Saturday 14 November 2015

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