Ulysses: leave the planetary plane to study the Sun

Ulysses was a joint ESA–NASA mission launched aboard Space Shuttle Discovery in October 1990. ESA supplied the spacecraft, while NASA supplied launch systems, the radioisotope power source and Deep Space Network support. European and American teams contributed instruments and operations. The original two-spacecraft concept became one flight spacecraft as the project evolved.

The mission flew outward to Jupiter for a February 1992 gravity assist that redirected its path into a steeply inclined orbit around the Sun. Jupiter was a means of changing the observing geometry, not the main long-term science destination. Ulysses then sampled the environment above the Sun’s southern and northern polar regions, far from the plane in which most planets orbit.

Ulysses carried instruments for particles, magnetic fields, radio and plasma waves, gas, dust and energetic radiation. Its solar investigation relied heavily on measurements of the environment along its path, rather than a familiar camera panorama of the solar surface. The deployment photograph shows the spacecraft leaving Discovery with its propulsion stages, not an image taken above a solar pole.

High-latitude sampling revealed how the solar wind and magnetic environment differed from what near-Earth observations alone could show. During solar minimum, fast wind was prominent at high latitudes. The mission’s long duration enabled repeated passages under different solar activity conditions, showing why one observation cannot define the Sun’s behavior for every phase of its cycle.

In a fictional comparison, a polar measurement and an equatorial measurement taken years apart differ in both location and time. A simple claim that latitude alone caused the difference is therefore incomplete. Use the trajectory, solar-cycle context and supporting observations to test that explanation. Repeated passes and multiple spacecraft help constrain the interpretation.

Ulysses operations ended in June 2009 after a mission far longer than originally planned. Its archive preserves the measurements behind the three-dimensional view of the heliosphere. Evaluating that legacy means connecting instrument data to geometry and uncertainty, while preserving the partners’ distinct contributions. A successful launch or gravity assist enabled the science; neither alone established every later scientific conclusion.