Date: October 6, 2009

Title: Crashing to Ground: Anniversary of the Impact of 2008 TC3


Podcaster: Emily Lakdawalla

Organization: Planetary Society

Description: A year ago today was the first time ever that a celestial object was discovered before it was to crash into Earth. 2008 TC3 was discovered by a telescope in Arizona and tracked by astronomers in Australia, Asia, and Europe before igniting as a fireball in the sky over the Nubian desert of northern Sudan. I’ll tell you about the discovery and how TC3 became the first direct link between asteroids and meteorites.

Bio: Emily Lakdawalla is a planetary geologist and writer who works for the world’s largest space interest group, The Planetary Society, as its blogger, web writer, and contributor to the weekly Planetary Radio podcast. She lives in Los Angeles with a 3-year-old who can list all the planets for you, a new baby who has yet to learn their names, and a husband who likes to pretend he doesn’t know anything about space.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the American Astronomical Society, the major organization for professional astronomers in North America, whose members remind everyone that One Sky Connects Us All. Find out more or join the AAS at


Hi, I’m Emily Lakdawalla, and I write for The Planetary Society’s website at planetary dot org slash blog. Today I’m remembering the impact of 2008 TC3, an asteroid that crashed into Earth a year ago today.

Both amateur and professional astronomers discover new near-Earth asteroids practically every day. And it’s also pretty common for people standing on the ground to see a fireball streaking through the sky as a small asteroid burns up in the atmosphere. But until last year, no one had ever discovered a near-Earth asteroid that was going to hit Earth with 100 percent certainty. TC3 was the first. And it all happened really fast.

On October 6, 2008, astronomers at the University of Arizona were doing routine sky surveys to look for near-Earth asteroids. It was nighttime in Arizona, of course, but in order to tell this story without hopeless confusion I’ll have to use Universal Time.

So it was almost 07:00 Universal Time when the Arizona astronomers detected something moving against the background of known stars. Some quick computer work revealed that the object was new. But the computer also spat out an orbital path for the object that was a huge surprise: the predicted path intersected the path of Earth. The new object was on a collision course. The University of Arizona team submitted their observation to the Minor Planet Center, a clearinghouse for orbital data on the thousands upon thousands of tiny rocks that wander around the solar system. A little later, two more Arizona observatories spotted the object, and all of them arrived at the stunning conclusion that it was going to hit Earth.

Soon the rotation of Earth carried the new object out of view from Arizona and California; its position was now over the vast Pacific Ocean. No one in Hawaii seems to have spotted it that night. More hours passed. The rotation of Earth finally brought sunset to Australia. In Moorook, near Adelaide in southern Australia, a remotely operated Global Rent-A-Scope instrument picked up the object too. With the Australian observation in hand, the Minor Planet Center finally had a long enough observational arc to issue a Minor Planet Electronic Circular. This was at about 15:00 hours Universal Time, eight hours after the initial discovery. The Minor Planet Center named the body 2008 TC3 and advised the astronomical community that, quote, “The nominal orbit given above has 2008 TC3 coming to within one earth radius around Oct. 7. The absolute magnitude indicates that the object will not survive passage through the atmosphere. Steve Chesley of the Jet Propulsion Laboratory reports that atmospheric entry will occur over northern Sudan.” End of quote. The impact was predicted to occur less than thirteen hours after the circular was issued.

TC3 was so close and so dim that astronomers knew it was small, no more than a few meters across and maybe 80 tons in mass. This was a relief; such a small object would probably burn up and pose no risk to anything on the ground, especially since it was predicted to hit Earth in a remote and relatively unpopulated area of the Nubian Desert. But this was the first time ever that an object had been seen before it was to hit Earth, and astronomers around the world scrambled to their telescopes to observe it.

They had to hurry; not only was the impact predicted to happen in less than thirteen hours, but TC3 would actually pass into Earth’s shadow an hour before it hit. The discovery was too late for anyone in the Americas to spot it; now it was up to Asians, Europeans, and Africans.

I think most astronomers racing to their telescopes on the night of October six and seven did so mostly for the sheer thrill of tracking and following a heavenly body that was going to crash into Earth. But the observations had a more important purpose: to refine the orbit of the object, which would, in turn, improve astronomers’ predictions of where it’d hit. Over the next eleven hours, twenty-four Minor Planet Electronic Circulars were issued with further details, pinning down TC3’s flight path with high precision. Some of the observers were professionals, but the majority of the observations were done by amateurs, people who might be businessmen or bankers during the day but who harbored a passion for stargazing at night.

Astronomers weren’t only gathering positional information. They were also racing to study TC3’s color. Asteroids in space are classified according to the different ways they reflect and emit light. Using the science of spectroscopy, careful measurement of their brightness at different wavelengths of light gives scientists clues about an asteroid’s composition, whether it’s rocky or metallic, old and weathered, or young and freshly broken. Professional astronomers at Armagh [ar-MAH] Observatory in Ireland scrambled to spot TC3 and managed to record its spectra. Shortly later, the William Herschel telescope in the Canary Islands managed the same feat.

TC3 was so close to Earth that the different observing angles of different observers on different parts of the globe made its position appear to shift against background stars due to parallax. Parallax allowed the Minor Planet Center and the professional asteroid watchers at the Jet Propulsion Laboratory, or JPL, to predict its orbit with much greater precision than usual. The initial impact prediction was formally confirmed by JPL scientist Paul Chodas [CHO-duss] just an hour before its predicted impact time, 02:46 UTC.

Astronomers in Spain were the last to record an image. In fact, they captured a movie that showed TC3 winking out as it crossed into Earth’s shadow, hiding it from view for its final hour of existence. With that, time had run out to observe the asteroid.

As TC3 entered the atmosphere, it began to compress the air in front of it. Do you remember the ideal gas law from high school chemistry? Pressure is proportional to temperature – so if you jack up the pressure, so goes the temperature. Pretty soon, the incoming asteroid made the air so hot that it incandesced, glowing in visible light. TC3 had started out as a dim dark rock, but now it was a brilliant fireball.

One resourceful astronomy enthusiast, an aviation meteorologist at the Dutch National Weather Service, called an official at the Amsterdam airport and alerted him about the incoming asteroid. He radioed crews of planes in the air over northeastern Africa and told them to look out for the fireball. Amazingly, the alert worked! A KLM airliner that was about 1400 kilometers to the southwest of the predicted atmospheric entry point observed a short flash just before the expected impact time.

Another detection came not from the flash but from the boom. A seismologist from the University of Western Ontario in Canada examined records from an infrasonic array, a set of remotely operated devices designed to listen for earthquakes. He found that he’d detected a one to two-kiloton blast just before the predicted impact time.

And that was it, for a while. TC3 had come in, excited the astronomical community, and then vanished.

But it turns out that wasn’t the whole story. The next morning, on October 7, the Muslim faithful of the Nubian desert emerged from morning prayers to see a sky filled with twisted, white, smoky streaks. Faced with this strange phenomenon, the Sudanese villagers did what any citizens of the twenty-first century would do – they took photos of the sky with their cell phones and shared them with their friends and relations.

A few months later, Peter Jenniskens, a meteor astronomer at NASA’s Ames Research Center, contacted Mauwia Shaddad, a physicist at the University of Khartoum. Together the two men organized an expedition to the Nubian desert to search for any fragments of the asteroid that might have survived the fireball.

In December, Jenniskens, Shaddad, and 45 students and staff set off from Station 6 of the local railway line along the flight trajectory of TC3. Quickly one of the students, Mohammed Alameen, found a black odd-looking rock that looked like a meteorite. It proved to be the first fragment of 2008 TC3, now named Almahata Sitta, Arabic for Station 6. Over several weeks, the expedition collected nearly 280 pieces of the asteroid strewn over 29 kilometers of desert. Altogether the meteorites weighed less than 5 kilograms – all that remained of the 80-ton asteroid.

It was a remarkable find: For the first time ever a celestial body that was observed in space was subsequently collected on the ground on Earth and made available for laboratory study. Spectral measurements were compared with the direct analysis of the meteorite shards. The two studies agreed closely, reassuring astronomers that spectral analysis really is a reliable way to understand the composition of small asteroids. The close correspondence suggested that small bodies like TC3 aren’t heavily coated with dust, which can distort spectroscopic measurements of larger objects.

All in all, I think the episode of 2008 TC3 has proven that the world’s astronomical community, made up of amateurs and professionals both, 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 TC3 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 now we have to ask ourselves: what would have happened if the object was much bigger than a few meters across? One reassuring fact is that a bigger object would most likely have been spotted sooner, because it would have been brighter. 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?

Here at The Planetary Society, we’re seeking answers to these questions. We’ve joined the Association of Space Explorers and the B612 Foundation in their efforts to develop an international framework for planetary defense. When the time is right, we’ll push for action on this issue from the United Nations’ Committee on the Peaceful Uses of Outer Space. In the meantime, we’re supporting amateur astronomers around the world with our Shoemaker Near Earth Object Grant program, which allows amateurs to buy equipment that helps them to make those all-important tracking and followup observations.

You can learn more about asteroid TC3 and The Planetary Society’s work to reduce the risk we face from near-Earth objects on our website at I’ll have lots of photos and links for you on my blog at planetary dot org slash blog. Thanks for listening!

End of podcast:

365 Days of Astronomy
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