Discovery of Jovian dust streams and interstellar grains by the Ulysses spacecraft

ON 8 February 1992, the Ulysses spacecraft flew by Jupiter at a distance of 5.4 AU from the Sun. During the encounter, the spacecraft was deflected into a new orbit, inclined at about 80^° to the ecliptic plane, which will ultimately lead Ulysses over the polar regions of the Sun¹. Within 1 AU from Jupiter, the onboard dust detector² recorded periodic bursts of submicrometre dust particles, with durations ranging from several hours to two days, and occurring at approximately monthly intervals (28 +/- 3 days). These particles arrived at Ulysses in collimated streams radiating from close to the line-of-sight direction to Jupiter, suggesting a jovian origin for the periodic bursts. Ulysses also detected a flux of micrometre-sized dust particles moving in high-velocity (>= =26 km s^-1) retrograde orbits (opposite to the motion of the planets); we identify these grains as being of interstellar origin.