Possible Replacement for Hitomi

JAXA and NASA are looking at the possibility of replacing the dead Hitomi X-ray observatory, with a potential launch around 2020.

Hitomi in space

An artist's conception of Hitomi in space.
JAXA

The failed Hitomi X-ray observatory may get a second chance. Over the past month, reports have surfaced via Japanese news and space agency officials that the Japanese Space Agency (JAXA) is considering building a replacement mission.

The loss of Hitomi ("pupil of the eye" in Japanese) earlier this year came as a blow to the astrophysics community. Launched on February 17, 2016 from Tanegashima Space Center in Japan, Hitomi experienced an anomaly on March 26, 2016, causing it to fragment and tumble in low Earth orbit. Like many other satellite watchers worldwide, we caught sight of the hapless Hitomi as it flashed and tumbled through the skies over southern Spain in the weeks following the event.

Originally slated for a three-year mission, Hitomi only lasted just over a month. The loss is still under investigation, but the leading suspect is a conflict in automated commands that ordered the spacecraft to counteract a fictional spin reported by a faulty reaction wheel. As JAXA lost communication with the spacecraft, the spin became an out-of-control tumble that tore the satellite to pieces. JAXA eventually declared the mission unrecoverable.

Most of the silent Hitomi is in fact still tumbling in orbit. The U.S. Joint Space Operations Center (JSpOC) has yet to issue a reentry prediction for the spacecraft, which will probably burn up in Earth's atmosphere in the coming year.

Designed to study the X-ray universe in the 0.3 to 600 keV (kiloelectron volts) range, Hitomi managed to return a few brief observations before its demise that demonstrated the enormous potential of its instruments — especially its X-ray calorimeter, the NASA-built Soft X-ray Spectrometer (SXS). For a brief time after launch, this instrument recorded the energy of each incoming X-ray photon with unprecedented precision.

X-ray astronomers had long awaited spaceflight for such a calorimeter, which promises to reveal the swirling motions of hot gas near black holes and at the cores of galaxy clusters. Two earlier attempts to fly such an instrument also failed, and the loss of Hitomi makes three. Astronomers were looking at another decade — the launch of the European Space Agency's Athena in 2029 — before getting another chance to fly such an instrument. So the announcement of a potential replacement within a few years comes as welcome news.

“NASA has been invited by JAXA to participate in a Hitomi recovery mission,” Felicia Chou (NASA) confirms. “NASA is meeting with representatives from JAXA as part of ongoing discussions of a potential Hitomi successor.”

The Return of Hitomi

The Kyodo News agency mentioned in late June that a potential second mission would launch around 2020 on JAXA's new H3 rocket, assuming the successor to the H-IIA launch vehicle is available. In mid-July a statement from Japan's Minister of Science and Technology, Hiroshi Hase, said that any formal funding for a second Hitomi mission would be included in the 2017 JAXA budget.

During a July 20th presentation, NASA's Dr. Paul Hertz mentioned that NASA's projected budget for a possible successor to Hitomi would be in the $70-90 million dollar range. The replacement would most likely be a copy of the original, including instruments and the extendable optical bench.

SXS instrument

Scientists perform a final check of the SXS instrument for Hitomi before closing.
NASA/GSFC

A replacement mission would also provide a valuable technical and observational pathfinder for Athena mission. If JAXA and NASA approve Hitomi's replacement, then NASA will complete its SXS instrument for Hitomi's successor, before building the X-ray Integral Field Unit (XIFU) microcalorimeter for Athena.

"The opportunity to build, launch and operate a precursor instrument would yield invaluable technical lessons learned for Athena," says Chou. "In addition, the scientific results from a Hitomi successor calorimeter would provide a major opportunity to devise an optimum observing strategy for Athena, which maximizes the scientific output of the mission for the scientific community.”

Hitomi was an essential link between the NuSTAR, Chandra, and XXM-Newton space-based X-ray observatories currently in operation, and X-ray missions of the future. Unlike earlier observatories, the spacecraft had been designed to conduct simultaneous observations of astrophysical targets across the X-ray spectrum as well as gamma-ray wavelengths.

The loss of Hitomi was a serious setback, but the mission may yet rise from the ashes in the coming years, a testament to the tenacity of scientific research.

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