dual-mode, liquid chemical propulsion system is integrated into
the spacecraft’s structure to make economical use of mass.
The structure is primarily composed of a graphite epoxy material.
This composite structure provides the strength necessary to survive
launch while offering lower mass. Two large solar panels, supplemented
with a nickel-hydrogen battery, provide MESSENGER’s power.
"brains" of the spacecraft are redundant integrated electronics
modules (IEMs) that house two processors each – a 25-megahertz
(MHz) main processor and a 10-MHz fault-protection processor.
Attitude determination – knowing
where the spacecraft is and in which direction it’s facing
– is performed using star-tracking cameras and an Inertial
Measurement Unit containing four gyroscopes and four accelerometers,
with six Digital Solar Sensors as a backup. Attitude control is
mostly accomplished using four reaction wheels inside the spacecraft
and, when necessary, MESSENGER’s small thrusters. MESSENGER
will receive commands and send data primarily through its circularly
polarized X-band phased-array antennas.
A key MESSENGER design element deals with the
intense heat at Mercury. The Sun is up to 11 times brighter than
we see on Earth and surface temperatures can reach 450 degrees Celsius
(about 840 degrees Fahrenheit), but MESSENGER will
operate at room temperature behind a sunshade made of heat-resistant
MESSENGER's science payload –
its instruments – was carefully chosen to answer the mission's
six key science questions. Most of the instruments are fixed rigidly
to the spacecraft’s body, so coverage of Mercury is obtained
by spacecraft motion over the planet. Instrument descriptions, and
how each instrument provides information needed to understand Mercury,
Mercury Dual Imaging System (MDIS): This instrument
consists of wide-angle and narrow-angle imagers that will map landforms,
track variations in surface spectra and gather topographic information.
A pivot platform will help point it in whatever direction the scientists
choose. The two instruments will enable MESSENGER to “see” much like our two eyes do.
Gamma-Ray and Neutron Spectrometer
(GRNS): This instrument will
detect gamma rays and neutrons that are emitted by radioactive
elements on Mercury's surface or by surface elements that have
been stimulated by cosmic rays. It will be used to map the relative
abundances of different elements and will help to determine if
there is ice at Mercury’s poles, which are never exposed
to direct sunlight.
X-Ray Spectrometer (XRS): Gamma rays and high-energy X-rays from
the Sun, striking Mercury's surface, can cause the surface elements
to emit low-energy X-rays. XRS will detect these emitted X-rays
to measure the abundances of various elements in the materials
of Mercury's crust.
Magnetometer (MAG): This instrument is at the end of a 3.6 meter(nearly 12-foot) boom, and will map Mercury 's magnetic
field and will search for regions of magnetized rocks in the crust.
Mercury Laser Altimeter (MLA): This instrument contains a laser
that will send light to the planet’s surface and a sensor
that will gather the light after it has been reflected from the
surface. Together they will measure the amount of time for light
to make a round-trip to the surface and back. Recording variations
in this distance will produce highly accurate descriptions of Mercury’s
Mercury Atmospheric and Surface Composition
This spectrometer is sensitive to light from the infrared to the
ultraviolet and will measure the abundances of atmospheric gases,
as well as detect minerals on the surface.
Energetic Particle and Plasma Spectrometer
(EPPS): EPPS measures
the composition, distribution, and energy of charged particles
(electrons and various ions) in Mercury's magnetosphere.
Radio Science (RS): RS will use the Doppler effect to measure
very slight changes in the spacecraft's velocity as it orbits
Mercury. This will allow scientists to study Mercury's mass distribution,
including variations in the thickness of its crust.