By Josh Roberts

Early on the morning of April 29, 2013, Commander Chris Hadfield’s residence got hit by a rock. Sounds like a story for a slow day on local news, except that Hatfield lives 225 miles above Earth in the International Space Station (ISS), and the offending rock was not thrown by vandals—it was merely passing by.

The rock in question is most likely just one of a huge number of similar pebbles that occupy our part of the Solar System. If you were to hold such an object in your hand, it would hardly seem threatening, but with gravity to accelerate it and without a thick layer of atmosphere to slow it down, these tiny objects can endanger spacefarers such as Hadfield.

Reaching speeds of 25,000 miles per hour, these rocks zip along between 12 and 20 times faster than a speeding bullet (depending on the bullet)!

Luckily, the object did not hit the main body of the space station, but it did punch a small hole in part of the ISS’s acre of photovoltaic solar panels. And while the station does sport various types of impact-resistant shielding, it could still be vulnerable to a larger object.

There have never been any lives lost due to an impact in space, but the Near Earth Object hunters at the B612 Foundation posted an ominous (and succinct) reminder to Facebook minutes after the event: “Definitely NOT good.”

It is estimated that hundreds of tons of similar objects fall to Earth every day, but with an ever-increasing presence beyond Earth’s protective atmosphere, current and future space explorers must be acutely alert to these potentially harmful rocks. We live in a densely populated part of the universe and have to remain vigilant.

Josh Roberts is a program presenter and astronomer at the California Academy of Sciences. He also contributes content to Morrison Planetarium productions.

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