[meteorite-list] MESSENGER Modifies Orbit to Prepare for Low-Altitude Campaign

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 18 Jun 2014 15:33:59 -0700 (PDT)
Message-ID: <201406182233.s5IMXxo5001839_at_zagami.jpl.nasa.gov>

http://messenger.jhuapl.edu/news_room/details.php?id=257

MESSENGER Mission News
June 17, 2014

MESSENGER Modifies Orbit to Prepare for Low-Altitude Campaign

MESSENGER successfully completed the first orbit-correction maneuver of
its Second Extended Mission this morning to raise its minimum altitude
above Mercury from 113.9 kilometers (70.8 miles) to 155.1 kilometers
(96.4 miles). This maneuver is the first of four designed to modify the
spacecraft's orbit around Mercury so as to delay the spacecraft's
inevitable impact onto Mercury's surface and allow scientists to
continue to gather novel information about the innermost planet.

During the primary phase of the MESSENGER mission, the spacecraft's
orbit around Mercury was highly eccentric, drifting between 200 and 500
kilometers (124 to 311 miles) above Mercury's surface at closest
approach, and between 15,200 and 14,900 kilometers (9,445 to 9,258
miles) above the surface at its farthest point, and completing an orbit
every 12 hours. Spacecraft operators at the Johns Hopkins Applied
Physics Laboratory (APL) in Laurel, Maryland, conducted several
spacecraft maneuvers to counter the perturbing forces that caused
MESSENGER's lowest orbital altitude to drift upward, away from its
preferred observing geometry, and early in MESSENGER's First Extended
Mission conducted a pair of maneuvers to reduce the orbital period to
eight hours.

"In this final phase of the mission, the opposite effect is happening,"
explained the mission trajectory lead Jim McAdams of APL. "To extend the
mission, we need to raise the minimum altitude by increasing the
Mercury-relative speed of the spacecraft when it is farthest from Mercury."

For this latest orbit adjustment, MESSENGER was 82.9 million kilometers
(51.5 million miles) from Earth and 69.8 million kilometers (43.3
million miles) from the Sun. The 3.2-minute-long maneuver -- which used
two of the four largest monopropellant thrusters, with a small
contribution from four of the 12 smallest monopropellant thrusters --
began at about 10:53 a.m., EDT. APL mission controllers verified the
start of the maneuver 4.6 minutes later, when the first signals
indicating spacecraft thruster activity reached MESSENGER's Mission
Operations Center via NASA's Deep Space Network tracking station outside
of Madrid, Spain.

"MESSENGER's results to date have revolutionized our knowledge of
Mercury's global geology, the nature of volcanism across the surface,
the cratering record, and the history of the planet's contraction, and
they have also revealed unexpected discoveries, such as hollows," said
APL's Nancy Chabot, who chairs MESSENGER's Geology Discipline Group.
During the remainder of MESSENGER's mission, the focus of geological
observations will narrow from a global perspective to views of selected
areas in unprecedented detail, she explained.

"With the low-altitude imaging campaign, MESSENGER will acquire the
highest-resolution images ever obtained of Mercury, enabling us to
search for volcanic flow fronts, small-scale tectonic features, layering
in crater walls, locations of impact melt, and new aspects of hollows.
Those detailed views will provide a new understanding of Mercury's
geological evolution," she said.

The measurement resolution of Mercury's gravity and internal magnetic
fields improves markedly as the altitude of the MESSENGER spacecraft
decreases. "This improvement means that smaller-scale and
weaker-amplitude features can be mapped, and in the case of magnetic
measurements, the external and internal fields can be separated with
greater fidelity," said Roger Phillips of the Southwest Research
Institute in Boulder, Colorado.

"MESSENGER will continue its downward march, interrupted by the final
three orbit-correction maneuvers, but also achieving altitudes lower
than 50 kilometers for the first time," said Phillips, who chairs
MESSENGER's Geophysics Discipline Group. "For the magnetic field, the
question of whether the crust has retained an ancient, frozen-in
(remanent) magnetic field, as have Mars and Earth, can be answered, and
a higher-resolution picture of the field generated by Mercury's liquid
outer core can be obtained. For the gravity field, the signatures of
large fold-and-thrust belts and of impact craters will shed light on the
structure of the crust and the early history of Mercury."

MESSENGER's geochemical measurements obtained during the upcoming months
will provide measurements with vastly improved spatial resolution,
according to APL's Patrick Peplowski, the instrument scientist for the
Gamma-Ray and Neutron Spectrometer. "Not only will the low-altitude
campaign help our overall efforts to map Mercury's surface composition
with the best possible spatial resolution, we will now be able to obtain
spatially resolved measurements of features that were previously too
small to resolve with the geochemical instruments," he said. "For
example, we have opportunities to study the chemical composition of
pyroclastic deposits and the mysterious low-reflectance material. We are
also hoping to spatially resolve individual ice-hosting craters for the
first time."

Even with today's maneuver, the spacecraft's altitude at closest
approach will continue to decrease until raised by additional maneuvers
in September and October of this year and January 2015. At that point,
MESSENGER will have spent its accessible propellant, and additional
altitude-changing maneuvers will not be possible. In March 2015, the
spacecraft will impact the surface of Mercury, having successfully
completed four years in orbit about Mercury.
------------------------------------------------------------------------
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 Wed 18 Jun 2014 06:33:59 PM PDT