The recent discovery of numerous lumps of metal-rich stones near Aletai, Xinjiang, puzzled geologists. Instead of being clumped together, the rocks rich in iron, copper, and gold are distributed in a long thin line stretching 430 kilometers (260 miles) and are clearly of extraterrestrial origin. To explain their distribution, scientists are proposing an asteroid came in at such a shallow angle that, rather than forming a single impact crater, it skipped over the Earth's atmosphere before falling in, like a flat stone bouncing over water.
Returning space missions must carefully tune the angle at which they hit the atmosphere, the window famously made narrower for Apollo 13 due to damage to the heat shield. Come in too steeply and the craft will burn up, too shallow and it will bounce off the atmosphere and wander endlessly in space. Asteroids experience something similar, although lacking parachutes there is no safety window in between.
With just the right entry angle, however, the asteroid could bounce off the atmosphere, losing some of its energy in the process. This would slow it down, leading to a subsequent entry a few hundred kilometers further along its path. There was no known evidence of this occurring, however, until the publication in Science Advances of a study of the Aletai deposits.
Meteorites frequently break up on hitting the Earth's atmosphere. In recent years this has sometimes been filmed, but even beforehand the evidence could be seen in meteorites of the same composition strewn across a local area. However, the paper notes, this normally involves, “A length of several to a dozen kilometers.” The longest confirmed strewn field for a meteorite line is 18.5 kilometers (11.5 miles), although debate continues over a proposed case stretching 275 kilometers (170.9 miles).
Consequently, the rocks from the Aletai meteorite were initially assumed to represent five different falls. The largest of these found so far weighed 28 tons, and the total mass found is already 74 tons. With 23 tons of this found only last year, there could be plenty more lying unnoticed.
Multiple asteroids hitting the Earth in a near perfect line was improbable enough. When Dr Ye Li of the Purple Mountain Observatory and co-authors examined the deposits, they found the same ratios of rare metals, indicating a common source asteroid.
The team modeled how this could occur and concluded by far the most likely explanation is an asteroid with an entry angle of just 6.5° to 7.3°.
In addition to causing the asteroid to skip, the initial shallow-angle encounter with the atmosphere would have caused it to break up. The pieces would then have had much more opportunity to spread out than in a more typical case where the meteorites fall directly to Earth, explaining why they were strewn over such a distance. A more remote possibility is an asteroid that had broken up prior to encountering the Earth, but continued on similar orbits.
The paper proposes the asteroid initially weighed between 280 and 3,440 tons, some of which burned up in the atmosphere, while other parts are waiting to be found. Given its composition it would probably have been 8-20 meters (26 to 65.5 feet) across – more comparable to a house than African wildlife. There is no sign of an impact crater, as post-breakup none of the pieces carried enough kinetic energy.
The Aletai asteroid would have hit the atmosphere at between 11.9 and 14.9 km/s (43,000-54,000 mph), the authors calculate, near the lower end of space rocks' impact velocities.