Fireball That Flew Over Japan in 2017 Was Tiny Piece of Giant Asteroid that Might One Day Threaten Earth

In the early morning of April 28, 2017, a small fireball crept across the sky over Kyoto, Japan. And now, thanks to data collected by the SonotaCo meteor survey, researchers have determined that the fiery space rock was a shard of a much larger asteroid that might (far down the road) threaten Earth. 

The meteor that burned over Japan was tiny. Studying the SonotaCo data, the researchers determined that the object entered the atmosphere with a mass of about 1 ounce (29 grams) and was just 1 inch (2.7 centimeters) across. It didn't threaten anyone. But small meteors like this are interesting because they can offer data on the bigger objects that spawn them. And in this case, the researchers tracked the little rock back to its parent: an object known as 2003 YT1.

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2003 YT1 is a binary asteroid, composed of one large rock about 1.2 miles (2 kilometers) across orbited by a smaller asteroid that's 690 feet (210 meters) long. Discovered in 2003, the binary system has a 6% chance of hitting Earth at some point in the next 10 million years. That makes the object what researchers call a "potentially hazardous object," even though it's unlikely to hurt anyone in your lifetime.

The binary didn't pass by Earth in 2017, so there wasn't an immediately obvious link between the meteor and its parent. But the researchers studied how the fireball moved across the sky and were able to reverse-engineer the object's orbit through space, pinning it to 2003 YT1 with a high degree of certainty.

The researchers said they aren't sure how the little rock split off from 2003 YT1 but believe it's part of a larger stream of dust that got flung off of the asteroid. And they offered a few potential explanations for how that stream formed: Maybe tiny micrometeorites routinely strike the bigger asteroid in the binary, fragmenting it like bullets striking a rock wall. Or maybe changes in heat cracked one of the asteroid's surfaces, spitting small pieces into the dark.

One scenario the authors offered is that the shards are a result of the process that formed the 2003 YT1 system in the first place.

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Most people likely imagine asteroids as great, big rocks, scaled-up versions of the stones they'd find here on Earth. But 2003 YT1, the authors wrote, is more likely a "rubble pile," a jumble of stuff loosely bound together by gravity that coalesced into two orbiting bodies at some point in the last 10,000 years. The forces holding the masses together as individual asteroids are likely weak, and as the two piles spin chaotically around one another every couple hours, they could fling more of themselves into space.

There are other, more exotic possibilities, the authors wrote. Water ice might be sublimating (turning from solid to gas) off one of the asteroids' surfaces and reforming as small balls of ice in open space. But that and other models are unlikely, the researchers wrote.

For now, we know that Earth has been visited by a little piece of a big asteroid. And that little piece is likely part of a stream of other little pieces that sometimes enter the Earth's atmosphere unnoticed. And at some point far down the road, that big asteroid might follow its small children and slam into Earth. That fireball would be much, much bigger.

The paper describing these findings has not yet been peer-reviewed. A draft was published Oct. 16 in the preprint journal arXiv.

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Originally published on Live Science.