Today, for the second time in less than a year, NASA's
MESSENGER spacecraft will swoop just
125 miles above the cratered surface of Mercury, snapping
hundreds of pictures and collecting a
variety of other data from the planet as it gains a
critical gravity assist that keeps the probe on track
to become the first spacecraft ever to orbit the innermost
planet, beginning in March 2011.
"The results from MESSENGER's first flyby of Mercury
in January resolved debates that are
more than 30 years old," said principal investigator Sean
C. Solomon, of the Carnegie Institution of
Washington. "Volcanic eruptions produced many of Mercury's
plains, its magnetic field appears to be
actively generated in a molten iron core, and the planet
has contracted more than we thought." This
second encounter, Solomon said, will uncover even more
information about the planet.
During MESSENGER's first flyby, its cameras returned
images of about 20 percent of
Mercury's surface, which had not previously been seen by
spacecraft, revealing new and unexpected
features.
Louise M. Prockter, instrument scientist for
MESSENGER's Mercury Dual Imaging System at
Johns Hopkins' Applied Physics Laboratory, said, "During
this second flyby, the cameras will take more
than 1,200 high-resolution and color images of the planet,
including 30 percent of Mercury's surface
that has never been seen by spacecraft. MESSENGER's second
flyby will show us a completely new
area of Mercury's surface, opposite from the side of the
planet we saw during the first flyby," she
said.
The probe's Mercury Laser Altimeter will measure the
planet's topography, allowing scientists to
correlate, for the first time, high-resolution topography
measurements with high-resolution images.
APL's Brian J. Anderson, MESSENGER's deputy project
scientist, said, "Unlike the topographic
data obtained during our first flyby — which covered
terrain we hadn't photographed from space —
these MLA range measurements will cover areas MDIS imaged
during that first pass. Moreover," he
said, "terrain sampled by MLA in the first flyby will in
turn be imaged by MDIS on the second
encounter."
The second flyby is also expected to yield more
surprises about the unique physical processes
governing Mercury's magnetosphere-exosphere system, as well
as additional information about the
charged particles located in and around Mercury's dynamic
magnetosphere.
A major goal of the orbital phase of MESSENGER's
mission is to determine the composition of
Mercury's surface, APL's Ralph L. McNutt, MESSENGER project
scientist, pointed out. The
instruments designed to make those compositional
measurements — the Visible-Infrared Spectrograph
on the Mercury Atmospheric and Surface Composition
Spectrometer, the X-Ray Spectrometer and
the Gamma-Ray and Neutron Spectrometer — will get
another peek at Mercury during this flyby.
"The VIRS high-spatial-resolution spectral
measurements of Mercury's surface will overlap with
high-resolution color images taken by MDIS, providing
complementary spectral information about
portions of Mercury's surface in unprecedented detail,"
McNutt said. "The second flyby will also
provide a first test of the spectral uniformity or
variability between the two hemispheres viewed by
MASCS in the two flybys."
MESSENGER is more than halfway through a 4.9
billion-mile journey to Mercury orbit that
includes more than 15 trips around the sun. It already has
flown past Earth once (Aug. 2, 2005), Venus
twice (Oct. 24, 2006, and June 5, 2007) and Mercury once
(Jan. 14, 2008). The upcoming flyby and an
additional pass of Mercury, in September 2009, will use the
pull of the planet's gravity to guide
MESSENGER progressively closer to Mercury so that orbit
insertion can be accomplished at the
fourth Mercury encounter, in March 2011.
"In addition to providing data that are already being
used to start answering the guiding science
questions of the mission, the observations made during the
Mercury flybys are critical to the science-
planning effort," said APL's Peter D. Bedini, MESSENGER
project manager. "The performance of the
spacecraft and instruments during the flybys helps us
prioritize and organize the observations to be
made during the orbital phase."
The MESSENGER project is the seventh in NASA's
Discovery Program of low-cost,
scientifically focused space missions. APL manages the
mission for NASA's Science Mission
Directorate and designed, built and operates MESSENGER. The
spacecraft's science instruments were
built by APL, NASA Goddard Space Flight Center, University
of Michigan, Ann Arbor and the
Laboratory for Atmospheric and Space Physics at the
University of Colorado, Boulder, with the
support of subcontractors across the United States and
Europe. GenCorp Aerojet and Composite
Optics provided MESSENGER's propulsion system and composite
structure.
For more on the mission, go to:
messenger.jhuapl.edu.
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