Mission Information
MISSION_NAME MARS GLOBAL SURVEYOR
MISSION_ALIAS
MISSION_START_DATE 1994-10-12T12:00:00.000Z
MISSION_STOP_DATE 2007-09-30T12:00:00.000Z
MISSION_DESCRIPTION
Mission Overview
    ================
 
      Mars Global Surveyor (MGS) was the successor to the failed
      Mars Observer (MO) spacecraft, which was lost as it approached
      orbit insertion in August 1993.  MGS carried five of the
      original seven MO investigations; the Gamma Ray Spectrometer
      (GRS) was reflown on the 2001 Mars Odyssey, and the Pressure
      Modulated Infrared Radiometer (PMIRR) eventually reached Mars
      in 2006 aboard Mars Reconnaissance Orbiter (MRO) under the
      name Mars Climate Sounder (MCS).  When the reports on the
      first year of MGS operations were published in 2001, MGS had
      already returned more information about Mars than all previous
      missions to that planet combined [ALBEE2001].  The mission to
      that point was described in an overview paper [ALBEEETAL2001]
      and a series of papers submitted by members of the science
      teams which will be cited below.
 
      The Mars Global Surveyor (MGS) spacecraft was launched from
      the Cape Canaveral Air Station in Florida on 7 November 1996
      aboard a Delta-2/7925 rocket.  The 1062-kilogram spacecraft,
      built by Lockheed Martin Astronautics, traveled nearly 750
      million kilometers over the course of a 300-day cruise to
      reach Mars on 12 September 1997 [JPLD-12088].
 
      Upon reaching Mars, MGS fired its main rocket engine for a
      25-minute Mars orbit insertion (MOI) burn.  This maneuver
      slowed the spacecraft and allowed the planet's gravity to
      capture it into orbit.  The initial MGS orbit was highly
      elliptical and had a period of 45 hours.
 
      After orbit insertion, MGS performed a series of orbit changes
      to drop the low point of its orbit into the upper fringes of
      the Martian atmosphere at an altitude of about 110 kilometers.
      During every atmospheric pass, the spacecraft slowed by a small
      amount because of air resistance.  This slowing caused the
      spacecraft to lose altitude on its next pass through the
      orbit's high point.  MGS was to use this aerobraking technique
      over a period of four months to lower the high point of its
      orbit from 56,000 km to near 400 km in altitude, resulting in
      a nearly circular orbit for mapping.  Aerobraking was
      complicated by discovery of a broken damper arm on one of the
      solar panels about three weeks into the procedure. After study,
      spacecraft engineers concluded that aerobraking could resume,
      but with less stress on the panel.  As a result, MGS reached
      its mapping orbit about a year later than planned; but science
      observations were interleaved with orbit adjustments.
 
      The spacecraft began its primary mission (the Mapping Phase)
      on March 9, 1999; at the same time its orbit counter was reset
      to 1, meaning that the first 1683 orbit numbers were repeated.
      During mapping operations, the spacecraft orbited Mars with
      a period of 117.65 minutes at an 'index' altitude of 378 km.
      The orbit parameters resulted in an 88 revolution near-repeat
      cycle of approximately 7 martian days.  With the true altitude
      ranging between 368 and 438 km and an inclination of 92.96
      degrees, navigators could use interactions between the orbit
      and the gravity field to maintain equator crossings at
      approximately 2 AM and 2 PM local time without human
      intervention or expenditure of fuel.  The mapping phase of
      the mission lasted for approximately one Mars year (687 days),
      ending January 31, 2001, on orbit 8505 [ALBEEETAL2001].
 
      A series of four extended mission phases began on February 1,
      2001, (orbit 8506), and continued until communication with the
      spacecraft was lost on November 2, 2006 (orbit 34202).  During
      that time operations continued in much the same way as during
      the primary mission, with ongoing data collection by the
      science instruments.  Failure of an oscillator in the laser
      altimeter reduced its capabilities, but it continued to
      function in a radiometry mode.  Performance of some other
      instruments was degraded over time; but the MGS camera system
      continued to provide high-quality imaging of potential
      landing sites, the radio science experiment was measuring
      atmospheric profiles, and the spacecraft was providing
      backup relay (communications) functions until the end.
 
      MGS was built of lightweight composite materials and divided
      into four sub-assemblies: the equipment module, the
      propulsion module, the solar array support structure, and
      the high-gain antenna support structure.  On board power
      was provided by the solar arrays; attitude was controlled
      by gyroscopes and small thrusters working in conjunction with
      celestial and sun sensors.
 
      Mars Global Surveyor carried four on-board science instruments.
      The Mars Orbiter Camera (MOC) had both a wide-angle mode for
      global coverage and a narrow-angle mode with resolution of 1.4
      meters [MALINETAL1992].  Results from the first Mars year of
      operation have been summarized by [MALIN&EDGETT2001]. The
      Thermal Emission Spectrometer (TES) measured infrared radiation
      [CHRISTENSENETAL1992]. TES was used to determine the general
      mineral composition of patches of ground as small as 9.0
      square kilometers; in addition, TES also scanned the Martian
      atmosphere to provide data for the study of the clouds and
      weather [CHRISTENSENETAL2001].  The Magnetometer and
      Electron Reflectometer (MAG/ER) were used to measure the global
      magnetic properties of Mars, which provided insight on internal
      structure [ACUNAETAL1992] [ACUNAETAL2001] [MITCHELLETAL2001].
      The Mars Orbiting Laser Altimeter (MOLA) gathered data that
      allowed calculation of surface feature heights to accuracies
      of 30 meters [ZUBERETAL1992] [SMITHETAL2001B].
 
      An ultra-stable oscillator (USO) in conjunction with
      the on-board telecommunications equipment and ground equipment
      at stations of the NASA Deep Space Network (DSN) made up the
      Radio Science Subsystem (RSS) [TYLERETAL1992]. RSS measurements
      included radio tracking of the spacecraft to improve the
      gravity field model of Mars, radio occultation observations to
      study the structure of the atmosphere and ionosphere, and
      surface scattering measurements to characterize potential
      landing sites [TYLERETAL2001].
 
      A sixth 'instrument' was the Mars Relay, a cylindrically
      shaped antenna that was used to collect data transmitted
      to MGS from landers on the Martian surface. These landers
      were carried to Mars by later spacecraft and operated after
      completion of the MGS primary mission.
 
      A seventh instrument was the Accelerometer, which measured
      the deceleration of the spacecraft as it passed through
      periapsis during the aerobraking orbits. The deceleration
      could be used to infer atmospheric drag and, thereby, density.
      From the atmospheric density and altitude, it was then
      possible to infer pressures and temperatures above 100 km
      altitude, a region not accessible to other instruments
      [BOUGHER&KEATING1999].
 
      The MGS Horizon Sensor, originally included for attitude
      monitoring and control, was also used for study of the
      martian atmosphere [MARTIN&MURPHY2001].
 
 
    Mission Phases
    ==============
      Six mission phases were originally defined for significant
      spacecraft activity periods. These were the Pre-Launch, Launch,
      Cruise, Orbit Insertion, Mapping, and Relay Phases.  The Cruise
      Phase included both Inner and Outer Cruise components.  Once
      every seven Martian days during the Mapping Phase, the
      spacecraft approximately retraced its ground track; these
      88-orbit intervals are known as 'repeat cycles.'
 
      The final mission phase, Relay, was intended to support the 1998
      Mars Polar Lander and possibly the Mars 2001 Lander. It was
      planned to run from February 1, 2001, through January 1, 2003.
      Since the Mars Polar Lander was lost and the 2001 mission was
      reconfigured without a lander, MGS no longer needed a Relay
      phase.  Instead, a series of Extended Mission phases replaced
      Relay.  Relay support, such as for the Mars Exploration Rovers
      starting in 2004, was woven into the Extended Mission planning.
 
      PRELAUNCH
      ---------
        The Prelaunch Phase extended from beginning of the MGS
        mission until the start of the launch countdown at the
        Kennedy Space Center.
 
        Mission Phase Start Time       : 1994-10-12
        Mission Phase Stop Time        : 1996-11-06
 
 
      LAUNCH
      ------
        The Launch Phase extended from the start of launch countdown
        until completion of the injection into the Earth-Mars
        trajectory.
 
        Mission Phase Start Time       : 1996-11-06
        Mission Phase Stop Time        : 1996-11-07
 
 
      CRUISE
      ------
        The Cruise Phase extended from injection into the Earth-Mars
        trajectory until Mars orbit insertion.  During the Inner
        Cruise sub-phase, MGS aimed its solar panels toward the Sun
        and communicated through its low-gain antenna; during
        the Outer Cruise sub-phase, the high-gain antenna could be
        used while the solar panels generated acceptable levels of
        power.  The transition occurred on 1997-01-09.
 
        Mission Phase Start Time       : 1996-11-07
        Mission Phase Stop Time        : 1997-09-12
 
      ORBIT INSERTION
      ---------------
        After orbit insertion, MGS performed a series of orbit changes
        to drop the low point of its orbit into the upper fringes of
        the Martian atmosphere at an altitude of about 110 kilometers.
        During every atmospheric pass, the spacecraft slowed by a
        small amount because of air resistance.  This slowing caused
        the spacecraft to lose altitude on its next pass through the
        orbit's high point.  MGS was to use this aerobraking technique
        over a period of four months to lower the high point of its
        orbit from 56,000 km to near 400 km in altitude, resulting in
        a nearly circular orbit for mapping.
 
        At the low point of orbit 15, on October 8, 1997, MGS
        experienced difficulties, later diagnosed as due to excess
        vibrations of one of the solar panels. The problem was
        associated with the fracture of a panel damper arm
        [ALBEEETAL1998]. While an evaluation of the solar array
        problem was underway, periapsis was raised to about 172 km on
        October 13, 1997, and remained near that altitude until
        November 7, 1997 (orbits 19 through 36). During this 26-day
        period the spacecraft instrument panel was pointed toward
        Mars during close approaches (i.e., near periapsis) and the
        first extensive set of science observations was conducted.
        Orbits 19-36 are known as the Assessment Orbits or the
        Aerobraking Hiatus. The science observations were acquired
        during the descending leg of each orbit -- that is, as the
        spacecraft moved from north to south.
 
        Aerobraking resumed on November 8, 1997 (orbit 37), but with
        a periapsis approximately 10 km higher.  Aerobraking was
        then conducted at about one-third the rate originally planned,
        placing the spacecraft in a 2 AM Sun-synchronous mapping
        orbit by March 1999 rather than the planned 2 PM mapping
        orbit in March 1998. The 2 PM orbit meant that the
        spacecraft would have crossed the equator in the descending
        leg of the orbit -- north to south -- at 2 PM, a desirable
        observing time for some instruments. This orbit could not be
        achieved given the new aerobraking constraints. However, a
        2 AM orbit was possible because, although the descending leg
        of the orbit crossed the equator at 2 AM, the ascending leg
        (south to north) crossed the equator at the desired time of
        2 PM.
 
        Aerobraking was halted again on March 27, 1998, and resumed
        on September 24, 1998. The period from March 27 through
        April 28 was known as Science Phasing Orbit 1 (SPO-1, orbits
        202 through 268). There was a break for solar conjunction
        (May 12) between April 29 and May 27. Then Science Phasing
        Orbit 2 (SPO-2, orbits 329-573) followed from May 28 through
        September 23. The two science phasing orbits were needed to
        synchronize the two-hour circular orbit period with the
        equatorial crossing time of 2 AM. A final period of
        aerobraking began September 24, 1998, and ended February 4,
        1999.  Another month was then used for the Gravity
        Calibration Orbit, other calibration activities, and final
        trajectory adjustments to put the spacecraft into its mapping
        orbit.  The period between arrival at Mars and completion of
        the orbit adjustment activities is known collectively as the
        Orbit Insertion Phase. It ended on March 9, 1999, with
        orbit 1683.
 
        Mission Phase Start Time       : 1997-09-12
        Mission Phase Stop Time        : 1999-03-09
 
        Subphases                 Dates                      Orbits
        ---------                 -----                     ---------
        Aerobraking Phase 1A      1997-09-12 to 1997-10-12  0001-0018
        Aerobraking Hiatus        1997-10-13 to 1997-11-07  0019-0036
        Aerobraking Phase 1B      1997-11-08 to 1998-03-27  0037-0201
        Science Phasing Orbit 1   1998-03-27 to 1998-04-28  0202-0268
           (SPO-1)
        Solar conjunction         1998-04-29 to 1998-05-27  0269-0328
        Science Phasing Orbit 2   1998-05-28 to 1998-09-23  0329-0573
           (SPO-2)
        Aerobraking Phase 2       1998-09-24 to 1999-02-04  0574-1284
        Transition to Mapping     1999-02-04 to 1999-03-09  1285-1683
 
 
      MAPPING
      -------
        The Mapping Phase was the period of concentrated science data
        acquisition.  At the beginning of this phase, orbit numbering
        was restarted at 1.  The Mapping Phase lasted for 687 days,
        approximately one Martian year.  Mars was at opposition with
        the Sun on April 24, 1999, and in conjunction with the Sun on
        July 1, 2000.
 
        As a risk reduction measure against possible problems with the
        deployment of the High-Gain Antenna, the first 20 days of the
        Mapping Phase were operated in so-called 'Fixed High-Gain
        Antenna' or FHGA mode.  In this mode, the undeployed HGA was
        pointed at Earth for four to five orbits out of every twelve
        to transmit data.  During data transmission, the science
        instruments were not pointed at Mars.  The HGA was deployed on
        March 29, 1999, and the first day of full mapping was
        April 3, 1999.
 
        Soon after the antenna was deployed (April 16, 1999) its
        azimuth gimbal jammed, causing an entry into contingency mode
        and interruption in the acquisition of science data.  This
        interruption lasted until April 29, 1999, and then data were
        acquired in a modified FHGA mode (HGA deployed, but boresight
        fixed in the spacecraft +x direction) until May 6, 1999, when
        normal mapping resumed.  It was determined that the restricted
        range of travel on the azimuth gimbal would allow normal
        mapping operations until early 2000.
 
        So-called 'beta supplement mode' operations, in which the
        antenna was reoriented to allow Earth tracking during data
        acquisition, were begun on February 7, 2000. But beta
        supplement mode required that the antenna be 'rewound' while
        the spacecraft was being tracked and precluded collection of
        egress (exit) radio occultations.
 
        Approximately three days of FHGA operation were inserted into
        the schedule (March 5, 2000, to March 7, 2000) to mitigate
        impact on radio science. Unexpected heating of MOLA resulted,
        and further FHGA operation was suspended pending resolution
        of the thermal problems. Radio science 'egress campaigns' were
        eventually resumed at a rate of approximately 24 hours every
        month.
 
        Mission Phase Start Time       : 1999-03-09   (orbit 0001)
        Mission Phase Stop Time        : 2001-01-31   (orbit 8505)
 
 
      EXTENDED MISSION E1
      -------------------
        The first Extended Mission phase (E1) began at the end of the
        Mapping Phase, 1 February 2001, and continued through 22 April
        2002.  Mars was at opposition with the Sun on June 13, 2001.
 
        Operations were much the same as during Mapping.  MGS
        supported orbit insertion and aerobraking for the 2001 Mars
        Odyssey spacecraft with rapid release of MOC and TES data in
        late 2001 and early 2002.
 
        A new type of spacecraft maneuver was designed for MGS
        targeted science observations during the extended mission:
        the Roll Only Targeted Observation (ROTO). The maneuver was
        constrained to occur primarily in the roll axis and could not
        exceed +/- 30 degrees off nadir. The spacecraft was
        rolled during selected orbits to acquire off-nadir contiguous
        MOC, MOLA, and TES data, which could be used to support
        landing site certification for future missions.
 
        Starting on August 16, 2001, the spacecraft was put into
        the 'Relay 16' or R16 attitude, in which it was pitched
        back along the velocity vector by 16 degrees.  This reduced
        gravity-gradient torques, slowing momentum buildup in the
        spacecraft reaction wheels and reducing fuel consumption.
 
        Mission Phase Start Time       : 2001-02-01   (orbit  8506)
        Mission Phase Stop Time        : 2002-04-22   (orbit 13960)
 
 
      EXTENDED-EXTENDED (E2) MISSION
      ------------------------------
        The E2 Mission phase began at the end of the E1 Extended
        Mission Phase, April 22, 2002, and continued until September
        26, 2004.  Mars was in conjunction with the Sun on August
        10, 2002, and on September 15, 2004, and was in opposition
        with the Sun on August 28, 2003.  There was a major maneuver
        in May 2004 to ensure aphelion power within local sun angle
        constraints.
 
        Operations were similar to those in the E1 phase.  ROTOs,
        the R16 attitude, and radio science 'egress campaigns' were
        continued.  MGS life expectancy was increased to 10 years
        based on the rate of fuel consumption.  Support of the Mars
        Exploration Rover (MER) missions was increased with both
        imaging of potential landing sites and testing of the UHF
        communication relay system.  The MGS orbit was synchronized
        to support descent and landing of both MER rovers; over
        6 terabits of surface science data were returned via the
        MGS Relay.
 
        Mission Phase Start Time       : 2002-04-22   (orbit 13961)
        Mission Phase Stop Time        : 2004-09-26   (orbit 24836)
 
 
      EXTENDED MISSION (E3)
      ---------------------
        The E3 Mission phase began at the end of the E2 phase,
        September 27, 2004, and continued until September 26, 2006.
        Mars was at opposition with the Sun on November 7, 2005.
 
        Operations were similar to those in the E2 phase except that
        a safe mode event in August 2006 showed that the high-gain
        antenna azimuth gimbal obstruction was no longer present and
        that Beta Supplement operation was no longer required.  This
        saved gimbal life, eased sequencing, reduced fuel consumption,
        and allowed collection of more radio occultation data.  A new
        estimate of attitude control fuel added 10 kg to the
        inventory, effectively removing fuel as a limiting factor in
        extending the mission lifetime.  MGS supported MRO orbit
        insertion and aerobraking after its arrival in March 2006.
 
        Mission Phase Start Time       : 2004-09-27   (orbit 24837)
        Mission Phase Stop Time        : 2006-09-26   (orbit 33815)
 
 
      EXTENDED MISSION (E4)
      ---------------------
        The E4 Mission phase began at the end of the E3 Extended
        Mission phase, September 27,2006, and continued until contact
        with the spacecraft was unexpectedly lost in early November
        2006. Normal operations were suspended between October 17
        and November 2, 2006, while Mars was in conjunction with the
        Sun (October 23).
 
        Operations were similar to those in the previous
        Extended Mission phases. During this phase the 250000-th
        image was transmitted to Earth, of which 1750 were collected
        using ROTOs and 250 were collected using motion compensated
        ROTOs (CPROTOs) with surface resolution as small as 50 cm.
        Spacecraft loss may have resulted from a solar panel hitting
        a hard stop during an eclipse rewind, leading to a cascade
        of other events which culminated in catastrophic battery
        failure within 10 hours.
 
        Mission Phase Start Time       : 2006-09-27   (orbit 33816)
        Mission Phase Stop Time        : 2006-11-02   (orbit 34202)
MISSION_OBJECTIVES_SUMMARY
One of the most intriguing, unanswered scientific questions is
    why do Earth and Mars appear different today?  At the time of
    their formation several billion years ago, Mars and Earth shared
    similar conditions.  Both planets harbored vast quantities of
    surface water, thick atmospheres, and climates warmer than at
    present.  Today, Earth is a lush world filled with a countless
    number of animal and plant species.  In contrast, data gathered
    from Mars prior to MGS showed that the planet was trapped in
    conditions reminiscent of a global ice age.  The dry and
    seemingly lifeless Martian surface makes the Sahara look like
    an ocean in comparison, and average daily temperatures make
    Antarctica seen balmy.  Comparing the history and evolution of
    the two planets yields clues into Earth's past and possibly
    its future.
 
    Science objectives for the failed Mars Observer Mission
    [ALBEEETAL1992] were essentially identical to those for Mars
    Global Surveyor [ALBEEETAL2001].
 
 
    Basic Measurements and Data Collection
    ======================================
      Although several spacecraft preceded MGS to Mars, fundamental
      measurements remained to be made.  No topographic model of
      the planet existed at the 100 meter level (and many areas were
      uncertain by kilometers); MOLA provided one with typical
      accuracies of 30 m.  Preliminary measurements on the magnetic
      field were carried out by early spacecraft; but MGS MAG/ER
      was the first instrument to carry out a systematic mapping
      effort.  Gravity models had been compiled from Mariner 9 and
      Viking data, but MGS RSS provided an order of magnitude
      improvement in these -- leading to improved understanding of
      the planet's interior.
 
 
    Atmospheric Processes
    =====================
      Despite its forbidding climate, surface temperatures on Mars
      resemble the Earth's more than any other planet.  These
      similarities in temperature result in part from the fact that
      Mars orbits the Sun only slightly farther out than the Earth as
      compared to other planets.  For example, the ground at some
      locations near Mars' equator may warm up to as high as 25C
      at noontime.  However, daytime temperatures still average well
      below freezing, and night temperatures dip much lower.
 
      Martian temperatures may seem almost inviting to the seasoned
      outdoors explorer, but the composition of the atmosphere leaves
      much to be desired from a human perspective.  Most of the
      martian air consists of carbon dioxide (CO2), similar to
      conditions on Venus.  If breathing carbon dioxide seems
      uninviting, the density of the air will appear worse.  Average
      barometric pressures on Mars are lower than those found at
      Earth's sea level by a factor of more than 125.  In other
      words, the air at the surface of Mars is thinner than that
      found on Earth at an altitude 19 times higher than Denver,
      Colorado.
 
      The extremely thin Martian air directly impacts the mystery of
      potential life on Mars, either in the past or present.  The
      reason is that almost all of the water lies trapped in the
      Martian polar ice caps or frozen beneath the surface.  Liquid
      water cannot exist on the surface because the thin atmosphere
      will cause melting ice to evaporate directly into water vapor.
 
      Despite the hostile composition, density, and temperature by
      today's standards on Earth, the atmosphere of Mars is both
      interesting and dynamic.  MGS objectives in this area included
      recording global daily images of the planet so that cloud
      patterns could be followed and the growth of dust storms could
      be monitored over a full martian year.  TES and RSS were both
      able to measure vertical structure within the atmosphere,
      another key to understanding transport of material within the
      atmosphere -- including precipitation of CO2 itself on the
      winter polar cap.
 
 
    Surface Processes
    =================
      Geologically, Mars is one of the most interesting planets in
      the Solar System.  Although only half the diameter of Earth,
      Mars maintains large water and CO2 ice caps at the poles, a
      canyon much deeper than the Grand Canyon and longer than the
      contiguous 48 United States are wide, crater valleys as
      large as the western United States, and a handful of monstrous
      volcanoes that make Mount Everest appear tiny in comparison.
 
      A study of Martian geology is crucial to deciphering clues
      about the history of the Earth.  Mars is the only planet in the
      solar system that both has an atmosphere and contains surface
      features that cover almost the entire range of history.  On
      Earth, pristine rocks and other surface features from
      the first billion years of our planet's existence do not exist
      because geological events, weather, and life have caused
      drastic alterations.  Because Earth and Mars shared similar
      conditions near the time of their formation, the MGS
      exploration of Mars allows us to take a peek into Earth's
      past in a way not possible by studying the Earth by itself.
 
      Although liquid water on Mars will quickly evaporate,
      photographs transmitted back to Earth by NASA missions prior
      to MGS revealed giant flood channels, dry river beds, and
      flood plains on the surface.  This evidence of past water on
      Mars led some scientists to consider Mars as the prime
      location in the Solar System to search for extraterrestrial
      life.  The speculation was that because Mars once possessed
      a thicker atmosphere and vast quantities of surface water
      billions of years ago, then the planet may have harbored
      conditions favorable to the formation of life despite its
      present forbidding climate.
 
      Viking and Mars Pathfinder returned information on elemental
      composition of some Mars surface materials at specific
      landing sites.  But regional and global information was needed
      to understand both the current state and history of rocky
      surfaces.  MOC provided high-resolution image data; and TES
      acquired spectral signatures of rock units so that thermal
      inertia, surface rock distributions, and composition could
      be inferred.
 
      MOC also revealed contemporary activity on the surface during
      the instrument's own lifetime including 20 new impact craters,
      numerous boulder trails, secular enlargement of south polar
      pits, and fresh channel outflows likely to be water-related.
 
 
    Search for Life
    ===============
      Sensors aboard various NASA spacecraft launched to Mars over
      the 30 years prior to MGS showed that advanced life forms
      almost certainly do not exist on the planet today.  However,
      many felt that the planet might hide bacterial forms of life
      or their fossil remains.  Although Mars Global Surveyor did
      not conduct a search for life on Mars, it gathered detailed
      data that will help in understanding the mystery of the
      missing water.  This type of study provides important
      background data to help scientists in their search for
      Martian life on future missions.
 
 
    Other Studies
    =============
      In addition to studying Mars, the spacecraft was also
      used for experiments of opportunity, such as searching for
      gravitational waves during cruise [ESTABROOKETAL1995] and
      probing the Sun's corona during solar conjunction [WOO1993].
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