[meteorite-list] MESSENGER Adjusts Orbit for a Closer Look at Mercury

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 17 Apr 2012 11:17:26 -0700 (PDT)
Message-ID: <201204171817.q3HIHQMq000977_at_zagami.jpl.nasa.gov>

http://messenger.jhuapl.edu/news_room/details.php?id=214

MESSENGER Mission News
April 16, 2012

MESSENGER Adjusts Orbit for a Closer Look at Mercury

The MESSENGER mission successfully completed the first of two maneuvers
designed to reduce the spacecraft's orbital period about Mercury. This
new trajectory will pave the way for more detailed measurements and
targeted observations of the Sun's closest neighbor.

The spacecraft was 124 million kilometers (77 million miles) from Earth
when the 188-second maneuver began at 3:13 p.m. EDT. Mission controllers
at the Johns Hopkins University Applied Physics Laboratory (APL) in
Laurel, Md., verified the start of the maneuver 6 minutes and 53 seconds
later, when the first signals indicating spacecraft thruster activity
reached NASA's Deep Space Network tracking station in Goldstone, Calif.

This maneuver -- which adjusted the orbital period from 11 hours, 36
minutes to 9 hours, 5 minutes -- was designed to deplete the remaining
oxidizer of the spacecraft's propulsion system in a final firing of the
large bi-propellant thruster. A second maneuver, scheduled for the
evening of April 20, will use the spacecraft's monopropellant system to
complete the transition to an 8-hour orbit.

The strategy to complete this transition involves the execution by the
MESSENGER flight team of carefully planned command sequences, says
MESSENGER Mission Design Lead James McAdams of APL. "The first
orbit-correction maneuver consumed the remaining oxidizer, which is one
of two propellants used for the higher-efficiency large thruster," he
explains. Although such an "oxidizer depletion" maneuver is not
uncommon, new procedures had to be developed and tested to make this
MESSENGER critical event possible and safe to perform.

After Friday's maneuver, the 8-hour orbit will remain highly eccentric,
with MESSENGER travelling between 278 kilometers (172 miles) and 10,314
kilometers (6,409 miles) above Mercury's surface. Reducing the orbital
period will increase from two to three the number of revolutions the
spacecraft will make about the planet each day, increasing the time that
the spacecraft will spend closer to the surface, says MESSENGER Mission
Systems Engineer Eric Finnegan, of APL.

The additional time at lower altitude, he says, will enhance the science
return. It will amplify the effectiveness of the high-energy
spectrometers used to determine the composition of the planet's surface
and will increase the number of altitude profiles that the laser
altimeter will be able to make in the northern hemisphere of the planet,
allowing for more detailed topographic maps. Operations at this lower
altitude will also enable higher-resolution imaging of Mercury's
southern hemisphere.

"The MESSENGER engineering and operations teams have once again made a
critical maneuver look easy," says MESSENGER Principal Investigator Sean
C. Solomon, of the Carnegie Institution of Washington. "The Science Team
is now looking forward to being able to address a host of scientific
questions on the composition, geological evolution, and environment of
Mercury that have been raised by earlier orbital observations. With our
new orbit, it feels as though we're embarking on a new mission."

------------------------------------------------------------------------

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.
Received on Tue 17 Apr 2012 02:17:26 PM PDT