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OSIRIS-REx view of Bennu
An image taken by the OSIRIS-REx probe last month shows the asteroid Bennu from a distance of about 40 miles. (NASA / Goddard / Univ. of Arizona Photo)

NASA’s OSIRIS-REx today made its official rendezvous with a promising and potentially perilous asteroid named Bennu, after two years of closing in on it.

“We have arrived,” telecommunications engineer Javi Cerna announced during a NASA webcast from mission control at Lockheed Martin Space in Colorado.

It’s a major step in OSIRIS-REx’s mission to study a near-Earth object at close range and snag samples for return to Earth in 2023.

The car-sized spacecraft has been creeping up on the 0.3-mile-wide (half-kilometer-wide), diamond-shaped space rock for weeks, but today a 28-second thruster firing stabilized its position at a point less than 12 miles (20 kilometers) away from the asteroid (and more than 75 million miles or 121 million kilometers from Earth).

Mark Fisher, chief spacecraft engineer for the OSIRIS-REx mission at Lockheed Martin Space in Colorado, said today’s maneuver cleared the way for the “precision flying regime that we’ll be doing for the next year and a half or so in order to map and discover all we can about OSIRIS-REx, and determine the best place to go get a sample of the asteroid and bring it back.”

The samples will be divvied up among researchers and studied for years. Bennu was named after an Egyptian deity linked with creation — which is apt, because scientists say the space rock could preserve compounds that go back to the creation of the solar system. Scientific data sent back from OSIRIS-REx’s sojourn, plus the results of on-the-ground analysis, could help unravel the processes that led to planetary formation and perhaps life.

There’s another angle to OSIRIS-REx, an Egyptian-sounding acronym that stands for “Origins, Spectral Interpretation, Resource Identification, Security – Regolith Explorer.” The “security” side of the acronym refers to the fact that Bennu’s orbit intersects with Earth and has been given a slight chance of posing a threat sometime in the next couple of centuries.

For years, scientists have debated how subtle long-term effects change the course of asteroids, and whether those effects could somehow be used to divert potentially hazardous asteroids. OSIRIS-REx’s up-close maneuvers could give scientists more of a sense about the best strategies.

OSIRIS-REx will study how the asymmetrical force exerted by sunlight causes Bennu’s orbit to change ever so slightly, due to a phenomenon called the Yarkovsky Effect. “The Yarkovsky Effect is key in being able to predict Bennu’s trajectory over that long span of time,” mission project manager Rich Burns from NASA’s Goddard Space Flight Center explained.

Some scientists have talked about using solar sails, mirrors or laser beams to take advantage of the Yarkovsky Effect and carefully shift potentially hazardous asteroids to a more benign orbital path.

Now that OSIRIS-REx has arrived, it will execute a series of flybys past Bennu, bringing it as close as 4.3 miles (7 kilometers) to the surface.

Over the next year and a half, the spacecraft will map the asteroid in detail. Then, in 2020, it will circle down and stick out a 10-foot-long robotic arm to touch the surface for several seconds at a time. The arm has an sampler head that’s designed to puff out bursts of nitrogen gas, stir up rocks and bits of grit and suck them in like a carpet sweeper.

The probe will get three chances to snag samples, and then the attachment will be tucked into a sample return capsule for safekeeping.

Japan’s Hayabusa probe set a precedent when it brought back bits of asteroid Itokawa in 2010, but this is NASA’s first asteroid sample return effort. NASA expects to get at least 2 ounces of material, and potentially as much as 4.4 pounds. “Ours will be by far the largest sample collection from a robotic spacecraft,” Fisher said.

In 2021, OSIRIS-REx will begin the journey back to Earth. The plan calls for the sample return capsule to be sent down to the Utah Test and Training Range in September 2023 for retrieval. The round-trip mileage is expected to amount to more than 4 billion miles, which exceeds the distance between Earth and Pluto.

Meanwhile, Japan is following up on its first asteroid mission with Hayabusa 2, which is currently studying the asteroid Ryugo and will be collecting samples over the coming months. Because of favorable orbital mechanics, Hayabusa 2 should return its samples in late 2020, well before OSIRIS-REx drops off its delivery.

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