2008 TC3 (The
first asteroid to be discovered
before impacting Earth)
2008 TC3 is the first minor
planet to be discovered and then found to be on a certain collision course
with Earth. It was very well observed on its approach and impacted in
northern Sudan 20 hours after discovery.
This page brings together information from various sources, including
Great Shefford Observatory. Links to the original material are given
On this page:
Last sightings before impact
A great demonstration of parallax
The moment of impact
The only visual sighting?
2008 TC3 was discovered at
06:39 UT on 6th October 2008 by Richard Kowalski working with the 1.5-m
reflector of the Mt. Lemmon Survey and was posted on
the Minor Planet Center's NEO Confirmation page an hour later.
The survey team released an animation of the four discovery images, showing
the object moving east to west in a field of view just under 6'x6' in
size, with a gap of about 15 minutes between each frame. It was moving
on the sky at about 6"/minute at magnitude +19 and was already only 1.25
Lunar Distances (L.D.) from Earth, or 480,000 Km.
copyright 2008 Catalina Sky Survey
(original Mt. Lemmon Survey animation available
It was followed for about 2.5 hours from Mt. Lemmon (G96) and also by James McGaha at Sabino Canyon Observatory near Tucson
(854). From the 15 positions
obtained it was already obvious that a very close approach was in progress, with an impact possible.
An orbit I determined
using FindOrb at around 13:00 UT gave close approach at 01:50 UT on 7th
Oct 2008 at a distance of 5,986 Km (Earth equatorial radius = 6357 Km).
At 13:39 UT I emailed the Minor Planet Center (08:39am local time) to make sure they were
aware of this possibility and at 13:54 UT Bill Gray (the author of FindOrb)
message 21070 on the Minor Planet Mailing List (MPML) with very
similar findings and estimated the chance of collision with the Earth at
Further positions obtained from the RAS Observatory, Moorook (D90) and from
the Siding Spring Survey (E12), both in Australia showed the object was
already closer than the Moon and just increased the estimated chances
of collision. The Minor Planet Center announced the new discovery as
2008 TC3 at 14:59 UT in
MPEC 2008-T50. A footnote to the circular indicated:
"The nominal orbit given above has 2008 TC3 coming to within one
earth radius around Oct. 7.1. The absolute magnitude indicates that the
object will not survive passage through the atmosphere.
Steve Chesley (JPL) reports that atmospheric entry will occur on
2008 Oct 07 0246 UTC over northern Sudan."
Andrea Milani (NEODys) wrote at 18:08 UT on the MPML that the
possibility of impact was now 99.8% or greater and that in practice a
collision was "sure".
As Europe fell into darkness later on 06th Oct, 2008 TC3 was well placed
for observation and many observatories measured positions and reported
them to the Minor Planet Center (MPC) in near real-time. The MPC kept
the NEO community up to date with the
latest information with 24 updates being published in the space of 8
hours, the last less than an hour before impact.
In the two animations
below, the small rock
is seen from Great Shefford in England, moving from West to East during
brief gaps in the clouds. The apparent speed as seen against the sky background doubled
in the intervening 80 minutes as it approached from
125,000 Km to 94,000 Km.
Note the "twinkling" of the minor planet, caused by rapid rotation,
exposing different aspects of its uneven surface to sunlight.
|4h 35m before impact.
22:11-22:15 UT 06 Oct 2008
mag +15.6, moving 33"/min
Distance 0.33 L.D.
45 x 4 second exposures
|3h 16m before impact.
23:26-23:30 UT 06 Oct 2008
mag +14.9, moving 60"/min
Distance 0.24 L.D.
45 x 2 second exposures
|Playback speed is approximately 80x faster than
0.4-m Schmidt-Cassegrain, Great Shefford Observatory
The images used in the animation on the right hand side are shown below,
this time combined so as
to keep the Minor Planet stationary, causing it to appear as a bright pinpoint of light in the
centre of the field of view and making the
stars trail during the 3 min 24 seconds the images were being recorded.
As indicated in the animations above, 2008 TC3
was showing regular variations in brightness during its approach and
long series of brightness measurements were obtained from a number of
observatories. Petr Pravec, well known for his work on asteroid spin
rates commented in message
21194 on the MPML that 2008 TC3
was definitely tumbling (spinning and precessing around two different
axes), with periods of 97 seconds and 49 seconds, making it one of the
fastest known rotators. His diagram showing
the solution can be seen by clicking
15 minutes before 2008 TC3
disappeared into the Earth's shadow the John J. McCarthy Observatory in
Connecticut, USA managed to obtain the following images. Due to the
geometry of observing from so far west, the apparent speed in the sky
was 704"/min., more than double the speed as seen from Europe at the same
copyright 2008 John J. McCarthy Observatory
Reiner Stoss, observing remotely with telescopes at La Sagra in Spain
arranged for a 0.45-m telescope to follow 2008 TC3 as it disappeared
into the Earth's shadow. As can be seen below, the minor planet,
streaking from right to left and showing some variation in brightness
due to rotation, fades completely from view as it is shaded from the Sun
by the Earth. The field of view is 20'x16' and the 6 minute exposure
started at 01:45:23 UT on 07 Oct 2008.
copyright 2008 Observatorio de La Sagra
See the original details on the La Sagra
2008 TC3 page
post on MPML