Mission Overview
    ================
      IRAS was launched on January 26, 1983 on a Delta rocket from
      Vandenburg Air Force Base in California at 02h 17m Greenwich
      Mean Time.  The project was initiated in 1975 as a joint
      program of the United States, the Netherlands, and the United
      Kingdom.  The satellite consisted of two main parts, the
      spacecraft and the telescope system.  The telescope system
      comprised the upper part of the satellite and was composed of a
      two mirror, Ritchey-Chretien telescope mounted within a
      toroidal superfluid helium tank, which in turn was mounted
      within the evacuated main shell.  The optical system was
      protected from contamination before launch and during the first
      week of the mission by an aperture cover cooled with
      supercritical helium.  After the cover was ejected, the
      sunshade limited heat flow to the aperture by blocking direct
      solar radiation and reflecting away terrestrial infrared
      radiation.  The telescope was cooled by contact with the
      superfluid helium tank to temperatures ranging from 2 to 5 K.
      The surfaces of the sunshade which could be viewed by the
      telescope aperture were cooled by a three-stage radiator to
      about 95 K.
 
      IRAS was succesfully placed into its planned 900 km altitude,
      99 degree inclination Sun-synchronous polar orbit with an orbital
      period of 103 minutes.  With the telescope pointing radially outwards
      from the Earth and perpendicular to the Sun vector, no Earth or
      sunlight could enter the telescope and all ecliptic latitudes
      would be swept out during one orbit while, as the line of nodes
      precessed at a rate of about 1 degree per day to remain
      perpendicular to the Sun vector, all ecliptic longitudes would
      be covered in a period of 6 months.  To allow mission
      flexibility, the attitude control system and telescope were
      designed to allow pointing away from the local vertical.
 
      The satellite attitude was controlled by three orthogonal
      reaction wheels; excess momentum was dumped via magnetic coils
      to the Earth's magnetic field as necessary.  The attitude, and
      changes in attitude, were sensed by a combination of a horizon
      sensor, a sun-sensor and three orthogonal gyros.  The z-axis
      gyro was used in all modes of control and was duplicated to
      provide a redundant backup.
 
      The telescope was constrained to point no further than 120
      degrees away from the Sun, since at greater angles the fine Sun
      sensor could no longer see the Sun well enough to function.  It
      was constrained also to angles no closer than 60 degrees
      towards the Sun in order to avoid solar radiation falling into
      the inside of the sunshade.  A third pointing constraint arose
      from prohibiting radiation from the Earth from falling upon the
      inside of the sunshield or the top of the telescope baffle
      system.
 
      Infrared radiation from the Moon and the planet Jupiter was
      sufficiently strong to affect the performance of the detectors
      for a significant time after being scanned.  An avoidance
      radius of 1 degree from Jupiter was set within which the
      telescope did not point.  For the Moon, an avoidance radius of
      25 degrees was used during the first two months of the survey,
      but was lowered to 20 degrees after April 3 except between
      August 26 and September 9 where it was lowered to 13 degrees.
      At 25 degrees significant 'Moon glints' were entered into the
      data stream.  Diffraction spikes from Jupiter were also
      introduced into the data stream.
 
      Another constraint was a region of high proton density known as
      the South Atlantic Anomaly (SAA).  Proton hits in the detectors
      when passing through the SAA increased the noise to such an
      extent that it was impossible to continue observations.  Data
      usually were not taken whenever the satellite entered a
      geographically fixed flux/energy contour shown in the IRAS
      Explanatory Supplement.  This contour was mapped out during the
      Presurvey portion of the mission.  On May 9, 1983, this
      avoidance contour was reduced slightly.
 
      Every 10-14 hours, as the satellite passed over its ground
      station at Chilton, England, observations would cease for
      typically 10 minutes as data from the preceding 10-14 hour
      observation period were being transmitted from the on-board
      tape recorders to the ground and the commands for the next
      10-14 hours of observations were being sent to the satellite.
 
      For more information see Neugebauer et al. 1984 [NEUGEBAUERETAL1984].
 
 
    Mission Phases
    ==============
 
 
      PRESURVEY
      ---------
        After launch, numerous checks were required to verify the
        health and safety of the satellite and to determine the best
        modes of operation.  The cooled aperture cover was kept on
        for the first six days to allow sufficient time for
        contaminants carried up with the satellite to outgas and
        disperse so that they would not freeze on the cold optics
        when the cover was ejected.  The eight days after cover
        ejection were used to test those aspects of the instrument
        that could not be tested with the cover on.
 
   SPACECRAFT_ID                  : IRAS
   TARGET_NAME                    : SKY
   MISSION_PHASE_START_TIME       : 1983-01-26
   MISSION_PHASE_STOP_TIME        : 1983-02-10
   SPACECRAFT_OPERATIONS_TYPE     : TEST
 
      MINISURVEY
      ----------
        The Presurvey period was followed by the repeated surveying of
        a limited region of sky to verify the survey strategy and the
        data processing facilities.  The scans of the minisurvey were
        hand-tailored for maximum efficiency coverage.  The area of the
        sky chosen, approximately 900 square degrees, consisted of two
        strips of sky centered approximately on ecliptic longitudes 60
        and 252 degrees.  The region of the sky was that area available
        immediately after cover ejection.  No part of the sky above
        galactic latitude 40 degrees was scanned.  Part of the
        minisurvey area was covered with four hours-confirming sets of
        scans to provide a basis for testing the processing of the
        survey.  Minisurvey scans included observations during SOPs 29,
        30, 33, 34, 37, 38, 41, and 43.  More information on the
        minisurvey may be found in the IRAS Explanatory Supplement
        by Beichman et al. 1988 [BEICHMANETAL1988] and the IRAS
        Minisurvey by Rowan-Robinson et al. 1984 [ROWAN-ROBINSETAL1984].
 
   SPACECRAFT_ID                  : IRAS
   TARGET_NAME                    : SKY
   MISSION_PHASE_START_TIME       : 1983-02-09
   MISSION_PHASE_STOP_TIME        : 1983-02-16
   SPACECRAFT_OPERATIONS_TYPE     : TEST
 
      SURVEY
      ------
        During this period, the strategy was to acquire four
        coverages (two sets of hours-confirming coverages, HCON 1
        and HCON 2) of the sky.  This was achieved by defining an area
        of sky ('lune') between two ecliptic meridians 30 degrees
        apart which was 'painted' by survey scans, one after another,
        as they passed through the viewing window of the telescope.
        Using two gyros the spacecraft scan rate was adjusted so that
        the sky was scanned at a rate of 3.85 arcminutes per second,
        independent of the solar elongation angle.  The first scan in
        a lune was placed so that it crossed the ecliptic at the
        lower longitude boundary of the lune.  Successive scans were
        laid down at increasing ecliptic longitudes, each one shifted
        over by 14.23 arcminutes, that is by half the width of the
        focal plane minus a safety margin.  The overlap ensured that
        measurements of the same area of sky were repeated within a
        few orbits (for hours-confirmation) and by generally
        restricting scans to be within 80 to 100 degrees solar
        elongation, the curvature of scans would not be too severe.
        The criterion for hours-confirmation was that the
        hours-confirming scan had to be made within 34-38 hours of
        each other.  After a lune was filled, a second lune in the
        same hemisphere was started.  It overlapped half of the first
        lune, ensuring that another hour's-confirming set of scans
        was repeated after about one to two weeks, thus providing the
        required repetition on the time scale of 7 to 11 days.
        During this phase, 95 percent of the sky was covered.
        See Beichman et al. 1988  [BEICHMANETAL1988].
 
   SPACECRAFT_ID                  : IRAS
   TARGET_NAME                    : SKY
   MISSION_PHASE_START_TIME       : 1983-02-10
   MISSION_PHASE_STOP_TIME        : 1983-08-26
   SPACECRAFT_OPERATIONS_TYPE     : STRIP SCAN
 
      SURVEY
      ------
        A third set of hours-confirming coverage of the sky (HCON 3)
        was undertaken.  Half circles rather than lunes were used
        during this period, beginning with solar elongations near 60
        and 120 degrees in general, and working towards solar
        elongations nearer 90 degrees on succeeding scans.  During
        this mission phase, 72 percent of the sky was covered.
        See Beichman et al. 1988 [BEICHMANETAL1988].
 
   SPACECRAFT_ID                  : IRAS
   TARGET_NAME                    : SKY
   MISSION_PHASE_START_TIME       : 1983-08-26
   MISSION_PHASE_STOP_TIME        : 1983-11-22
   SPACECRAFT_OPERATIONS_TYPE     : STRIP SCAN
 
      POINTED OBSERVATIONS
      --------------------
        Roughly 40 percent of the IRAS mission time was devoted to
        pointed observations of selected objects.  Nearly 10,000 of
        these observations were made of virtually every kind of
        astronomical object.  Raster scans were made of these objects
        with scan lengths ranging between 1.6 and 6 degrees,
        different numbers of scan legs, different sizes of cross-scan
        steps between legs, and different scan rates.
        See Young et al. 1985 [YOUNGETAL1985].
 
   SPACECRAFT_ID                  : IRAS
   TARGET_NAME                    : POINT SOURCES
   MISSION_PHASE_START_TIME       : 1983-02-10
   MISSION_PHASE_STOP_TIME        : 1983-11-22
   SPACECRAFT_OPERATIONS_TYPE     : RASTER SCAN

Mission Objectives Overview
    ===========================
      The primary mission of IRAS was to conduct a sensitive and
      unbiased survey of the sky in four wavelength bands centered at
      12, 25, 60, and 100 microns.