On February 17th, a new X-ray satellite observatory launched from Japan’s Tanegashima Space Centre. Named "Hitomi" meaning "Eye pupil" in Japanese, the observatory was set to explore the nature of super-massive black holes and even the origin of the mysterious dark matter.
Five weeks after launch everything was going smoothly. Hitomi was making trial observations and transitioning from the highly supervised commissioning mode to regular operation. Just after 3:00 AM JST on Saturday March 26th, Hitomi performed its first maneuver without continual monitoring from Earth. Instead, scientists at the Japanese Aerospace Exploration Agency (JAXA) waited for Hitomi to re-establish radio contact as its orbit swept over Japan’s ground stations in Spain and Australia. The satellite was scheduled to check in with the Australia station at 4:40 PM. But the call never came.
Telescopes on the ground scanned the skies for Hitomi, and finally spotted debris close to the observatory’s expected location—not a good sign. Still, JAXA engineers kept trying to reach Hitomi over the next few weeks. But in the end, they were forced to admit defeat. Just two months after its launch, the $273 million satellite was declared lost.
What followed was a month-long investigation by JAXA, culminating in a 103- page report released early this month. The verdict: Hitomi wasn’t hit by a stray meteor or a piece of space junk. Instead, the document detailed a fatal cascade of problems in scrupulous detail. The satellite was doomed by short-cuts and human error.
Hitomi’s problems began after the fateful maneuver on March 26th that swung the telescope from looking at the Crab Nebula to observing the nucleus of a galaxy thought to harbor a supermassive black hole. Directly after the move, the satellite’s gyroscope-based attitude sensor erroneously believed that the spacecraft was still rotating. Hitomi’s designers had anticipated that this would happen, so they’d added a second attitude monitor that oriented the observatory by tracking the positions of known bright stars.
Here was where the errors began to creep in. The star tracker was set to notice only stars above a certain minimum brightness—but the threshold was set too high, which meant it couldn’t find enough stars to do a proper orientation. By the time enough bright stars appeared in its detector, the star tracker and the gyroscope were reporting major discrepancies in which way they thought Hitomi was pointing. In cases like this, Hitomi was programmed to issue a warning to ground controllers, but reject the star tracker data in favor of the gyroscope.
Unfortunately, since Hitomi had just graduated from commissioning mode, controllers had stopped watching so carefully, and the warning flag was overlooked. Accepting the gyroscope’s conclusion that Hitomi was spinning, the observatory’s electric reaction wheels kicked in, attempting to counter the rotation. But since it wasn’t spinning in the first place, this put the satellite into an actual spin, which got faster and faster. At this point, Hitomi went into safe mode and awaited instructions from Earth.
Hitomi still could have been saved by onboard rocket thrusters that were capable of realigning the satellite—and now came a second set of errors. When controllers uploaded instructions to the thrusters, they turned out to be incorrect. This could have been caught if the instructions had been tested on a computer simulation of Hitomi, but no checks were in place to confirm this had been done. The thrusters fired, making Hitomi spin even faster—and ultimately beyond its breakup point. The observatory disintegrated.
The loss of a major mission is not without political consequences. Three JAXA executives have chosen to take a salary cut for four months as a direct result of Hitomi’s failure, and there may be a further backlash in the form of reduced funds for future missions. But by publicly dissecting the problems, JAXA hopes to correct issues within their own program and prevent this happening to any other mission.
Hitomi was part of an international collaboration that included NASA and ESA, and neither of those agencies is a stranger to lost missions. Space is unforgiving and any mistake can be fatal. The best we can do is share these errors to minimize the chance of them happening again.