OSIRIS-REx finds water on asteroid Bennu

by Chris Gebhardt

In its first major scientific find, NASA’s OSIRIS-REx spacecraft has found water on asteroid Bennu.  The discovery was made via observations during OSIRIS-REx’s approach to the Near Earth Asteroid between mid-August and early-December 2018 and confirms previous thoughts that Bennu might be a place harboring water in the inner solar system.

The discovery also holds significant potential in terms of the material from Bennu OSIRIS-REx hopes to return to Earth early next decade.  The return of such water-bearing material could help solidify or distance scientists from a theory that asteroids seeded Earth billions of years ago with the water that now dominates our planet’s surface.

An amazing start to OSIRIS-REx’s primary science mission:

Last week, when all eyes were on OSIRIS-REx for the spacecraft’s arrival at asteroid Bennu, mission scientists were ending a multi-month long data comparison between OSIRIS-REx (Spectral Interpretation, Resource Identification, Security-Regolith Explorer) instruments to confirm spectral data showing the presence of water on the Near Earth Asteroid.

During the 3.5 month approach phase of OSIRIS-REx to Bennu, mission scientists aimed three of the spacecraft’s instruments toward the asteroid and began making the mission’s first scientific observations.

Of those three instruments, two of OSIRIS-REx’s spectrometers, the OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) and the OSIRIS-REx Thermal Emission Spectrometer (OTES), revealed the presence of molecules containing bonded oxygen and hydrogen atoms, known as hydroxyls – revealing a global distribution of water-bearing clay minerals on Bennu.

Bennu, composed of 12 PolyCam images collected on Dec. 2 by OSIRIS-REx (Credit: NASA/Goddard/University of Arizona)

While the discovery of water is not a discovery of liquid water pooling on Bennu’s surface, it is a discovery of water containing clay minerals.  And since Bennu is too small to have ever hosted liquid water on its own, the discovery indicates that the parent body from which Bennu originated – a larger asteroid – did contain liquid water itself.

Speaking at the American Geophysical Union’s Fall 2018 meeting, members of the OSIRIS-REx team spoke to the fact that a major mission objective was accomplished so quickly, but iterated that there is still a great deal of work ahead for the mission – which is also tasked with retrieving a sample from the surface of Bennu and returning that sample back to Earth.

“The presence of hydrated minerals across the asteroid confirms that Bennu, a remnant from early in the formation of the solar system, is an excellent specimen for the OSIRIS-REx mission to study the composition of primitive volatiles and organics,” said Amy Simon, OVIRS deputy instrument scientist at NASA’s Goddard Space Flight Center.

“When samples of this material are returned by the mission to Earth in 2023, scientists will receive a treasure trove of new information about the history and evolution of our solar system.”

Getting those materials back to Earth will be tricky, but it’s something the team and spacecraft are well prepared for.

From the beginning, OSIRIS-REx was designed to not only arrive at and fly in formation with Bennu – something the craft achieved on 3 December – but also to enter orbit of the asteroid (scheduled for 31 December 2018) and then touch down on its surface sometime in 2020.

That touchdown – more of a “docking” than a landing, as described by the OSIRIS-REx team – will enable the craft to robotically retrieve a sample from Bennu’s surface, store it in a sample return container, and bring the sample back to Earth for deeper investigation.

But touching down on any surface requires a good understanding of the local terrain as well as an ability to see hazards that need to be avoided.

Bennu – before OSIRIS-REx’s launch – was thought to be less bolder-y than is now understood thanks to high resolution imaging of the asteroid, imaging that will only get better the closer OSIRIS-REx gets to the asteroid.

Based on initial close observations from OSIRIS-REx, we now know that Bennu’s surface is a mix of very rocky, boulder-filled areas with few relatively smooth regions that lack boulders and that the number of boulders on the surface is higher than expected.

This means that a very detailed survey of Bennu’s surface will be needed to determine a safe touchdown location for OSIRIS-REx and that the craft will have to get much closer to Bennu before the science teams can accurately assess where a sample can be taken from the surface.

OSIRIS-REx performs its Touch-And-Go sample retrieve on Bennu. (Credit: NASA’s Goddard Space Flight Center)

“Our initial data show that the team picked the right asteroid as the target of the OSIRIS-REx mission.  We have not discovered any insurmountable issues at Bennu so far,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson.

To ensure that no issue occurs during the 2020 touchdown and sample collection part of the mission, scientists will spend 2019 gathering detailed observations of the surface before selecting a landing site later that same year.

After the site is chosen, the teams will run multiple simulations of the touchdown sequence – drawing heavily on NASA’s experience in docking spacecraft together.  Bennu’s low mass and weak gravity field make OSIRIS-REx’s landing far more similar to a docking than to the landing of a craft on the surface of the Moon.

OSIRIS-REx’s touchdown on Bennu is slated to occur at some point in 2020, as is the actual collection of the sample to be returned to Earth.

If all goes to plan, the sample return will land back on Earth in September 2023.

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