NASA’s OSIRIS-REx Is About to Touch an Asteroid

By Daniel Oberhaus

For nearly two years, a small spacecraft called OSIRIS-REx has been orbiting an asteroid more than 100 million miles away, patiently biding its time by studying the rock’s surface. Scientists believe that this asteroid, Bennu, is a piece of a much larger one that formed just a few million years after Earth. It’s a perfectly preserved cosmic time capsule that could reveal the secrets of the ancient history of our solar system. Tomorrow, OSIRIS-REx will make a daring plunge to Bennu’s surface and use a robotic arm to vacuum up some of its space dust, which it’ll bring back to Earth. The encounter will last for just a few seconds, but it is a technological feat that has been more than a decade in the making.

Shortly before 2 pm ET on Tuesday, OSIRIS-REx will fire its thrusters and begin a torturously slow descent from its orbit above Bennu. It will take the craft four and a half hours to travel just a kilometer to the surface, and by the time it gets there, it will be moving only about 4 inches per second. “Everything around this asteroid happens slowly,” says Richard Burns, the OSIRIS-REx project manager at NASA’s Goddard Flight Center. Although the team has done two practice approaches earlier this year, getting the spacecraft as close as 120 feet above the asteroid, this will be the first physical contact. “This is the one thing we haven't done, so we don’t know what’s going to happen,” says Burns. “We’re cautiously optimistic that this will be the only time we touch the surface.”

The planned sample collection area, dubbed Nightingale, is a rugged 66-foot-wide crater near Bennu’s north pole. It was selected primarily because the crater appears to be young, which means that the exposed rock is likely to consist of pristine remnants from when the asteroid was formed billions of years ago. As OSIRIS-REx approaches Bennu, it will extend an 11-foot-long arm capped with a sample collection device that looks like a large shower head. When the arm contacts the surface, it will blow a small burst of nitrogen gas onto it to stir up some dust and rocks. This asteroid dirt will be collected in a ring around the head, which can store a little more than 4 pounds of material.

The maneuver requires extreme precision. The surface of Bennu turned out to be a lot more rugged than researchers expected, but the sample collection arm works best on a flat, sandy surface. If the arm touches down on top of rocks more than a few inches in diameter, it could limit how much material it’s able to collect.

“We weren’t certain what the surface was going to look like,” says Mark Fisher, a space engineer at Lockheed Martin who worked on building the sample collection arm. “We thought it was going to have a lot more fine-grain material, but there’s lots of big rocks on the surface. And that makes it hard to get some smaller rocks into the sampler.”

Fisher says he’s not too concerned about the spacecraft’s ability to pick up at least some material. He says the nitrogen jet that will be used to stir up rocks on the surface has “a lot of kick,” so even if the sampler head touches down on a rocky area, it should be able to blow out some loose dirt lying below the rocks. The outside of the sampler head also acts a bit like velcro, which means that as long as the arm makes contact with Bennu, researchers are guaranteed to at least get some material stuck to the collector plate.