The life and death of WT1190F (= UDA34A3 = UW8551D = 9U01FF6)

An object discovered by the Catalina Sky Survey and assigned the designation WT1190F initially looked very much like a near-earth asteroid when added to the NEO Confirmation Page (NEOCP) on 3rd October 2015 but it was removed by the Minor Planet Center two days later with the note "Was not a minor planet". The astrometry at that time showed it was orbiting Earth every 21 days in a very unusual and elongated orbit taking it to within one Earth radius of the surface of the Earth at perigee but 1.65 times further away than the Moon at apogee. The eccentricity (e) of the orbit was 0.96.

WT1190F captured during its penultimate close pass with the Earth, three weeks before impact

Even while it was still on the NEOCP I had been in contact with Bill Gray, the author of Guide and also of FindOrb, because the orbit appeared similar to an unidentified object picked up in October 2009 (9U01FF6) and also to an object discovered in February 2013 (UDA34A3) that Bill had subsequently managed to link to another discovery in November later the same year (UW8551D).

9U01FF6 was in a ~25 day orbit with e=0.83 at discovery and had been followed for a couple of days by a number of observatories. I had then managed to follow it at a further five perigees up to May 2010 and had written about it in Oct, Nov and Dec 2009.

UDA34A3 (= UW8551D) was in a 29 day orbit with e=0.86 in February 2013. Bill's linkage between the two 2013 objects was remarkable, as UDA34A3 was only observed for 5 hours in February and UW8551D for less than 2 hours in November! 

WT1190F less than 1 hour from impact with the Earth, showing regular rotational light variations every 3/4 second

Bill pointed out on 5th Oct. 2015 that calculating the orbit of WT1190F forwards showed that it would impact the Earth on 13th November 2015! Subsequent astrometry confirmed this, impact occurring less than 10 miles south of Sri Lanka at 06:19 UT on 13 Nov. 2015, in daylight, around noon local time.

Bill eventually managed to show that WT1190F was the same object as UDA34A3 = UW8551D from 2013, though he faced considerable difficulties due to the effects that solar wind has on this apparently very low mass object, together with close approaches to the Moon. Bill then requested if both the Catalina and PanSTARRS surveys could search their archives using the new linkage to see if they could find any further images of WT1190F going back that might link up with 9U01FF6 from 2009. PanSTARRS managed to locate and measure images going back to December 2012 but even with these, attempts to link the 2012-2015 apparitions back to the 2009/10 observations of 9U01FF6 proved impossible.

However, Bill then figured that maybe the prediction from the 6 months of observations of 9U01FF6 from 2009 might be good enough for the surveys to locate images extending the arc forwards from 2009 to try and join the two arcs up that way and indeed both Catalina and PanSTARRS managed to find it in their archives from January and April 2011. The April positions were crucial as a lunar flyby in February 2011 had made the orbital elements very uncertain after that date. With the new positions Bill was able to calculate 9U01FF6 forwards to another lunar flyby on 24 May 2012 and similarly calculate WT1190F back to the same flyby and prove they were indeed the same object. Have a look at the very comprehensive FAQ provided by Bill Gray here.

The streak on the left is the final image captured from Great Shefford before WT1190F disappeared below the horizon, 20 minutes before impact over the Indian Ocean

WT1190F is thought likely to have been part of an old lunar mission and from the way it was easily blown around by the solar wind it is probably hollow and 1-2 meters in size, possibly something like the Lunar Prospector TLI stage from 1998.

A team from ESA observing from an aircraft over the Indian Ocean witnessed it burn up in the atmosphere exactly as predicted, this image and more details can be found at their page here.

When WT1190F struck this atmosphere over the Indian Ocean around 6:20 Universal Time (12:20 a.m. CST) today , it broke apart into multiple fireballs against the blue sky. The object came down around 1:20 p.m. local time. Credit: IAC/UAE Space Agency/NASA/ESA

2009 December notes: 2009 WZ104, 9U01FF6, 2009 observing statistics

December provided a reasonable amount of observing time but the number of NEOs discovered by the surveys was much reduced, 34 new objects compared to the 116 they picked up in November, so NEO follow-up work was quieter than of late.

As well as following objects on the NEO Confirmation page, the opportunity was taken to follow 2009 WZ104 on several nights to try and determine its light curve. This is an Aten discovered on Nov. 25 by the Catalina Sky Survey and was brighter than 17th mag. for the first two weeks in December. However, not enough coverage was obtained to be able to find an unambiguous period, but it is likely to be longer than 6-8 hours.

The unusual artificial satellite 9U01FF6 mentioned last month in TA Vol 46 No 548 p211 (2009) which is in a 30+ day, very elongated orbit, taking it as close to us as 5 Earth diameters and about twice as far as the Moon at apogee, was picked up again on Dec. 27, three days before perigee. It was only 5" from the prediction calculated with FindOrb using positions from the previous two returns and taking into account Solar Radiation Pressure. Unfortunately the weather stopped any further observations but it should be visible next on the night of 6th Feb. 2010 and then in March a relatively long apparition, from the 11th-15th.

2009 overall was an exceptionally good year for observing at Great Shefford, producing the best observing figures since my observatory was commissioned in May 2002. Several statistics follow, with the previous best figures and year in parentheses: The most nights used (202 vs. 183 in 2003), the most hours spent at the telescope (938h vs. 838h in 2005), the largest amount of time the CCD shutter was open (24d 00h vs. 22d 03h in 2007) and far and away the largest number of images taken in a year (148,361 vs. 92,025 in 2007). That last statistic is in part to do with more Minor Planet photometry being done in 2009 than before, with many objects being followed for long periods of time, collecting many images during a night. The total number of images taken since 2002 passed the 1/2 million mark during the year and now stands at 570,575.

2009 generally seems to have beaten my previous records by 8-12% all round. Lets hope 2010 doesn't let us down!

2009 November notes: 2009 VX, 2009 VZ, 2009 WJ6, 2009 WV51 and 9U01FF6

Although many nights in November had interruptions from cloud, observing time was logged on 18 nights and there were plenty of Near Earth Objects to be followed that had been discovered during the month by the surveys.

Apollo 2009 VX had been discovered on Nov. 9th by the Catalina Sky Survey (CSS) as a 19th mag. object but had brightened to 17th mag. by the time it was picked up from Great Shefford early on the morning of Nov 12th, moving at 150"/min. It was then at 2.8 Lunar Distances (LD) and would pass at 2.59 LD at 13h UT the same day.

Another relatively bright new discovery was Apollo 2009 VZ and although it didn't get any closer than 15 LD it remained above 17th magnitude for 12 nights, I recorded it at 15th mag. on Nov. 12 & 14th. 2009 VZ was intrinsically about 5 magnitudes brighter than 2009 VX, which translates to 2009 VZ having a diameter about ten-times larger than 2009 VX (approx. 300 meters vs. 30 meters), assuming similar albedos for the two objects.
2009 WJ6 was another CSS discovery from Nov 19th and was followed from Great Shefford later that same day. By then it was already about mag. +17.0, at 2.1 LD and moving at 58"/min. It reached its closest to Earth at 11am UT the next day at 0.46 LD and was reported from the Westfield site of the Astronomical Research Observatory, Illinois three hours before perigee, at a distance of 0.82 LD.

One other close approach Apollo object was 2009 WV51. Discovered at 7am on Nov 23 by the CSS it was due to pass at just 0.39 LD the next day. I picked it up on the evening of Nov 23 at 17th magnitude when it was at a distance of 2.8 LD and moving at 'just' 22"/min. Watching individual images download it was easy to see it rising and falling in brightness by approximately 1 magnitude in just a few minutes, though the lightcurve has yet to be reduced. It was last reported from the Magdalena Ridge Observatory, New Mexico on Nov. 24 at 05:43 UT with the 2.4-m reflector, at a range of about 1.8 LD. It went unobserved from Great Shefford that night at its closest approach due to bad weather, but would have been 14th magnitude and moving at over 1,000"/min in the early evening of Nov. 24.

The probable artificial satellite 9U01FF6 mentioned last month was due back at perigee early on Nov. 28 after being followed for only 44 hours by 7 observatories at the end of October. I started searching for it on the evening of Nov 23 but given a likely positional uncertainty of up to 1°, the waxing Moon only about 30° to the south and the predicted mag. being +21.2 I was not expecting much success. However, it was picked up only 5' from the predicted position and ranging in magnitude from +20.5 up to +18.9, the standard asteroid magnitude formula obviously not providing a good fit for this object! Further positions obtained on three subsequent nights allowed the effect of Solar Radiation Pressure (SRP) to be determined using FindOrb using all the available observations (26 October - 27 November 2009). SRP effectively pushes an object away from the Sun and the smaller the mass and larger the surface area of the object exposed to the Sun, the larger the discrepancy from Newtonian motion may be observed. Without taking into account SRP the RMS residual for 78 positions over the two apparitions is a very unsatisfactory 16", but taking into account SRP this drops to 0.8". The value of SRP (or Area/Mass Ratio) determined was 0.011 m2/kg which is similar to those determined using FindOrb for other distant artificial satellites, e.g.

LCROSS (incl. Centaur) size 14.5m x 4.7m, mass ~3,200kg, SRP = 0.015 m2/kg
IMP8 size 1.4m x 1.6m, mass 371kg, SRP = 0.010 m2/kg
ASTRON (1983-020A) size 6m long, mass 3250kg, SRP = 0.008 m2/kg

and is 10 times larger than the 0.0011 m2/kg value derived for 2006 RH120 (=6R10DB9), the tiny (natural) minor planet that was temporarily trapped in Earth orbit during 2006-2007. All this points to 9U01FF6 being man-made and from a posting on the SeeSat-L (Visual Satellite Observers) mailing list here http://www.satobs.org/seesat/Oct-2009/0176.html it is suggested that 9U01FF6 is very likely to be from a lunar transfer mission of some kind, but the perturbations on the orbit make identification of the specific launch very difficult. It is noted that some of the Agena rockets for the Ranger missions of the 1960s were in very similar orbits.

2009 October notes: 2009 TM8, 2009 UD, 2009 UV18, LCROSS and 9U01FF6

Apollo 2009 TM8 made a very close approach to the Earth early on Oct. 17th about 48 hours after discovery by the Catalina Sky Survey. It was followed from Great Shefford from soon after sunset on Oct. 16th when it was moving at an apparent speed of 180"/min, through to 04:20 UT the next morning by which time it had accelerated to 285"/min. It reached its closest to Earth at 03:37 UT at a distance of 0.9 LD. The brightness stayed fairly constant, around mag. +17.5 all night, the decreasing distance from Earth being offset by increasing phase angle but the large increase in apparent speed made it a more difficult target as the night wore on.

Another newly discovered Apollo, 2009 UD passed Earth at 2 LD on Oct. 20 and was observed on the morning of Oct. 17th for 2 hours and again for 3 hours the next morning when it was at about 3 LD and mag +17.0. Over 1,000 images were obtained in those 5 hours and allowed the determination that this ~15 metre diameter object rotates very fast with a period of 83.7 seconds with an amplitude of about 0.7 magnitudes.

2009 UV18 was an interesting LINEAR discovery from Oct. 22, relatively bright at mag. +18 and unusually large for a new discovery, estimated at over 2 km diameter. With a low inclination and the Earth approaching the ascending node of the orbit the size of the orbit was initially rather indeterminate and attempts at trying to identify it with a known minor planet failed, even though it ought to have been relatively bright in earlier apparitions. Then, on Oct. 29, Rob Matson, well known for precovery work with online image databases managed to find it on old NEAT images from Jan 2004, fortunately only 2.5° from the predicted place, even though the uncertainty was estimated at up to 90°.

The orbit of 2009 UV18 is similar to a Jupiter family comet and although it appeared stellar in images taken on Oct.23 and Oct. 26th it is probably worth keeping an eye on as it approaches perihelion this January in the morning sky. The last favourable return appears to have been in May 1993 when it should have reached mag. +16 and before that spring 1976 at mag +17.

The LCROSS mission was followed one final time on the night of Oct 8/9th. The spacecraft was only 6.4° from the gibbous Moon and internal reflections in the telescope made it a difficult target. Unfortunately the sky clouded over before the LCROSS shepherding spacecraft separated from the Centaur at 02:50 UT, the last images obtained that show LCROSS were taken at 01:56 UT but were so light polluted they could not be measured. The image of LCROSS was taken about an hour earlier, before the glare from the Moon became too strong, less than 11 hours before impact with the Moon.

What initially appeared to be a Near Earth Asteroid discovery was reported by the Catalina Sky Survey on Oct 26 and received their temporary designation 9U01FF6. It was 19th magnitude and moving at 10"/min but was already closer to Earth than the Moon. After follow-up from other observatories that same day it was found to be in a very unusual 31.5 day period, highly eccentric geocentric orbit, taking it to within 82,000 km of Earth (0.2 Lunar Distances) at perigee and out to 761,000 km (2.0 LD) at apogee. With an absolute magnitude of +30.9 it would only be 1-2 metres in diameter if it were a natural object, but smaller if artificial (with a higher albedo assumed). Further observations were made from Great Shefford on the evening of Oct. 27th and the last positions reported this perigee came from the OAM Observatory at La Sagra in Spain early on Oct. 28th.
9U01FF6 is fainter than mag. +21 for most of its orbit but will next be at perigee around Nov. 28th and should reach +16th magnitude but moving as fast as 250"-500"/min on the evening of Nov. 27th. Hopefully it will be recovered in the days leading up to that but observability is probably limited at most to only 2-4 nights each perigee. It is likely to be an unusual artificial satellite but further observations at the end of November are encouraged.