On an asteroid called Bennu, things are about to get weird.

It's been quiet on Bennu for a long time. It wasn't always this way: Back in the old days, Bennu was a wild place. More than four and a half billion years ago, when the solar system was just getting going, Bennu was growing up fast with the rest of the big rocks, picking up minerals, ice, and organic molecules as it rambled through the cloud of raw material circling the proto-Sun.

Over time, the planets settled into their orbits, and Bennutook up position in asteroid belt. A few billion uneventful years followed, until...splat! A collision tore Bennu apart, shattering it into spray of rubble. From the shards emerged a new, smaller version of Bennu, measuring about a half-kilometer across. Over time, Bennu, Jr. nudged its way closer to the Sun, taking on the shape of a squashed meatball and adopting a new orbit, one that's very similar to Earth's.

But now, there's a strange speck in the sky over Bennu, tiny but getting bigger every day. That speck is us.

Well, not us, exactly, but OSIRIS-REx, a NASA spacecraft that's on its way to study Bennu. The size of a small motorhome, OSIRIS-REx is outfitted with cameras and lasers to map the asteroid's surface, plus a suite of spectrometers to figure out Bennu's make-up, which researchers believe is a time-capsule of the early solar system. After a journey of two years and more than a billion miles, OSIRIS-REx is now beginning its final approach. On Aug. 17, the spacecraft snapped its first images of the asteroid — a bright dot edging across the sky — and in October, OSIRIS-REx will start slowing down for its December rendezvous.


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But the climax of the mission, the thing that really sets it apart from NASA's previous asteroid expeditions, will come in July 2020, when OSIRIS-REx picks up a piece of Bennu to bring home. OSIRIS-REx will extend an 11-foot robotic arm down to the asteroid's surface, blow out a puff of nitrogen gas, and collect the surface particles kicked up by the little gust. Then, it will tuck the sample into a capsule and begin the long journey home, leaving Bennu alone again.

OSIRIS-REx is NASA's first asteroid sample return mission, but the practice of bringing home space souvenirs goes back to the Apollo-era moon rocks. In the decades since, spacecraft have collected and returned samples of interstellar dust, solar wind, comet-stuff, and natural and artificial debris orbiting Earth. Japan's Hayabusa mission was the first to deliver asteroid samples to Earth, in 2010, and that probe's successor, Hayabusa2, is currently hovering over an asteroid called Ryugu.

Of course, you don't have to go to space to get a piece of an asteroid: they drop down on Earth routinely, as meteorites. But after making the fiery trip through Earth's atmosphere and spending some time exposed to the elements, those meteorites are less than pristine.

Yet those mostly-harmless encounters hint at one major motivator for the OSIRIS-Rex mission: Bennu is a "potentially hazardous asteroid," meaning that it could hit us and do some real damage. In fact, some time in the 22nd century, Bennu will veer so close that there's a 1 in 2,700 chance it will collide with Earth. If you're a glass-half-full type, you can think of that as a 99.96% chance that Bennu won't hit us. Still, it makes Bennu one of the most threatening known asteroids, and researchers charged with defending the planet are eager to know more about Bennu and the forces that steer it through the solar system.

After all, Bennu's story started with a smash-up. Hopefully it won't end that way, too.