Mission Overview
  ================
    The GRAIL mission placed two spacecraft (GRAIL-A and GRAIL-B),
    flying in formation, into orbit around the Moon to study its
    internal structure. By very precisely measuring the distance of
    one orbiter relative to the other, the orbital perturbations
    caused by the Moon could be observed. Combining this with the
    orbiter position as determined from Earth-based observations, the
    mass distribution on the Moon could be determined.
 
    The GRAIL spacecraft were launched side-by-side on a single Delta
    II vehicle during a 26-day launch window that opened on September
    8, 2011. The actual launch was two days later.  The mission
    timeline was constructed to ensure continuity in operations by
    avoiding lunar eclipses on December 10, 2011, and June 4, 2012,
    when spacecraft power from the solar panels would be interrupted.
    The Goldstone DSN complex acquired initial spacecraft signals and
    confirmed solar array deployment.
 
    The trans-lunar cruise phase consisted of a 3.5-month low-energy
    transfer via the Sun-Earth Lagrange point 1 (EL1). Compared to a
    direct trajectory, this low-energy transfer reduced spacecraft
    fuel requirements (by approximately 130 m/s), allowed more time
    for spacecraft check-out and out-gassing, and increased the number
    of days available in the launch window each month.
 
    Both spacecraft approached the Moon under the South Pole where they
    each executed a 41-minute Lunar Orbit Insertion (LOI) maneuver to
    put them into an elliptical orbit with a period of 11.5 hours. The
    LOIs for the two spacecraft were separated by one day. Each LOI
    was simultaneously visible from the Goldstone and Canberra DSN
    complexes.
 
    A series of four maneuvers were then performed to reduce the orbits
    to nearly circular with a mean 55-km altitude and 113-minute
    period. Further maneuvers positioned the spacecraft to the desired
    initial separation distance, which then drifted between 65 km and
    225 km.
 
    The 90-day Science Phase was divided into three 27.3-day
    nadir-pointed mapping cycles. Two daily 8-hour DSN tracking passes
    acquired science and 'E/PO MoonKam' data.
 
    Following the Extended Mission Phase (August 29 - December 12,
    2012), there was a 6-day decommissioning period, after which the
    spacecraft impacted the lunar surface.
 
    For more information on the GRAIL mission, see [RONCOLI&FUJII2010].
 
  Mission Phases
  ==============
    The GRAIL Mission was divided into eight phases: Launch, Cruise,
    Orbit Insertion, Transition to Science, Science, Hiatus, Extended
    Mission, and Decommissioning.
 
    LAUNCH
    ------
    The Launch Phase extended from the start of the countdown to the
    initial acquisition, by the DSN, of the orbiters in a safe and
    stable configuration.
 
    The twin orbiters were launched from Cape Canaveral Air Force
    Station, Florida, on a Delta II Heavy shortly after the opening of
    the launch window in September 2011. There were at least 21 launch
    opportunities within this 26-day window. The Goldstone DSN complex
    acquired the initial signal, as well as confirmation of solar
    array deployment.
 
      Mission Phase Start Time : 2011-09-10
      Mission Phase Stop Time  : 2011-09-10
 
    CRUISE
    ------
    The Cruise Phase extended from after DSN initial acquisition of
    the orbiters in a safe and stable configuration until the first
    Lunar Orbit Insertion (LOI) maneuver.
 
    The orbiters followed a low-energy trajectory to the Moon, which
    took them near the Sun-Earth Lagrange Point 1 (EL1). The low-energy
    trajectory minimized the velocity change (dV) required for
    insertion into lunar orbit. During the 112-day cruise, several
    maneuvers were executed on each orbiter to adjust the trajectory.
    One maneuver placed the orbiter on the low-energy trajectory and
    another separated the Lunar Orbit Insertion (LOI) maneuvers for
    each orbiter by about a day. The remaining maneuvers were used to
    fine tune the trajectory.
 
    The low-energy transfer was chosen to reduce the spacecraft fuel
    requirements (by ~130 m/s), to allow more time for spacecraft
    check-out and out-gassing, and to increase the number of days
    available in the launch window each month.
 
      Mission Phase Start Time : 2011-09-10
      Mission Phase Stop Time  : 2011-12-31
 
    ORBIT INSERTION
    ---------------
    This phase included the propulsive maneuvers to insert the two
    spacecraft into lunar orbit. The maneuvers were separated by about
    25 hours, with the GRAIL-A maneuver occurring on December 31, 2011,
    and the GRAIL-B maneuver occurring on January 1, 2012.
 
    Both spacecraft approached the Moon under the South Pole where
    they each executed a 41-minute Lunar Orbit Insertion (LOI)
    maneuver to put them into an elliptical orbit with a period of
    about 11.5 hours. Each LOI was simultaneously visible from the
    Goldstone and Canberra DSN complexes.
 
      Mission Phase Start Time : 2011-12-31
      Mission Phase Stop Time  : 2012-01-01
 
    TRANSITION TO SCIENCE
    ---------------------
    Once in lunar orbit, and after checkout of the orbiter systems,
    several maneuvers were performed on each orbiter to reduce the
    orbit period and lower the altitude to the science orbit. These
    maneuvers were grouped into two clusters, with each cluster
    spanning about a week; clusters alternated between GRAIL-A and
    GRAIL-B. Each maneuver cluster used the same maneuver design,
    which reduced the workload on the operations teams. The orbit
    period was reduced to 113 minutes.
 
    A series of maneuvers was then performed to establish the proper
    spacecraft formation at the start of the Science Phase. These
    maneuvers positioned the spacecraft to the desired initial
    separation distance, which would then drift between 65 km and 225
    km.
 
      Mission Phase Start Time : 2012-01-01
      Mission Phase Stop Time  : 2012-03-01
 
    SCIENCE
    -------
    The 90-day Science Phase was divided into three 27.3-day
    nadir-pointed mapping cycles. Two daily 8-hour DSN tracking passes
    acquired science and 'E/PO MoonKam' data.
 
    The Science Phase of the mission was conducted from a
    near-circular, near-polar orbit with a mean altitude of 55 km.
    There were three different mapping cycles during the Science
    Phase, each covering a sidereal month (27.32 days). The first
    mapping cycle started while the science orbit had low periapsis
    altitudes with an orbiter separation distance of approximately 85
    kilometers. Over the course of the first mapping cycle, and into
    the second mapping cycle, the periapsis altitudes of the science
    orbit increased, and the mean separation distance slowly drifted
    to 225 kilometers. A small Orbit Trim Maneuver (OTM) was performed
    during the second mapping cycle to decrease the separation drift
    rate. Following this OTM to the end of the third mapping cycle,
    the periapsis altitudes decreased, and the mean separation between
    the orbiters slowly drifted from 225 km back to 65 km.
 
    During the Science Phase, data were stored onboard and downlinked
    as bandwidth became available. Each orbiter received an average of
    twelve hours of tracking coverage per day.
 
      Mission Phase Start Time : 2012-03-01
      Mission Phase Stop Time  : 2012-05-29
 
    HIATUS
    ------
    In the Hiatus Phase the beta-angle, which determines the amount of
    sunlight on the solar panels and controls the amount of energy
    available to the spacecraft, dropped below its threshold.  Science
    operations were suspended and the two spacecraft executed a series
    of maneuvers to re-position themselves for beta-angles above the
    threshold prior to the start of the Extended Mission.
 
      Mission Phase Start Time : 2012-05-29
      Mission Phase Stop Time  : 2012-08-29
 
    EXTENDED MISSION
    ----------------
    GRAIL proposed (and NASA approved) an Extended Mission, since
    there was sufficient fuel in the two spacecraft to acquire
    additional science data.  The mean spacecraft altitude was
    lowered by about a factor of 2 to increase the sensitivity of the
    gravity measurements, and GRAIL defined a new set of science
    objectives.  As in the Science Phase, the spacecraft separation
    distance was variable.  DSN coverage was comparable.
 
      Mission Phase Start Time : 2012-08-29
      Mission Phase Stop Time  : 2012-12-12
 
    DECOMMISSIONING
    ---------------
    Science data were collected during the Decommissioning Phase as in
    the Extended Mission phase. After final calibrations, the
    Decommissioning Phase ended with impact of the spacecraft on the
    lunar surface.
 
      Mission Phase Start Time : 2012-12-12
      Mission Phase Stop Time  : 2012-12-18