The mothership of the Japanese probe Hayabusa2 will make its first touchdown on the Ryugu asteroid in January, instead of this month as originally planned.

The Japan Aerospace Exploration Agency’s mission team has determined that the asteroid’s surface is rougher than expected and has decided to take more time to plan the landing, according to a statement JAXA published on 14 October.

This part of the mission — which will collect a sample of the asteroid to be brought back to Earth — is the most important, but also the riskiest. Hayabusa2, which also carries several smaller probes , launched in late 2014 and reached Ryugu in June of this year, aiming to return samples of the space rock’s surface to Earth by 2020.

Hayabusa2 is hovering over the space rock at varying altitudes, countering the gentle gravitational attraction with its ion thrusters, as the 1-kilometre-wide Ryugu rotates underneath it once every 7.5 hours. In late September and early October, the spacecraft descended to altitudes of a few tens of metres to deploy three small landers , which have sent back images and data from the surface.

For the sample-collection phase, the project team had been hoping to identify a region at least 100 metres wide that would be relatively boulder-free — meaning without rocks higher than 50 centimetres. Otherwise, higher boulders might strike the main body of the craft as it deploys its 1-metre arm to collect a sample, a JAXA statement says.

But detailed maps of the surface have shown that the best such area is only about 20 metres wide. The agency now wants to make sure that it can hit such a narrow target on the rotating surface.

JAXA says they will be performing a touchdown rehearsal from 14-16 October, lowering the craft to an altitude of about 25 metres — the lowest to date — to test the probe’s altitude measurements at short distances.

“Although the spacecraft can be controlled with a position error of 10 m at an altitude down to 50 m, there remains the question of whether this accuracy can be retained as the spacecraft descends to the surface,” the JAXA statement says. This is what the mission team will try to establish in the next months.

The postponement will not affect the schedule for the sample return, which is still planned for 2020, mission manager Makoto Yoshikawa at JAXA’s Institute of Space and Astronautical Science in Sagamihara told Nature .