The spacecraft traveled over 200 million miles and four years to briefly bump into Bennu, blast it with compressed gas and collect bits of its surface. The space agency on Wednesday shared the first batch of images from the daring operation, revealing a delicate-yet-explosive moment between rock and robot.
When the spacecraft's robotic sampling arm, named Touch-And-Go Sample Acquisition Mechanism, or Tagsam, touched down on Bennu, it performed what amounts to a cosmic pickpocketing maneuver. Mission planners expected that the total time of contact between the arm and asteroid would be less than 16 seconds. When preliminary data was released, it showed that the period of contact was just six seconds, with much of the sample collection happening in only the first three.
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The spacecraft, which operates largely autonomously due to the 18-minute communications delay with mission control on Earth, fired a canister of gas through Tagsam that disrupted the surface of Bennu and forced a sample into the arm's collector head.
Photos taken of the head on Thursday showed that so much sample was collected that some larger rocks seemed to fail to make it all the way inside, wedging a mylar flap meant to seal the container partially open, allowing some small bits of dust and pebbles to escape back out into space.
"Bennu continues to surprise us with great science and also throwing a few curveballs," said Thomas Zurbuchen, NASA's associate administrator for science, in a statement. "And although we may have to move more quickly to stow the sample, it's not a bad problem to have. We are so excited to see what appears to be an abundant sample that will inspire science for decades beyond this historic moment."
Osiris-Rex was designed to touch down on a flat, even surface, but Bennu is so rocky the team found no suitable space. Fortunately, Osiris-Rex outperformed its design and was able to perform its sampling on a site dubbed Nightingale, which is only about as big as a few parking spaces.
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Osiris-Rex tags a boulder
As the spacecraft approached and then spent two years orbiting and surveying Bennu, it became clear this tiny world is different from what scientists expected. The team hoped to find a number of sandy surfaces ideal for sampling, but it turns out Bennu is a rubble pile, with a rugged terrain strewn with boulders.
Around 24 hours after the operation, NASA shared the first images of the touchdown operation captured by the spacecraft. The Tagsam moves into position and its sampling head makes contact with Bennu's surface before the explosive burst of nitrogen is fired. The operation kicks up a ton of debris that flies around the acquisition arm. It's really something!
Although the above GIF appears relatively fast, the operation proceeded much more delicately. The arm was lowered at around 10 centimeters per second, much slower than walking pace, when it contacted the sample site.
The team's goal is to collect about 60 grams of dust, dirt and pebbles from the surface of Bennu. It reported on Friday that it believes Osiris-Rex collected a sufficient sample and moved to begin to stow it quickly, skipping a planned sample mass measurement and canceling a braking burn to keep acceleration of the spacecraft to a minimum.
"We are working to keep up with our own success here, and my job is to safely return as large a sample of Bennu as possible," said Dante Lauretta, Osiris-Rex principal investigator at the University of Arizona.
The mission joins Japan's Hayabusa and Hayabusa-2 missions in the annals of asteroid exploration. Hayabusa sampled and returned a tiny bit of material from asteroid Itokawa, and Hayabusa2 is in the process of returning a significant sample of space rock Ryugu.
Once the sample is stowed, the team will begin preparations for a long journey back to Earth, with a planned landing in the Utah desert in September 2023.