NASA will return to Mercury for the first time in
almost 33 years today, Jan. 14,
when the MESSENGER spacecraft makes its first flyby of the
sun's closest neighbor,
capturing images of large portions of the planet never
before seen. The probe will make
its closest approach to Mercury at 2:04 p.m., skimming 124
miles above its surface. This
encounter will provide a critical gravity assist needed to
keep the spacecraft on track for
its 2011 orbit insertion around Mercury.
"The MESSENGER Science Team is extremely excited about
this flyby," said
principal investigator Sean C. Solomon, from the Carnegie
Institution of Washington. "We
are about to enjoy our first close-up view of Mercury in
more than three decades, and a
successful gravity assist will ensure that MESSENGER
remains on the trajectory needed
to place it into orbit around the innermost planet for the
first time."
During the flyby, the probe's instruments will make
the first up-close
measurements of the planet since Mariner 10's third and
final flyby of Mercury on March
16, 1975, and will gather data essential to planning the
mission's orbital phase. The
spacecraft's seven scientific instruments will begin to
address the mission goals of
mapping the elemental and mineralogical composition of
Mercury's surface, imaging
globally the surface at a resolution of hundreds of meters
or better, determining the
structure of the planet's magnetic field, measuring the
planet's gravitational field
structure and characterizing exospheric neutral particles
and magnetospheric ions and
electrons.
The cameras onboard MESSENGER will take more than
1,200 images of Mercury
from approach through encounter and departure.
APL's Louise M. Prockter, instrument scientist for the
Mercury Dual Imaging
System, said, "When the Mariner 10 spacecraft did its
flybys in the mid-1970s, it saw
only one hemisphere--a little less than half the planet.
During this flyby we will begin to
image the hemisphere that has never been seen by a
spacecraft and at resolutions that
are comparable to or better than those acquired by Mariner
10 and in a number of
different color filters so that we can start to get an idea
of the composition of the
surface."
One site of great interest is the Caloris basin, an
impact feature about 808 miles
in diameter and one of the largest impact basins in the
solar system. "Caloris is huge,
about a quarter of the diameter of Mercury, with rings of
mountains within it that are up
to three kilometers high," Prockter said. "Mariner 10 saw a
little less than half of it;
during this first flyby, we will image the other side.
These impact basins act like giant
natural drills, pulling up material from underneath the
surface and spreading it out around
the crater. By looking through different color filters we
can start to understand what
the composition of the Caloris basin may be and learn
something about the subsurface of
Mercury."
MESSENGER instruments will provide the first
spacecraft measurements of the
mineralogical and chemical composition of Mercury's
surface. The visible-near infrared
and ultraviolet-visible spectrometers will measure surface
reflectance spectra that will
reveal important mineral species. Gamma-ray, X-ray and
neutron spectrometer
measurements will provide insight into elemental
composition.
During the flyby, Doppler measurements will provide
the first glimpse of Mercury's
gravity field structure since Mariner 10. The
long-wavelength components of the gravity
field will yield key information on the planet's internal
structure, particularly the size of
Mercury's core.
The encounter provides an opportunity to examine
Mercury's environment in ways
not possible from orbit because of operational constraints.
The flyby will yield low-
altitude measurements of Mercury's magnetic field near the
planet's equator. These
observations will complement measurements that will be
obtained during the later orbital
phase.
The flyby is an opportunity to get a jump-start on
mapping the exosphere with
ultraviolet observations and documenting the energetic
particle and plasma population of
Mercury's magnetosphere. In addition, the flyby trajectory
enables measurements of the
particle and plasma characteristics of Mercury's
magnetotail, which will not be possible
from Mercury's orbit.
MESSENGER is slightly more than halfway through a 4.9
billion-mile journey to
Mercury orbit that includes more than 15 trips around the
sun. It has already flown past
Earth once (2005) and Venus twice (2006 and 2007). Three
passes of Mercury, in
January 2008, October 2008 and September 2009, will use the
pull of the planet's
gravity to guide MESSENGER progressively closer to
Mercury's orbit, so that orbit
insertion can be accomplished at the fourth Mercury
encounter, in March 2011.
"The complexity of this mission, with its numerous
flybys and multitude of
maneuvers, requires close and constant attention," said
MESSENGER project manager
Peter D. Bedini, of APL. "MESSENGER is being driven by a
team of extremely talented
and dedicated engineers and scientists who are fully
engaged and excited by the
discoveries before them."
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 the
spacecraft. The 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. The propulsion system and composite
structure were provided by
GenCorp Aerojet and Composite Optics, respectively. The
MESSENGER Science Team
includes 46 scientists from 26 institutions.
For more about the mission, go to
messenger.jhuapl.edu.