NASA's OSIRIS-REx probe only arrived at the asteroid Bennu a week ago, and it's already made a startling discovery. After analyzing data gathered by the craft, the team found that Bennu is rich in water molecules, locked away in clay minerals.

After a two-year journey from Earth, OSIRIS-REx (short for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer), came within 19 km (12 mi) of Bennu on December 3. The science team kicked off studies of the asteroid by pointing three of the craft's instruments towards Bennu, including the camera suite, a visible and infrared spectrometer (OVIRS) and thermal emission spectrometer (OTES).

Analysis of data from the two spectrometers showed that the space rock was rich in hydroxyls, molecules made up of bonded oxygen and hydrogen atoms. These appear to be locked away in water-bearing clay minerals across and throughout Bennu. It's not exactly liquid water – the asteroid is too small for that to happen – but it does indicate that liquid water was present in the past, perhaps on the much larger parent asteroid that Bennu broke off from.

"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," says Amy Simon, OVIRS deputy instrument scientist. "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."

The craft has also taken some clearer snaps of the space rock and confirm some predictions made about it from Earth. The team found that Bennu's shape, diameter, rotation rate and inclination closely match models calculated in 2013. But not everything was as expected – the surface is littered with far more boulders than was thought, which could make taking a sample tricky. One particularly big boulder near the south pole was found to measure about 50 m high and 55 m wide (164 ft x 180 ft), which is roughly five times bigger than the models predicted.

Currently, OSIRIS-REx is performing a series of flybys over Bennu, getting as close as 7 km (4.3 mi), to get a better idea of its mass. That will allow the science team to understand the gravitational pull of the asteroid, which is vital information for them to be able to steer the craft into orbit.

OSIRIS-REx is due to enter Bennu's orbit on December 31, when it will get as close as 1.4 km (0.9 mi). That will make it the closest orbit and smallest object ever orbited by any spacecraft.

A 3D model of Bennu can be seen in the video below.

Source: NASA

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