor Planet Center was +19.1 and the likely uncertainty area given as over 8 degrees long, aligned almost North-South.

I covered the central 4 degrees with my 18'x18' field of view during 24/25 Jan and extended this to the N by another degree on 26 Jan but searching was cut short by clouds. Only 0.5 degrees was added to the S the next night before clouds interrupted again, but finally on 28 Jan, on the first attempted field that night the NEO was finally caught, 2.35 degrees off track to the south. It had taken 814 images taken on the four nights, of 30 separate fields to locate it.

The next night was cloudy again in the UK so I contacted Sergio Foglia (IAU code 147) and Luca Buzzi (code 204) in Italy to see if either of them had good skies to try and get a confirming second night of positions for it. Sergio was not able to observe but Luca managed to get some astrometry before he too was clouded out. With the positions from 2008 Sergio then searched for possible images from the old SkyMorph image archives and managed to locate it faintly on two images from 07 Jan 2002 taken with the Palomar 1.2-m Schmidt. All three sets of positions were sent off to the MPC and within the hour MPEC 2008-B60 was issued formally announcing the recovery.

Another extensive search that didn't end up with such a positive result was also a LINEAR discovery found at 03h UT on 16 Jan and posted on the NEO Confirmation page with temporary designation BI10604 just after midnight on 17 Jan. Although relatively bright at mag +17, by the time I could observe on the evening of 17 Jan the uncertainty area was a 1 deg x 2.5 deg oval. I covered the whole area with multiple images being taken of 31 different fields, but did not find it. By the end of the 4 hour session the area had already expanded by a further 0.5 degrees. It never did get recovered by any observatory and two nights later was removed from the NEOCP...

A good set of luck came my way on 7th Jan while I was following up on one of the main belt discoveries I had made in December (2007 XS16). I found a very faint moving object, about mag +21, that did not match any known object and so measured positions of it. I had hoped to try for it again in the coming nights and send in two nights of astrometry to the Minor Planet Center, but clouds stopped any further attempt for over a week. By 17th Jan I decided to just send in the positions as a one night stand (ONS), expecting that I would not hear any more about it. However, by the next day the MPC had managed to match it up to a ONS from the 1.5-m reflector of the Mt Lemmon Survey obtained 5 days after mine on 12 Jan. It was designated as 2008 AV72 and I was given discovery credit for having the earlier of the two ONS!

With this extra data I now realised that other images taken of 2007 XS16 from the previous month might include 2008 AV72 and on re-examining these managed to find it on images taken 29 Dec and then 19 Dec 2007. After these pre-discovery positions were sent in, the MPC quickly managed to find various other ONS back to May 2000 and even two occasions where the object had been given a designation on the basis of two nights of observations (not enough to determine a proper orbit), these being 2000 JF4 and 2006 PH11. Because 2008 AV72 had a 2 month observed arc in 2007/8 it was retained as the primary designation and with observations at 5 oppositions and an MPC uncertainty code of 1 it is now in a state where it could potentially get numbered - not bad for just sending in the ONS from 07 Jan having lost hope of being able to find it again!

2007 July notes: Floods, 2007 DT103, 2007 MB4 and recovery of 2001 RV17, 2005 XW77 and 2003 CG11

The continued bad weather here culminated in flash flooding on 20th July which left us with two rooms under water, but fortunately the observatory stayed dry. Since then the weather has noticably improved with a run of good clear nights in the last week of the month.

During the month three NEOs were recovered at their second apparition - 2001 RV17 at the beginning of the month with 2005 XW77 and 2003 CG11 towards the end, both of those two in collaboration with Luca Buzzi at the Schiaparelli Observatory in Italy.

A couple of moderately fast movers were followed over several nights - 2007 DT103 which was detected at the end of the month by RADAR from Goldstone and found to be a binary, and 2007 MB4 that passed at about 8 Lunar Distances on July 5th.