2011 August notes: An amateur discovered tumbling NEO

August continued the poor run of weather from July and most of the time it was a case of making the best of what clear sky there was.

One opportunity came early in the morning on August 9th when a new discovery was posted on the NEO Confirmation Page, eventually designated as 2011 PE2, this one picked up by the amateur run La Sagra Sky Survey in Spain. The discovery images were from 24 hours earlier which was quite unusual, often La Sagra post their discoveries in near real-time. Jaime Nomen from the survey mentioned later that their reduction software had crashed during the previous evening and those images that had not been processed at the time were put into the queue for the next night, so it was only detected a day later.

Because of the delay the positional uncertainty was growing, with the Minor Planet Center's uncertainty map indicating the likely area it might be found in was 1.75 degrees long. I eventually picked it up 20' from the nominal prediction and then followed it for 20 minutes to get enough images to measure astrometry to send to the MPC. The ephemeris indicated it should be about mag +18.3 but during that 20 minutes it could be seen to vary in brightness from barely visible to very obvious within about 10 minutes. Because of this, I decided to continue to follow it until dawn to try and obtain a lightcurve, ending up with nearly three hours of measurements and showing the total variation to be very large at about 2 magnitudes (a factor of 6). 2011 PE2 was about at its brightest when discovered and on subsequent nights had already faded too much for any more useful photometry to be obtained.

Canopus was used to reduce the brightness measurements obtained that first night and to plot a lightcurve. Although the rises and falls from max. to min. in approx 10 minutes that were noticed at the time the images were taken were visible, the variations were not regular, indicating that it may not be simply rotating in one axis, but probably tumbling. In the diagram below, "beating" can be seen in the maxima and minima as two competing periods cause constructive and destructive interference to the overall curve, indeed at about 0.57 on the x-axis a minimum is almost completely absent.

Solving for a single period gives a value of 43.8 minutes for the main variation (so 11 minutes between each maxima and minima). However, the second period has not yet been satisfactorily determined and so the final value of the main period is still undetermined and could be somewhat different to that given above.

Raw (unfolded) lightcurve of 2011 PE2 from 348 data points obtained 2011 Aug. 09 00:14-03:08 UT

Bright Supernova 2011fe in galaxy M101

Discovered automatically by the The Palomar Transient Factory on August 24 at magnitude 17, this was quickly determined to be a very young type 1a supernova, expected to brighten by possibly 6 magnitudes within the next week or two.

The animation below consists of two frames, the first taken on 2003 Jan 28 using a 0.3-m telescope, before the supernova appeared and the second on 2011 Aug 27 using a 0.4-m telescope, with the supernova at magnitude +13.0. This image was obtained during a 4 minute gap with the object passing between trees, when M101 was almost at its lowest point over the north horizon, poor weather preventing any attempt in better circumstances.

The Sky At Night comes to Great Shefford

The BBC's Sky at Night team visited Great Shefford Observatory on July 6th and filmed a short interview following the very close approach of NEO 2011 MD at the end of June (see the June and July notes).

The interview was included in the program "Dawn at Vesta" and was first transmitted in the early hours of Monday 8th August on BBC1.

Watch the interview with Paul Abel here:

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

 

Confirming the discovery of close approach asteroid 2011 MD

The Minor Planet Center (MPC) added two new LINEAR discoveries to the NEO Confirmation page (NEOCP) just after 01am UT on 23 June while I was taking images of another object discovered a day and a half earlier by the SPACEWATCH team (which would eventually be designated 2011 MF). With only 45 minutes left before twilight would get so bright that imaging would have to be abandoned, I decided to stop what I was doing and try for one of the LINEAR discoveries.

The two new objects were posted on the NEOCP with the temporary designations assigned to them by the LINEAR team, BZ52584 and BZ52587.

BZ52584 was in reasonably dark sky, about 4 degrees north of M13, the Great Globular star Cluster in Hercules while BZ52587 was much further east, about 3 degrees west of M31, the Andromeda galaxy and already in the glow of the approaching dawn. As BZ52584 was moving twice as fast as BZ52587 and better placed I decided it would be more useful and more likely to succeed to try and confirm it in the short time left before dawn. (BZ52587 would turn out to be a comet and be designated C/2011 M1 LINEAR a couple of days later).

However, the MPC prediction for BZ52584 was a bit odd - LINEAR had only observed it for 60 minutes some 18 hours earlier and normally the MPC would provide both a predicted position for the new object as well as an uncertainty map showing the likely area of sky the new object might be found in. However, this time only the predicted position was given, no uncertainty map. I took a set of images centered on the predicted position between 01:20 - 01:40 UT but when examined, there was no trace of the new object. With only about 20 minutes of usable sky left I started to hunt for BZ52584. With time only to take one or two more fields of view I chose to start with the field of view immediately to the east of the MPC predicted position.

As there was so little time left, the images were examined as soon as they were downloaded from the camera and after 7 minutes collecting 13 images it looked like there was a probable candidate, moving with the right motion, over 1/4 degree from the original prediction and just 17 pixels from the bottom of the images!

Two fields taken during the hunt for BZ52584. White denotes the first, centered on predicted position, yellow indicates the field where the object was found, very close to the bottom of the frame

One or two of the images had been spoiled by clouds that had started to thicken up and in a desperate attempt to positively confirm the new discovery before being clouded out I repositioned the telescope to centre the suspected object in the field of view and get some more images. The clouds continued to thicken and only 5 of 23 images taken after repositioning were at all usable, but fortunately they did show the new object in the expected place.

All of the good images were then measured and positions sent off to the MPC at 02:05 UT.



Animation of 2 sets of 5 x 20 second stacked exposures, showing motion of 2011 MD
01:41-01:45 UT 23 June 2011


The three positions I had just measured and the four provided by LINEAR were then put into FindOrb to work out an orbit and to provide an early view of where the new object was going to be in the next few days. It was immediately apparent that it was headed for a very close approach to Earth in 4 or 5 days time and so, to alert other observers and the MPC, I posted on the MPC's NEOCP blog at 02:16 UT:

"J95: BZ52584 probable v. close approach on June 27.2 UT"

"FindOrb gives ~23,000Km on June 27.2 UT (leaving out 1 of 704 positions). Worth
some more follow-up."

With further positions measured from other observatories in the following hours, the new object was announced by the MPC as 2011 MD later on June 23 and the close approach turned out to be somewhat closer than that first prediction, at 18,700 km from the Earth's centre on June 27.7 UT, or just under 1 Earth diameter from the Earth's surface.