<P> Experiment Boom . A third type of boom, shorter and much more rigid, extended from the last side of the spacecraft, opposite the RTG . This was a hollow carbon - fiber tube, of 50 mm (2 in .) diameter . It can be seen in the photo as the silver rod stowed alongside the body . It carried four types of instruments . A solid - state X-ray instrument, which was composed of two silicon detectors to study X-rays from solar flares and Jupiter's aurorae . The GRB experiment consisted of two CsI scintillator crystals with photomultipliers . Two different magnetometers were mounted: a vector helium magnetometer and a fluxgate magnetometer . A two axis magnetic search coil antenna measured AC magnetic fields . </P> <P> Body - Mounted Instruments . Detectors for electrons, ions, neutral gas, dust, and cosmic rays were mounted on the spacecraft body around the quiet section . </P> <Dl> <Dd> SWOOPS (Solar Wind Observations Over the Poles of the Sun) measures positive ions and electrons . </Dd> </Dl> <Dd> SWOOPS (Solar Wind Observations Over the Poles of the Sun) measures positive ions and electrons . </Dd>

What object did the european space probe ulysses set off for in 1991