NASA’s OSIRIS-REx spacecraft is a cosmic rock collector. Cheers erupted from mission control at 6:12 p.m. EDT on October 20 as scientists on Earth got word that the spacecraft had gently nudged a near-Earth asteroid called Bennu, and grabbed some of its rocks to return to Earth.
“The spacecraft did everything it was supposed to do,” said mission principal investigator Dante Lauretta of the University of Arizona in Tucson on October 20 on a NASA TV webcast. “I can’t believe we actually pulled this off.”
OSIRIS-REx arrived at Bennu in December 2018, and spent almost two years making detailed maps of the 500-meter-wide asteroid’s surface features and composition (SN: 10/8/20). Observations from Earth suggested Bennu should be smooth and sandy, but when OSIRIS-REx arrived, it found a treacherous, rocky landscape.
The team selected a relatively smooth patch in a crater named Nightingale. The spot was not without hazards, though — the team was so worried about a particularly large rock nearby that they named it “Mount Doom” (SN: 12/12/19).
Luckily, the spacecraft did not need to fully land in the crater to complete its mission. As it hovered just above the surface, OSIRIS-REx reached out a robotic arm with an instrument called TAGSAM at the end, for Touch-And-Go Sample Acquisition Mechanism. The instrument tapped the asteroid lightly for six seconds, and released a burst of nitrogen gas to disturb the surface dust and pebbles. Once those small rocks were lofted, some were blown into the sample collector.
The team stayed up late analyzing the spacecraft’s pictures of the sampling sequence, Lauretta said in a news conference on October 21. Based on those images, it looked like the head of sampling device pushed down into the dust of Bennu’s surface a bit. That’s good news, Lauretta said. In experiments performed on Earth, the spacecraft grabbed the most rock when the sampling head was surrounded by dust.
It also looks like the spacecraft destroyed a relatively large rock on its descent. “Literally, we crushed it,” Lauretta said. “That rock appears to fragment and shatter.” Studying the asteroid from orbit suggested that type of smashing was possible (SN: 10/8/20), the team reported on October 8, and it was exciting to see it actually happen, Lauretta said in the news conference.
Because signals from Earth took 18½ minutes to reach Bennu, the spacecraft performed the sampling sequence autonomously. When the mission team got the signal that the spacecraft had finished its job and retreated to a safe distance from Bennu, team members pumped their arms in the air, cheered and sent each other socially distant high-fives and hugs.
OSIRIS-REx is not the first spacecraft to grab samples from an asteroid. That distinction goes to Japan’s Hayabusa mission, which brought back grains of asteroid Itokawa in 2010 (SN: 6/14/10). An encore to that mission, Hayabusa2, collected samples of asteroid Ryugu last year, and is on track to land in Australia in December (SN: 2/22/19).
But OSIRIS-REx has collected much more material than Hayabusa2 did. Hayabusa2 hoped to collect 100 milligrams, although Hayabusa2’s scientists have no way to know how much material it actually collected until the spacecraft returns to Earth. OSIRIS-REx aimed for a minimum of 60 grams, or a little more than two ounces.
Images that the spacecraft took of itself first suggested that OSIRIS-REx easily cleared that bar, snagging an estimated 400 grams, or 14 ounces. The sample collector was so full of rock and dust that small asteroid particles were escaping into space, the team reported on October 23.
Then, on October 29, the team announced that they actually think the collector was full to capacity when it pulled away from Bennu, which means it carried about two kilograms of material. “Imagine a sack of flour at the grocery store, that’s what we were pulling away from the surface with,” Lauretta said in a news conference.
Some tens of grams of rock and dust escaped as the spacecraft moved the collector and stowed it safely in the capsule that will bring it back to Earth. While the part of the collector that was visible to the spacecraft’s cameras probably holds about 400 grams, that’s only 17 percent of the collector’s whole volume. “We’re probably pretty close to in excess of a kilogram of material,” Lauretta said.
Originally, the spacecraft was supposed to extend its arm and spin its whole body. The difference in the way it spun after the sample collection, versus before, would have revealed the mass of the sample. But the team cancelled that plan, fearing that the motion would send more asteroid dust flying into space.
If the team decides to go back for a second sample, the spacecraft won’t return to Nightingale crater, said project manager Rich Burns of NASA’s Goddard Space Flight Center in Greenbelt, Md., at the October 21 news conference. Any second attempt would touch down in a backup site called Osprey, and would happen in January.
OSIRIS-REx will return to Earth in 2023, where scientists will analyze the rocks in hopes of unlocking details of the history of the solar system and the origins of water and life on Earth (SN: 1/15/19).