18 October 2008

NASA SpaceGuard Shacks Asteroid Strike Prediction

In an extraordinary dress rehearsal of future events to come, the SpaceGuard Survey, for the first time ever discovered an asteroid before impact, and tracked it to impact. Discovered only 20 hours in advance, within 8 hours and 21 minutes, using only four observations, it accurately predicted the time of impact to less than a minute. The asteroid, 2008 TC3, a 5 meter charbonacious chondrite, created an atmospheric detonation (bolide) of 1.1 to 2.1 kilotons over a remote region in Sudan.
Serendipidy played an important role, as a week later, the survey would have been looking at a different section of sky.
The event set off a flurry of notifications through US and international agencies. Such notifications are important not only in the case that a larger impact could cause casualties, but even a smaller impact such as this one might be mistaken in some tense regions as a nuclear test or strike.

7 10 2008 Posted by Dee Norris
A recently discovered Apollo Asteroid, 2008 TC3, exploded over Sudan at about 1046 EDT on October 7, 2008. 2008 TC3 was discovered on Monday by an observer at the Mt Lemmon Observatory near Tucson, Arizona. 2008 TC3 is notable in that it is the first Asteroid of its size that was identified before impact and tracking it put the entire Spaceguard tracking system to an extreme test. TC3 is estimated to be only two to five meters in diameter but exploded with the force of a one kiloton nuclear device. Asteroids of this size hit the Earth every few months according JPL scientists. No deaths have been reported yet. The important lesson here is that Spaceguard is able to identify and track these smaller objects as well as the larger ones. A 20 to 50 meter asteroid exploding over a major city could result in a significant loss of property and life. The most imagined dire consequences of AGW could never stack up to the actual consequences of a larger asteroid actually impacting nearly anywhere on the Earth. If for this reason alone, funding for space exploration needs to be continued.
2008 TC3 (Catalina Sky Survey temporary designation 8TA9D69) was a meteoroid 2 to 5 meters (7 to 16 ft) in diameter that entered Earth's atmosphere on October 7, 2008, at 02:46 UTC (5:46 a.m. local time) and burned up before it reached the ground.
The meteoroid was discovered by an observer at the Catalina Sky Survey (CSS) 1.5 meter telescope at Mount Lemmon, north of Tucson, Arizona, USA, about a day before the impact.[2][3] The meteoroid was notable as the first such body to be observed and tracked prior to reaching Earth.[4] The process of detecting and tracking a near-Earth object, an effort sometimes referred to as Spaceguard, was put to a test. In total, 586 astrometric and almost as many photometric observations were performed by 27 amateur and professional observers in less than 19 hours and reported to the Minor Planet Center, which issued 25 Minor Planet Electronic Circulars with new orbit solutions in eleven hours as observations poured in. Impact predictions were performed by University of Pisa's CLOMON 2 semi-automatic monitoring system[5][6] as well as Jet Propulsion Laboratory's Sentry system. Spectral observations that were performed by astronomers at the 4.2 meter William Herschel Telescope at La Palma, Canary Islands are consistent with either a C-type or M-type asteroid. The meteoroid, also considered a bolide[7] due to its fiery explosion, is confirmed to have entered Earth's atmosphere above northern Sudan at a velocity of 12.8 kilometres per second (8.0 mi/s). Estimated trajectory has the object coming out of the western sky at an azimuth of 281 degrees, and an altitude angle of 19 degrees to the local horizon.

Meteosat 8 / EUMETSAT IR image of the 2008 TC3 explosion. Copyright 2008 EUMETSAT
It exploded tens of kilometers above the ground with the energy of around one kiloton of TNT, causing a large fireball in the early morning sky.[8] Very few people inhabit the remote area of the Nubian Desert where the explosion took place; The Times, however, reported that the meteoroid's "light was so intense that it lit up the sky like a full moon and an airliner 1,400 km (870 miles) away reported seeing the bright flash."[9] A low-resolution image of the explosion was captured by the weather satellite Meteosat 8.[10] The Meteosat images place the fireball at 21°00′N 32°09′E / 21.00, 32.15.[11] Infrasound detector arrays in Kenya also detected a sound wave from the direction of the expected impact corresponding to energy of 1.1 to 2.1 kilotons of TNT.[12] Meteoroids of this size hit Earth about two or three times a year.[13]
The trajectory showed intersection with Earth's surface at roughly 20°18′N 33°30′E / 20.3, 33.5[14] though the object was expected to break up perhaps 100–200 kilometers west as it descended, somewhat east of the Nile River, and about 100 kilometers south of the Egypt–Sudan border.
According to U.S. government sources[15][16] U.S. satellites detected the impact at 02:45:40 UT, with the initial detection at 20°54′N 31°24′E / 20.9, 31.4 at 65.4 km altitude and final explosion at 20°48′N 32°12′E / 20.8, 32.2 at 37 km altitude.
Don YeomansNASA/JPL Near-Earth Object Program OfficeOctober 6, 2008
A 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.
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 at around 2:45:54 UTC. 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.

Okay, so Asteroid 2008 TC3 wasn’t an Earth-killer, but rather a crowd-thriller. It wasn’t miles across-not even tens of meters across. It was, perhaps, a few meters in size, similar in volume to mid-size car. In fact, it didn’t even hit the Earth’s surface, but vaporized in the atmosphere.
Sounds a bit anticlimactic-and that’s not the half of it. It’s not even a rare event! Objects of this size are believed (and sometimes observed) to enter Earth’s atmosphere a few times each year. So what’s the blog deal? Observers on the ground reported the fireball lit up the skies with the intensity of the Full Moon. A nearby airliner (not in danger, as the fireball exploded tens of kilometers above the ground, well above the airliner’s flight path) reported seeing a bright flash.
In a sense, this event was kind of a dress rehearsal for the international system of predicting, and possibly defending against, impacts on Earth by much larger asteroids and comets. We already know of thousands of Near Earth Objects (NEOs-asteroids and comets that cross Earth’s orbit and are large enough to cause a catastrophe should they strike the Earth). It is also expected that there are many thousands more that we haven’t yet detected, being small enough to “fly under the radar” of our NEO detecting network.
Early detection and sustained tracking of NEOs is key to the protection plan against impact disaster. If we can accurately predict an impact far enough in advance, we could potentially send a spacecraft to it and gently “nudge” it off course and deflect the eventual impact.
So ends the existence of another chunk of rock that had, up to that point, been serenely orbiting the Sun for billions of years…

An asteroid with a size of a few meters in diameter hit the Earth a few hours ago. The news is reported by the Central Bureau for Astronomical Telegrams, circular 8990. Below is the expected trajectory of the body. The small asteroid was discovered yesterday at Mt. Lemmon by R.A.Kowalski, as an object with a visual magnitude of about 30.4. The object was then at about 450,000 kilometers from our planet. Those who were able to look up this night might have spotted it before it entered our atmosphere only with a telescope, since its expected magnitude was probably around 11. A bright fireball might have been observed over northern Africa, and a possible fall might have resulted, depending on the composition of the rock.
More information is available at the CBAT site.
UPDATE: the body was 5 meters in diameter. It was a carbonaceous chondrite, and its darkness explains why the diameter had been underestimated by luminosity measurements before the impact. It is quite likely that many small bodies will be found in northern Sudan, which is above the impact point of the asteroid. Also worth noting is that dr. Peter Brown, from West Ontario University, detected a sound wave from the impact with detectors located in Kenya.
The fireball made by the body at 4.46AM yesterday has been spotted by a airplane pilot from Air France-KLM, according to Jacob Kuiper, a meteorologist from the Netherlands who had informed the pilots of the possibility before the impact. The fall has freed an energy of about two kilotons of TNT, about a tenth of the energy of the first atomic bomb over Hiroshima.
UPDATE: thanks to the Meteosat, we now have a picture of the event. It is a temperature scan, which shows the impact of the object with the atmosphere. It is the first time that the impact of a body with the Earth is predicted and then observed. Near-Earth object surveys are getting better and better…
By Emily Lakdawalla
Oct. 7, 2008 16:15 PDT 23:15 UTC
Over the last 24 hours it has been tiring but really fun to watch the drama of asteroid 2008 TC3. It has happened so quickly that it's necessary to convert all times to UTC in order to see how events have unfolded across the globe. Fortunately for my sanity, nearly all of the events are neatly collected on the Minor Planets Mailing List. To briefly review: the night before last (my time), or at 06:38 UTC on October 6, astronomers at the University of Arizona discovered an object provisionally called 8TA9D69 that appeared to be on a collision course with Earth. Three other observatories reported sightings within the next few hours -- Sabino Canyon and Siding Spring in Arizona and a Royal Astronomical Society site in Moorook, Australia. Together these four observers provided enough data on the object so that a Minor Planet Electronic Circular was issued at 14:59 UTC the same day, giving 8TA9D69 the more formal name 2008 TC3, and advising the astronomical community that "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."
The object wouldn't be more than a big meteor, but even so, it represented the first time ever that an object had been observed before it was to hit Earth, and, clearly, astronomers around the world scrambled to their telescopes to observe it before it was to pass into Earth's shadow (and, therefore, invisibility) just before 01:50 UTC. The observations were partly for the thrill -- seeing an object in its last hours, before it met a fiery fate in Earth's atmosphere -- but they also had a more important purpose: to refine the orbit of the object, which would, in turn, improve the predictions of where it would hit. Over the next 11 hours, fully 24 Minor Planet Electronic Circulars were issued with further observations, pinning down 2008 TC3's final path with high precision. One terrific set of images, from Eric Allen, is shown at right. Like many of the amateur astronomers participating in the impromptu 2008 TC3 observing campaign, Allen's work has been aided by Planetary Society members, through the Gene Shoemaker Near Earth Object Grant Program. With the object so close to Earth, the parallax of different observers on different parts of the globe allowed much greater precision than is usual, given the short observing arc. The initial impact prediction was confirmed by JPL scientist Paul Chodas at 01:45 UTC: "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." After positional information, the next challenge was to obtain spectral data -- information on the color of the object, which would help to classify it and determine its origin. The first I heard of such data being captured successfully was from this item on the MPML by Alan Fitzsimmons and coworkers at Armagh Observatory: "We obtained optical spectra of 2008 TC3 using the 4.2m William Herschel Telescope and ISIS spectrograph on Oct 6.93-6.94UT. The spectra cover the range 546-995nm at a resolution of 4nm. Initial analysis of the spectra via comparison with the solar analogue 16CygB reveals a featureless reflectance spectrum with no indication of the silicate absorption feature longward of 800nm." Presumably there were other observers who were able to obtain spectral data, though not as many as were gathering positional information. Time quickly ran out to observe the asteroid, however. Astronomers in Spain recorded 2008 TC3's entry into Earth's shadow, hiding it from view for its final hour of descent. The image is below:
2008 TC3 disappears into Earth's shadow, on its way to impactTraveling from right to left in this image, asteroid 2008 TC3 entered first the umbra (at 1:47:30 UTC) and then the penumbra (at 1:49:50) of Earth, disappearing into shadow for the final hour of its approach to burning up in the atmosphere over Sudan. The view is a six-minute exposure tracked at sidereal rate (so stars stay fixed). Exposure start time was October 7, 2008, at 01:45:23 UTC. The view is half a degree wide, comparable to the size of the full Moon. At the start of the exposure 2008 TC3 was at a distance of 29,600 kilometers, approaching at a speed of 7.61 kilometers per second. The periodic light variation along the early part of the trail indicates a fast rotation of the intruder around its spin axis. Credit: La Sagra Sky Survey, Spain The atmospheric entry occurred over an extremely remote location on Earth, just 20 hours after it was first discovered. As yet there are no confirmed images of the fireball -- it's possible there may never be any. There is one possible sighting: one resourceful enthusiast, Jacob Kuiper, the General Aviation meteorologist at the National Weather Service in the Netherlands, called an official of the Air-France-KLM airline at the Amsterdam airport to inform him "about the possibility that crews of their airliners in the vicinity of impact would have a chance to see a fireball. And it was a success! I have received confirmation that a KLM airliner, roughly 750 nautical miles southwest of the predicted atmospheric impact position, has observed a short flash just before the expected impact time 0246 UTC. Because of the distance it was not a very large phenomenon, but still a confirmation that some bright meteor has been seen in the predicted direction." (This via spaceweather.com.) And there's more than one way to detect an asteroid impact. Even in relatively unpopulated areas, there are seismological stations scattered around the world, using infrasound to record seismic events. One such station seems to have detected a 1 to 2-kiloton blast associated with the impact. This is according to Peter Brown, of the University of Western Ontario [hey, that's the same university that's home to asteroid mapping genius Phil Stooke]. Brown said:
A very preliminary examination of several infrasound stations proximal to the predicted impact point for the NEO 2008 TC3 has yielded one definite airwave detection from the impact. The airwave was detected at the Kenyian Infrasonic Array, (IMS station IS32), beginning near 05:10 UT on Oct 7, 2008 and lasting for several minutes. The signal correlation was highest at very low frequencies – the dominant period of the waveform was 5-6 seconds. The backazimuth of the signal over the entire 7 element array is shown in the attached map – it clearly points to within a few degrees of the expected arrival direction. Moreover, assuming a stratospheric mean signal speed of 0 28 km/s, the arrival time corresponds to an origin time near 02:43 UT, which is consistent with the expected impact time near 02:45:40 UT given expected variations in stratospheric arrival speeds. The dominant period of 5-6 seconds corresponds to an estimated energy (using the AFTAC period at maximum amplitude relationship from ReVelle, 1997) of 1.1 – 2.1 kilotons of TNT. The five other closest infrasound stations were briefly examined for obvious signals and showed none – more detailed signal processing of these additional data are ongoing in the search for additional signals.
Detection of the 2008 TC3 impact by infrasoundAn infrasound station (intended for the detection of seismic events) in Kenya recorded the blast associated with the atmospheric entry of asteroid 2008 TC3 over Sudan. Credit: Peter Brown, University of Western Ontario All in all, I think the episode of 2008 TC3 has proven that the world's astronomical community, at least, is prepared to respond when an object on a collision course is detected. Within just a few hours of its discovery, the digitally connected world knew exactly where and when the object would hit, and also that it posed no threat. It was a wonderful simulation of the first part of the call to arms when a truly threatening object is detected. But of course we now have to ask ourselves: what would have happened if the object was much bigger than 2 meters in diameter? Reassuringly, the first thing that would have happened is that the detection most likely would have happened much earlier. The bigger and more hazardous an object is, the brighter it is, and the sooner we will detect it. We will likely have way more than 20 hours' warning of an incoming dangerous object. Still, though, the warning time for a tens-of-meter-diameter object could only be measured in days. If we'd had three days' warning of a dangerous impactor heading for Sudan, what could the world have done? The remote location of the impact would have been fortunate for humanity in general, but disastrous for the few people who lived out in that remoteness. Could the developed world have done anything to prevent yet another humanitarian disaster from befalling the Sudanese? The Planetary Society is seeking answers to these questions. We have joined the Association of Space Explorers and the B612 Foundation in their efforts to develop an international framework for planetary defense, and we plan to hold both an invited workshop and a public meeting on these issues in the summer of 2009, as a part of the International Year of Astronomy. When the time is right, we will push for action on this issue from the United Nations' Committee on the Peaceful Uses of Outer Space. A final plea to wrap up this story: if you'd like to support these efforts to protect our planet from hazards from space, please vote with your wallet by sending a donation our way. Any amount is helpful!

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