The coldest instrument ever to fly into space has unexpectedly lost its liquid helium coolant ā rendering it useless. The mishap occurred just a month after it was launched on a Japanese X-ray satellite on 10 July 2005.
The instrument, called the X-ray Spectrometer (XRS), was one of three X-ray detectors to launch aboard Japanās Astro-E2 mission. The mission has since been renamed Suzaku.
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XRS was to operate at temperatures of just 0.06 kelvin (0.06Ā°C above absolute zero) in order to detect the heat deposited by an X-ray photon on its detector. But on 8 August, mission operators realised that the liquid helium coolant it used to maintain this temperature had all evaporated.
āItās a major loss,ā says Richard Mushotzky, an XRS team member at NASAās Goddard Space Flight Center in Greenbelt, Maryland, US. āPsychologically, itās even worse. Weāve been working on this for 20 years.ā
Ill-fated instrument
Scientists at Goddard first designed the XRS in the early 1980s and have used it for years in atomic physics experiments on Earth. But the instrument has had no luck operating in space, where it was designed to be used.
Budget problems prevented it from being launched as originally planned on NASAās Chandra X-ray Observatory in 1999. Then, the XRS was put aboard the Japanese Astro-E satellite in 2000. But the first stage of its M5 rocket failed before the spacecraft could reach orbit. Now, an unexplained problem has caused the instrumentās crucial liquid helium to disappear.
Most X-ray detectors measure X-ray photons by converting their signal to electricity, but the XRS directly measures their heat. It uses several insulating layers to keep it colder than space, which is 2.7 K (2.7Ā°C above absolute zero).
Liquid helium at 1.3 K surrounds a refrigerator that uses magnetic fields to keep the XRS super cold. A layer of solid neon at 15 K surrounds the helium, followed by many layers of insulation.
Hard X-rays
The XRS would have been the āpremier instrumentā for measuring the velocity and temperature of hot gas in space, allowing astronomers to study āthe dynamics of how things move aroundā, Mushotzky told Ā鶹“«Ć½.
It would have offered 10 times the resolution of Chandraās grating spectrometers and five times their sensitivity at high energies. āIt would also have allowed us to look at objects we hadnāt looked at before,ā says Mushotzky. That is because Chandra can measure high-resolution X-ray spectra of point-like sources but not extended sources like galaxies and clusters of galaxies.
But, he says, the mission āis not a complete lossā. Suzakuās two other instruments should also study high-energy phenomena such as supernova remnants, and black holes that are gobbling up material around them. One of those instruments is also the first of its kind to fly.
Called the Hard X-ray Detector (HXD), it will study high-energy X-ray photons such as those produced when material is accelerated to 10,000 kilometres per second in supernova shock waves. It is designed to be several times more sensitive than the detectors on Europeās Integral spacecraft. āWeāre optimistic that we can still carry out a very interesting science programme ā but not as revolutionary as before,ā says Mushotzky.
āItās very disappointing,ā he says. āWe believe the XRS is the wave of the future for X-ray astronomers.ā No suitable spacecraft are scheduled to launch within the next decade, meaning it will be quite some time before the XRS has another opportunity to fly.
