The interstellar neutral-gas experiment on ULYSSES

The properties (density, bulk velocity relative to the solar system, and temperature) of the local interstellar gas, represented by neutral helium penetrating the heliosphere, will be measured in-situ for the first time by the ULYSSES GAS instrument. By employing the solar gravitational field as a natural velocity analyser, the bulk velocity relative to the solar system and the temperature of the gas can be derived from the angular distributions of the particles measured in at least two widely separated points in the heliosphere. The gas density can be determined if a composition corresponding to cosmic abundances is assumed.