Ulysses

ESA/NASA Solar Probe
Courtesy of ESA and NASA

## Table of Contents

- Introduction
- Objectives
- Mission Operations
- Archiving
- The Future
- Views of Ulysses
- References

Introduction

Ulysses is a solar probe launched in 1990 that has made the first-ever measurements of the Sun from a polar orbit. In route to solar polar orbit, it made a close flyby of Jupiter and made measurements of the solar system's largest planet.

Formerly called the International Solar-Polar Mission (ISPM), Ulysses is an exploratory mission carried out jointly by ESA and NASA. The European contribution to the Ulysses program consists of the provision and operation of the spacecraft, and about half of the experiment. NASA provided the launch aboard the Space Shuttle Discovery (together with the upper-stage motor) and the spacecraft power generator, and is responsible for the remaining experiments. NASA also supports the mission using its Deep Space Network ( DSN). The original mission planned for two spacecraft, one built by NASA and the other by ESA. NASA canceled its spacecraft in 1981.

## Objectives

The 55-kilogram (121-pound) payload includes:

- two magnetometers;
- two solar wind plasma instruments;
- a unified radio/plasma wave instrument;
- three energetic-charged particle instruments;
- an interstellar neutral gas sensor;
- a solar X-ray/cosmic gamma-ray burst detector;
- and a cosmic dust sensor.

The primary objectives of the Ulysses mission encompass the properties of the solar wind, the structure of the Sun/wind interface, the heliospheric magnetic field, solar radio bursts and plasma waves, solar X-rays, solar and galactic cosmic rays, and the interstellar interplanetary neutral gas and dust, all as 3 function of solar latitude.

Secondary objectives of the mission include interplanetary-physics investigations during the in-ecliptic Earth/Jupiter phase, measurement in the Jovian magnetosphere during the Jupiter encounter, the detection of cosmic gamma-ray bursts, and a search for gravitational waves.

Besides the on-board experiments, the spacecraft and ground telecommunications systems have been used to conduct radio-science investigations. In addition, interdisciplinary investigations that use data from more than one Ulysses experiment to address specific questions of out-of-ecliptic science are being carried out.

## Mission Operations

The spacecraft, launched in October 1990, used a Jupiter swingby in February 1992 to transfer to a heliocentric orbit with high heliocentric inclination. It passed over the rotational south pole of the Sun in mid-1994 at 2 AU, and over the north pole in mid-1995. The Ulysses spacecraft is powered by a single radio-isotope generator. It is spin stabilized at a rate of 5 rpm and its high-gain antenna points continuously to the earth.

All spacecraft systems and the nine sets of instruments that make up the scientific payload have been functioning well and has survived the potentially dangerous flight through Jupiter's magnetosphere. Spacecraft operations, conducted by the joint ESA-NASA Mission Operations Team located at the Jet Propulsion Laboratory, have proceeded smoothly, with the exception of four occurrences of a 'Disconnect Non-Essential Loads' (DNEL) condition on board the spacecraft. DNELs occur when the on-board protection logic places the spacecraft in a minimum current demand mode by disconnecting the scientific payload in response to a violation of overcurrent criteria. It is likely that the DNEL condition, which has on each occasion occurred in conjunction with a routine Earth-pointing maneuver, is associated with operation of the latching valve when this coincides with unpredictable peaks in payload current demand. In all cases, the spacecraft was restored to its nominal operational configuration without incident, with the majority of instruments suffering less than 24 hours outage.

The only other problem of any significance has been a nutation-like motion which built up following deployment of the 7.5-meter (25-foot) axial radio wave experiment antenna shortly after launch. This disturbance was apparently driven by asymmetric solar heating of the axial boom as the spacecraft spun and was of a temporary nature, damping out once the boom was permanently in the shadow of the spacecraft body. The nutation is predicted to return during the high-latitude phase of the mission, and operational measures utilizing the on-board Conscan system have been developed to suppress the motion. In order to exploit the damping provided by Conscan, continuous ground station coverage is needed. As a result, the ESA ground station in Kourou has been upgraded to augment the DSN during periods of potential nutation, the DSN having only one station in the southern hemisphere.

Continuity of the scientific data--a prime scientific requirement for the mission--has been very good, data coverage exceeding 95% for the greater part of the mission to date.

## Archiving

The creation of a European archive for Ulysses data was formally approved in June 1993. A two-phase approach has been adopted, the first phase of which has been the implementation of the Ulysses Data System (UDS). The UDS provides on-line electronic access within the Ulysses community to calibrated data from all the Ulysses instruments, with a view to facilitating the exchange of data between Principal Investigator (PI) teams and encouraging correlative studies. In the second phase, the UDS will form the foundation for the European Ulysses data archive which will be accessible to the general space science community. The final archive will be developed in coordination with the National Space Science Data Center (NSSDC) and the Planetary Data System (PDS)

## The Future

The prime mission will terminate on 30 September 1995, at the end of the north polar pass. A technical evaluation has been carried out to assess the feasibility of continuing spacecraft operations for a second complete solar orbit (6.2 years) beyond this date. The scientific benefit to be derived from an extended mission arises as a result of the approximately 11-year solar activity cycle. While the polar passes of the prime mission occur during a period of minimum solar activity, the corresponding passes of an extended mission would take place at near-maximum conditions. This offers the unique opportunity to extend our knowledge of these hitherto unknown regions by studying their properties under widely varying conditions, thereby greatly enhancing the scientific return of the mission as a whole. The consumables on board the spacecraft are sufficient to support a second orbit and ESA's scientific advisory bodies have endorsed the extended mission on condition that NASA continues its participation.

Views of Ulysses

Ulysses l
This image shows the Sun as seen by an X-ray telescope, the Ulysses spacecraft, and the Earth-Moon system. (Courtesy NASA/JPL)
Ulysses at Sun's Pole
The Ulysses is the first spacecraft to explore the third dimension of space over the poles of the Sun. Scientists have found some surprising new discoveries about the polar regions of the Sun when the spacecraft passed over the regions in 1994 and 1995. The spacecraft revealed two clearly separate and distinct solar wind regimes, with fast wind emerging from the solar poles.

Scientists were surprised by observations of how cosmic rays make their way into the solar system from galaxies beyond the Milky Way. The magnetic field of the Sun over its poles turned out to be very different from previous expectations using ground-based observations. In addition, Ulysses found a beam of particles from interstellar space that was penetrating the solar system at a velocity of close to 80,000 kilometers per hour (50,000 miles per hour).

On September 29, 1995, Ulysses completed its first solar orbit and began its voyage back out to the orbit of Jupiter, where it will loop around and return to the vicinity of the Sun in September 2000. At that time the Sun will be in a very active phase of its 11-year solar cycle and Ulysses will find itself battling through the atmosphere of a star that is no longer docile. (Courtesy NASA/JPL)

References

ESA's Report to the 30th COSPAR Meeting, Hamburg, Germany, July 1994, European Space Agency, Paris, 1992, pp. 47-57.

Planetary Science Database, National Space Science Data Center (NSSDC), National Aeronautics and Space Administration, Greenbelt, Maryland. World Wide Web: http://nssdc.gsfc.nasa.gov/nssdc/nssdc_home.html.

Space History

##### Views of the Solar System Copyright © 1997 by Calvin J. Hamilton. All rights reserved.