| MISSION_DESCRIPTION |
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'
|
| MISSION_OBJECTIVES_SUMMARY |
Mission Objectives Overview
===========================
The primary mission of the Ulysses spacecraft was to
characterize the heliosphere as a function of solar latitude.
The heliosphere is the vast region of interplanetary space
occupied by the Sun's atmosphere and dominated by the outflow
of the solar wind. The periods of primary scientific interest
are when Ulysses was at or higher than 70 degrees latitude at
both the Sun's south and north poles. On 26 June 1994, Ulysses
reached 70 degrees south. There it began a four-month
observation from high latitudes of the complex forces at work
in the Sun's outer atmosphere -- the corona.
Scientists have long studied the Sun from Earth using Earth-
based sensors. More recently, solar studies have been
conducted from spaceborne platforms; however, these
investigations have been mostly from the ecliptic plane (the
plane in which most of the planets travel around the Sun) and
no previous spacecraft have reached solar latitudes higher than
32 degrees. Now that Ulysses high latitude data is available,
scientists from the joint National Aeronautics and Space
Administration (NASA)-European Space Agency (ESA) mission are
obtaining new and better understanding of the processes going
on at high solar latitudes.
Scientists have long been aware of differences between the
polar regions of the Sun and lower latitudes. Sunspots are
only seen at lower latitudes, and photographs of the solar
corona take during solar eclipses often showed dark regions
over the poles. The solar corona consists of hot gasses (over
1,000,000 degrees); at this temperature the gravitational field
of the Sun can not prevent escape of coronal gas as the solar
wind. However, the Sun has a global magnetic field. Many of
the solar magnetic field lines that leave the solar surface
return to the surface, but some of the field lines,
particularly those over the poles, extend deep into
interplanetary space. The solar wind expands into
interplanetary space along these field lines, and the regions
(known as coronal holes) of the corona from which the hot gas
escapes are dark because of the low gas density. The
properties of the Sun's polar magnetic field are poorly
understood, and they have an important influence on the escape of
the solar wind. The complex processes that heat and accelerate
the solar wind are not well understood, and Ulysses
observations over the poles should provide important new
information on how the solar wind expands from the Sun that
will aid scientists in understanding these processes. The
magnetic field also exerts a crucial influence on matter
arriving near the Sun from the Milky Way galaxy and from the
nearby interstellar medium. Incoming cosmic rays are subjected
to forces exerted by the magnetic field. The structure of the
Sun's magnetic field is thought to favor entry of cosmic rays
by way of the Sun's polar regions. Scientists hope that
Ulysses can shed some light on the extent to which the galactic
cosmic rays observed at Earth use this route and on the ways in
which their properties are modified as a result. Scientists
also hope to gain more knowledge of the intensity and
properties of the cosmic rays far from the Sun.
Jupiter Encounter
-----------------
The primary aim of the flyby was to place the spacecraft in
its final heliocentric out-of-ecliptic orbit with a minimum
of risk to the onboard systems and scientific payload.
Scientific investigations at Jupiter are a secondary
objective of the mission. Nevertheless, the opportunity to
study Jupiter's magnetosphere was exploited to the greatest
extent possible.
Jupiter is a strongly magnetized, rapidly rotating planet.
Its magnetosphere is the largest object in the solar system,
a fact reflected in the long interval of 12 days from 2 to 14
February (days 033 to 045 of 1992) that it took for Ulysses
to travel through it. The large Galilean satellites are
embedded within the magnetosphere and Io is known to be a
prolific source of ions and neutral particles. Ions,
predominantly of sulfur and oxygen, are distributed around
the orbit of Io to form a large torus. Electrons and ions
from Io, Jupiter's ionosphere and the solar wind are all
present and are transported throughout the magnetosphere. A
substantial fraction of these particles are accelerated to
extremely high energies to form intense radiation belts.
Upstream of the magnetosphere, in the free-streaming solar
wind, a detached bow shock forms which slows the solar wind
and allows it to be deflected around the magnetosphere. A
wide variety of complex physical phenomena are available for
study.
References
==========
Description of the Jupiter Encounter Mission Objectives adapted
from [MARSDEN&WENZEL1992]).
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