Five lessons from the OSIRIS-REx asteroid sample return mission
By Daniel Stolte, University Marketing and Communications
An illustration of NASA's OSIRIS-REx spacecraft, with its sampling arm extended, approaching the surface of the asteroid Bennu.
NASA/GSFC/University of Arizona
A University of Arizona-led NASA mission that helped transform how scientists understand asteroids and the history of our solar system officially came to an end Sept. 30.
The OSIRIS-REx mission fulfilled its scientific objective with flying colors, scooping up a pristine sample of rock and dust from near-Earth asteroid Bennu in October 2020 and returning it to Earth three years later.
While the asteroid sample return portion is winding up, the OSIRIS-REx spacecraft keeps on going – again under U of A leadership. Redesignated as OSIRIS-APEX, the van-sized space probe has embarked on a new mission to study near-Earth asteroid Apophis. Led by Dani Mendoza DellaGiustina, an assistant professor at the university's Lunar and Planetary Laboratory, the OSIRIS-APEX mission aims to observe Apophis during the asteroid's close approach to Earth in 2029, which could provide unprecedented data on how Earth's gravity affects asteroids.
The majority of the 4.3 ounces of material tucked safely inside the capsule that parachuted down to the Utah desert on Sept. 24, 2023, remains under careful curation at NASA's specialized facilities, but portions have been distributed to research institutions, including the U of A. The mission has revolutionized what we know – or thought we knew – about asteroids and the history of our solar system.
Explore the OSIRIS-REx mission through images of asteroid Bennu, the spacecraft and the scientists who helped bring samples back to Earth in the gallery below.
Five of the mission's most interesting findings
Bennu's surface is much softer than scientists expected
Asteroid Bennu turned out to be "fluffier" than the rocky body it was believed to be. When the OSIRIS-REx spacecraft descended to the surface – sampling arm extended – on Oct. 20, 2020, the mission team braced for an experience resembling a pogo stick bouncing off a gravel road. To everybody's surprise, that didn't happen. Instead, when the sampling head contacted the surface, it dove right in with almost no resistance. Had the spacecraft not fired its thrusters to back away immediately after grabbing dust and rock from the asteroid's surface, it would have sunk right into Bennu, and the asteroid might have swallowed it whole. As OSIRIS-REx Principal Investigator Dante Lauretta explained, "The particles making up Bennu's exterior are so loosely packed and lightly bound to each other that they act more like a fluid than a solid."
Dante Lauretta
Bennu was covered in boulders, not smooth sand
In one of the biggest surprises of the mission, Bennu turned out to be a jagged world covered in large boulders, rather than the relatively smooth surface scientists had expected based on observations from Earth. "We expected some boulders, but we anticipated at least some large regions with smoother, finer regolith that would be easy to collect," said Andrew Ryan, a scientist with the U of A Lunar and Planetary Laboratory. "Instead, it looked like it was all boulders, and we were scratching our heads for a while."
Andrew Ryan
Courtesy of Andrew Ryan
Asteroids may have helped bring water to Earth
Long before oceans formed on Earth, oxygen and hydrogen – the building blocks of water – were already drifting through the void, trapped inside stone. While Bennu is too small to have ever hosted liquid water, scientists believe it once existed on Bennu's parent body, a much larger asteroid. The sample confirmed that the asteroid is saturated with water locked inside clay minerals. This discovery supports the theory that a steady drumbeat of asteroid bombardments during Earth's infancy delivered the water for the world's oceans.
The ingredients for life existed before Earth was formed
Bennu didn't just contain water; it preserved the raw chemistry of life itself. Scientists found abundant carbon and organic molecules, including stardust from other stars that are long dead and a high concentration of extraterrestrial amino acids, suggesting the some of the ingredients necessary for life were already floating in the interstellar dark long before our own planet consolidated into a solid ball.
Bennu’s composition suggests the ice and organic compounds in its parent asteroid came from the cold outer reaches of the disk of gas and dust that gave rise to the solar system. "We think this parent body was struck by an incoming asteroid and smashed apart," said LPL Associate Department Head Jessica Barnes. "Then the fragments re-assembled and this might have repeated several times."
Scientists also found evidence that some pieces of asteroid Vesta ended up on Bennu, suggesting that many asteroids participate in a complex orbital dance that sometimes results in cosmic mashups. In addition, minerals called evaporites, never before found in extraterrestrial samples, indicate that the interior of Bennu's ancestor was warm enough to support liquid water for possibly thousands of years.
Tracking Bennu helped scientists better predict asteroid threats
Researchers used precise tracking data from the OSIRIS-REx spacecraft to better understand Bennu's movements. The findings confirm that Bennu's estimated risk of impacting Earth late in the next century is much lower than previously thought and improve our ability to predict orbits of many other asteroids. Scientists have pinpointed Bennu's future orbits with unprecedented accuracy, determining its impact probability through the year 2300 is about 1 in 1,750, or 0.057%. Although the chances of hitting Earth are low, Bennu is still one of the two objects with the highest known impact risks in the solar system, along with another asteroid called 1950 DA.