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Posted by on October 30, 2007, 3:49 pm
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http://messenger.jhuapl.edu/news_room/status_report_10_30_07.html
MESSENGER Mission News
October 30, 2007
Sun Cuts into MESSENGER's Dance Around the Solar System
MESSENGER entered solar conjunction on October 26, when the
spacecraft's
trajectory moved it behind the Sun and out of clear view from Earth
for
several weeks. The team has just a limited time left before the Sun's
interference with the probe's radio transmission severely limits
communication with mission operations at the Johns Hopkins University
Applied Physics Laboratory in Laurel, Md.
"We expect we'll be able to communicate with MESSENGER for about
another
week or two before we completely lose contact," says APL's Andy
Calloway, the mission operations manager.
Although this is the longest solar conjunction of the mission - 47
days
- it's not the first. The previous solar conjunction period began
October 17, 2006 (just before the first Venus flyby on October 24,
2006),
and lasted about a month, including about two weeks with no communic
ations at all. "We learned a lot from that first solar conjunction,"
Calloway says. "Our planning for this solar conjunction is based, in
part, on our experience with that event."
Prior to a deep-space maneuver this past October 17, engineers began
shutting down the instruments, with the exception of the Gamma-Ray
Spectrometer (GRS) sensor on the Gamma-Ray and Neutron Spectrometer
(GRNS) instrument. The GRS has been left on but placed in a "sleep
mode"
that allows it to maintain a safe temperature without help from
mission
operators on Earth.
Operators also programmed MESSENGER's onboard computer to go 54 days
without receiving a command from Earth. Typically, MESSENGER's
autonomy
system will put the spacecraft into a safe state if it goes a week
without successfully receiving a command from home. This week, the
team
transmitted a series of commands that will carry MESSENGER through its
basic operations until mid-December.
The extra-long upload also includes commands to rotate MESSENGER on
roughly a daily basis during the solar conjunction. "The guidance and
control team developed an effective strategy to rotate the spacecraft
each day to keep the spacecraft's momentum from building up too
quickly
because of the combined effects of all the natural torques a
spacecraft
experiences," Calloway says.
MESSENGER's navigation team has asked operations to conduct a series
of
tracking observations called delta differential one-way ranging or
DDOR
(pronounced "Delta Door"). These measurements have been used
effectively
on the project since the first Venus encounter, and they improve
spacecraft navigation accuracy in a direction not observable with
ranging and Doppler observations alone. The technique uses distant
celestial objects known as quasars for reference points. The quasars,
along with the separation of two DSN complexes and highly accurate
clocks, combine to determine the angular position of the spacecraft in
the plane of the sky.
"DDOR is a supplement to the Doppler and ranging data that the
navigation team normally uses and helps them shrink their error
ellipse
so that they know much more precisely where MESSENGER is in space,"
Calloway says. 'We would like to execute trajectory correction
maneuver
19 soon after coming out of conjunction on December19, so we have
included many DDOR observations so that if the Sun remains quiescent
we
can use those data to plan and execute the maneuver.'
The operations team will also use the conjunction period to test the
final encounter sequence for the January 14 Mercury flyby on ground
simulators and review contingency plans and simulations so the team is
prepared for any outcome at the end of the conjunction period. This is
important because the team has three opportunities in December and two
opportunities in January to execute the next trajectory correction
maneuvers as MESSENGER's final approach to Mercury is fine-tuned.
------------------------------------------------------------------------
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.
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