Just outside Hiroya Yamaguchi’s office is a blackboard filled with exploded stars, schematics of spaceships, and spectral lines. The A4 printouts cover almost all the free space, except for a tiny corner where he sometimes scribbles with white chalk. Right now, Yamaguchi, an associate professor at the Japan Institute of Space and Astronautical Sciences, is standing in front of this blackboard facing me.
He’s giving me a crash course on the X-ray Imaging and Spectroscopy Mission, or XRISM, a partnership between NASA, the Japan Aerospace Exploration Agency (JAXA), and the European Space Agency (ESA). The first thing I learn is that I’ve been saying the name of the telescope wrong this whole time. Fortunately, I was mostly repeating the incorrect “ex-ris-um” in my head. It’s actually pronounced “crisis-um”.
The second is that this space telescope was launched on September 6, 2023, carrying the biggest burden of all: Expectation.
Connected: JAXA, NASA reveal first images from XRISM X-ray Space Telescope
JAXA’s two previous X-ray telescopes, Suzaku and Hitomi, had problems after launch. Suzaku’s spectrometer malfunctioned after launch, but managed to complete a decade-long imaging mission. Hitomi was disastrous: after making its first light image, the spacecraft went into an uncontrolled spin and disintegrated. So far, XRISM is performing well, Yamaguchi says, and has already provided scientists with a wealth of data since the first glow in January — including some discoveries no one expected to find.
“There are so many surprises,” laughs Yamaguchi, looking at the various printouts taped to the blackboard.
However, there is a small problem.
First, the good news: The telescope’s main instrument, a soft X-ray spectrometer known as Resolve, is working as expected. The slightly worse news: the hatch door covering the Resolve didn’t open. Multiple attempts to open the door – or “valve” – are unsuccessful. Despite reports suggesting JAXA and NASA are decided to “operate the spacecraft as is for at least 18 months”Yamaguchi told me it “hasn’t been officially decided.”
A NASA spokesperson confirmed that “NASA and JAXA continue to have ongoing discussions about the best way forward for XRISM operations; The current, leading option is to gather scientific data for the next 18 months before making another attempt to open the valve, but the agencies will continue to evaluate alternatives.”
Behind closed doors, an intriguing “What if?” the situation for mission specialists and X-ray astronomers is presented. On the one hand, the spacecraft is performing superbly and showing that it is capable of delivering a wealth of new, exciting data. Trying to open the door risks damaging the spacecraft. On the other hand, opening the door could fundamentally change our understanding of the universe.
Decide on “X”
X-rays provide a way to study some of the most energetic phenomena in the universe, but because the Earth’s atmosphere blocks X-rays, space telescopes are a prerequisite.
“We are revealing the composition of the universe,” Aurora Simionescu, an astrophysicist at the Netherlands Institute for Space Research, tells me. “That’s what X-rays do.”
There are currently more than a dozen X-ray telescopes in space, such as NASA’s Chandra Observatory, one of the so-called Large Observatories, perhaps best known for amazing views he has provided the x-ray universe. XRISM, with its ability to see the most detailed X-ray spectra yet, hopes to create a similar legacy. However, Yamaguchi points out that although Chandra and XRISM observe the same part of the electromagnetic spectrum, they must do so in different ways. It comes down to the tools on board.
Resolve is what is known as a microcalorimeter spectrometer. The detector converts the X-rays into heat, measuring tiny changes in temperature — we’re talking millikelvin changes — to determine the number and energy of observed X-rays coming from a particular region of space. Energy is measured in electron volts (eV).
Therefore, the instrument only needs to be cooled to a few degrees above absolute zero. This is colder than even the cosmic microwave background, which is residual radiation from the beginning of time. This radiation is still scattered throughout our universe hidden from the human eye because of how absolutely cool it is. “You’re basically almost 30 times colder than the coldest part of space,” says Simionescu. The extreme cooling effect is achieved by chemical and mechanical means.
Chandra uses a different style of X-ray detector that includes an array of charge-coupled devices, or CCDs. This converts the X-ray photons into electrons rather than heat.
Measuring energy is particularly useful because you can plot the number of X-rays that reach your telescope against their energy level—creating what researchers call a “spectrum.” XRISM’s Resolve takes precedence in this case. It can measure energies about 20 to 30 times higher than Chandra and with greater resolution. “This allows XRISM to study much more detail about the atomic physics and velocity structure of X-ray sources,” says Patrick Slane, director of the Chandra X-ray Center.
However, Chandra has its advantages. It’s also constructed with the highest-quality X-ray mirrors ever built, Slane says, meaning its image quality far exceeds that of XRISM. The key here is that the mirrors give Chandra an angular resolution of 0.5 arcseconds, essentially allowing Chandra to distinguish between objects in the sky that are close together. Compare this to XRISM, which has an angular resolution of 1.7 arcsecminutes.
Thanks to this engineering feat, Slane says Chandra can pick up X-ray point sources about 200 times easier than XRISM. In practice, this makes NASA’s telescope extremely useful for focusing on these point sources – distant, smaller targets such as neutron stars, planets and comets. XRISM is good for “extended” targets, such as the diffuse gas between and within galaxies.
Which finally brings us to the XRISM valve: The closed gate effectively blocks low-energy X-rays from reaching the detector. For now, the telescope continues to probe the high-energy X-ray universe because those wavelengths are not affected by the gate dilemma—in fact, both Yamaguchi and Simionescu say it already produces fantastic results at higher energies.
But if the door is blocked forever, scientists will have to contend with parts of our cosmos remaining inaccessible… at least until another X-ray telescope comes along, likely to be the Athena mission in the mid-2030s.
XRISMgate-gate
The valve was designed to maintain a near-vacuum inside the telescope’s cryostat — essentially the refrigerator that ensures its instruments stay extremely cold — while XRISM was stationed on Earth.
Once the telescope reaches orbit, maintaining this kind of vacuum is not a problem. In space, space itself creates the vacuum. For this reason, the damper is designed to open in a two-step process after start-up via a set of actuators. In short, the actuators would slide back to allow the door – made of a beryllium window and steel mesh – to open. That didn’t happen.
JAXA tried to perfect the device, opened on three different occasions, but it wouldn’t budge. The next attempt would be much riskier, potentially requiring the spacecraft to warm up from its extremely cold temperatures and be shaken. The goal? Displacing actuators with force. This is a risk that the space agencies operating XRISM will have to weigh. With the valve closed, they are now bank data. And that’s very good data.
“The most beautiful thing is when you look at the data and it doesn’t look like what you expected – and that’s what’s happening with the current XRISM data,” says Simionescu.
Still, it’s a tough break for Simionescu. She is particularly interested in studying X-rays from “galactic atmospheres”—the stuff XRISM was designed to look at with the valve open. With the gate closed, this part of the X-ray universe remains locked. She is completely on board with the decision not to risk trying to open the gate – at least for now. But that doesn’t mean it isn’t painful, knowing what it can be.
“I’m absolutely gutted that we can’t see below 2 keV,” says Simionescu.
And what might be hiding underneath?
Some X-ray space telescopes, such as ESA’s XMM-Newton, can see lower-energy X-rays, down to below 2 keV. For example, he observed the Coma cluster, which contains over 1000 galaxies, at energies as low as 0.3 keV. And XRISM’s other instrument, Xtend, can also reach lower energies. But these are also CCD detectors and are not as useful for obtaining spectra.
Outside of XRISM, there is no X-ray telescope orbiting Earth capable of looking through “extended” objects at low energies with high resolution, which is particularly important for Simionescu’s work.
During an online chat, she shares a wide-field X-ray image of M87, the first black hole imaged by humans in visible light. The image was snapped Chandra in 2019
“It’s my favorite subject in the world,” she says excitedly.
The space around this black hole is a vortex. Simionescu’s cursor bounces across the sky as she points out the large jet emanating from the black hole, as well as regions of dense gas and a long filament that stretches light-years into space. She plots the spectra observed by Chandra at M87 – all below 2 keV – and notes how it’s all a “mambo jumbo” of emission lines from oxygen, neon, nickel and other gases.
With the gate open, that would change.
“You can tell what the composition of the gas is, how it’s moving, how it’s being pushed out of the black hole — all of that is information you can’t get at the moment,” she says.
It is interesting to consider the leap forward with XRISM amid the uncertainty surrounding NASA’s Chandra mission.
Unfortunately, the field of X-ray astronomy may be without Chandra in the near future. The space telescope’s 25-year operations face extreme budget cuts in 2024. Astronomers say the proposed budget would cancel the mission.
“If Chandra is cancelled, we will lose a huge resource for all of modern astrophysics,” says Slane.
It would be an ignominious end for the Large Observatory, which remains invaluable for future discoveries, including working in tandem with XRISM. If JAXA shuts the door, Chandra will be an important follow-up to the XRISM observations.
Meanwhile, the ghosts of Suzaku and Hitomi will remain until the next attempt to open the door. For now, the field of X-ray astronomy is excited about what lies ahead. The worst case scenario isn’t that bad, depending on how you look at it.
“We’re taking fantastic data that no one has been able to take before,” says Simionescu. “All the spectra are absolutely spectacular.”
