MESSENGER Mission Overview
==========================
The MErcury Surface, Space ENvironment, GEochemistry,
and Ranging (MESSENGER) spacecraft was launched from
the Cape Canaveral Air Station on 2004-08-03, on an
approximately 8 year mission to become the first
probe to orbit the planet Mercury.

The MESSENGER payload consists of seven instruments
and a radio science (RS) experiment. The instruments
are the Mercury Dual Imaging System (MDIS), the Gamma-
Ray and Neutron Spectrometer (GRNS), the X-Ray
Spectrometer (XRS), the Magnetometer (MAG), the
Mercury Laser Altimeter (MLA), the Mercury Atmospheric
and Surface Composition Spectrometer (MASCS), and the
Energetic Particle and Plasma Spectrometer (EPPS).

The MESSENGER mission is fully described in
[SOLOMONETAL2007].

MDIS
----
The MDIS instrument includes both a wide-angle (WA)
and a narrow-angle (NA) camera and both are capable
of summing pixels. This provides for images of the
surface that are of nearly uniform horizontal
resolution (125 m per pixel or better throughout
MESSENGER's elliptical orbit). The WA and NA
cameras (WAC and NAC) are mounted on opposite sides
of a pivot platform, making MDIS the only MESSENGER
instrument capable of pointing independent of
spacecraft attitude.

MDIS contributes to the understanding of the
geological landforms and processes that shaped
Mercury's surface.

GRNS
----
The GRNS instrument includes two sensors, a Gamma-
Ray Spectrometer (GRS) and a Neutron Spectrometer
(NS). The GRS is a germanium detector with an
active shield capable of measuring the elemental
abundances of O, Si, S, Fe, H, K, Th, and U. The NS
sensor consists of two lithium glass scintillators
separated by a thick slab of borated plastic
scintillator. The glass scintillators measure
thermal neutrons, while the borated-plastic
scintillator counts fast neutrons.

GRNS contributes to the understanding of surface
elemental abundances and the composition of polar
deposits.

XRS
---
The XRS detects solar-induced X-ray fluorescence to
measure the surface abundances of Mg, Al, Si, Ca, Ti
and Fe. Three proportional counters measure low-
energy X-rays from the planet, while a Si-PIN
detector located on the spacecraft sunshade measures
the solar X-ray input. The XRS has a field of view (FOV)
of 12 degrees and covers an energy range from 1 to 10 keV.

XRS contributes to the understanding of surface
elemental abundances.

MAG
---
The MAG instrument is a miniaturized three-axis,
ring-core, fluxgate magnetometer mounted on a
lightweight 3.6 m carbon-fiber boom extending from
the spacecraft in the anti-sunward direction. It
samples the field at a 20-Hz rate with selectable
readout intervals between 0.04 s to 1 s. Readout
intervals of greater than 1 s generate a 0.5 s
average.

MAG contributes to the mapping of Mercury's internal
magnetic field and to understanding the
magnetospheric structure.

MLA
---
The MLA consists of a 1064 nm laser transmitter and
four sapphire lens receiver telescopes. It is
capable of measuring altitudes to a 30-cm precision
at ranges up to 1000 km. Because of this range, the
MLA will operate for about 30 minutes around the
periapsis of each orbit.

MLA contributes to the mapping of the northern
hemisphere topography and the altimetry of polar
craters and is instrumental in determining Mercury's
gravity field, obliquity and libration amplitude.

MASCS
-----
The MASCS instrument combines a movable-grating
Ultraviolet-Visible Spectrometer (UVVS) and a
Visible-Infrared Spectrograph (VIRS) into one
package. Both instruments share a single front-end
telescope. UVVS spans the spectral range from 115
to 600 nm with an average spectral resolution of 1
nm, has a 25 km altitude resolution, and is
optimized for measuring very weak exospheric
emissions. VIRS measures the visible (300-1025 nm)
and infrared (0.95-1.45 um) spectral ranges
utilizing a 512 element detector for the visible and
a 256 element detector for the infrared.

MASCS contributes to the understanding of the
composition of Mercury's surface in association with
particular geological units, and to the
understanding of neutral species in the exosphere
especially near the polar regions.

EPPS
----
EPPS consists of an Energetic Particle Spectrometer
(EPS)and a Fast Imaging Plasma Spectrometer (FIPS).
The EPS measures the time-of-flight and residual
energy of ions from 10 keV/nucleon to ~3 MeV and
electrons to 400 keV. Its FOV, 160 degrees by 12
degrees, is divided into six segments of 25 degrees
each. The FIPS measures thermal and low-energy ions
and is sensitive over nearly a full hemisphere, with
energy per charge (E/q) up to > 15 keV/q.

EPPS contributes to the understanding of the solar
environment associated with Mercury and its
magnetosphere.

RS
--
The spacecraft's radio frequency (RF)
telecommunications system is for communications,
navigation and radio science (RS). Precise
observation of the spacecraft's Doppler velocity and
range are used to assist in navigating the
spacecraft. These observations will be inverted to
determine the effect of the planet's gravitational
field on the spacecraft. Occultation observations
of the spacecraft's RF signal will provide necessary
measurements of Mercury's shape in the southern
hemisphere.

RS contributes to the understanding of Mercury's
gravity field, obliquity and libration amplitude
(Doppler observations) and its global topography,
especially the southern hemisphere (occultation
observations).

Mission Phases
==============
Nineteen mission phases were defined for significant
spacecraft activity periods. The large number of
phases is due to the complex sequence of gravitational
assists necessary to bring the spacecraft into orbit
around Mercury while maintaining a minimal mass due to
fuel. This consideration lead to one Earth flyby, two
Venus flybys, and three Mercury flybys before orbit
insertion at Mercury.

The mission phases are defined naturally by the
various planetary encounters and their intervening
cruise periods. Given the short encounter times for
each MESSENGER flyby, we define encounter phases on
the basis of a 4 week period centered on the closest
approach to each target body (two weeks before and two
after) and separate such encounter segments by cruise
phases. The cruise periods and flybys are named
according to the planetary body involved. Also defined
are a launch and an orbit phase.

The mission phases are: Launch, Earth Cruise, Earth
Flyby, Venus 1 Cruise, Venus 1 Flyby, Venus 2 Cruise,
Venus 2 Flyby, Mercury 1 Cruise, Mercury 1 Flyby,
Mercury 2 Cruise, Mercury 2 Flyby, Mercury 3 Cruise,
Mercury 3 Flyby, Mercury 4 Cruise, Mercury Orbit,
Mercury Orbit Year 2, Mercury Orbit Year 3,
Mercury Orbit Year 4, and Mercury Orbit Year 5.

Launch
------
The launch phase has been defined to capture
instrument data produced between launch and the
beginning of Phase E.

Mission Phase Start Time : 2004-08-03 (2004-216)
Mission Phase Stop Time : 2004-09-12 (2004-256)

Earth Cruise
------------
Earth Cruise is the period of time between launch
and the week before closest approach to Earth.

Mission Phase Start Time : 2004-09-13 (2004-257)
Mission Phase Stop Time : 2005-07-18 (2005-199)

Earth Flyby
-----------
Earth Flyby is defined as the four week (28 day)
period centered on closest approach to Earth.

Mission Phase Start Time : 2005-07-19 (2005-200)
Mission Phase Stop Time : 2005-08-16 (2005-228)

Venus 1 Cruise
--------------
Venus 1 Cruise is defined as the period between the
Earth flyby and the first Venus flyby.

Mission Phase Start Time : 2005-08-17 (2005-229)
Mission Phase Stop Time : 2006-10-09 (2006-282)

Venus 1 Flyby
-------------
Venus 1 Flyby is defined as the four week (28 day)
period centered on the first of the mission's two
closest approaches to Venus.

Mission Phase Start Time : 2006-10-10 (2006-283)
Mission Phase Stop Time : 2006-11-07 (2006-311)

Venus 2 Cruise
--------------
Venus 2 Cruise is defined as the period between the
first and second Venus flyby.

Mission Phase Start Time : 2006-11-08 (2006-312)
Mission Phase Stop Time : 2007-05-22 (2007-142)

Venus 2 Flyby
-------------
Venus 2 Flyby is defined as the four week (28 day)
period centered on the second of the mission's two
closest approaches to Venus.

Mission Phase Start Time : 2007-05-23 (2007-143)
Mission Phase Stop Time : 2007-06-20 (2007-171)

Mercury 1 Cruise
----------------
Mercury 1 Cruise is defined as the period between
the second Venus flyby and first Mercury flyby.

Mission Phase Start Time : 2007-06-21 (2007-172)
Mission Phase Stop Time : 2007-12-30 (2007-364)

Mercury 1 Flyby
---------------
Mercury 1 Flyby is defined as the four week (28 day)
period centered on the first of the mission's three
closest approaches to Mercury.

Mission Phase Start Time : 2007-12-31 (2007-365)
Mission Phase Stop Time : 2008-01-28 (2008-028)

Mercury 2 Cruise
----------------
Mercury 2 Cruise is defined as the period between
the first and second Mercury flyby.

Mission Phase Start Time : 2008-01-29 (2008-029)
Mission Phase Stop Time : 2008-09-21 (2008-265)

Mercury 2 Flyby
---------------
Mercury 2 Flyby is defined as the four week (28 day)
period centered on the second of the mission's three
closest approaches to Mercury.

Mission Phase Start Time : 2008-09-22 (2008-266)
Mission Phase Stop Time : 2008-10-20 (2008-294)

Mercury 3 Cruise
----------------
Mercury 3 Cruise is defined as the period between
the second and third Mercury flyby.

Mission Phase Start Time : 2008-10-21 (2008-295)
Mission Phase Stop Time : 2009-09-15 (2009-258)

Mercury 3 Flyby
---------------
Mercury 3 Flyby is defined as the four week (28 day)
period centered on the third of the mission's three
closest approaches to Mercury.

Mission Phase Start Time : 2009-09-16 (2009-259)
Mission Phase Stop Time : 2009-10-14 (2009-287)

Mercury 4 Cruise
----------------
Mercury 4 Cruise is defined as the period between
the third Mercury flyby and Mercury orbit insertion.

Mission Phase Start Time : 2009-10-15 (2009-288)
Mission Phase Stop Time : 2011-03-03 (2011-062)

Mercury Orbit
-------------
The Orbit phase begins at Mercury orbit insertion
and continues until the end of mission. This phase
begins the most intensive science portion of the mission
with full instrument utilization throughout the
period.

Mission Phase Start Time : 2011-03-04 (2011-063)
Mission Phase Stop Time : 2012-03-17 (2012-077)

Mercury Orbit Year 2
--------------------
The Orbit phase year 2 begins the extended mission. This phase
continues the most intensive science portion of the mission
with full instrument utilization throughout the period.

Mission Phase Start Time : 2012-03-18 (2012-078)
Mission Phase Stop Time : 2013-03-17 (2013-076)

Mercury Orbit Year 3
--------------------
The Orbit phase year 3 continues the extended mission. This phase
continues the most intensive science portion of the mission
with full instrument utilization throughout the period.

Mission Phase Start Time : 2013-03-18 (2013-077)
Mission Phase Stop Time : 2014-03-17 (2014-076)

Mercury Orbit Year 4
--------------------
The Orbit phase year 4 continues the extended mission. This phase
continues the most intensive science portion of the mission with
full instrument utilization throughout the period.

Mission Phase Start Time : 2014-03-18 (2014-077)
Mission Phase Stop Time : 2015-03-17 (2015-076)

Mercury Orbit Year 5
--------------------
The Orbit phase year 5 continues the extended mission through to
the end of orbital operations. This phase continues the most
intensive science portion of the mission with full instrument
utilization through near the end of the period which ended when
the spacecraft impacted Mercury as expected on 30 April 2015.

Mission Phase Start Time : 2015-03-18 (2014-077)
Mission Phase Stop Time : 2015-04-30 (2015-120)