osiris rex.jpg

An illustration of the spacecraft OSIRIS-REx preparing to contact the asteroid Bennu, set to take place Oct. 20. 

On Oct. 20, scientists at the University of Arizona will monitor a outer-space heist that is more than 10 years and one billion miles in the making. 

That date is when the OSIRIS-REx spacecraft will brush against on the asteroid Bennu to collect rocks and dust from its surface. If successful, it will be the first American spacecraft to retrieve a sample from an asteroid—all without humans aboard. 

While OSIRIS-REx is a NASA mission, it has long been a topic of interest here in Tucson due to lead researcher Dante Lauretta, a professor of planetary science at UA. Lauetta describes OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) as the most ambitious program the university has ever led, with the potential for us to better understand the origins of life on our planet. 

“This will be a culmination, for some of the people on the team, of 20 years of work,” said Erin Morton, communications lead for the OSIRIS-REx mission. “It all boils down to this first attempt. The entire team is excited and is barreling toward our target date, getting ready for that sample collection.”

The spacecraft launched from the Earth in September 2016 and is planned to return in 2023. Even before reaching the asteroid in December 2018, images sent from the spacecraft indicated the mission would be more difficult than once thought; the asteroid’s surface was far rougher than planned, with more than 200 boulders larger than 33 feet in diameter, and is described as a “rubble pile” asteroid. At roughly 1,600 feet across, the asteroid is about the same size as Pusch Ridge in the Catalina Mountains, but because of its uneven surface, the spacecraft only has an area the size of three parking spaces to collect samples in. 

After examining the asteroid’s surface, the team selected a site nicknamed “Nightingale” as the best to collect a sample from. Lauretta admitted he thought it would be obvious from initial imaging which spot on the asteroid would be best to collect from, but it turned out to be “nothing of the sort.”

“This [site] really came out on top because of the scientific value,” Lauretta said upon Nightingale’s selection last year. “The high latitudes means it stays relatively cool, and the primary objective of OSIRIS-REx is to bring back organic material and water-bearing material from the early solar system. And being in those high latitudes we think gives us the best chance to preserve that kind of material.”

Because the spacecraft and asteroid are millions of miles from Earth, it takes nearly 20 minutes for scientists to receive signals. This lag means OSIRIS-REx cannot be commanded in real-time, and must instead perform the entire sample collection sequence autonomously.

To prepare for the sample retrieval, the science team staged two rehearsal events earlier this year. The rehearsals involved descent and navigational maneuvers, as well as a chance to verify that the spacecraft’s imaging and ranging systems operated as expected. 

“It’s not so much we’re altering anything as we go through each rehearsal, but we’re learning exactly how the spacecraft responds in real-life situations,” Morton said. “The hardest thing for us is having to know how to do this without having done it before.”  

During the four-and-a-half-hour sample collection event, the spacecraft will descend some 2,500 feet from orbit toward the asteroid’s surface. While the spacecraft will “touch down” to collect the samples, it will not land on the asteroid. Instead it will extend a robotic arm to the asteroid, firing a jet of pressurized nitrogen onto the surface to kick up dust. The spacecraft will collect a minimum of 2 oz. (60 grams) of rocky material in its sampling chamber. Should OSIRIS-REx fail to capture sufficient samples, another attempt will take place “no earlier than January 2021.”

After the samples are retrieved, the spacecraft will calculate the amount of samples collected. Scientists should know if they’ve gathered sufficient samples within a week. 

While the central goal for OSIRIS-REx is to collect a sample from the surface of Bennu, researchers have already highlighted several key findings from the mission.

In late 2018, scientists announced they had discovered “water-bearing clay minerals” on the asteroid. While these hydrated minerals do not mean Bennu contains liquid water, they indicate that liquid water was present at some point in the past on Bennu’s parent body, a much larger asteroid.

New data also indicates Bennu’s rotational speed is increasing, only at about one second per century however. This increase is due to the YORP effect, where differences on the surface of the asteroid unequally scatter solar radiation. Due to Bennu’s aforementioned roughness, it is catching and reflecting sunlight at different angles, causing its rotational rate to drift. 

Thanks to OSIRIS-REx, scientists are also seeing for the first time how this “bombardment” of solar radiation can physically wear down asteroids. According to a paper led by Jamie Molaro of the Tucson-based Planetary Science Institute, rocks on Bennu appear to be cracking as sunlight heats them up during the asteroid’s day and they cool down at night. According to Lauretta, how quickly this occurs relative to other weathering processes tells us how and how quickly the surface has changed.

“Another thing we learned that was really shocking were the particle ejection events that were happening,” Morton said. “For a long time scientists have thought you have asteroids, which are lifeless bodies, and you have comets which eject all sorts of ice and rocks as they go. But over the years we’ve seen maybe it’s not so cut and dry. But this is the first time we can see these asteroid ejections within a kilometer. We were able to watch the birth and death cycle of a natural satellite. We’ve watched a piece of the asteroid come off, orbit a couple of times, and then fall back down.”

Beyond discoveries, OSIRIS-REx has also achieved two Guinness World Records: In June 2019, OSIRIS-REx became the spacecraft to orbit closest to an asteroid, at a distance of about 2,200 feet. Bennu is also the smallest object to ever be orbited by a spacecraft.

While local in-person viewings are limited due to COVID, NASA will be broadcasting live during Oct. 20 on nasa.gov/multimedia/nasatv. In addition, OSIRIS-REx’s Twitter will confirm each step of the sample retrieval. There will also be a livestream with mission representatives on asteroidmission.org.