A very, very small recently discovered asteroid, 2008 TC3, was predicted to hit the Earth's atmosphere over Africa in the early morning hours of 7 October 2008 (evening 6 October U.S. time). It was small enough that it apparently completely disintegrated in the upper atmosphere, with no risk to people on the ground. Still, this is the first time that such an event has been predicted ahead of time. As of 7 October, there is one possible sighting of the fireball by a commercial pilot over Africa, plus confirmation of the midair explosion by infrasound instruments.
posted 6 October:
2008 TC3 was discovered from Mt. Lemmon Observatory in Arizona by Richard Kowalski on 6 October at 6:39 GMT (1:39 AM Central time), when it was about as far away as the Moon. Its provisional designation was 8TA9D69 before receiving the designation 2008 TC3 from the Minor Planet Center. Announcements went out through a variety of channels used by astronomers that observe potentially hazardous asteroids, allowing many astronomers to observe the object and help determine its orbit. By the afternoon of that day, observations were sufficient to confirm both a very small size for this asteroid (large boulder, really) and that it would hit the Earth's atmosphere.
The Minor Planet Center gives about a 99.9% chance of 2008 TC3 encountering the Earth. JPL predicts that 2008 TC3 will enter the Earth's atmosphere over northern Sudan at 0246 GMT on 7 October (9:46 PM Central time 6 October). Impact point is predicted as 20.6° N, 33.1° E, at an angle of 18° above the horizon, by astronomers on the Minor Planets Mailing List bulletin board. This is approximate, and because of the low impact angle the fireball will pass over a significant downrange distance, moving west to east.
JPL reports the absolute magnitude of 2008 TC3 as 30.51±0.50, corresponding to a size of 2-6 meters across (depending on the object's reflectivity) making it between the size of a car and the size of a trailer home. Based on its orbit, its speed relative to the Earth at impact will be about 12.8 km/sec. This yields a kinetic energy of 0.1-2 kilotons TNT equivalent for a low density (1 g/cm3) object, or 0.3-9 kilotons for a more dense (4 g/cm3) object (which is less likely).
2008 TC3 will become superheated as it strikes the thin upper atmosphere of the Earth to the point that it explodes far above the ground, probably at an altitude of tens of kilometers. This will occur high enough that no damage will occur on the ground. Some small fragments may survive and fall downrange (from Egypt and Sudan possibly to the Red Sea and Saudi Arabia) to reach the ground as meteorites. People over a large area should be able to see a spectacular fireball.
Similar objects routinely strike the Earth's atmosphere at a rate of a few per year. Many have been observed in recent decades, given the use of various technologies and funding for observing programs.
posted 7 October:
Shortly prior to impact, of the many observatories worldwide observing 2008 TC3, at least one (La Sagra Sky Survey in Spain) was able to image the asteroid as it passed into the Earth's shadow shortly before impact. Predictions including passing 100 km altitude at 21.10° N, 30.37° E at 2:45:29 GTM, and reaching maximum deceleration around 2:45:54 (± 15 sec) at an altitude of about 14 km.
The impact of 2008 TC3 was detected by an infrasound array (measuring the sound waves from the high altitude explosion of the meteor) in Kenya, giving a result of about a 1 kiloton energy release at 02:45:45 GMT. The crew of a KLM airliner over Chad about 1400 km southwest of the predicted impact site observed a short flash just before 0246 GMT, the first eyewitness report.
posted 8 October:
The fireball from 2008 TC3 was detected by Meteosat 8, a weather satellite in geosynchronous orbit, at 02:45 GMT; the fireball was gone at the time of the next image, 02:50 GMT.
Releases, 6/7 October: From NASA's JPL Near Earth Object Program:
very small, few-meter sized asteroid, designated 2008 TC3, was found Monday morning by the Catalina Sky Survey from their observatory near Tucson Arizona. Preliminary orbital computations by the Minor Planet Center suggested an atmospheric entry of this object within a day of discovery. JPL confirmed that an atmospheric impact will very likely occur during early morning twilight over northern Sudan, north-eastern Africa, at 2:46 UT Tuesday morning. The fireball, which could be brilliant, will travel west to east (from azimuth = 281 degrees) at a relative atmospheric impact velocity of 12.8 km/s and arrive at a very low angle (19 degrees) to the local horizon. It is very unlikely that any sizable fragments will survive passage through the Earth's atmosphere.
Objects of this size would be expected to enter the Earth's atmosphere every few months on average but this is the first time such an event has been predicted ahead of time.
From NASA's JPL Near Earth Object Program:
Confirmation has been received that the asteroid impact fireball occurred at the predicted time and place. The energy recorded was estimated to be 0.9 to 1.0 kT of TNT and the time of detection was 02:45:45 on October 7 (Greenwich Standard Time). More details on this detection will be forthcoming. An additional confirmation was apparently reported by a KLM airliner (see: http://www.spaceweather.com/). As reported by Peter Brown (University of Western Ontario, Canada), a preliminary examination of infrasound stations nearest to the predicted impact point shows that at least one station recorded the event. These measurements are consistent with the predicted time and place of the atmospheric impact and indicate an estimated energy of 1.1 - 2.1 kT of TNT.
From the Harvard-Smithsonian Center for Astrophysics:
Boulder-sized Asteroid Will Burn Up in Earth's Atmosphere Tonight
Cambridge, MA - A tiny asteroid discovered just hours ago at an Arizona observatory will enter Earth's atmosphere harmlessly at approximately 10:46 p.m. Eastern time tonight (2:46 a.m. Greenwich Mean Time). There is no danger to people or property since the asteroid will not reach the ground. It is between 3 and 15 feet (1-5 m) in diameter and will burn up in the upper atmosphere, well above aircraft heights. A brilliant fireball will be visible as a result.
"We want to stress that this object is not a threat," said Dr. Timothy Spahr, director of the International Astronomical Union's Minor Planet Center.
"We're excited since this is the first time we have issued a prediction that an object will enter Earth's atmosphere," Spahr added. Odds are between 99.8 and 100 percent that the object will encounter Earth, according to calculations provided by Andrea Milani of the University of Pisa.
When a meteoroid (small asteroid) enters the atmosphere, it compresses the air in front of it. That compression heats the air, which in turn heats the object, causing it to glow and vaporize. Once it starts to glow, the object is called a meteor.
"A typical meteor comes from an object the size of a grain of sand," explained Gareth Williams of the Minor Planet Center. "This meteor will be a real humdinger in comparison!"
The meteor is expected to be visible from eastern Africa as an extremely bright fireball traveling rapidly across the sky from northeast to southwest. The object is expected to enter the atmosphere over northern Sudan at a shallow angle.
"We're eager for observations from astronomers near the asteroid's approach path. We really hope that someone will manage to photograph it," said Williams.
The Minor Planet Center, which is located at the Harvard-Smithsonian Center for Astrophysics, serves as the worldwide clearinghouse for asteroid and comet observations. It collects, checks and disseminates observations and calculates orbits.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.
From the Minor Planet Center MPEC 2008-T50:
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.
From the Minor Planet Center MPEC 2008-T58:
Earth MOID = 0.0000 AU(MOID=minimum orbital intersection distance!)
From NASA's JPL:
Update - 6:45 PM PDT (1 hour prior to atmospheric entry)
Since its discovery barely a day ago, 2008 TC3 has been observed extensively by astronomers around the world, and as a result, our orbit predictions have become very precise. We estimate that this object will enter the Earth's atmosphere at around 2:45:28 UTC and reach maximum deceleration around 2:45:54 UTC at an altitude of about 14 km. These times are uncertain by +/- 15 seconds or so. The time at which any fragments might reach the ground depends a great deal on the physical properties of the object, but should be around 2:46:20 UTC +/- 40 seconds.
© 2008 by Wm. Robert Johnston.
Last modified 8 October 2008.
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