Image of Bennu from OSIRIS-REx.
Photo: NASA/Goddard/University of Arizona

NASA’s OSIRIS-REx arrived today at its target asteroid, Bennu, an important step on its mission to collect a sample from an asteroid and return it to Earth.

OSIRIS-REx launched on September 8, 2016 from Cape Canaveral. It carries five data-taking instruments, and scientists hope to learn more about the Solar System’s origins and even what resources an asteroid might hold. Today’s milestone marks the end of a two-year journey to Bennu, and the start of a 1.5-year study period.

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“It’s been a long time coming for the arrival, and we’re really looking forward to the next chapter of this mission,” Heather Enos, the OSIRIS-REx deputy principal investigator from the University of Arizona, said in a NASA press video.

Today, scientists burned the engines to place OSIRIS-REx safely in its orbit around Bennu, 7 kilometers from the asteroid at closest approach. The spacecraft has undertaken maneuvers since the summer to change its velocity and prepare it for this approach and arrival.

Orbiting asteroids (and rendezvousing with them) isn’t easy—these rocks have very little gravity, and several asteroid-orbiting missions have run into issues. Bennu will be the smallest astronomical object ever orbited by a spacecraft; it’s only around 1,600 feet from end to end, on average. “The low-gravity environment is one of the foremost challenges in conducting this mission,” Rich Burns, OSIRIS-REx project manager at NASA’s Goddard Space Flight Center, said in a NASA video. Even the Sun’s energy heating the spacecraft can change its trajectory.

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Asteroids are of extreme interest to scientists. They’re thought to contain an unaltered record of the early Solar System, and they could be the way that some of Earth’s water arrived at our planet in the first place. That water or the asteroid’s metals might one day serve as useful resources, so space explorers wouldn’t need to bring these heavy materials with them. And Bennu’s orbit makes it a “potentially hazardous” asteroid, meaning it’s big and could possibly threaten the Earth in the distant future, so scientists hope to further characterize it.

Scientists look forward to having some real bits of asteroid back here on Earth to analyze. “I’m particularly excited about the moment when the sample will be retrieved,” Neyda Abreu, associate professor of geoscience and mathematics at Penn State DuBois, told Gizmodo. “We have only scratched the surface when it comes to understanding asteroids.” Abreu hopes that Bennu will reveal a variety of different materials and terrains, while also offering a safe place to land.

OSIRIS-REx is NASA’s first asteroid sample return mission, and the largest sample return since Apollo returned moon rocks. It will hopefully snatch up to 2 kilograms of material. It’s not the first asteroid sample return, though—the first “troubled” Hayabusa mission from Japan brought back 1,500 dust grains from the asteroid Itokawa after considerable difficulties. Its predecessor, Hayabusa2, is in the midst of its own mission to touch down on another asteroid, Ryugu.

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The OSIRIS-REx team will now map the asteroid in extremely fine detail and measure its mass. This will help them determine future orbits and where on the asteroid to collect the sample, Coralie Adams, OSIRIS-REx flight navigator, said during the NASA press event.

The spacecraft is slated to return to Earth in September 2023.