It took only a few days of collecting X-rays from the distant Perseus galaxy cluster, 240 million light years away, for the Japanese Hitomi satellite observatory to conclude that the cluster contains proportions of elements that are nearly identical to our Sun, a new revelation that could tell us more about our own solar system.
The device responsible for collecting the data is the product of three decades of research and development by Goddard Space Flight Center (GSFC), in partnership with University of Wisconsin–Madison scientists led by astronomer Dan McCammon.
According to McCammon, in the 1980’s NASA was trying to develop new technology for satellites to use for measuring the energy of X-rays. The precise measurement of X-ray energies can reveal much about conditions around the black holes, neutron stars, and supernovae that are the brightest sources of X-rays in the sky.
“Harvey Moseley, an infrared astronomer at Goddard Space Flight Center suggested that the X-ray group there could detect X-ray photons thermally, and get much more accurate measurements of the photon energy,” McCammon says, referring to the previous method of measuring X-rays by converting their energy into an electric charge.
So their team set to work making the new detector, producing what is called an X-ray calorimeter. The device uses several tiny silicon detectors that absorb the energy of a single X-ray beam. To detect the miniscule temperature increase, the calorimeter is kept inside a high-tech refrigerator and cooled to incredibly low temperatures, near absolute zero.
The inner workings of an X-ray calorimeter. Source: JAXA
The science for creating these improved X-ray calorimeters was pioneered at Goddard Space Flight Center, with assistance from a team of scientists at UW–Madison led by McCammon, who spent time working at both the space agency and the university as they worked together, sharing research to improve the science of the microcalorimeters.
“Here at (the University of) Wisconsin it was me and probably five graduate students and some postdocs,” McCammon says. “There were probably half a dozen different grad students and three times that many undergraduates that worked on the project,” McCammon says. For eight years, McCammon and his students worked to achieve more and more accurate measurements from the detectors.
Henry Guckel, a UW engineering professor, guided McCammon and his team through the fabrication process while they completed the first round of the detectors, and in 1995 an array of 36 calorimeters, weighing one gram out of a 935-pound payload, took their first flight in New Mexico on a sounding rocket, a small rocket designed to test the space-bound instruments before sending them into orbit.
But that same year the project experienced its first hiccup. In a series of budgeting decisions, the United States Congress canceled NASA’s funding for the AXAF-S mission, which would have carried the microcalorimeter that the team in Wisconsin was assisting on.Â
Video produced from data collected by AXAF-I, a spin-off mission of the canceled AXAF-S mission, which was renamed Chandra once in flight. The video shows a Kelvin-Helmholtz wave disrupting the gas in the Perseus cluster. Source: NASA Goddard.
However, the Institute of Space and Aeronautical Science, the Japanese space exploration research agency that existed at the time, decided to include the calorimeter on their own satellite X-ray observatory mission called Astro-E. NASA agreed to continue funding for the instrument. McCammon continued to work for another five years with GSFC to improve the calorimeter before the project’s launch.
(ISAS, the National Space Development Agency, and the National Aerospace Laboratory or NAL, Japan’s other two space agencies that handled technology development and launching of vessels, merged in 2003 to form JAXA, the Japanese Aerospace Exploration Agency.)
Finally, in 2000 came Astro-E’s scheduled launch from Kagoshima Space Center in Japan. On the morning of Feb. 10, the satellite was installed onto its rockets, placed atop the launch table, and sent away into the atmosphere.
But it didn’t stay there for long. The first stage rocket failed, sending the satellite plummeting out of the sky and to the bottom of the Pacific Ocean, X-ray calorimeter and all.
Nevertheless, JAXA remained optimistic, and decided that they would try again, commissioning a near-identical satellite to be fabricated and launched in 2005. That project would include another X-ray calorimeter.
For another five years after the crash, the team at UW helped fine-tune and improve the replacement detectors, in partnership still with GSFC and ISAS.
When it came time for the launch of the Astro-E2 on July 10, 2005, the launch proceeded without a hitch, and the satellite reached orbit. Once in space, it was renamed Suzaku. But shortly into the mission, a misaligned helium vent caused the X-ray calorimeter to fail, erasing its capacity to take measurements. But the stroke of bad luck only affected the X-ray detector, one of three scientific instruments aboard. Suzaku failed about a year ago after working well for ten years.
After a long and careful evaluation process, JAXA and NASA agreed to produce an even more ambitious spacecraft with the X-ray calorimeter as the main feature. So McCammon spent the next eight years with the Goddard team recreating the detector for the next satellite observatory, Astro-H, traveling back and forth to Japan and Goddard working out kinks in the project.
Then on Feb. 17, 2016, the satellite Astro-H was successfully launched from Tanegashima Space Center in Japan, without incident, much to the team’s relief. While orbiting Earth, JAXA renamed the satellite Hitomi, Japanese for “the pupil of the eye.” After three weeks of successful orbit, the calorimeter was activated and began observing X-rays. (McCammon specifically forbade me from using the phrase “third time’s the charm”).
Diagram of the Hitomi satellite, composed by JAXA. Source: http://www.nature.com/news/software-error-doomed-japanese-hitomi-spacecraft-1.19835
But despite the successful launch and initial operation, only days after the X-ray calorimeter began recording data, the spacecraft began to spin. The onboard software told the satellite to fire its jets and stabilize itself. But because the wrong command was uploaded, the spacecraft fired its jets in the wrong direction, accelerating the spin and eventually causing the satellite to break apart. On March 26, JAXA lost contact with Hitomi, and on April 28 declared it lost.
Yet the data that the calorimeter collected in the short period of operation was so beautiful that work had already begun on a publication overnight, even before the specialists in Tokyo awoke the next morning to the news that Hitomi was in “emergency mode.”
Because the calorimeter performed so well, JAXA has already begun planning a replacement mission, and NASA has ordered an exact replica of the Hitomi’s calorimeter.
It took three decades of work and three tragic malfunctions to finally record the data, but McCammon hopes that the technology will “provide a revolutionary new capability for the study of black holes and neutron stars,” and that the information will help advance our understanding of the massive gas clouds wafting through the stars.
“Of course, this was only one cluster that Hitomi was able to observe,” McCammon says. “It was supposed to view 30 to 40.”