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Salute to MINERVA

Dr. Tetsuo Yoshimitsu of ISAS, JAXA, wrote a story of MINERVA on JAXA/ISAS Mail Magazine issued on the 22nd. MINERVA (MIcro/Nano Experimental Robot Vehicle for Asteroid) is a coffee-can size robot designed to hop around the surface of Itokawa. Unfortunately it was released from Hayabusa (formally known as MUSES-C) on the 12th at the wrong timing, when Hayabusa turned into ascent mode, and it never reached to Itokawa. Yoshimitsu is a "manager" of his one-man operation team of MINERVA.

Eight years have passed since he joined the MUSES-C project. He was a doctoral graduate student in 1997. He got a position at ISAS in 2000 and started the development of MINERVA in full time. It's been two and a half year since the probe is launched.

Originally NASA planned to develop a 2.2 lb (1kg) more advanced rover for MUSES-C. The project had to be cancelled five years ago because of budgetary issues. Soon ISAS decided to develop an alternative rover of 600g. The total budget for MINERVA is not disclosed but according to Matsuura's guesstimate it should be much less than 1 oku-yen (roughly 1 million USD).

Unlike other sophisticated rovers, MINERVA used as much commercially available electronics products as possible. The CCD cameras on board were the ones you can buy at Akihabara shops, not the expensive radiation-proof ones. It was designed to "hop" around with a group of small pins hitting around the ground counter torque by quickly rotating the balance weight inside the rover, to exploit the extremely low gravity of Itokawa (escaping speed from the asteroid is just about 8 15cm per second).

In the morning of 12th, Yoshimitsu was sitting alone in front of a personal computer in an operation room across a corridor from the main control room of Hayabusa. It was going to be a long day. Hayabusa had started its descent towards Itokawa on the day before, and approaching gently to 70 m from Itokawa, the scheduled altitude to release MINERVA.

At 8:15 he sent a command to turn on the rover. The distance between the Earth and the probe was 290 million kilometers. It would take 16 minutes for the command to reach. 32 minutes later he confirmed the command was received. All conditions were GO. He was not so busy for the rest of 8 hours because the rover was designed to work autonomously. Another two scientists were on duty to survey the scientific data sent from the rover.

At 14:30 the communication via Usuda station ended, and switched over to the NASA DSN station in Spain. Original plan was to release MINERVA by a command sent from Usuda, but the descent maneuver timeline was slightly lagged behind.

At 15:07:38 the command was sent to release MINERVA. At 15:40 he confirmed the signal from the spacecraft that the rover was indeed deployed. He rushed off to the control room to tell the status to the Hayabusa team.

People showed perplexed feelings. Unfortunately at the very timing Hayabusa was turned into its autonomous ascent mode at the altitude of 200m, the velocity 15cm per second. It was too fast for the gravity of Itokawa to capture the rover.

There were some estimates that the solar wind may eventually push back MINERVA near Itokawa, close enough for eventual "landing". Calculations show it might take several days or weeks for the rover to come back near Itokawa.

The radio contact between the rover and Hayabusa lasted until 09:32:20 JST, November 13. He speculates the contact was lost because the rover went too far away from Hayabusa. MIRNERVA had appeared healthy just before the loss of the contact, and he believes the rover is still alive.

Although MINERVA did not achieve its objectives, it did capture a part of Hayabusa image 212 seconds after its release. It was the first picture of the spacecraft ever taken in deep space, from outside of the spacecraft.

"Also it achieved various engineering milestones," says Yoshimitsu. "the rover had to survive extremely low temperature, minus 65 deg C. We have demonstrated the dual layer electrolytic capacitor(*) to work under extreme conditions. We have also demonstrated the software for the autonomous operation, ultra-light release mechanism, and functionality of the rover as a very small satellite. We have accumulated the temperature readings for 18 hours after the release, and this may reveal yet unknown scientific aspects of the asteroid."

(*) exact English term is unknown to the author. See the references below.

Yoshimitsu keeps the communication relay circuits onboard Hayabusa alive. He will continue to look for the signals from MINERVA until Hayabusa leaves the asteroid to come back to Earth in early December.


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