[meteorite-list] MESSENGER Completes Its 3, 000th Orbit of Mercury, Sets Mark for Closest Approach

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 22 Apr 2014 14:20:16 -0700 (PDT)
Message-ID: <201404222120.s3MLKGfx024561_at_zagami.jpl.nasa.gov>


MESSENGER Mission News
April 21, 2014

MESSENGER Completes Its 3,000th Orbit of Mercury, Sets Mark for Closest

On April 20, MESSENGER completed its 3,000th orbit of Mercury and moved
closer to the planet than any spacecraft has been before, dropping to an
altitude of 199 kilometers (123.7 miles) above the planet's surface.

"We are cutting through Mercury's magnetic field in a different
geometry, and that has shed new light on the energetic electron
population," said MESSENGER Project Scientist Ralph McNutt, of the Johns
Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. "In
addition, we are now spending more time closer to the planet in general
-- and that has, in turn, increased the opportunities for all of the
remote sensing instruments to make higher-resolution observations of the

MESSENGER has been completing three orbits of Mercury every day since
April 2012, when two orbit-correction maneuvers reduced its orbital
period about Mercury from 12 hours to 8 hours. The shorter orbit has
allowed the science team to explore new questions about Mercury's
composition, geological evolution, and environment that were raised by
discoveries made during the first year of orbital operations.

APL's Carolyn Ernst, the deputy instrument scientist for the Mercury
Laser Altimeter (MLA), said the change from a 12- to an 8-hour orbit
provided her team with 50% more altimetry tracks. "MLA coverage takes a
long time to build up, and because of the small footprint of the laser,
a lot of coverage is needed to obtain good spatial resolution. The more
data we acquire, the better we resolve the topography of the planet,"
she said. "The 8-hour orbit has also allowed us to make more MLA
reflectivity measurements, which have provided critical clues for
characterizing Mercury's radar-bright deposits at high northern latitudes."

The probe has been edging closer and closer to Mercury since March 2013,
at about the time that the spacecraft orbit's minimum altitude passed
closest to Mercury's north pole.

APL's David Lawrence, a MESSENGER Participating Scientist, said he is
excited about what the low-altitude orbits will reveal about Mercury's
surface composition. "To date our compositional measurements with
neutron, X-ray, and gamma-ray data have resolved only very large regions
on Mercury's surface. Altitudes of less than 100 kilometers will enable
us to pinpoint the compositional signatures of specific geologic
features, which in turn will help us to understand how the surface
formed and has changed over time."

MESSENGER's periapsis altitude will continue to decrease until the first
orbit-correction maneuver of the low-altitude campaign, scheduled for
June 17.

"The final year of MESSENGER's orbital operations will be an entirely
new mission," added MESSENGER Principal Investigator Sean Solomon, of
Columbia University's Lamont-Doherty Earth Observatory. "With each
orbit, our images, our surface compositional measurements, and our
observations of the planet's magnetic and gravity fields will be higher
in resolution than ever before. We will be able to characterize
Mercury's near-surface particle environment for the first time. Mercury
has stubbornly held on to many of its secrets, but many will at last be
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and
Ranging) is a NASA-sponsored scientific investigation of the planet
Mercury and the first space mission designed to orbit the planet closest
to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and
entered orbit about Mercury on March 17, 2011 (March 18, 2011 UTC), to
begin a yearlong study of its target planet. MESSENGER's first extended
mission began on March 18, 2012, and ended one year later. MESSENGER is
now in a second extended mission, which is scheduled to conclude in
March 2015. Dr. Sean C. Solomon, the Director of Columbia University's
Lamont-Doherty Earth Observatory, leads the mission as Principal
Investigator. The Johns Hopkins University Applied Physics Laboratory
built and operates the MESSENGER spacecraft and manages this Discovery-
class mission for NASA.
Received on Tue 22 Apr 2014 05:20:16 PM PDT

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