Tuesday, 11 December 2012

NEO 2012 XE54 eclipsed by Earth shadow

As predicted by Pasquale Tricarico on the Minor Planet Mailing List in message 27642, NEO 2012 XE54 was eclipsed by the Earth's shadow during its close approach to Earth on 11 December 2012, a relatively rare event. More information from Pasquale here.

A very preliminary lightcurve of the eclipse as measured from images taken at Great Shefford Observatory is given below, with the details subject to revision later. The measurements are from individual images apart from the points at mid-eclipse, measured from stacks of multiple images.

Tuesday, 4 September 2012

Distant Comet C/2012 Q1 (Kowalski)

Comet C/2012 Q1 (Kowalski) taken 2012 Sep 02.01UT
2 images showing movement in 23 minutes.
Each frame is a 16 minute exposure with 0.40-m Schmidt-Cass. telescope & CCD
Original scale 2.14"/pixel, enlarged x4, field size 3.5x3.5 arcmin, North up
(c) P. Birtwhistle 2012, Great Shefford Observatory (J95)
Comet C/2012 Q1 (Kowalski) was discovered by Richard Kowalski using the 1.5-m Reflector on Mt Lemmon as a 19th magnitude object in Pegasus on 28 August 2012, 3 days before full moon. He described it as having a faint round coma approximately 20" in diameter (about double the diameter visible in the animation above).

The full moon hampered the recovery of the discovery and the images above were obtained 2 days after full moon, the very bright sky and interference from high cloud causing a very bright and uneven background to the images.

At the time of writing, with just a week of positions measured for the new comet, it appears to be between 8-10 AU from Earth and 9-11 AU from the Sun, though any determination of its orbit is necessarily very uncertain without a much longer span of observations. However, at such a great distance, the coma size must be impressive, equating to about 150,000 km diameter.

Sunday, 2 September 2012

The faint tail of Comet P/2012 NJ (La Sagra)

Comet P/2012 NJ (La Sagra) was originally designated as an asteroid when announced by the Minor Planet Center on MPEC 2012-N19 as 2012 NJ. It had been discovered by the La Sagra Sky Survey team from the mountains of Andalucia, Southern Spain early on 13 July 2012 and appeared to be physically unusually big for a newly discovered NEO these days, estimates of its size, derived from assumed asteroidal values of its albedo or reflectivity indicating a diameter of approximately 4-12 km. Coupled with an unusual orbit, inclined almost perpendicular to the Earth's orbit and very elongated, with perihelion just inside Mars' orbit at 1.29 AU but with aphelion further out than Saturn's orbit, it could well be expected to be a comet but none of the initial observers reported any cometary activity, several observers using telescopes of 0.5 - 0.6-m diameter.

However, on July 18th circular CBET 3178 (subscription required) was issued, announcing that the object was indeed a comet and re-designating it as Comet P/2012 NJ (La Sagra). Gerhard J. Hahn (Institute of Planetary Research, German Aerospace Center, Berlin) had reported it as showing a 35" tail in p.a. 235° on stacked and single images taken by Stefano Mottola using the 1.23-m telescope on Calar Alto on July 16, 17, and 18 UT. A post by Gerhard on the Minor planet Mailing list on July 19th gave further details and a link to a document containing images from July 17, 18 and 19th, showing a thin straight tail. An animation of portions of the individual images from that document has been made, used with permission and is shown here:

Comet P/2012 NJ (La Sagra) on July 17 ,18 & 19th 2012
Images taken by Stefano Mottola using the 1.23-m Calar Alto telescope
(c) Institute of planetary Research, German Aerospace Centre, Berlin

During the days following a few other images showing the tail were posted, including a good sequence  covering July 21-25 by Jean-François Soulier from France here, showing a very faint thin tail.

The comet reached perihelion at a distance of 1.29 AU from the Sun on 2012 June 14 and reached its closest to the Earth (0.59 AU) on 2012 July 22.

At Great Shefford Observatory, my first opportunity to image the new comet was on July 21 but with clouds affecting images it was difficult to detect the tail at all. However, on the nights of July 22, 24 and 25 the skies were better and 30 minute sequences of exposures were made once or twice per night to examine the appearance of the tail. Here, 30 minute stacks from the three nights are shown, the original field of view being cropped to 7'x9' and the scale doubled from the original 2.2"/pixel:

Comet P/2012 NJ (La Sagra) on July 22, 24 & 25th 2012
0.40-m Schmidt-Cassegrain
Each of the three frames is a stack of 90 x 20 second exposures
(30 minutes total)
(c) P. Birtwhistle, Great Shefford Observatory 

On the night of July 24/25th, 180 images were combined for a total exposure of 1 hour and in the resulting image below the very faint tail can be traced for 6' 30" in p.a. 210°

Comet P/2012 NJ (La Sagra) July 25-26 2012
7'x9' field of view, 1 hour exposure
0.40-m Schmidt-Cassegrain
(c) P. Birtwhistle, Great Shefford Observatory

The tail showed a marked drop in brightness between the nights of July 24/25th and 25/26th. An initial sequence of 90 images exposed on the evening of July 25th did not show the tail at all, further sets were obtained later in the night, eventually revealing the faint straight tail. Similar sets the night before had revealed the tail much more readily.

On 31 July 2012 Artyom Novichonok posted on the Comets Mailing List message 19737 that he and Otabek Burhonov had imaged the comet using the 1.5-m f/8 telescope at Majdanak in Uzbekistan with a total exposure of 2.5 minutes a few hours earlier than the image above.

The weather stopped any more attempts from Great Shefford before the full moon on 02 Aug 2012 and in the subsequent dark of the moon no further deep attempts were made, the comet itself receding from both Sun and Earth and predicted to be a full magnitude fainter than in the last half of July.

However, Artyom posted again on the Comets ML with message 19774 on 27 Aug 2012 with details of an image taken 2 weeks earlier on 13 Aug 2012 showing a very faint trace of the tail pointing in the anti-solar direction (p.a. 150 deg, down and to the left) and what appears to be a separate tail in p.a. 96 deg, though this second tail has yet to be confirmed.
Comet P/2012 NJ
From Majdanak Observatory in Ukbekistan
The tail observed in July is now very faint
and points down and to the left.
Another possible (shorter) tail points to the left,
though this tail is yet to be confirmed.

It is understood that the team at Calar Alto may target P/2012 NJ in mid-September, it will be interesting to see whether any trace of the tail is detectable by then.

During the current apparition, the anti-solar direction (the expected direction for an ion tail) rotated from p.a. 242° at discovery, 238-234° during July 16-18, 215° on July 25 and 147° on Aug 14th and those directions agree to within a few degrees of the measured angles from the images above. By mid September 2012 the anti-solar direction will have reached p.a. 100 to 90°.

Wednesday, 4 July 2012

A decade of operations and a few close approachers

The end of May marked 10 years of operation at Great Shefford Observatory, first light occurring on 26 May 2002 with the first astrometry being  measured on 30 May 2002, of Comet C/2000 WM1 (LINEAR). In the intervening 10 years I've taken 831,500 images and measured 30,600 astrometric positions of minor planets and comets, with many of the images being taken for photometry rather than astrometry.

The weather in April and May this year was particularly poor and looking at the stats over the last 10 years, this April was the worst April in all that time, with only 11 nights usable, nearly half of what would normally be expected. Indeed the 18-night cloudy period from April 23 - May 10 was easily the longest endured here, the previous worst being a period of 12 consecutive cloudy nights in January 2011.

In what remained of the month there were a number of notable close approachers observed, first up was 2012 JU, discovered at 6am UT on May 12th by the Catalina Sky Survey, 31 hours before passing by at 0.53 Lunar Distances (LD). David Briggs at South Observatory, Clanfield (code J84) reported positions during the evening of the 12th at mag +17 and I first picked it up at 23:15 UT, following it for just over 2 hours. In the 3 hours that David and I had it under observation its distance from Earth decreased by 1/5th, to 1.6 LD and its apparent speed accelerated from 60"/min to 90"/min. It was last reported from the Astronomical Research Observatory, Westfield, USA (observatory code H21) at 6h UT on 13th at 1.04 LD, moving at 208"/min at mag +16.

A much better observed fly-by was 2012 KP24, discovered from Mt. Lemmon with the 1.5-m reflector on May 23rd, giving NEO observers the luxury of 5 days warning before closest approach, due at 15:21 UT on May 28th, just 4 Earth diameters from the Earth's surface! At discovery it was mag +21 and 15 LD from Earth. I observed it on four consecutive nights, initially moving at just 1"/min and at mag +20 on May 24/25 but brightening about a magnitude each night. My last measurement was mag +16 in strong twilight at 02:40 UT on May 28th, with it still only moving at 30"/min even though by then it was only 1.5 LD away. It was last reported at 10:24 UT that morning, by the Catalina Sky Survey at mag +14 and moving at 187"/min, 5 hours before closest approach. Sergio Foglia posted on the Minor planet Mailing list a lightcurve derived from observations from H21 that shows a rotation period of 150 seconds and an amplitude of about 1 magnitude for this object, estimated to have a diameter between 13-19 metres.

However, even before 2012 KP24 was out of the way, the Mt. Lemmon telescope had picked up the next incoming object, 2012 KT42. The discovery image was taken just 23h 58m before 2012 KT42 would skim the Earth's surface by a mere 1.1 Earth diameters. As with 2012 KP24 the timing of the close approach would favour observatories in the USA more than those in the UK, with closest approach at 07:07 UT on May 29.

At discovery 2012 KT42 was at 3.6 LD and within 1.3° of the opposition point, about midway between Antares and Beta Sco and headed almost exactly towards Earth. From Great Shefford it was low in the south that night and I had to wait for it to pass into gaps between trees but eventually managed to image it for a 13 minute period a few minutes after midnight early on May 29th and again from 01:17-02:05 UT. It was mag +15 but only at 13° altitude by 2am.
2012 KT42, mag. +15.5, moving 49"/min, Distance from Earth 0.8 LD
Stack of 77 x 4 second exposures, 01:31:30 - 01:45:56 UT 29 May 2012
0.40-m Schmidt-Cassegrain at f/6, 2.1"/pixel, field 9.4'x7.7', North up.
Even though it was nearly 19 hours after discovery it had only moved about 3° from the discovery position, still headed almost straight at us, but had approached to just 0.76 LD by then. Mt. Lemmon remarkably managed to keep it under observation to within an hour of closest approach, their last reported astrometry was at 06:17 UT when 2012 KT42 would have been about 12th mag and just 3.6 Earth diameters above the Earth's surface, screaming along at 1,630"/min, equivalent to the apparent diameter of the moon in less than 70 seconds!

The alignment of 2012 KT42 during this approach was quite interesting, as already mentioned, it was nearly at the opposition point during most of the approach, but was actually in the Earth's penumbra from about 14:10 to 21:40 UT on May 28th. After close approach the line-up was still perfect and 2012 KT42 was in transit in front of the Sun for about an hour either side of 10:12 UT on May 29th, though far too small at about 0.006" diameter to be visible from Earth.

Wednesday, 6 June 2012

Understanding the MPC's discovery credit rules

In last month's Planetary notes in the April 2012 The Astronomer magazine (TA Vol 48 No 576 p327 [2012]) sub-editor Mark Kidger commented that I had discovered 3 Main Belt asteroids during March but that's not quite true(!) so it gives me an opportunity to try and explain the Minor Planet Center's (MPC) new rules relating to assigning minor planet discovery credit, published in an editorial notice in MPEC 2010-U20 on 2010 Oct. 19th.

In my astrometry report submitted to Mark I had indicated that minor planets 2012 FU1, 2012 FW1 and 2012 FX1 were Great Shefford "designations", rather than discoveries. These three objects, along with two others had been picked up on the night of March 17/18th, on images exposed for three NEO Confirmation Page objects. As a standard practice I check for any other moving objects in the field of view and if any are found, use the "Known Object Overlay" tool of Astrometrica to identify them, then measure positions and send in astrometry to the MPC. However, none of the five objects could be matched with known objects and a subsequent check against the MPC's Minor Planet Checker also did not come up with any exact matches.

Positions for all five objects were measured and, as they had not been identified with known objects I gave them my own observer-assigned temporary designations of GSC3HA - GSC3HE and immediately sent the astrometry in to the MPC. Four minutes later an automated message was received back identifying GSC3HB with main-belt minor planet 2006 BQ34, but nothing was received back for the other four objects.

The next night was clear and along with other NEO work, follow-up images were taken for the remaining four objects and positions measured. For each object the positions from the previous night were then combined with the newly measured positions and new observer-assigned temporary designations given to each object. The astrometry for the two nights for each object were then sent off to the MPC. So for instance, from the first night, three positions were sent in for object GSC3HA, on the second night another three positions were obtained for the same object and all 6 positions were sent to the MPC with new observer-assigned temporary designation of GSC3JA.

By sending in observations from just a single night the MPC's automated linking routines might have a hard task to match those positions with other observations of potentially the same object that might have been made many days or even weeks earlier. Submitting two nights at the same time, with the same ID used for both nights gives the matching routines a head start to locate other relatively recent observations. As soon as the matching routines put more than one night of observations together and don't match with any known object then they will automatically allocate a new MPC provisional designation and this is what happened for the four remaining objects, receiving provisional designations 2012 FT1, 2012 FU1, 2012 FW1 and 2012 FX1.

This would have been the first step towards achieving discovery credit if following the MPC rules prior to  2010 Oct. 19th - if further observations allowed identification to an earlier apparition for the object where a good orbit had been determined then that earlier apparition would take precedence, but otherwise, the observations that had allowed the provisional designation to be assigned would eventually lead (when the orbit was determined well enough to be numbered) to that observer being given discovery credit and the right to name the minor planet.

However, with the new rules there is nothing special about the assigning of a provisional designation such as 2012 FT1. Every single observation needs to be associated with a designation and if it appears that an object cannot immediately be identified with a known designation then a new one is automatically assigned by the MPC. In this way it is quite possible that the same object gets more than one provisional designation during a single opposition and very likely, if it has been observed at earlier oppositions it will have received one or more provisional designations from those earlier apparitions in the process too.

As the orbit gets more and more refined (as new observations are made, or older observations are identified from previous oppositions), eventually the orbital uncertainties are determined to be so small that the object is ready for permanent numbering by the MPC. It is only at this point that discovery credit is assigned. Central to this assignment is a new MPC procedure, introduced with the new rules on 19th Oct. 2010 that stores the date & time that the MPC received the astrometry from the observer, along with the more familiar details of Observatory code, date & time of observation, RA and Dec etc. These dates & times of receipt are called timetags by the MPC. Observations sent in before the new rules came into play have timetags constructed that are "related to the date of observation". Unfortunately, there is currently no means for an observer to see the timetags associated with observations that have been sent in to the MPC.

The MPC rules state in MPEC 2010-U20 :

"Discoverers will be defined only when an object is numbered.  At that time, the timetags on all the observations included in the solution will be examined.  The discovery observation will be that observation which is the earliest-reported observations at the opposition with the earliest-reported second-night observation.  The discovery observation will then define the discoverer."


"Objects that have multiple-opposition orbits as of now (2010 Oct. 19) will be grandfathered into the old scheme of assigning discovery credit"

There are several points to note in these statements:
a) For discovery credit to be assigned there must be at least two nights of positions reported, no assignment is possible where only a single night is available during an opposition.

b) The early reporting of astrometry matters. If an observer detects a new object on night 1 and sends in his astrometry on night 3, but in the meantime, another observer detects the same object on night 2 and sends his astrometry in immediately, then the second observer would be selected for discovery credit in preference to the first because the night 2 astrometry would have been received by the MPC first.

c) By the time an object is numbered it will quite likely have been observed on at least 3 oppositions, but the discovery credit will be assigned to the opposition that has the earliest-reported second-night observation, not necessarily the earliest opposition. So for example, if old NEAT images from 2003 are mined for moving objects and positions measured and submitted today for an object on several nearby nights back in 2003 they might end up being the only observations from the chronologically earliest observed opposition, but would have been reported last and therefore would not be considered for discovery credit.

d) An object observed at more than one opposition before the new rules came into effect will have the previous rules applied and therefore discovery credit is likely to go to observations from one of those old oppositions. An object observed at only one opposition before the new rules will have the new rules applied (but with timetags for the old observations based on the date of observation, not date submitted) so credit is also likely to go to observations made at the old opposition.

So, returning to the four objects I observed on March 17th that had received provisional designations, at the time of writing (6th June 2012):

2012 FT1: Now identified with observations on 3 nights in 2005 obtained by Spacewatch (code 691) and Moletai Astronomical Observatory in Lithuania (code 152) so discovery credit is likely to go to one of those observatories.

2012 FU1: Observations from Spacewatch (code 691) on March 16 and Mt Lemmon (code G96) on March 17 pre-date my positions from March 17th and very likely 691 and G96 submitted their observations in near-real time, so if no earlier observations are uncovered, discovery credit is likely to go to either 691 or G96, depending on which submitted first.

2012 FW1: Observations from Pan-STARRS (code F51) on Feb. 27 and Mt Lemmon (code G96) on March 13 pre-date my positions from March 18th and again, F51 and G96 are likely to have submitted their observations in near-real time, so if no earlier observations are uncovered discovery credit is likely to go to F51.

2012 FX1: Observations from Pan-STARRS (code F51) on March 16.5UT pre-date my positions from March 18.11UT and again, F51 probably submitted their observations in near-real time, so if no earlier observations are uncovered discovery credit is likely to go to F51.

Without being able to examine the timetags for 2012 FU1 and especially 2012 FX1, it can't be completely ruled out that my positions were in fact the first submitted, even though it is very, very unlikely! Additionally, as mentioned earlier, discovery credit is only assigned when the objects are finally numbered and therefore these guesses at how the credit will be assigned may all change if still unidentified observations that have been reported possibly years earlier are linked as further observations accumulate in upcoming oppositions. So I am fully expecting that all of those four objects will be assigned to other observatories by the time they each get permanent numberings.

So my advice to observers wanting to try and get discovery credit for moving objects they find in their images is:

- Measure and report unidentified objects as soon as possible, preferably on the same night as the exposures are obtained

- Try to get at least a second night of astrometry and preferably a third, just in case no other positions have been reported from anywhere else this opposition

- Assume the worst, it is very rare now that an object will be found without any earlier 2 or more night oppositions and very difficult to find an object that has not already been picked up and reported by one of the big surveys earlier in the same opposition. Difficult but not impossible.

- Don't hold your breath! Even if you have a promising candidate, it could be some years before you know whether your observations will definitely receive discovery credit or not.

[This post has minor updates from the originally published article that appeared in the May 2012 The Astronomer magazine (TA Vol 48 No 577 p19-21 [2012])]

Thursday, 10 May 2012

NEO 2008 PG1 recovered

Earlier today the Minor Planet Center (MPC) published circulars for four Near-Earth asteroids, each one having already received a designation two or more years ago. In each case the MPC had managed to link old positions with newer ones that had until now managed to evade detection as referring to the same object. These new linkages allowed each of their orbits to be substantially improved:

MPEC 2012-J26 1999 OQ3 (2012 Apr positions linked to 1999 Jul-Oct positions)
MPEC 2012-J27 2012 FB57 = 2008 VO12 (2012 Mar-Apr positions linked to 2008 Nov positions)
MPEC 2012-J28 2010 VZ71 = 2010 MB113 (2010 Nov-2011 Jan positions linked to 2010 Jun positions)
MPEC 2012-J29 2009 EV = 2008 PG1 (2009 Mar positions linked to 2008 Aug positions)

It looks like the MPC may have recently introduced some enhanced routines to search for linkages in their archive of old observations, the positions linking 2009 EV with 2008 PG1 for instance have been available for three years and only now has the connection been made that they are the same object.

2008 PG1 was only observed on two nights in August 2008, discovered by LINEAR on August 3 and observed 16 hours later from Great Shefford on the morning of Aug 4th, the discovery was announced the same day in MPEC 2008-P14. Attempts were made from Great Shefford to recover it on the next two clear nights but without success and it was the subject of a note here in September 2008. With a very uncertain orbit it was listed by JPL as a Virtual Impactor with the first (very low probability) impact with Earth predicted to happen as early as 2013.

Update 15 May 2012: Following the linkage and subsequent orbital improvement, the JPL Sentry system has been able to remove 2009 = 2008 PG1 from their list of potential future Earth impact events.

Following the linkage of 2009 EV to 2008 PG1 by the MPC, the images taken at Great Shefford on the mornings of 2008 Aug. 9th and 13th were re-examined to check whether 2008 PG1 would have been in the field of view during those search attempts. On Aug. 9th, four fields were taken, spanning 1° but further searching had to be abandoned because of the approaching dawn. Unfortunately, just one more field would have picked it up, close but not close enough. With a rapidly expanding uncertainty area, fields taken on the morning of Aug 13th were all more than 2.5° away from the actual position.

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.

Saturday, 10 March 2012

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 recovery field on 2012 Feb 02
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.