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Asteroid explorer Hayabusa2’s “tail-up”: The Attitude and Orbit Control Subsystem (AOCS) in action

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Hayabusa2 in the tail-up posture
From a computer-generated video of Hayabusa2’s first touchdown (Feb. 22, 2019)

Have you ever wondered how the asteroid explorer Hayabusa2 controls its orientation in space? Think of Hayabusa2 in terms of human anatomy. Its sensors serve as eyes, determining its orientation and gauging how far it is from the asteroid Ryugu. That information is relayed to the onboard computer, which corresponds to the brain. The computer then orders the thrusters, wheels, and other propulsion systems, which function as muscles, to make the necessary maneuvers to keep the craft steady. The set of equipment needed to maintain the spacecraft’s orientation is referred to as the Attitude and Orbit Control Subsystem (AOCS). There are nine different types of sensors alone, all working in sync.
Hayabusa2 spent most of its stay at Ryugu in “home position” 20 km above the asteroid’s surface. Ryugu’s gravitational pull is so weak — only 1/80,000th that of Earth — that a spacecraft can easily hover above it. But gravity, however weak, is still gravity, and if Hayabusa2 had done nothing, it would have been inexorably pulled toward the asteroid. Moreover, in outer space, where there was nothing around Hayabusa2 to support it, the sun’s rays could also have had a disruptive effect on its orientation. The AOCS system therefore constantly adjusted Hayabusa2’s orientation by the sun and stars.

Lateral view of Hayabusa2 in the tail-up posture
Hayabusa2 was tilted slightly to match the slight slope of the asteroid’s surface and the elevation of the rocks, so that it would be less likely to hit a boulder.
Image courtesy of JAXA

The engineers at JAXA and NEC had, since before Hayabusa2’s launch, worked together on planning the protocols for this set of maneuvers, and they drilled them over and over. Ten different modes were developed, including “maintaining home position,” for keeping Hayabusa2 in place above the asteroid’s surface, “global mapping,” for conducting observations of the asteroid, and “pinpoint touchdown,” for landing the craft precisely at the designated point.
The most challenging of these maneuvers was the “pinpoint touchdown.” This involved approaching Ryugu’s surface with thrusters firing as the onboard cameras tracked the target marker indicating the landing site; tilting the craft slightly; and then touching down and immediately taking off again. Each of these steps had to be performed with the greatest precision. Ryugu’s surface is covered in boulders, which could have damaged the spacecraft. The engineers therefore decided to make the descent with Hayabusa2 parallel to Ryugu’s surface and then reposition the spacecraft by slightly elevating the end where the ion engines were. In terms of animal anatomy, it was like alighting on the ground with the tail raised; hence it was dubbed the “tail-up” posture. That was certainly an elegant way to avoid the rocks and keep Hayabusa2 safe.
To ensure the mission’s success, the engineering team thought of every possible contingency. First, prior to launch, they drew up plans for controlling the spacecraft’s orientation; then they added more detailed safety protocols once Hayabusa2 reached the asteroid. The two pinpoint touchdowns, the most challenging part of the Hayabusa2 mission, were both successful because the AOCS system worked exactly as the AOCS team had planned in positioning the craft — right down to the perfectly executed tail-up.

Article by Ayano Akiyama
Published November 4, 2020

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