ULYSSES

The Solar Polar Mission

	Mission Overview
	The Ulysses Spacecraft
	Scientific Payload
	Ulysses Mission Timetable
	The EPAC Instrument (Energetic PArticles Composition)
	EPAC publications
	The GAS Instrument
	GAS publications

Mission Overview

The Ulysses mission is unique in the history of the exploration of our solar system by spacecraft. Its orbit enabled the scientists, for the first time, to explore the heliosphere within 5 astronomical units of the Sun over all heliospheric latitudes. This provides the first characterisation of the third heliospheric dimension. Highly sophisticated scientific instruments carried on board the spacecraft are designed to measure the properties of the solar wind, the heliospheric magnetic field, solar radio bursts and plasma waves, solar X-rays, solar and galactic cosmic rays, and interplanetary/interstellar neutral gas and dust. Ulysses detected cosmic gamma-ray bursts and searches also for gravitational waves. The mission is a collaboration between ESA and NASA.

Within the project, the responsibilities for Ulysses mission operations have been defined as follows:

• NASA launched the spacecraft and provided telemetry acquisition, tracking, and command transmission through its DSN ground stations.
• ESA provided data processing for spacecraft control, spacecraft manoeuvring, and scientific quick-look data examination, and provide spacecraft control personnel to control the mission from the JPL Misssion Operations Centre in Pasadena.

The Ulysses spacecraft was launched on October 6, 1990 by means of the NASA Space Shuttle "Discovery", using a Centaur upper-stage motor to inject the spacecraft into an ecliptic transfer orbit. The spacecraft arrived at Jupiter 14 months after launch, passed by the planet slightly above its equatorial plane. The Jovian gravitational field deflected the spacecraft into a high-inclination orbit, taking it south of the ecliptic plane. The elliptical out-of-ecliptic trajectory, which has aphelion near 5.3 AU and perihelion near 1.3 AU, is such that, 2.5 years after Jupiter encounter, the spacecraft passed over the southern solar polar region at a distance 2 AU above the ecliptic. Following this polar passage, it then crossed the plane of the ecliptic, and headed for a pass over the north polar region eight months after the first. The maximum latitude was limited to 80 degrees due to the rquirement, that it should not pass Jupiter closer than 6 RJ in order to avoid the high radiation fields nearer to the planet. All hardware was designed to withstand the radiation dose accumulated during the Jupiter flyby.

The Ulysses Spacecraft

The Ulysses spacecraft is dominated by the large-diameter (1.65 m), parabolic, High-Gain Antenna providing the communication link to Earth. The second prominent feature of the spinning spacecraft (5 rpm) is the Radioisotopic Thermoelectric Generator (RTG) supplying the spacecraft's electrical power. This design is necessary because of the long distances from the Earth and the Sun at which the spacecraft is operating. Experiment requirements for electromagnetic cleanliiness and for minimisation of the RTG radiation environment resulted in a 5.6 m-long radial boom. It carries several experiment sensors and is mounted on the side of the spacecraft opposite to the RTG. A 72 m tip-to-tip dipole wire boom and a 7.5 m axial boom serve as electrical antennas for the Radio and Plasma-Wave Experiment. Most scientific instruments are mounted on the main body, as far as possible removed from the RTG, and in compliance with the field-of-view requirements of the instrument sensors. The high gain antenna serves as the radio link to Earth with 20 W X-band and 5 W S-band transmitters. The uplink S-band carries commands and ranging code. The downlinks in X- and S-band carry telemetry and turnaround ranging, respectively. This simultaneous ranging and telemetry is a basic feature of the spacecraft communication system.

At launch Ulysses weighed 366 kg. Spacecraft mass properties and balance have been a driver in the spacecraft design to meet the requirements both for the launch configuration (stowed booms) and for deployed boom configuration. Continuous coverage by ground stations was considered impossible for such a long-duration mission; therefore data were stored on-board for 16 hours and replayed, interleaved with real-time data, during 8 hour periods of coverage; the NASA deep Space Network has been used for commanding and data reception. A variety of downlink bit rates 128 and 8192 bit/s are selectable. The spacecraft also provides autonomous system capabilities for failure-mode detection and for safe spacecraft reconfiguration. This is required during unexpected and/or predicted periods of nontracking and because of the long signal travel-time between ground and spacecraft. The preprogrammable functions include searchmode initiation to reacquire the Earth if no command is received after a preselectable time up to 30 days, switch-over to redundant units, and preprogrammed attitude manoeuvres at superior conjunction.