MESSENGER Beams Back First Approach Images of Mercury
- From: baalke@xxxxxxxxxxxxx
- Date: Fri, 3 Oct 2008 15:01:38 -0700 (PDT)
http://messenger.jhuapl.edu/news_room/details.php?id=107
MESSENGER Mission News
October 3, 2008
MESSENGER Beams Back First Approach Images of Mercury
MESSENGER mission operators at the Johns Hopkins University Applied
Physics Laboratory (APL) in Laurel, Md., have received the first
optical
navigation images
<http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?
gallery_id=2&image_id=206>
from the spacecraft. "We will be taking seven additional sets over the
next three days as the spacecraft approaches the planet,' said APL's
Eric Finnegan, the Mission Systems Engineer.
Optical navigation is commonly used to tie the position of a
spacecraft
to the position of a target body to ensure a safe and well-positioned
flyby, particularly when the position of the target body is uncertain
or
if the navigation process has not been validated in flight. "During
the
first encounter with Mercury, both of these issues were of concern to
mission planners," Finnegan explained. "However, following the highly
accurate flyby in January, the necessity of these images for critical
trajectory planning was removed."
"For successful optical navigation, we need to see the target body in
the same image sequence as the background star field,' said
MESSENGER's
Navigation Team Chief Ken Williams of KinetX, Inc. "Stars are far
away,
so to us, it appears that their positions are fixed in space. By
comparing where Mercury is in the field-of-view with the stars visible
behind it, and by controlling where the camera is pointing, we can
estimate the position of the spacecraft."
The Mercury Dual Imaging System (MDIS) instrument consists of two
imagers, a wide-angle camera (WAC) with a 10.5º field of view, and a
narrow-angle camera (NAC), with a 1.5º field of view. These imagers
are
always pointed at the same place, and the NAC footprint falls in the
center of the WAC footprint. The WAC has a filter specially designed
for
imaging stars, most of which are so faint that long (up to 10-second)
exposures are required.
The MESSENGER team employs both cameras for optical navigation, taking
a
star image with the WAC, and then quickly switching to the NAC for an
image of the planet limb. Because the images are taken within seconds
of
each other, they can be used to see where the planet is compared with
the star field.
The navigation images snapped during this flyby will also help the
team
plot MESSENGER's yearlong orbital survey of Mercury, which begins in
March 2011. MESSENGER will fly very close to the surface of
Mercury - within 200 kilometers (124 miles) - during the October 6
flyby, as
it did in January. However, during this encounter, the navigation team
will rely only on radiometric tracking data during closest approach.
------------------------------------------------------------------------
As the flyby continues toward closest approach, additional information
and features will be available online at
http://messenger.jhuapl.edu/mer_flyby2.html, so check back frequently.
Following the flyby, be sure to check back to see the latest released
images and science results!
------------------------------------------------------------------------
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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