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Posted by baalke on September 27, 2007, 5:42 pm
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http://messenger.jhuapl.edu/news_room/status_report_09_27_07.html
MESSENGER Mission News
September 27, 2007
MESSENGER Team Wraps Up Radio Science Test
MESSENGER's Radio Science (RS) team recently completed a month-long
orbit simulation test to measure Doppler performance using the probe's
low-gain radio frequency (RF) antennas. The test was conducted in an
environment similar to one MESSENGER will encounter during its Mercury
orbit phase in 2011-12.
The Radio Science team will use MESSENGER's RF and telecommunications
systems to study several aspects of Mercury. The frequency of
MESSENGER's radio signal, as received by the Deep Space Network (DSN)
ground stations, depends on the velocity of the spacecraft as seen
from
Earth. The large, powerful DSN antennas in California, Spain, and
Australia are operated by NASA's Jet Propulsion Laboratory.
"By monitoring small changes in radio frequency, we can determine the
velocity and acceleration of the MESSENGER spacecraft,' explains Radio
Science Instrument Scientist Mark Perry of the Johns Hopkins
University
Applied Physics Laboratory in Laurel, Md., adding that the frequency
changes are called Doppler shifts. "Measuring the very slight changes
in
MESSENGER's velocity allows us to derive variations in Mercury's
gravity
with great precision. The gravity field then tells us how mass is
distributed on and within Mercury, particularly variations in the
thickness of the crust, the deep structure of such features as craters
and mountains, and even the size of the planet's core."
During the orbit simulation test - conducted from August 20 to
September
19 - MESSENGER's trajectory carried it nearly along the path of
Mercury's orbit. On 10 separate occasions the team assessed the
quality
of Doppler observations by collecting an hour of tracking data using
the
low-gain antennas.
"This campaign has given us hard data on the performance of MESSENGER
and the DSN ground system in the configuration that they will have
during the orbital phase of the mission," says Perry. The team also
hopes to see how variations in solar activity affect the Doppler
measurements. The radio science team will use the data collected to
refine MESSENGER's orbital operations plans.
RS Experiments to Reveal Mysteries of Mercury's Interior
One of the highest-level science goals for MESSENGER is to elucidate
the
structure of Mercury's deep interior. A major surprise from the first
Mariner 10 flyby of Mercury in 1974 was that Mercury has an internal
magnetic field, which may indicate that the planet has a liquid outer
core. At that time, theories for planetary evolution predicted that
Mercury's core would be frozen solid.
Earth-based radar measurements recently detected a variation in
Mercury's spin rate over the planet's 88-day year. The amplitude of
that
variation, according to a theory worked out more than 30 years ago by
MESSENGER Co-Investigator Stanton Peale, provides strong evidence that
at least the outer part of Mercury's core is molten.
"MESSENGER's Radio Science investigation will measure the gravity
field
of Mercury and, in conjunction with other data, is expected to answer
major questions about the size of Mercury's core, the extent to which
the core is molten, and how the core is coupled to the overlying
mantle,
which in turn will help in understanding the thermal evolution of the
planet," says David Smith, a planetary geodesist at NASA's Goddard
Space
Flight Center and a MESSENGER Co-Investigator.
Radio Science Cruise-Phase Calibrations
Throughout MESSENGER's cruise phase, the team has calibrated the radio
frequency system to achieve the most accurate velocity measurements.
Perry says that part of this calibration is to determine how much the
Doppler frequency shifts are affected by the solar wind.
Because Mercury is so close to the Sun, this determination is more
important for MESSENGER than for most missions. "A second part of the
calibration is to verify that we can measure accurate Doppler shifts,
even when using MESSENGER's small low-gain antennas," Perry explains.
"We also tested our analysis techniques by extracting the shape of
Venus' gravity field using the data from the second Venus flyby."
Mercury Flyby Operations
During MESSENGER's first flyby of Mercury on January 14, 2008, and the
two subsequent passes of Mercury on October 6, 2008, and September 29,
2009, the Radio Science team will use the Doppler shifts to get an
early
read on Mercury's gravity field.
"MESSENGER's radio frequency system and the upgraded DSN antennas
enable
an accuracy 10 to 100 times better than obtainable from Mariner 10,"
Perry notes. "Our first glimpse of the shape of Mercury's gravity
field
next January may improve considerably our determination of the
character
of Mercury's core, achieving important science three years before
MESSENGER's primary orbit phase begins!"
------------------------------------------------------------------------
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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