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Japan’s Hitomi spacecraft measured the movements of a galaxy cluster just before it died


Earlier this year, Japan’s space agency suffered a major blow when its new X-ray astronomy satellite, known as Hitomi, broke apart in space just one month after the vehicle had launched. But before the spacecraft met its premature demise, it was able to squeeze in a bit of valuable science. By observing X-ray emissions coming from a cluster of galaxies 250 million light years away, Hitomi measured just how fast interstellar gases moved between the galaxies within the cluster.

The gas is not as turbulent as the researchers expected

The measurements defied some scientists’ expectations: the gas is moving at about 102 miles (164 kilometers) per second, according to a new study published today in Nature. That’s not as turbulent as the researchers expected, since galaxies within the cluster are moving much faster, at thousands of miles per second.

It’s an interesting finding, but perhaps the most valuable aspect of this study is that it hints at all the science that the spacecraft could have done. Hitomi’s loss was painful for the astronomy community, as it was supposed to tell us more about the structure of the Universe — by observing turbulent galaxy clusters and high-energy black holes. "It was very exciting to get the science, but it’s devastating to lose the spacecraft," study author Andrew Fabian, director of the Institute of Astronomy at the University of Cambridge, told The Verge. "It feels like a door was opened and we can see through it and then immediately the door is slammed shut."

But today’s study could serve as a good advertisement for sending up another Hitomi-like spacecraft to fill the void the satellite left behind.

The death of Hitomi

Hitomi, which translates to "pupil of the eye," launched from the Tanegashima Space Center in Japan on February 17th. The satellite was transported into orbit by one of JAXA’s H-IIA rockets, but the spacecraft wasn’t functional for very long.

The problem started with a glitch

The problem started with a glitch. Hitomi had two ways to determine its position in space: a star tracker, which measures the positions of stars, and a sensor that measures the spacecraft’s inertia. For some reason, the latter sensor, known as the inertial reference unit (IRU), detected that Hitomi had started to roll slowly, even though that wasn’t the case. It wasn’t rotating at all. Normally if the star tracker and IRU data don’t match up, the spacecraft is supposed to default to whatever the star tracker is saying. But at this point, the star tracker had been turned off and wasn’t capturing any data.

An artistic rendering of the Hitomi spacecraft. (JAXA)

Based on the IRU data, Hitomi tried to counteract this non-existent spin by rotating in the opposite direction. That actually initiated a spin, which wasn’t supposed to occur. As the rotation got worse, the satellite tried to go into a type of safety mode, where it uses rocket thrusters to stabilize itself and point its solar panels at the Sun. How those thrusters fire depends on the position of the spacecraft’s center of gravity. But one of Hitomi’s instruments had been extended, changing the overall shape of the spacecraft and moving its center of gravity. The flight controllers hadn’t updated the spacecraft’s knowledge of this new shape, so the thrusters fired in the wrong direction. This spun the spacecraft even faster.

"It resulted in a positive feedback situation where it kept on getting worse and the rocket motors kept firing until essentially the whole spacecraft was spinning about once every five seconds," said Fabian. Eventually that caused the satellite to break apart; its solar panels ripped away, cutting off Hitomi’s source of power. On March 27th, JAXA announced that it had lost communication and wasn’t able to get it back. "What a tragedy that was," said Jonathan McDowell, an astrophysicist working at the Harvard-Smithsonian Center for Astrophysics, who was not involved in the study. "And really, really unfortunate, and very much human error I’m afraid. They had inadequate planning for likely failure modes."

Observing Perseus

Prior to the failure, Hitomi had been looking at X-ray emissions from the Perseus cluster — specifically those coming from the hot gases that lie between the cluster’s galaxies. Most of the gas in the Universe is found between galaxies, but the gases in clusters are particularly hot. Clusters are thought to contain a lot of dark matter — a mysterious type of material that doesn’t reflect or emit light but has a gravitational pull. "[The dark matter] causes compression of the gas between the galaxies and heating of that gas so it ends up at a temperature of 50 million degrees Kelvin," said Fabian. The gas in clusters is also particularly dense, leading to constant collisions that emit X-rays.

That movement was thought to be particularly messy. At the center of galaxy clusters is a supermassive black hole, which releases a ton of energy that interacts with the gases in the cluster. This interaction, known as black hole feedback, helps keep the gases hot by churning them around. Originally, it was thought to be a very turbulent process, but today’s study shows that chaos to be a little less chaotic than expected. "The fact that we see a relatively quiet or low velocity is telling us something about how violent this feedback process is," said Fabian.

The part of the Perseus cluster viewed by Hitomi. (Hitomi Collaboration/JAXA, NASA, ESA, SRON, CSA)

Today’s study also shows the incredible amount of precision of Hitomi’s Soft X-ray Spectrometer, which measured the X-ray emissions. The researchers argue that their measurements of gas speed could vary by about 6 miles (10 kilometers) per second. That makes their measurements pretty spot on. "It demonstrates the potential of this kind of instrument," said Fabian. "It really can perform in space and make these precision measurements."

Fabian hopes that such precise measurements will inspire scientists to create another Hitomi with a Soft X-ray Spectrometer sooner rather than later. The European Space Agency is planning on launching a much larger spectrometer in 2028 with its ATHENA spacecraft. And other satellites, like the Chandra and XMM-Newton telescopes, can do high-resolution X-ray mapping, but they can’t get the same kind of precise measurements of galaxy clusters that Hitomi could capture.

"There’s been no instrument that has a precision within a factor of 20 to 50 close to that of that Hitomi," said Fabian.

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