Mission Overview
================

Introduction------------
Launched in October 1990, Ulysses was an exploratory mission carried out jointly by ESA and NASA and had as its primary objective the study of the inner heliosphere in three dimensions. The importance of such a mission was recognized even at the dawn of the space era (e.g., [SIMPSONETAL1959]), since it was generally accepted that the conditions found in the narrow band of heliographic latitudes available to observers in the ecliptic plane were not representative of the global structure of the inner heliosphere. Nevertheless, prior to Ulysses, attempts to understand the basic physical processes occurring within this environment had, by necessity, been based for the most part on observations made in or near the ecliptic plane. The Ulysses mission provided, for the first time, comprehensive in-situ measurements of the heliospheric particles and fields at distances from 1 to 5 AUfrom the Sun, and at essentially all solar latitudes.

Within the framework of the Ulysses project, ESA was responsible for the operation of the European-built spacecraft, while NASA provided the launch, the spacecraft's radioisotope thermoelectric power source, and was responsible for acquisition of data using the Deep Space Network of tracking stations. The scientific payload comprising nine hardware investigations was provided by international teamsof scientists from Europe and the United States.

The mission provided results addressing many aspects of solar and heliospheric science. In addition to numerous individual publications, as of 1996 seven collections of papers had appeared as special issues or sections of journals, each one focusing on a specific part of the mission ([GRLV19N121992],[SCIENCEV257N50761992], [JGRV98NA121993], [PSSV41N11/121993],
[SSRV72N1/21995], [SCIENCEV268N52131995], [GRLV22N231995]).


General aspects of the mission------------------------------
Following launch by the Space Shuttle, a combined IUS/PAM-S upper-stage was used to inject Ulysses into a direct Earth/Jupiter transfer orbit. A gravity-assist maneuver at Jupiter in February 1992 placed the spacecraft in its final Sun-centered out-of-ecliptic orbit, with a perihelion distance of 1.3 AU and an aphelion of 5.4 AU. The orbital period was 6.2 years. Ulysses' trajectory took the spacecraftliterally into the uncharted third dimension of the heliosphere.

The prime mission, covering the period from launch up to the end of September 1995, included two polar passes, which are defined to be the parts of the trajectory when the spacecraft was above 70 degrees heliographic latitude in either hemisphere. The first polar pass (over the south solar pole) commenced on 26 June 1994 and ended on 5 November, the second pass (north) occurred one year later (19 June - 29 September), making a total of 234 days(or approximately 9 solar rotations) above 70 degrees latitude.
The maximum heliographic latitude reached by Ulysses was the same in both hemispheres, namely 80.2 degrees. Based on the scientific success of the mission in the first solar orbit, and the excellent health of the spacecraft and its payload, both ESA and NASA undertook to continue operating the spacecraft for a second orbit of the Sun. Constituting what was essentially a new mission, the so-called Second Solar Orbit brought Ulysses back over the solar poles in 2000 and 2001. In contrast to the high-latitude phase of the prime mission, which took place under quiet solar conditions,the second set of polar passes occurred close to solar maximum.


Scientific investigations-------------------------
Phenomena studied by Ulysses include the solar wind, the heliospheric magnetic field, solar radio bursts and plasma waves, solar and interplanetary energetic particles, galactic cosmic rays, interstellar neutral gas, cosmic dust, solar X-rays and gamma-ray bursts. The prime goal of all of these studies was to characterize the heliographic latitude dependence of the physical parameters involved. In addition, however, Ulysses' unique interplanetary orbit was highly suitable for carrying out measurements that are difficult to perform from the relative proximity of the Earth's orbit to the Sun. An important example of such measurements was the detection of interstellar pick-up ions (atoms of interstellar gas that have become singly ionized). Other investigations carried out by Ulysses included detailed interplanetary-physics studies during the in-ecliptic Earth-Jupiter phase, measurements in the Jovian magnetosphere during the Jupiter encounter, and radio-science investigations of the Sun's corona and the Io Plasma Torus using the spacecraft andground telecommunication systems.


Scientific highlights---------------------
Summaries of the key findings from the southern polar pass andpole-to-pole transit have been reported elsewhere (e.g.
[SMITH&MARSDEN1995], [MARSDEN&SMITH1996A], [MARSDEN&SMITH1996B]).
The papers in [A&AV316N21996], in addition to addressing new aspects of the data obtained during these periods, also focus on the results from the first northern polar pass. Of particular interest in this context are the north-south asymmetries reportedby various authors.


Nutation Anomaly----------------
About two hours following deployment of the 7.5 meter axial boom on 4 November 1990, a small nutation of the spacecraft was observed. This gradually grew until it reached ahalf-cone angle of 3 degrees with a pronounced periodicity.
At that distance from Earth, data transmission was still in S-band with its wide-angle beam. If significant nutation had occurred at larger geocentric distance, with the narrowbeam (2 degree beamwidth) X-band system operating, datatransmission would have been seriously hampered.

Investigations to find the cause of the nutation and to find ways to reduce and control it showed that the principal, but not the only, cause was solar energy entering the axial boom as the spacecraft rotated at 5 rpm. This induced a periodic bending of the boom which coupled into the entire spacecraft to cause nutation. As the spacecraft traveled further from the Sun and the solar aspect angle diminished (putting the boom into the shadow of the spacecraft), the effect was reduced and on 17 December 1990 the nutation disappeared completely. In the period when nutation was still active, it was experimentally established that use of the automatic earth seeking attitude control system (closed loop CONSCAN) was extremely efficacious in keeping nutation at very lowlevels (see [WENZELETAL1992]).

Correct interpretation enabled the preparation for the return of nutation in 1994 to be carried out in an orderly manner followed by the successful control of nutation during a period of over one year. These activities stretched to the limit the NASA and ESA ground facilities used for the task as well as the operational staff located at JPL and at the ground stations. The fact that the acquired scientific datawere not degraded is proof of success of these operations.

In 1995, as expected, the joint ESOC-JPL Mission Operations Team located at JPL had to contend with a renewed build-up of the nutation disturbance. Following the procedures developed during previous periods when nutation was present, the onboard Conscan system was operated in closed-loop mode in order to keep the spacecraft's high gain antenna pointed at the Earth. In line with predictions, the nutation, which peaked in early May, had decayed by early September. The cooperation of NASA's Deep Space Network in providing the round-the-clock tracking support needed to operate Conscan throughout this period is gratefully acknowledged. Nutation is not expected to return until January2001.


Mission Phases
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Launch------
The spacecraft was launched on Oct. 6, 1990 by the shuttle Discovery with two upper stages. To reach high solar latitudes, the spacecraft was aimed close to Jupiter so that Jupiter's large gravitational field would accelerate Ulysses out of the ecliptic plane to high latitudes; no man-made launch vehicle could by itself provide the needed velocity for Ulysses to achieve highlatitudes.

Spacecraft ID : ULY Mission Phase Start Time : 1990-10-06 Mission Phase Stop Time : 1990-12-03 Spacecraft Operations Type : LAUNCH/CHECK-OUT

Earth-Jupiter Cruise--------------------
Spacecraft ID : ULY Mission Phase Start Time : 1990-12-04 Mission Phase Stop Time : 1992-01-24 Spacecraft Operations Type : CRUISE

Jupiter Encounter-----------------
Ulysses arrived at Jupiter 16 months after departing from Earth,having traveled nearly 1 billion kilometers in the ecliptic.
Closest approach to the planet occurred at 12:02 UT on 8 February,1992.

The inbound trajectory was rather similar to those of the four spacecraft which flew past Jupiter previously: Pioneer 10, 11 (1972, 1973) and Voyager 1, 2 (1979). In contrast to these missions, however, Ulysses reached high latitudes (40 degrees north of Jupiter's equator) near closest approach. Ulysses' outbound flight path was through the hitherto unexplored dusk sector (18:00 hours local time) of the magnetosphere, this time at high southern latitudes. Another unique aspect of the flyby was the penetration of the Io Plasma Torus (IPT), a few hours after closest approach, in a basically north-south direction which contrasted with the nearly equatorial traversals of the Voyager 1 and Galileo spacecraft. In addition to this direct penetration, the spacecraft radio signal passed through the IPT for a significant length of time making it possible to probe theelectron density distribution in the Torus.

Spacecraft ID : ULY Target Name : JUPITER Mission Phase Start Time : 1992-01-25 Mission Phase Stop Time : 1992-02-17 Spacecraft Operations Type : FLYBY

Sequence of Events during the Ulysses Jupiter Flyby.

Event Time Distance (Day/Hour/Min) (Rj)-------------------------------------------------------------
Bow Shock Crossing (In) 033/17:33 113
Magnetopause Crossings (In) 033/21:30-035/04:00 110-87
Magnetodisc/Plasmadisc 036/06:30-037/22:00 67-36 Crossings
High Latitude Polar Cap 038/22:30 15 (or possibly Cusp) 039/06:30 8.7
Closest Approach 039/12:02 6.31
Observations of Io Plasma 039/13:00-18:00 6.4-9.0 Torus
Observation of Field Aligned 041/01:00-043/13:00 35-82 Currents, Electron and Ion Streaming
Magnetopause Crossings (Out) 043/13:57-045/21:40 83-124
Bow Shock Crossings (Out) 045/00:37-047/07:52 109-149

First Solar Orbit-----------------
The launch energy provided by the space shuttle and three upper stage rockets, combined with the gravity assist maneuver at Jupiter, placed the Ulysses spacecraft in a Sun-centered, elliptical orbit inclined at 80 degrees with respect to the Sun's equator. Important design requirements for the mission were to maximize the time spent at high solar latitudes and to achieve the highest possible latitude. Owing to the relative positions of the Earth, Sun and Jupiter at the time of the planetary swing-by, a south-going out-of-ecliptic trajectory best met theserequirements.

Spacecraft ID : ULY Target Name : HELIOSPHERE Mission Phase Start Time : 1992-02-18 Mission Phase Stop Time : 1995-09-30 Spacecraft Operations Type : ORBITER
Aphelion (5.40 AU) : 1992-02-15 Perihelion (1.34 AU) : 1995-03-12 Ecliptic Crossing : 1995-03-13

First South Polar Pass: On 26 June 1994, 28 months after leaving Jupiter, Ulysses began its passage over the Sun's southern polar cap. The Ulysses polar passes are defined to be the segments of the trajectory corresponding to solar latitudes greater than or equal to 70 degrees in either hemisphere. The south polarpass lasted 132 days, equivalent to 5 solar rotations.
During this time, the distance from the spacecraft to the Sun decreased from 2.8 AU to 1.9 AU (1 AU = 150 million km). The spacecraft reached its most southerly point, 80.2 degrees south of the solar equator, on 13 September 1994, at adistance of 2.3 AU from the Sun.

Polar Pass (latitude > 70 deg. S) : 1994-06-26 to 1994-11-05 Max. Latitude (80.2 deg. S) : 1994-09-13

First North Polar Pass: On 30 September, five years after launch, Ulysses completed the first phase of its exploratory mission to study the Sun'senvironment from the perspective of a solar polar orbit.
Between 19 June and 29 September, one year after its south polar pass, the spacecraft flew over the Sun's northern polar regions, reaching a maximum latitude of 80.2 degrees north ofthe equator on 31 July.

Polar Pass (latitude > 70 deg. N) : 1995-06-19 to 1995-09-29 Max. Latitude (80.2 deg. N) : 1995-07-31

Second Solar Orbit------------------
Ulysses' out-of-ecliptic orbit has a period of 6.2 years, corresponding to approximately half a solar cycle. A consequence of this is that the high-latitude passes of the second solar orbit occurred close to the maximum of solar cycle 23. The conditions in the polar regions are expected to be dramatically different from those encountered during the prime mission. In particular, the rather simple configuration of the corona found at solar minimum, with large coronal holes over the polar caps, will have been replaced by a much more complex arrangement, probably including high-latitude streamers. Transient events (solar flares, coronal mass ejections, etc.) related to the increase in solar activity will dominate, greatly disturbing the underlying structure of the solar wind and influencing the transport of cosmic rays andenergetic solar particles.

Spacecraft ID : ULY Target Name : HELIOSPHERE Mission Phase Start Time : 1995-10-01 Mission Phase Stop Time : 2001-12-31 Spacecraft Operations Type : ORBITER
Aphelion (5.41 AU) : 1998-04-17 Ecliptic Crossing : 1998-05-09 2nd South Polar Pass (Lat.>70 deg. S) : 2000-09-06 to 2001-01-16 Max. Southern Latitude (80.2 deg. S) : 2000-11-27 2nd North Polar Pass (Lat.>70 deg. N) : 2001-08-31 to 2001-12-10 Max. Northern Latitude (80.2 deg. N) : 2001-10-13 Perihelion (1.34 AU) : 2001-05-23

References
==========
Text for the MISSION_DESC has been adapted from the following sources:
[MARSDENETAL1996] - 'Mission Overview' [MARSDEN&WENZEL1992] - 'Jupiter Encounter' [MARSDEN1995A] - 'First Solar Orbit' [MARSDEN1995B] - 'Second Solar Orbit', 'Nutation Anomaly' [WENZELETAL1992] - 'Nutation Anomaly'