[meteorite-list] MESSENGER Modifies Orbit to Prepare for Extended Mission

[Ron Baalke]

http://messenger.jhuapl.edu/news_room/details.php?id=195

MESSENGER Mission News
March 2, 2012

MESSENGER Modifies Orbit to Prepare for Extended Mission

MESSENGER successfully completed an orbit-correction maneuver this
evening to lower its periapsis altitude - the lowest point of
MESSENGER's orbit about Mercury relative to the planet's surface - from
405 to 200 kilometers (251 to 124 miles). This is the first of three
planned maneuvers designed to modify the spacecraft's orbit around
Mercury as science operations transition from MESSENGER's primary
orbital mission to its extended mission.

MESSENGER's orbit around Mercury is highly eccentric, taking it from 200
kilometers (124 miles) above Mercury's surface to 15,200 kilometers
(9,445 miles) altitude every 12 hours. Since orbit insertion nearly one
year ago, spacecraft operators at the Johns Hopkins Applied Physics
Laboratory (APL) in Laurel, Maryland, conducted five earlier maneuvers
to counter the perturbing forces that pull MESSENGER away from its
preferred observing geometry, including those arising from solar gravity
and Mercury's slight oblateness (the flattening of its spherical shape
at the planet's poles).

For this latest orbit adjustment, MESSENGER was 148 million kilometers
(92 million miles) from Earth when the 171-second maneuver, which used
all four of the medium-sized monopropellant thrusters on the deck
opposite most of the science instruments, began at 8:44 p.m. EST. APL
mission controllers verified the start of the maneuver 8 minutes and 12
seconds later, when the first signals indicating spacecraft thruster
activity reached NASA's Deep Space Network tracking station near
Canberra, Australia.

In mid-April the team will conduct two additional maneuvers designed to
reduce the period of MESSENGER's orbit around Mercury from 12 to eight
hours. According to MESSENGER Principal Investigator Sean Solomon, of
the Carnegie Institution of Washington, "This reduction in orbital
period will mean that MESSENGER spends a greater fraction of its time
close to Mercur's surface than during the primary mission that is now
nearing a successful completion. Moreover, the accomplishment of the
global mapping carried out during the primary mission will free many of
MESSENGER's instruments for a new mix of measurements to address a fresh
set of scientific objectives designed to answer questions raised by the
findings from orbital observations to date."

"The eight-hour orbit will provide 50% more low-altitude observation
opportunities of Mercury's north polar regions, including permanently
shadowed craters," explains MESSENGER Mission Design Lead Jim McAdams of
APL. "A one-third reduction in maximum altitude relative to the 12-hour
orbit will enable higher-resolution imaging of the southern hemisphere."

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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
after flybys of Earth, Venus, and Mercury will start a yearlong study of
its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie
Institution of Washington, 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.