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.

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.

Confirming the recovery of Comet P/2006 T1 (Levy)

Comet P/2006 T1 (Levy), due back to the Sun's neighbourhood in mid-January 2012, its first return to the Sun following its discovery in 2006 was eventually recovered by Richard Kowalski from the Mt. Lemon Survey early on 17 Dec 2011 during a search of the object's entire uncertainty region.

It was over 2° from the predicted position at the time and magnitude +19, about 9 magnitudes fainter than predicted, based on its brightness during the 2006 apparition, though it is thought that the comet underwent a brightness outburst just before being discovered.

The recovery was confirmed by further positional measurements from observatories Farra d'Isonzo (595) and Great Shefford (J95), both observatories noting the object as diffuse and with an elongated coma or tail pointing to the North-East (upper left in the image below).

Images taken from Great Shefford Observatory of the comet at its 2006 discovery apparition can be found here.

2011 November notes

Poor weather in the first half of November (that stopped the very close fly-by of 2005 YU55 from being observed from Great Shefford) was replaced by a decent last two weeks and plenty of newly discovered NEOs were found to be in need of observation, although a fair number of these were rather faint.

However, there were a few close passes observed and probably the best of those were by 2011 WP4 and 2011 WQ4. My last sighting of 2011 WP4 was on the morning of Nov 23 at mag +17.6 with it moving almost due south at 48"/min. At that time it was just over 3 Lunar Distances (LD) from Earth. Two other observatories observed it in the few hours following but no one reported it at its closest just over 1 day later when it had approached to within 1.5 LD and was moving 5 times faster but had reached high southerly declinations of -60 to -70 degrees. 2011 WQ4, although a similar absolute magnitude to 2011 WP4 (H=+27.4 vs H=+27.2 for WP4, equating to likely diameters in the range of 8-13 meters) was always a fainter target and only approached to 2 LD. It was also last seen on the morning of Nov 23, heading out from its close approach a day and a half earlier but by then was only mag +19 and moving at 55"/min made it a much harder target than 2011 WP4 had been.

At 10pm on the night of Nov 17th I received an email from Michel Ory (discoverer of Comet P/2008 Q2) requesting help to try and recover a fast moving 19th mag. object that Claudine Rinner had discovered two nights earlier from station J43 with a 0.5-m f/3 reflector (she would go on to discover Comet P/2011 W2 less than 2 weeks later with the same instrument). With only three positions spanning just 41 minutes from Nov 15th, the uncertainty in the position predicted by FindOrb for Nov 17th was expected to be fairly large and so a number of fields would need to be taken to stand any chance of recovery. Although Michel had sent the positions into the Minor Planet Center, the object had not yet appeared on the NEO Confirmation page (NEOCP). I started taking images for it within the hour, but already it was getting low in the southwest, the altitude of the field was only 27 degrees and getting lower but even more of a problem was thickening cloud coming in from the west. I managed to search for nearly 45 minutes before being completely clouded out but could not positively identify anything, especially difficult with the poor quality, cloud affected images.

The next night was clear and so I started searching again for the object, earlier in the evening and in better conditions than the night before. By now it was listed on the NEOCP, together with an uncertainty map showing its likely location was expected to be anywhere within a strip of sky over 2.5 degrees long. After 1h 45mins I eventually found the telltale expected movement in blinked images about 3/4 degree ahead of the NEOCP nominal predicted position. To help other observers, at 10:30pm I posted a comment on the NEOCP Blog (http://minorplanetcenter.net/NEOCPblog/) stating where I'd found it and about 15 minutes later, a set of positions from the Catalina Sky Survey Mt. Lemmon telescope (station code G96) were posted from images taken some 18 hours earlier.

In the meantime I had used FindOrb to improve the orbital elements, combining the discovery positions with those I had just measured and worked out an ephemeris for the previous night. With the accurate ephemeris and much improved values for the rate of motion, I was able to re-stack the images from the night before and managed to identify weak images of the object. These were quickly measured and both nights of measures sent in to the Minor Planet Center and by 10:52pm the Minor Planet Center announced the new discovery as Amor object 2011 VP12 in MPEC 2011-W18, containing the discovery positions from J43 together with those from G96 and my two nights. Congratulations to Claudine on this discovery as well as for P/2011 W2!

On Nov 26, NASA launched the Mars Science Laboratory (MSL) on top of an Atlas V rocket and, using an ephemeris from the JPL Horizons web page available from http://ssd.jpl.nasa.gov/horizons.cgi I picked it up on the morning of Nov 28 at mag 16.8-17.0, moving at a fairly gentle 5"/min. The next day a 16th mag object was added to the NEOCP with very similar RA and Dec to where I had observed MSL the night before and moving in the same direction with similar speed. It was quickly determined however that this object was NOT MSL, but rather the Centaur second stage of the Atlas V rocket, about 3/4 degree away from the MSL spacecraft. The Centaur stage is used twice, once to achieve a low Earth orbit after launch, then again to take the MSL out of Earth orbit and accelerate it towards Mars and although not part of the experiment is accompanying MSL on its journey to Mars. I imaged both the MSL and the Centaur 2nd stage on the morning of Nov 30th and by then MSL had more than doubled its distance from Earth since my first observation, 2.8 LDs compared to 1.3 LDs two days earlier and was also a magnitude fainter than before at +18.1. The Centaur stage was much brighter than MSL, maybe by a magnitude or more and was also varying rapidly in brightness, very obviously changing between consecutive images, in marked contrast to the steady light from the more stable MSL spacecraft.

2011 September notes

A big improvement over recent months, September ended with a remarkable and record breaking heatwave in the UK and more than half the nights in the month being at least partially usable at Great Shefford, though fog and mist stopped work several hours before dawn on a number of occasions.

After a quiet summer, both PANSTARRS and the Catalina Sky Survey (CSS) telescopes were in full survey mode, looking for new Near Earth Objects and astrometry was contributed for 43 of the objects posted on the NEO Confirmation Page.

PANSTARRS picked up 2011 SP25 on September 20th, relatively faint at mag +20 and slow moving, I observed it 2 nights later and measured it at (red) mag +19.7. It is not a NEO and at the time of writing (Oct. 5th) there are now two weeks of astrometry available and the elements show it to be in a highly eccentric retrograde Halley-type orbit with period 89 years and perihelion at 2.27 AU due this coming November. It looked completely stellar in my images and will probably stay that way, but may be worth keeping an eye on in the next couple of months.

A couple of moderately close NEO passes were well observed, 2011 LJ19, discovered back in June by CSS came closest mid-month, at about 17 Lunar Distances (LD) and maintained 16th mag or brighter for about 2 weeks. A CSS discovery from September, 2011 SR5, passed about 22 LD towards the end of the month and peaked at mag +16 for about 5 days, reaching a speed against the sky slightly faster than 2011 LJ19 at 35"/min.

The closest observed approach though was 2011 SE58, a very faint mag +20.8 discovery made early on September 23 by the Spacewatch team using the 1.8-m reflector on Kitt Peak, when it was moving slowly at 0.8"/min. Spacewatch observed it again the next night by which time it had tripled in speed and was about 0.7 mags brighter, closing in fast at a range of about 14 LD.

I observed it 11 hours later on September 24 at 21:00 UT and found it more than a magnitude brighter at +18.7 and moving at 4"/min. Its distance had approximately halved to 8 LD in that time and a very close approach to 0.6 LD was predicted for September 27.1 UT. The next night was cloudy in Great Shefford but the night of the close approach started out clear. By 23:00 UT 2011 SE58 had risen to an altitude of 14 degrees, just high enough in the east to clear my house roof but it was moving so fast due east and still accelerating that it almost kept pace with the Earth's diurnal motion and stayed in approximately the same place just above the rooftop, edging in azimuth towards the south for the next hour, until mist came down and stopped any further observation. Although registering between mags +14-15 it was moving so fast and at a low altitude that it was unfortunately never recorded strongly enough for any accurate photometry to be attempted.

Here 2011 SE58 is seen passing from west to east through the 18'x18' field of view in less than 2 minutes

When first picked up that night it was moving at 480"/min at 0.80 LD and when last recorded 58 minutes later had accelerated to 600"/min, had approached to 0.72LD and moved a total of 9 degrees against the sky background. The Minor Planet Center Daily Orbit Update published the next morning included the measures I had made that night and also included some pre-discovery PANSTAARS astrometry from 2 days before the Spacewatch discovery observations, when it was 22nd magnitude and had only moved 9" in the hour it was under observation, quite a change in a week!