Asteroid 2008 TC3 impacts over northern Sudan

By HansPeter Roesli (EUMETSAT), Zdenek Charvat (CHMI ) and Maria Putsay (Hungarian Meteorological Service)

Asteroid 2008 TC3 was first detected on 6 October 2008 and it soon became clear that it would collide with the Earth. Its atmospheric entry, predicted some 20 hours after detection, was expected to be in the early morning on 7 October 2008 over northern Sudan. For general information on asteroid 2008 TC3 see Wikipedia and NASA Near Object Program; for details of the orbit see the NASA Jet Propulsion Laboratory website.

On 7 October 2008 Zdenek Charvát emailed "I was very sceptical about the possibility that anything could be seen in satellite images, but I was WRONG!!! ". In fact, on the rapid-scan imagery of Meteosat-8 (images at 5-minute intervals) he detected a tiny bright spot at 02:45 UTC (see HRV image). Time and geographical location matched the impact prediction very well.

The response of the IR channels is interesting. All channels show a warm spot of varying intensity, well illustrated by the IR10.8 difference image (time difference 2:45–2:40 UTC). The detectors of the IR3.9 (see image below and IR3.9 difference image) and WV6.2 channels become saturated.

They show the typical 'ringing' pixel pattern, well known from observations of small but very hot (fire) or reflective (sunglint) spots in IR3.9. The radiance spectrum of the warmest spot deduced from the IR channels (see radiance spectrum) is sharply peaked around 1000 cm-1 (10 micron), whereas the solar channels closely follow a Planck curve at 3800 K (slightly below the melting point of carbon).

On the geolocated SEVIRI level 1.5 data used for this case study, the strongest signal in the HRV and VIS0.6/0.8/1.6 channels appears some 23 km WNW of the strongest signal in the IR10.8 channel (see HRV/IR10.8 composite image). This is due to the fact that for thermo-mechanical reasons the solar and the IR detectors are mounted in different positions in the focal plane of the SEVIRI telescope, i.e. SEVIRI is 'cross-eyed' and its detectors do not look simultaneously at the same spot on the Earth disc.

The time of registration of the same spot differs by some three seconds between the HRV/VIS/NIR and IR detectors. Whereas for slow-moving (meteorological) objects the SEVIRI strabismus is insignificant (and is not considered in processing the data to level 1.5), the signal of fast moving objects (in the order of km/s) appear displaced when remapped to the reference geography.

The trajectory calculated by Steve Chesley (JPL, lower white line) and the Meteosat-8 parallax shifted trajectory (upper white line) are shown on the IR10.8 image. The parallax-shifted trajectory matches the satellite signal rather well. The white circles on both trajectories mark the decreasing heights at 10-km steps from WNW to ESE. The two red marks on the parallax-shifted trajectory give the position of the HRV flash and its predicted position 3.0 seconds later. The predicted flash position is not collocated with the warmest IR signal but slightly ahead of it at its forefront.

The strong signals from the explosion were followed by an elongated weak signal drifting slowly WSW at a height of 20–30 km (according to the trajectory calculations), most probably resulting from debris of the explosion (see Animation, 02:45–03:50 UTC). The debris cloud is first picked up in the IR10.8 channel that initially shows a cold pattern with a maximum cooling of 2 K against the background.

The cool signal is strongest in IR10.8 getting weaker both at longer and shorter wavelengths. As the IR10.8 signal becomes weaker a fortunate coincidence has the debris cloud picked up in the HRV channel by the scattered light from the rising Sun (initially at 5° below the horizon). Note that, with with a signal strength of <1% of the maximum measurable intensity, the HRV signal is also extremely weak, and is outshone as soon as the ground below is illuminated.