The Japan Aerospace Exploration Agency (JAXA) has delayed the touchdown of the main section of the Hayabusa2 probe on the asteroid Ryugu from this month until January 2019, Nature reported, after mission scientists determined that the Ryugu’s surface is rougher than expected and the landing needs more planning time.
While Hayabusa2 carries a number of smaller rovers, some of which have already been deployed to Ryugu’s surface, the mothership of the craft needs to touch down on the asteroid’s surface to collect samples. According to Nature, JAXA scientists have determined that there are few sites on the asteroid clear of boulders, and the best one is very tiny:
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.
Hayabusa2 will collect samples via a variety of methods; the first run, which is the delayed descent in question, will be conducted by flying the craft to the surface to fire a tantalum bullet into Ryugu. After that, loose material will be collected by a sampling horn and the craft will fire its thrusters to escape the asteroid’s surface.
A final sample collection scheduled for 2019 will involve firing a kinetic impactor towards the asteroid from a distance and remotely detonating a shaped charge within. (According to Popular Science, the 10kg of explosives within will fire a copper disk into Ryugu in a manner similar to military anti-tank weapons). After spending about two weeks avoiding debris from the impactor, it will descend to the surface to collect samples from the impact site.
“Although the spacecraft can be controlled with a position error of 10 meters at an altitude down to 50 meters, there remains the question of whether this accuracy can be retained as the spacecraft descends to the surface,” JAXA told Nature in a statement.
However, JAXA mission manager Makoto Yoshikawa of the agency’s Institute of Space and Astronautical Science in Sagamihara told the journal that the delay is not expected to affect Hayabusa2's return date to Earth in 2020, so that’s good.
According to Nature, JAXA will conduct a rehearsal in October to verify whether the craft is capable of descending to the surface safely:
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.
One close approach by Hayabusa2, coming within 25m, was already conducted this week.
Hayabusa-2 got to within 35 m of Ryugu this morning before beginning its ascent.
— Jonathan McDowell (@planet4589) October 15, 2018
[TD1-R1-A] This rehearsal also successfully tested the altitude range measurement from the LRF.
・Lowest altitude arrival time: October 15 at 22:44 JST.
・Lowest altitude: 22.3 m (altitude calculated from the LRF range measurement)
The current height of the spacecraft is 1.8km.
JAXA made the call to delay the landings last week, with project manager Yuichi Tsuda telling Agence France-Press, “The mission... is to land without hitting rocks.” This would be “most difficult,” he added, because “We had expected the surface would be smooth... but it seems there’s no flat area.”
In late September, Hayabusa2's MINERVA-II1A and MINERVA-II1B rovers (which use torque generated by internal motors to hop rather than rely on wheels) made a successful landing on Ryugu and began surveying its surface. They soon began transmitting back fascinating shots of the meteor’s surface.
Ten days later, Hayabusa2 successfully deployed its Mobile Asteroid Surface Scout (MASCOT) observation robot, which contains a camera, spectrometer, magnetometer and radiometer, though its non-rechargeable lithium-ion battery ran out of juice as planned a little over 17 hours after touchdown.