The near-Earth asteroid, Bennu, has thrown NASA's OSIRIS-REx mission a curve ball as its rough surface littered with boulders had not been anticipated, making the collection of samples to analyse much more complicated than originally planned.

Bennu is a diamond-shaped asteroid that is about 500 metres wide and may offer answers to long-standing mysteries about the origins of our solar system and the sources of water and organic molecules on Earth.

Professor Trevor Ireland, from ANU, will analyse samples of the asteroid once they are returned to Earth in 2023.

"From Earth-based observations, the team expected a generally smooth surface with a few large boulders and so the OSIRIS-REx spacecraft was designed accordingly," said Professor Ireland from the ANU Research School of Earth Sciences.

"A big question is whether the roughness and high number of boulders corresponds to the age of the surface. Hopefully with samples back in the lab, we can determine the ages of these surfaces through solar wind and cosmic ray exposure.

"What we don't know is what underlies the surface. Are we looking at material redeposited through rotation and impacts from other moving objects? Is there a solid body underneath or is it like another near-Earth asteroid Itokawa - just a rubble pile?"

Professor Ireland said the higher-than-expected density of boulders meant the mission's plans for sample collection needed to be adjusted.

"The original mission design was based on a sample site that is hazard-free, with a 50-metre diameter," he said.

"Now the team is identifying candidate sites that are much smaller in diameter where the OSIRIS-REx spacecraft can take samples."

Professor Ireland said the smaller sample site and high number of boulders will mean the spacecraft will have much less margin for error during its descent to the surface than originally planned.

"Bennu's rough surface has thrown up a big challenge to the OSIRIS-REx spacecraft, but the team is confident that it is up to the task," he said.

Professor Ireland is a co-author as part of the OSIRIS-REx team on a paper about Bennu's surface published in Nature: https://www.nature.com/articles/s41586-019-1033-6

More information about NASA's Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) mission: https://www.nasa.gov/osiris-rex