| MISSION_DESCRIPTION |
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
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| MISSION_OBJECTIVES_SUMMARY |
The Moon is the most accessible and best studied of rocky, or
'terrestrial', bodies beyond Earth. Unlike Earth, however, the
Moon's surface geology preserves the record of nearly the entirety
of 4.5 billion years of solar system history. Orbital observations
combined with samples of surface rocks returned to Earth show that
no other body preserves the record of geological history so
clearly as the Moon.
The structure and composition of the lunar interior (and, by
Inference, the nature and timing of internal melting and heat loss)
hold the key to reconstructing this history. Longstanding
questions such as the origin of the maria, the reason for the
nearside-farside asymmetry in crustal thickness, and the
explanation for the puzzling magnetization of crustal rocks, all
require a greatly improved understanding of the Moon's interior.
Deciphering the structure of the interior will bring understanding
of the evolution of the Moon itself and also extend knowledge of
the origin and thermal evolution of the Moon to other bodies in
the inner solar system. The Moon was once thought to be unique in
developing a 'magma ocean' shortly after accretion; now such a
phenomenon has been credibly proposed for Mars as well.
GRAIL Primary Mission science objectives were (1) to determine the
structure of the lunar interior from crust to core and (2) to
further understanding of the thermal evolution of the Moon. GRAIL
accomplished these goals by performing global, regional, and local
high-resolution (30x30 km), high-accuracy (less than 10 mGal, or
0.0001 m/s/s) gravity field measurements with twin, low-altitude
(55 km) polar-orbiting spacecraft using Ka-band ranging instruments
and radio tracking links to the Deep Space Network. For the
Extended Mission, the objectives were refined to reflect the lower
mean altitude (reduced by a factor of 2 from 55 km).
Science Questions Addressed
===========================
The GRAIL Science Team conducted six lunar science investigations
[RONCOLI&FUJI2010]:
1. Map the structure of the crust and lithosphere -
Determine the Moon's global gravity field with global average
Surface resolution of 30 km and with an accuracy of +/- 10 mGal.
2. Understand the Moon's asymmetric thermal evolution. Determine
large regional gravity with global average surface resolution
of 30 km and with an accuracy of +/- 2 mGal.
3. Determine the subsurface structure of impact basins and the
origin of mascons - Determine small regional gravity that
resolves basins and rings to a global average surface
resolution of 30 km to a precision of +/- 0.5 mGal.
4. Ascertain the temporal evolution of crustal brecciation and
Magmatism - Determine high-resolution local gravity fields with
global average surface resolution of 30 km to a precision of
+/- 0.1 mGal.
5. Constrain deep interior structure from tides -
To constrain the deep interior from tides, determine the Love
Number, k2, to an accuracy of 6 x 10^-4.
6. Place limits on the size of the possible inner core -
To place limits on the size of a possible solid inner core,
determine the Love Number, k2, to an accuracy of 2 x 10^-4 and
determine the seconddegree and first-order gravity coefficients
to an accuracy of 1 x 10^-10.
GRAIL's Extended Mission included six additional investigations:
1. Structure of impact craters
Utilize high resolution gravitational field to examine the
structure of simple crater structure and central peaks.
2. Near-surface magmatism
Investigate the processes and extent of magmatism on the moon.
3. Mechanisms and timing of deformation
Develop a possible record and timing of faulting.
4. Cause(s) of crustal magnetization
Investigate sources of magnetization on the Moon.
5. Estimation of upper-crustal density
Examine variation of upper crustal density and porosity.
6. Mass bounds on polar volatiles
Study the relationship, if any, of neutron suppression regions
to sub- surface structure.
Science Instruments
===================
To measure the inter-spacecraft range-rate, each spacecraft had a
Ka-band Lunar Gravity Ranging System (LGRS), derived from the
Earth-orbiting Gravity Recovery and Climate Experiment (GRACE)
instrument. Timing and frequency within the LGRS were referred to
its on-board Ultra-Stable Oscillator (USO), which also served as
a frequency reference for the X-Band Radio Science Beacon (RSB)
and a timing reference for the S-Band Timing Transfer Assembly
(TTA) on each spacecraft. The RSB sent a one-way signal to DSN
stations on the ground. The TTA synchronized timing between the
two spacecraft LGRS units.
Also included on the spacecraft was an 'E/PO MoonKam' assembly
that provided images and video of the lunar surface as part of the
Education/Public Outreach and Student Collaboration segment of
GRAIL.
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