MESSENGER is a scientific
investigation of the planet Mercury, the least explored terrestrial
planet. Understanding Mercury and how it was formed is essential to
understanding the other terrestrial planets and their evolution. Mercury
has been visited by only one other spacecraft, Mariner 10, so we know little
more than its average density (the second greatest of all the planets),
the composition of its atmosphere (thinnest of the terrestrial planets),
the fact that it posses a global magnetic
field, and its extreme variations in temperature. MESSENGER will serve
to lift some of the uncertainty about this innermost planet of our solar
system.
The MESSENGER mission will orbit Mercury after making three flybys
of the planet, using data collected during the flybys as an initial
guide to perform a more focused scientific investigation of this
mysterious world. MESSENGER will investigate six key scientific
questions about Mercury’s characteristics and environment
with a set of miniaturized space instruments. The spacecraft will
enter Mercury orbit in March 2011 and carry out comprehensive measurements
for one full Earth year.
Mission Design
It is expensive to place an object in orbit around Earth, let
alone reach another planet such as Mercury. To reduce the cost
of putting the probe in orbit around Mercury, scientists and engineers
will exploit the laws of physics to minimize the amount of fuel
needed. Mission planners will use the gravity of Venus and Mercury
to help adjust the velocity of MESSENGER so that it can enter
an orbit around the inner planet. This will be done in two flybys
of Venus, and three of Mercury.
During its three flybys of Mercury, MESSENGER will map nearly
the entire planet in color, collecting images of most parts of
the planet not seen by Mariner 10, and will take measurements
of surface, atmosphere and magnetosphere
composition. The flyby results will then be used to plan the yearlong
portion of the mission in which MESSENGER orbits the planet.
Spacecraft Design
Spacecraft design is a critical part of any mission. For each kilogram of
the spacecraft, many kilograms of rocket fuel are needed to reach
escape velocity. So engineers
and scientists have worked to keep the spacecraft mass (including structure,
engines and fuel for orbital maneuvers, instruments, and power generation,
navigation and communication equipment) at an absolute minimum. MESSENGER's
propulsion system is integrated into the spacecraft structure to make the
most economical use of mass. The miniaturized instruments are located on a
science deck that will face Mercury, while a lightweight thermal shade
shields the spacecraft from the blistering light of the nearby Sun. Most
of the instruments are fixed rigidly to the body of the probe, so coverage
of Mercury is achieved by spacecraft motion over the planet. The imaging
system uses a miniature scanning mirror so it can quickly collect image
mosaics.
Onboard Instruments
The eight instruments aboard MESSENGER, described below, have been carefully chosen to
answer the key scientific questions of the mission.
Mercury Dual Imaging System (MDIS)
This instrument consists of two cameras that will map landforms, track
variations in surface spectra
and gather topographic information. A scanning mirror will help point
it in whatever direction is chosen. The two instruments will enable
MESSENGER to “see” depth 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.
Magnetometer (MAG)
This instrument 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 laser light after it has
been reflected back 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 topography.
Mercury Atmospheric and Surface Composition Spectrometer (MASCS)
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.
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.
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.
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