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.
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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.
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 µs 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.
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