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Asteroid Gallery

2007 EH (A Very Fast Moving Object)

2007 EH was discovered 44 hours before passing the Earth closer than the Moon. However, in the hours immediately following discovery, it could not be found at its predicted location, eventually being picked up again nearly 20 hours after discovery. The next night it was then tracked right through its close approach with Earth when it was moving across the sky at a speed of 1 in less than 3 minutes, or the apparent diameter of the Moon in about 90 seconds.

On this page:

Discovery
Confirmation
Close Approach
Radar Detection
Issues and techniques for astrometry of VFMOs

Discovery

2007 EH was discovered by Ed Beshore using the 0.68-m Schmidt of the Catalina Sky Survey on 9th March 2007 on four exposures made between 05:30 and 06:05 UT. A further four positions were obtained between 09:12 and 09:29 UT and it was added to the NEO Confirmation page (NEOCP) at 16:31 UT the same day with temporary designation 7E17639.

It was immediately obvious to the Minor Planet Center (the MPC) that the object was already very close to Earth and rapidly approaching, the nominal prediction posted on the NEOCP having the object making a very close pass to Earth around 14:00 UT the next day.

Confirmation

Conditions were clear at Great Shefford on the evening following discovery and the MPC prediction for 21:00 UT gave the likely uncertainty area as being a narrow band just 5' x 1' in size. This was predicted to expand to 9' long four hours later, still much smaller than the 18'x18' field of view of the telescope and CCD at Great Shefford and at magnitude +18 the object should have been an easy object to confirm.

The first set of exposures was made at 20:55 UT but only a very faint suspect was found, much fainter than the predicted brightness of the minor planet. Further runs with more exposures were taken from 21:41 to 22:34 UT to try and confirm this but in the end nothing was found. Starting at 23:25 UT more exposures were taken, of fields that extended each end of the uncertainty area in case the MPC prediction was too conservative. Eight overlapping areas were searched during the next 90 minutes, covering an area approximately 1 x 0.3  but still nothing was found.

At this point the original eight positions from the Catalina Sky Survey were examined, by assuming the object was moving with constant speed and in a straight line, then determining how far from the line the positions fell. The 5th and 8th position appeared to be somewhat out of step with the rest and so FindOrb was used to work out an orbit ignoring these possibly rogue positions. It gave a prediction nearly 1 to the west of the area previously searched and so this area was searched next, starting at 01:01 UT on 10th March 2007.

At the same time Tim Spahr from the MPC was in e-mail contact, requesting assistance to try and locate the object and, because of the close approach indicated from the initial observations he had also contacted Steve Chesley from JPL. At 1am Steve sent through two ephemerides, one for the nominal prediction, similar to the MPC's nominal prediction on the NEOCP and also another which had the object somewhat more distant from the Earth. This second ephemeris was only 2' different from the revised prediction generated with FindOrb, which was already being used for the search. The object was found in the images taken using the Findorb prediction 7' East and 1' North of the centre of the field of view!

Positions were measured and sent off at 01:44UT to the MPC and to Steve Chesley. Tim Spahr issued MPEC 2007-E34 announcing the discovery at 02:18UT. Steve e-mailed that he had passed the details on to the radar team at Arecibo as there was a half hour window they could use to detect 2007 EH around 15:30UT on 11th March 2007, although rescheduling the telescope time at such short notice would be difficult.

The revised orbit showed that the object had been discovered 7 lunar distances (L.D.) away and that close approach would occur some 12 hours later than first predicted, at 01:37UT on 11 March 2007, passing Earth at less than half the distance of the Moon. Fortunately 2007 EH would be well placed from Great Shefford almost all night on 10/11 March, giving an almost perfect opportunity to follow the object when at its closest to Earth.

Close Approach

When darkness fell on the evening of 10th March 2007 at Great Shefford 2007 EH was just rising and already just outside the distance of the Moon. By the time the first images were taken between 19:15 - 19:21UT it was magnitude +16.1 , 47,000 Km outside the Moons orbit and moving at 204"/minute, 15 times faster than when confirmed, just 18 hours earlier.

Getting a set of positions early in the evening was important to be able to correct the prediction for the rest of the close approach so that telescope pointing would be accurate. The positions measured were used together with the positions from Catalina and Great Shefford from the previous night to generate a new orbit and ephemeris for the rest of the night.

Further exposures were taken between 21:31 - 21:35 UT by which time 2007 EH was mag. +15.6, had crossed inside the Moons orbit and was moving at 404"/minute. To reduce the amount of trailing of the fast moving minor planet, individual exposures were limited to just 1 second long, with a 3 second gap between each exposure. As the individual images were taken it was an impressive sight watching the object moving across the field of view in real time. Then suddenly, a brighter, faster moving object entered the field of view, moving in approximately the same direction as 2007 EH (see animation below), streaking from one side to the other in just 30 seconds! This turned out to be Navstar 48 (USA 151) = NORAD 27407U = 2000-040A, a navigation satellite in the GPS constellation, launched from Cape Canaveral on 16 July 2000.

2007 EH being overtaken by Navstar 48

Navstar 48 was 21,600 Km distant, some 14 times closer than 2007 EH and was moving at 1,927"/minute. Just 4 hours later 2007 EH itself was to reach a maximum speed of 1,250"/minute.

A run was taken between 23:21 - 23:23 UT with the object now at mag +15.4 and moving at 769"/minute. The apparent speed was now increasing very rapidly, reaching a maximum acceleration of +5" per minute near 00:00 UT.

Another run between 00:43 - 00:46 UT on 11th March caught the minor planet moving at 1,150"/minute, it was by now less than half the distance of the Moon from Earth and still approaching. By this time the phase angle (see box below) was increasing by 1 every three minutes, causing the brightness to fall, even though it was still approaching the Earth.

Phase angle is zero when a body appears fully illuminated (therefore brightest), e.g. full moon and increases to 90 when half the body is illuminated, e.g. the first quarter moon and is 180 when between Earth and Sun (so no reflected sunlight reaches Earth) e.g. new moon.

Although always too far away to see the actual shape of 2007 EH, as the aspect of the minor planet changed from being face on like the full moon at discovery, to like a first quarter moon at closest approach the effect of the increasing phase angle would cause the brightness to plummet by about 1.5 magnitudes per hour immediately after closest approach, as it became a thin crescent and additionally started to recede from Earth.

The closest approach was at 01:34 UT on 11th March 2007 when it came to within 171,035 Km of Earth, or just 44% of the distance of the Moon and reached its maximum apparent speed of 1,250"/minute. At this speed it was passing through the 18.4'x18.4' field of view of the CCD camera in just 53 seconds, covering 10 pixels in the CCD each second! Exposures were now limited to 0.3 seconds to stop the very fast moving asteroid from trailing. Note how the motion has changed from north-easterly to south-easterly direction in the four hours since the animation above was taken.

2007 EH at closest approach to Earth

Within 10 minutes of the moment of closest approach the sky clouded over, clearing about half an hour later. A further set of exposures were taken from 02:15 - 02:25 UT on 11th March, but by then it was fading rapidly, 10 lower in the sky but still moving at over 1,100"/minute and with deteriorating transparency was much more difficult to detect. It was last recorded in images taken 02:22 - 02:23UT.

2007 EH was at the time of its close approach the third closest observed fly-by ever, only 2004 FH and 2006 DD1 having had astrometry measured at a closer distance. See the diagram here showing all observed minor planet approaches closer than the Moons distance.

Radar Detection

Mike Nolan managed to detect radar echoes bounced off 2007 EH from Arecibo at 15:40 and 15:49 UT later the same day when the minor planet had receded to about 2 lunar distances from Earth and was moving at a relatively sedate 44"/minute. It was within 15 of the Sun by then, so completely unobservable by visual means. Mike commented on the Minor Planet Mailing list that because it was so close at the time, the Doppler resolution was poor, limiting the precision of size and rotation estimates, but that the rotation period was likely to be no longer than 20 minutes, not a really fast rotator. He later added by e-mail that the range measurement was also limited by the proximity of 2007 EH to Earth:

"... it takes a finite amount of time (about 4s) to change the telescope from "transmit 106 W" mode to "listen to 10-21 watt" mode, and the object was pretty close to that far away. That affects the Doppler resolution but not the range resolution, which in this case was 4 microseconds = 600m. I had to disable some of the telescope interlocks to get it as it was, which took me a while of frustration, which is part of why I was only able to measure it to 4 s accuracy rather than 50 ns accuracy."

The echo from 2007 EH was measured by Mike to have a round trip time of 5.979606 0.000004 seconds (1 sigma uncertainty) which equates to 896,320.39 0.60 Km.

At the time of detection it was the smallest NEO ever detected by radar, with an absolute magnitude of H = +27.6 (translating to a size estimate of just 8 - 19 meters diameter, though there are necessarily large uncertainties in this figure).

Issues and techniques for astrometry of VFMOs

Telescope positioning

Setting the telescope at the correct position for a very fast moving object can be challenging. Ephemerides are used at Great Shefford for pointing the telescope (rather than using orbital elements to directly calculate positions) and it is very important when interpolating positions from an ephemeris that the ephemeris interval is not too long. If the VFMO is accelerating or decelerating rapidly which is very often the case at a close approach, then using an hourly spaced ephemeris can be completely inadequate, causing the calculated position of the object to be outside the field of view of the equipment. Use a shorter interval, say 10 minutes or shorter when the object is very close to the Earth.

The time it takes the telescope to point to a new location and the elapsed length of time needed to take the exposures all need taking into account to determine the VFMO's predicted position. The software used at Great Shefford uses the following simple calculation to determine the time of the middle of the run of exposures for fast movers which is then used to calculate the object's ephemeris position:

Time = # Exposures * (Exposure_Length + Image_Download_Time) + Telescope_Repositioning_Time

where:

  • Image download time is the time taken for a freshly taken image to download to the computer, up to the point when the next image is taken.
  • Telescope repositioning time is the average time taken for the telescope to slew to a new nearby position (assuming the telescope is already pointing at where the object was just a minute or two before).
Locating as early as possible on the night

It often happens that objects making a close approach to Earth are small and only discovered hours or a day or two before passing the Earth. It is very important to try and detect the object as early as possible on the night of closest approach so that any uncertainties in position can be eliminated before those uncertainties become so large that the object is difficult or impossible to locate. Use existing (probably discovery) positions together with those made on the night to improve the prediction using software such as FindOrb. 

Interlaced stacking

If the object is faintly recorded and moving very fast it can be difficult to correctly identify in the images, especially if there are many stars also recorded in the field of view. Interlaced stacking is a method especially suited to very fast moving objects that can make positive identification much easier and was used to help identification of 2007 EH in the runs immediately after close approach when it was still moving very fast but fading rapidly and also in the image above to reduce the apparent movement in the frame.

Time management

Time management is crucial to achieving accurate astrometry of a VFMO. With 2007 EH during its close approach the residuals from the astrometry were better than 2" at all times, equivalent to a timing error in the exposures of better than 0.1 seconds of time. Several mechanisms are in place to achieve this:

  • The pc running the CCD and telescope is running Windows XP. Older operating systems such as Windows 95/98 or ME are very poor at multitasking and should not be used. Windows 2000 and XP do not suffer from the same problems and work well. Windows Vista has not been used but is expected to also work well.
  • Accurate time is maintained on the pc taking the images by running Dimension4 software during images, keeping the pc clock set to time over the internet every 60 seconds. With a broadband connection the individual adjustments made are in the range of +/- 0.05 seconds of time, with a dial-up connection achieving corrections of the order of +/- 0.5 seconds. The Synchronisation history graph from Dimension4 during the time of close approach of 2007 EH is reproduced below, with the periods when exposures were being taken marked with grey rectangles. As can be seen, for the majority of the time corrections were significantly smaller than 1/100th of a second.

  • With the pc clock accurate to better than 1/10th second of time it is then important to know accurately when the exposures start. Software is used to wait for the second of the pc's clock to change and then as soon as possible after that, start the exposure, so that each exposure can be taken as starting very close to an integer second. The amount of time taken for the program to regain control after requesting the start of an exposure is measured and recorded, but very rarely exceeds 0.01 seconds. Shutter latency (the time taken for the shutter to open after it has been requested to be opened) of the AP47P in use until July 2005 was measured to have an average delay of 0.02 seconds 0.02 seconds. The shutter of the U47+ CCD in use since then has not yet been measured but is expected to be similar.
Exposure length v.s. Object trailing v.s. Reference star registration

A VFMO  will be recorded as a streak of light if the exposure time is too long and very elongated streaks are difficult to measure accurately. It is easier to achieve accurate astrometry by keeping the exposure duration short enough so that the object fits completely in the measurement aperture. The object does not have to be completely untrailed, as long as the measurement software can find the centre of light accurately. However, a very short exposure may cause the stars in the field of view to be only weakly recorded and cause a problem matching the stars in the field of view to the stars in the reference catalogue being used. So a balance has to be struck when taking the images, 

Astrometric Catalogue Relaxation

As mentioned previously, with the reduction in exposure time comes problems with catalogue star matching and it may be necessary to change software settings to get a plate solution to solve. Not enough catalogue stars identified when using Astrometrica to solve very short exposure time images can be alleviated for instance by, changing from the UCAC2 catalogue to USNO B1 (which contains many more stars), or reducing the number of stars required for a match, or even relaxing the acceptable residuals (both astrometric and photometric) when the software tries to identify catalogue stars. This last point needs to be used with care, relaxing the acceptable residuals when matching catalogue stars does not mean relaxing the residuals for the final astrometric solution, just letting the software have a better chance of choosing catalogue stars in the field of view.

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