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The satellite launched in February, but in March, it malfunctioned, spun out of control, and broke into pieces. The mission was declared a failure. But! Before it failed, Hitomi managed a couple days’ worth of observations.

Hosted by: Hank Green
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Hank: One of the studies published this week in the journal Nature is kind of... unusual. It’s an analysis of some satellite data of a far-off galaxy cluster -- which might not seem all that weird at first. But these are the only observations that the Japanese satellite, called Hitomi [he-TOE-me], managed to make.

The satellite launched in February, but in March, it malfunctioned, spun out of control, and broke into pieces. The mission was declared a failure. But! Before it failed, Hitomi managed a couple days’ worth of observations. The results are helping astronomers understand the history of the universe a little better... but they’re also a reminder of just how much we could have learned.

Down here on Earth, we use X-rays for all kinds of things, from checking for broken bones to studying the structures of tiny molecules. But X-rays are also very important to astronomers. They take a lot of energy to make, so they’re emitted by gases that are really, really hot -- like, tens or hundreds of millions of degrees.

Long-term exposure to X-rays can be dangerous, since they’re energetic enough to damage DNA and cause mutations that can lead to cancer. Luckily for us, Earth’s atmosphere blocks just about all of the X-rays that the universe throws at us. But that makes X-ray astronomy much harder -- when astronomers want to scan for X-rays, they have to send something out of the atmosphere and into space.

Right now, NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton are two satellites in Earth orbit that scan for X-rays. But they mostly look at smaller objects, like really hot stars and black holes.

This is where Hitomi was supposed to come in. The satellite was meant to be the future of X-ray astronomy. The plan was for it to spend three years probing the centers of galaxies and galaxy clusters, which would help astronomers learn how matter has moved around and between galaxies over time. It was specially designed to look at some of the bigger things in the universe like clusters of galaxies.

One of Hitomi’s main goals was to map out the hot, diffuse gas between galaxies in a cluster, which is hard for astronomers to probe in any other way. And for three days in March, that’s precisely what it did. Hitomi’s first task was to measure the X-rays coming from the Perseus cluster of galaxies, centered roughly 250 million light-years from Earth.

Then, scientists looked at Doppler shifts in the X-rays -- the changes in the light’s frequency caused by the gas either moving toward or away from Earth. They found that the Doppler shift was small, meaning that the gas was moving relatively slowly, and that it also wasn’t moving along with the nearby galaxies like astronomers had expected it to. With more observations like this, astronomers might have been able to piece together what’s causing these unexpected movements, and they could have found out whether this is typical of galaxy clusters in general.

And if it is typical, that would’ve taught scientists more about how much dark matter and dark energy there is in the universe, and how that dark matter and energy affected the formation of galaxies and galaxy clusters. But there was no time to study anything else before the satellite failed. Eventually, the mission team figured out what happened: Hitomi used a bunch of different systems to keep itself oriented in space: there were gyroscopes, star-tracking systems, reaction wheels, and thrusters.

But first the star tracker failed, so Hitomi didn’t know where it was pointing. That wouldn’t be such a big deal on its own, but then the gyroscopes started telling the satellite that it was rotating much faster than it actually was -- and in the wrong direction. Using the reaction wheels and thrusters, Hitomi tried to fix the rotation.

But because the readings were backwards, it was putting its foot on the gas instead of hitting the brakes. It kept trying to slow down, but that only made things worse. Eventually, the satellite was spinning so quickly that as many as ten pieces broke off of the main ship.

This wasn’t the first Japanese X-ray satellite to fail, either. There was ASTRO-E, which crashed just after takeoff in 2000. Five years later, there was Suzaku [SOO-zah-koo], which got into orbit but started leaking the liquid helium that it used to keep its X-ray sensors cold -- though the rest of its instruments kept working until the mission ended in 2015.

There was no way to keep Hitomi working, though. Scientists pretty much have to start from scratch, making sure that what happened in March can’t happen next time. Meanwhile, we’ll have to wait for future missions -- like the ESA’s Athena X-ray Observatory, set to launch in 2028 -- to get our answers about the history of the universe.

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