Almahata Sitta
The first asteroid spotted in space before it hit the Earth – and the meteorites later collected from the Sudanese desert.
Key facts
- Type
- Polymict ureilite, with many other meteorite types
- Fell or found
- Fell (predicted and observed)
- Year
- 2008 (7 October)
- Country
- Sudan
- Mass
- about 10 kg recovered, in some 600 fragments
- Official name
- Almahata Sitta
On 6 October 2008, the astronomer Richard Kowalski, working with the Catalina Sky Survey in Arizona, spotted a faint moving point of light. Calculations quickly showed something astonishing: the object, a small asteroid a few metres across, was on a collision course with Earth and would hit within about 20 hours. It was named 2008 TC3. For the first time ever, astronomers had found an asteroid in space before it struck our planet.
A predicted impact
Observatories around the world rushed to track it. Its predicted path led to the sky over northern Sudan in the early hours of 7 October. Right on schedule, it entered the atmosphere and exploded about 37 kilometres above the Nubian Desert. A weather satellite recorded the flash, infrasound sensors detected the blast, and the crew of an airliner hundreds of kilometres away saw a flash on the horizon. The asteroid was far too small to cause any harm, but there was a big question: had any pieces survived?
Searching the desert
In December 2008, the astronomer Peter Jenniskens joined Muawia Shaddad of the University of Khartoum and a large team of students and staff to search the desert along the calculated track. Walking in long lines across the sand, they found their first meteorite within hours. Over several expeditions, around 600 fragments were recovered, with a total mass of about 10 kilograms. The meteorites were named Almahata Sitta, meaning 'Station Six' in Arabic, after a railway station in the area.
A cosmic jumble
The fragments were a big surprise. Most were ureilites, a rare type of meteorite made mainly of olivine and pyroxene, with carbon in the form of graphite and tiny diamonds. Ureilites are thought to come from a parent body that partly melted and was later shattered by a collision. But among the Almahata Sitta stones were also many other types of meteorite, including enstatite chondrites, ordinary chondrites and carbonaceous material.
This means that asteroid 2008 TC3 was not a single uniform rock. It was a jumbled mixture, a breccia, made of fragments from different parent bodies that had come together over time through collisions in space. Many scientists think it was a fragment of a loosely bound 'rubble-pile' type asteroid. Its stones show that asteroids can be far more mixed than meteorite collections alone might suggest. In 2018 one study argued that tiny inclusions in Almahata Sitta's diamonds formed under high pressure inside a large planetary body, perhaps as big as Mercury or Mars, which was later destroyed; the idea is still debated.
Linking asteroids and meteorites
Because astronomers had observed 2008 TC3 with telescopes before it hit, scientists could compare its light spectrum, the way it reflected different colours of sunlight, with the meteorites recovered on the ground. This allowed a direct link between a particular type of asteroid seen in space and the rocks that fell from it, something that had never been possible before.
Since 2008, a handful of other small asteroids have been detected shortly before impact, and in several cases meteorites have been found afterwards. Almahata Sitta was the first, proving that it could be done.
Why it matters
Almahata Sitta was a triumph for planetary defence and for meteorite science. It showed that sky surveys can spot small objects before impact, that predictions of their arrival time and place can be accurate, and that meteorites can be recovered from a predicted fall. It also revealed that some asteroids are much more complex than expected.
It is also a wonderful story of collaboration, linking astronomers, desert searchers and university students in Sudan to answer a question no one had been able to ask before.
Did you know?
Asteroid 2008 TC3 was discovered about 20 hours before it struck Earth's atmosphere.
The name Almahata Sitta means 'Station Six', after a railway station in the Nubian Desert.
Its fragments include many different meteorite types, showing that the asteroid was a jumbled mixture.