At 15.4 kilometres per second, it became the fastest space probe ever, propelled by the Inertial Upper Stage and the Payload Assist Module (the spherical motor immediately below Ulysses). Ulysses is a joint ESA/NASA mission studying the interplanetary medium and solar wind in the inner heliosphere beyond the Sun's equator for the first time. Its high-inclination solar orbit took it over the Sun's south pole in 1994 and then the north pole in 1995. It made a second southern solar pass above 70° latitude during September 2000 to January 2001, and a similar northern pass during September to December 2001.