In 1996 the X-ray sky was enhanced with an unexpected class of objects: for the very first time X-ray emission from comets was detected. This surprising discovery was made with the ROSAT satellite, first on comet C/1996 B2 (Hyakutake) during its close approach to Earth, then a few weeks later on other comets, observed by chance during the ROSAT all-sky survey in 1990/91.

These comets are C/1990 K1 (Levy), C/1990 N1 (Tsuchiya-Kiuchi), 45P/Honda-Mrkos-Pajdusakova, and C/1991 A2 (Arai). The last two were optically more than 10000 times fainter than Hyakutake. In the case of comet C/1991 A2 (Arai), the X-ray observations took place six weeks before the comet was discovered. Comet Tsuchiya-Kiuchi was in the field of view on two and Levy on three occasions. All comets observed within 2 AU from the sun and brighter than 12 mag were detected with ROSAT. Thus, comets represent a new class of X-ray sources.

The spectral resolution of the ROSAT PSPC allows specific models to be tested, and the unlimited field of view provided by the all-sky survey makes it possible to trace the full extent of the X-ray emission. The ongoing research activities indicate that charge exchange between highly charged heavy ions (e.g. C⁵⁺, C⁶⁺, O⁶⁺, O⁷⁺) in the solar wind and cometary neutrals is the dominant process for the X-ray emission. Comets may thus be utilized as probes for monitoring the heavy ion content of the solar wind.