[meteorite-list] MESSENGER Endures Its First Hot Season

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 17 Jun 2011 09:34:34 -0700 (PDT)
Message-ID: <201106171634.p5HGYYGV010697_at_zagami.jpl.nasa.gov>

http://messenger.jhuapl.edu/news_room/details.php?id=172

MESSENGER Mission News
June 13, 2011

MESSENGER Endures Its First Hot Season

Yesterday the MESSENGER spacecraft successfully completed the first of
four "hot seasons" expected to occur during its one-year primary mission
in orbit about Mercury. During these hot seasons, the Sun-facing side of
the probe's sunshade can reach temperatures as high as 350?C.

These hot conditions are the result of two concurrent circumstances,
says MESSENGER Mission Systems Engineer Eric Finnegan, of the Johns
Hopkins University Applied Physics Laboratory in Laurel, Md. "Mercury is
in an eccentric orbit, and its distance from the Sun varies over 88
days, from 43,689,229 miles to 28,816,300 miles," he explains. "On May
13, Mercury began heading closer to the Sun in its orbit. The planet
reached its closest distance from the Sun on June 12."

The second contributor to this heat is the geometry of MESSENGER's orbit
relative to the hot dayside of Mercury. The spacecraft is in a highly
eccentric orbit around the planet, approaching to within 310 miles of
the surface every 12 hours.

"During this hot period, the closest point of approach of the spacecraft
to Mercury's surface occurs on the sunlit side of the planet, so for
almost one hour per orbit the spacecraft must pass between the Sun on
one side and the hot dayside surface of the planet on the other,"
Finnegan says. "To add further extremes, this season is also when the
spacecraft passes over the nightside of the planet at high elevations
and experiences the longest solar eclipses of the mission. During this
period, when eclipses last as long as 62 minutes per orbit, the solar
arrays are not illuminated and the spacecraft must derive its power from
its internal battery."

High temperatures are always a risk to mechanical and electronic
systems, and the geometry of this portion of the orbit severely
constrains the ability of the spacecraft to cool itself by radiating
heat to cold space. MESSENGER engineers have taken several steps to
ensure that the spacecraft remains safe.

"We rotated the solar arrays off the Sun through some of the hottest
points so they do not have a view to either the Sun or the hot, dayside
surface of the planet," Finnegan says. "We are power cycling some of the
more sensitive instruments to reduce their internal heat dissipation. In
a manner similar to the treatment of the solar arrays, we are also
adjusting the attitude of the spacecraft to keep some of the more
sensitive parts of the spacecraft from seeing the hottest parts of the
planet's surface."

All of the instruments have been operating during this period. Finnegan
says that there have been times during each orbit when instruments are
turned off, however, mostly to conserve power during eclipses.

These conditions are expected to recur approximately every 88 days
(i.e., the time it takes Mercury to orbit the Sun). MESSENGER can
therefore look forward to three more hot seasons during the course of
its primary 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
entered orbit about Mercury on March 17, 2011 (March 18, 2011 UTC), to
begin a yearlong study of its target planet. 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 Fri 17 Jun 2011 12:34:34 PM PDT