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Posted by on February 28, 2010, 7:46 pm
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http://messenger.jhuapl.edu/news_room/details.php?id=142
MESSENGER Mission News
February 27, 2010
MESSENGER's Odometer Reading: Four Billion Miles!
Today the MESSENGER spacecraft crossed the four-billion-mile mark since
its launch. The probe has completed about 81 percent of its journey
toward its destination to be the first spacecraft inserted into orbit
about Mercury.
That MESSENGER's odometer reading has reached another major milestone
reminds us of the long and complex route that our spacecraft must
follow. Mercury orbits deep within the Sun's gravity well. So, even
though the planet can be as close as 82 million kilometers (51 million
miles) from Earth, getting the probe into orbit around Mercury depends
on an innovative trajectory that uses the gravity of Earth, Venus, and
Mercury itself to slow and shape the probe's descent into the inner
solar system.
On its 4.9 billion-mile journey to becoming the first spacecraft to
orbit the planet Mercury, MESSENGER has flown by Earth once, Venus
twice, and Mercury three times.
"Four billion miles, more than 43 times Earth's distance from the Sun,
is an impressive figure," says MESSENGER Principal Investigator Sean
Solomon, of the Carnegie Institution of Washington. "But MESSENGER is a
well-built vehicle, with many more miles of productive work ahead. The
Mercury orbital phase of our mission is barely one year ahead, and the
team is hard at work to ensure that we are ready for the intensive
activity that awaits."
All of MESSENGER's instruments are on except for the Mercury Laser
Altimeter (MLA), which was turned off after its recent Earth-ranging
exercise. On February 19, the Earth-orbiting Ice, Cloud, and land
Elevation Satellite (ICESat) and MESSENGER changed their orientations to
point at one another for a three-part ranging exercise with ICESat
receiving signals sent by the MESSENGER MLA instrument. The ICESat team
is now examining the data to confirm that the MLA signals were indeed
detected.
On February 22, the team conducted the first in a series of short
solar-array-offset-characterization tests. "These exercises are designed
to improve our model of solar-array performance prior to orbit, and each
is a simple test that can be executed in a few minutes without load
management," explained MESSENGER Mission Project Manager Peter Bedini,
of The Johns Hopkins University Applied Physics Laboratory.
The test measures the output of each solar array wing individually by
placing it at a 72° Sun-offset angle. During the 5-minute measurement,
the other wing is rotated off of the Sun at 95° so that it contributes
no power. The measurement will be repeated at several solar distances to
allow for correlation to previous tests at ~0.5 astronomical units (AU),
as well as additional insight into orbit performance at ~0.3 to 0.45 AU.
This solar-array exercise also presented an opportunity to test for the
effect of jitter on the camera system. The Mercury Dual Imaging System
collected 66 images while the solar panels were re-positioned. These
images have been downloaded and are being analyzed.
------------------------------------------------------------------------
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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