Although such small impact events occur several times per year around the globe, this case was unprecedented because the asteroid was actually discovered the day before it reached the Earth and the impact location and time were for the first time predicted in advance.

At 6:39 UT (UT = GMT) on the morning of October 6, 2008, Richard Kowalski, at the Catalina Sky Survey, discovered this small near-Earth asteroid using the Mt. Lemmon 1.5 meter aperture telescope near Tucson, Arizona.

When the discovery observations were reported to the Minor Planet Center (MPC) in Cambridge Massachusetts, a preliminary orbit computation immediately indicated that the object was headed for an Earth impact within 21 hours.

The MPC quickly made the discovery and subsequent "follow-up" observations available to the astronomical community and contacted the NASA/JPL Near-Earth Object Program Office. The MPC also notified NASA Headquarters of the impending impact so that subsequent US government interagency alerts and inter-governmental notifications could begin.

By the time this object (now designated as 2008 TC3) entered the Earth's shadow 19 hours after discovery, some 570 astrometric (positional) measurements had been reported from 26 observatories around the world, both professional and amateur.

Within an hour of receiving the initial data set, JPL predicted that the object would enter the Earth's atmosphere above northern Sudan around 02:46 UT on October 7. As the day progressed and more and more data arrived from the MPC, JPL continued to improve the orbit for 2008 TC3 and forwarded updated predictions to NASA Headquarters.

On the afternoon of Oct 6th, NASA Headquarters alerted officials at the National Security Council, the Office of Science and Technology Policy, the Department of State, and the Department of Defense Northern Command and Joint Space Operations Center. NASA also issued a press release at approximately 21:30 UT announcing the predicted impact later that night.

Detections of the actual atmospheric impact event suggested that it was an airburst explosion at an altitude of 37 km with an energy equivalent to about one kiloton of TNT explosives.

The time and place of the predicted impact agree very well with a number of atmospheric entry observations including those from U.S. government satellites, infrasound signals from at least one ground station, images from the Meteosat 8 weather satellite and a sighting by a KLM airline pilot flying over Chad.

The latest JPL trajectory estimate, which carefully considers all available data, including some measurements not available until after the event, is accurate to within a few kilometers at the time of atmospheric entry.

This dramatic prediction of an actual impact underscored the successful evolution of the Near-Earth Object (NEO) Program's discovery and orbit prediction process. The discovery was made, observations were provided by 26 international observatories, the orbit and impact computations were determined, verified and announced well before the impact, which took place only 20.5 hours after the discovery itself.

While improvements to the impact prediction process still need to be made, the system worked well for the first predicted impact by a near-Earth object.

Circumstances of the Earth impact:

The relative velocity of the 2008 TC3 as it entered the atmosphere was 12. 4 km/s. It was on a shallow, near-grazing impact, arriving from a direction 20.0 degrees above horizontal and from an azimuth of 281.6 degrees, which is nearly west to east.

Based on the measured brightness of the object in space, and assuming typical reflectance properties of near-Earth asteroids, it was estimated to be in the range of 2-5 meters in diameter. This places the kinetic energy at around 1 kiloton of TNT, give or take a factor of two.

Analyses of detections of the entry by US government satellites and ground-based very low frequency infrasound measurements also suggest a total kinetic energy of about 1 kT (Brown, 2008a,b,c; Johnson, 2008).

Harris (2008) provided information on the object's light curve, which indicates a rapidly tumbling spin state. Marek Kozubal and Ron Dantowitz, two Massachusetts amateur astronomers, observed 2008 TC3 with a fast read out photometer.

They supplied these photometric data to Al Harris who de-trended these data for 3 magnitudes of brightening, and then sent his results to Petr Pravec who ran the light curve analysis and determined the object was in a non-principal axis rotation state (i. e., it was tumbling or wobbling) with the two principal rotation periods being 97 and 49 seconds.

Related Links
Catalina Sky Survey
Asteroid and Comet Mission News, Science and Technology

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