© P Jenniskens/SETI Institute/NASA
Peter Jenniskens of NASA's Ames Research Center and Muawia Shaddad of the University of Khartoum (standing, centre) and volunteers combed the Nubian Desert in the Sudan to find remnants of asteroid 2008 TC3. This meteorite was found 8 December 2008 during the first of three search campaigns.
Last October, astronomers found the first asteroid on a certain collision course with Earth, observing the 4-metre-long rock as it hurtled towards the planet and then exploded in the sky some 37 kilometres above the Nubian Desert in Sudan.

At the time it was unclear whether the blast would leave anything but dust behind, but a team of scientists and volunteers has managed to recover fragments of the 80-tonne asteroid, called 2008 TC3, during several searches that began in December (see First tracked space rock recovered after impact). So far, meteorites weighing a total of about 5 kg have been found.

They will provide a crucial piece of ground truth to test how well observations of asteroids in space match up with their actual compositions.

New Scientist talks with Peter Jenniskens of the SETI Institute in Mountain View, California, about how the rocks were found.

How did you get started in the search?

I immediately realised if it was possible to recover some meteorites that it would be spectacular, because we could for the first time link a particular type of meteorite with a particular class of asteroids.

So I established contact with Dr Muawia Shaddad at the University of Khartoum and gave him as much advice as I could on what sort of information to collect from observers. In the days and weeks after the event, he asked around to people who had seen the fireball and sent me spectacular images of a big cloud of dust in the sky that were photographed by people's cell phones just before dawn on the morning of October 7 (see image).

It turns out thousands of people along the Nile [in northern Sudan] had seen this because it happened just after morning prayer. And they all described a really bright light, as if somebody just behind them had turned on their car lights.

The pictures themselves were not immediately usable to get information on how high in the sky the asteroid had exploded, because there were no reference stars in the pictures. So after a while I thought maybe I should go and visit, interview eyewitnesses, and take some star background images from the very spots where these pictures were taken.

[Shaddad] organised for us to travel all the way up to the border of Egypt to interview the eyewitnesses that saw the fireball and for a group of 45 students and staff from Khartoum to join us a few days later to help comb the desert for meteoroids.

Once you got to the Sudan, was it clear you would find meteorites?

Initially things looked pretty bleak, because the reports we were getting from the observers was that nothing big had came out of the explosion. We didn't know if anything would have survived. Never before has a meteoroid been recovered for something that exploded this high up in the atmosphere. When it's high up it's not appreciably slowed down by the atmosphere, and that makes the explosion very energetic, turning most of the thing into dust.

The few scraps that come out are small pieces and they fall very close to where the explosion happened. The big pieces just keep going. In the end, we had to walk 29 kilometres to cover this debris field.

How did the search work?

On the first day, we spent most of the day driving 28 kilometres inland from the railroad to the area where the smaller pieces would have fallen (see map). Then we basically lined up the students maybe 20 metres apart over the span of a kilometre and started to comb the desert.

We didn't quite know what to look for. Based on the spectra that were obtained of the object in space, there was talk that the rock might have been a C-type asteroid. C-types are linked to carbonaceous chondrites [primitive, unaltered meteorites], so I was looking for something dark. But I didn't know whether the crust would be recognisable, so it was quite difficult to explain what it might look like.

Were the pieces hard to find?

If we didn't have so many eyes and legs to go walk the desert, we would never have found them. [On the first survey] I walked in the desert for three days and I didn't find a single one. We had 45 people and only 15 meteorites were found the first time, and that was after 29 km of hiking and searching. It took a lot of patience.

Students came with various sorts of rocks, none of which were meteorites. As the Sun was low on the horizon, yet again someone came back and said a student had found something. I distinctly remember thinking, "Oh no, not again", and then I was brought to the student and he showed me this meteorite. It had a beautiful fusion crust - a nice layer of glass - and the rock had broken so you could look inside (see meteorite image).

It was still quite possible it was something that had lain in the desert for a long time, but in hindsight it was indeed part of our asteroid.

Very gradually we started finding bigger pieces, and at the end the pieces we were finding were chicken-egg size.

What did you find when you analysed the meteorites?

[The meteorites belong to a known type, but they are] different from the ones that have been recovered before, in the sense that [they're] very fragile and very dark, because there's lots of carbon. It's material that was heated so much that part of the rock became fluid but not the whole rock. It illustrates what happens in a certain phase of planet evolution before things get all molten (see Magma oceans sloshed across early asteroids).

We can now say with certainty that this dark variety corresponds to F-class asteroids. There are many ideas about how these rocks formed, so we're hoping that this meteorite will be able to differentiate between them. That's sort of a next step in the study.

Do you think you'll be able to recover more such meteorites in the future?

I really hope this would happen again sometime somewhere. Asteroids of this size hit the Earth about once a year, but for astronomers to see the object coming in, it has to move through the area of searches [by astronomical surveys], and that's only a small part of the sky, so we have to be lucky for this to happen. In the future, when more [astronomical surveys] come online, maybe our chances will improve.