Roughly every two weeks a piece of space hardware plunges to Earth. Last weekend it was ROSAT, the German-US-UK x-ray astronomy mission. Launched in 1990, ROSAT collected data through late 1998, until problems with the attitude control system developed, and the satellite tipped away from the Sun, lost power, and went quiet.
In low-Earth orbit, atmospheric drag slows objects. Without a power source to keep objects at speed, they will inevitably fall to Earth. ROSAT's impending crash landing was awaited with some suspense because of the possibility that large, heavy chunks of the satellite might make it through the atmosphere and inflict damage upon impact. People were worried about a 1.7-ton mirror made of zero-expansion glass, says Günther Hasinger, a member of the ROSAT team at its home institution, the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, and now director of the Institute for Astronomy at the University of Hawaii. In the end, on 23 October ROSAT reentered the atmosphere over the Indian Ocean's Bay of Bengal.
In its first six months, ROSAT collected the first—and so far best—all-sky imaging survey at soft x-ray wavelengths; the satellite's x-ray and extreme UV instruments spanned the spectral range 0.06–2.4 keV. The next eight years were devoted to pointed observations. ROSAT counts among its findings more than 150 000 x-ray and 479 UV sources. Observations with ROSAT have resulted in some 4300 papers in refereed journals.
One surprise discovery was that comets emit x rays. "Nobody expected x-ray emissions from dirty snowballs," says ROSAT scientific director Joachim Trümper. Astrophysicists then figured out that they were seeing charge exchange between the hot solar wind and the water cloud around the comet. Solar wind atoms nab electrons from the cloud's water molecules, which then emit x rays as they relax back to ground state. A similar charge exchange explains the x rays that ROSAT spotted in the Moon's shadowed side. Here, the charge exchange is between the solar wind and neutral gas emanating from Earth's atmosphere.
Other ROSAT highlights include the discovery of steady nuclear fusion on the surfaces of white dwarfs as they accrete matter from a binary companion. ROSAT observations showed that millisecond pulsars emit x rays, and that neutron stars emit thermal x rays, from which, with other data, their diameters (around 25 km) could be determined. In mapping the diffuse x-ray emission from the hot interstellar medium, which is heated by supernova explosions to about 1 million kelvin, ROSAT discovered shadows cast by cool (~100 K) clouds.
Observations with ROSAT solved the mystery of the extragalactic x-ray background. Most of the background is from accreting black holes throughout the universe, says Hasinger. That finding, he adds, "started a whole new industry studying galaxy and black hole evolution." Determining the amount of dark matter that binds galaxy clusters was another result of cosmological importance, Trümper notes.
A clever save vastly prolonged ROSAT's lifetime. After only six months, one of the satellite's four gyroscopes failed, leaving it with no spares. At that point, Hasinger began exploring reprogramming onboard software to control the satellite with magnetometers. Working with engineers in industry, he says, "We found out the magnetic field was stable enough that we could go back to compass navigation." Their work paid off: After about a year in orbit another gyroscope died. ROSAT was the first satellite to use magnetometers for three-axis stabilized navigation. People from the Italian satellite BeppoSAX "came to us alarmed," recalls Hasinger, "and then they reprogrammed in the same way. Later people realized you could build in the magnetometer navigation from the beginning." ROSAT's mirrors were polished so smooth—to a roughness of less than a nanometer—that they got a mention in the Guinness Book of World Records.
A successor to ROSAT, called eROSITA, is set for launch in late 2013. The German–Russian project will do a sky survey that goes up to 40 times deeper than ROSAT's did. "It will see 5 million black holes, and clusters of galaxies," says Hasinger, who calls the satellite "his baby" although he is no longer directly involved in it. By studying the large-scale structure that eROSITA will reveal, he says, "We can say something about dark energy."
Toni Feder
