Nasa's first asteroid sample return mission, OSIRIS-REx, reached the asteroid Bennu only a week ago, but it's already learning more about this time capsule from the early solar system.

OSIRIS-REx, which stands for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer, has found water within the clays on Bennu.

Bennu was worth the two-year wait to reach it.

As OSIRIS-REx travelled 1.4 million miles from Earth and approached Bennu, three of its instruments were pointed toward the asteroid to make scientific observations between mid-August and early December.

Two spectrometers on the spacecraft, OVIRS and OTES, discovered hydroxyls, or molecules of oxygen and hydrogen atoms bonded together. The mission scientists believe that these hydroxyls exist across the asteroid, locked in clay minerals.

This means at some point over its lifetime, Bennu interacted with water - although it's too small to host water itself. But Bennu was probably once part of a latger asteroid, and the liquid water would have been present on this" parent" body.

"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 OSIRlS-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."

The mission scientists were encouraged to learn that they were also accurate in predicting and modelling Bennu's actual shape, including diameter, rotation rate and inclination, almost exactly.

But now that OSIRIS-REx is up close and personal with Bennu, it has found the asteroid's surface more rocky than expected, with more boulders. This means the mission scientists will need to make more observations at closer range to ensure from where a sample can be taken from the surface in 2020.

OSIRIS-REx will continue to survey the asteroid by flying by its north pole, equator and south pole, sometimes as close as 4.4 miles away, to determine mass.