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 AU
      from 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 teams
      of 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 spacecraft
      literally 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 and
      ground telecommunication systems.
 
 
    Scientific highlights
    ---------------------
      Summaries of the key findings from the southern polar pass and
      pole-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 reported
      by 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 a
      half-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, data
      transmission 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 low
      levels (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 data
      were 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 January
      2001.
 
 
  Mission Phases
  ==============
 
    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 high
      latitudes.
 
      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 the
      electron 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 these
      requirements.
 
      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 polar
      pass 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 a
      distance 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's
      environment 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 of
      the 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 and
      energetic 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'