MESSENGER Team Receives First Optical Navigation Images
- From: baalke@xxxxxxxxxxxxx
- Date: Wed, 9 Jan 2008 12:24:54 -0800 (PST)
http://messenger.jhuapl.edu/news_room/status_report_01_09_08.html
MESSENGER Mission News
January 9, 2008
MESSENGER Team Receives First Optical Navigation Images
MESSENGER mission operators at the Johns Hopkins University Applied
Physics Laboratory (APL) in Laurel, Md., have received the first eight
optical navigation images from the spacecraft. "We're going to be
taking
these images every day, up until just before the flyby, to make sure
that we are on target for our aim point above the surface of Mercury,"
said Louise Prockter of APL, the instrument scientist for the Mercury
Dual Imaging System (MDIS).
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. Because MESSENGER will fly very close to the surface of Mercury
during the January 14 flyby, optical navigation is used to provide an
independent method for finding and correcting subtle errors in the
trajectory. "This information will enable the navigation team to
verify
that the spacecraft is in the correct position for the flyby, or
whether
a last-minute maneuver will be needed to avoid either coming too close
to the planet or missing it by so much that a large amount of fuel
would
be needed to return the probe to its optimum trajectory," Prockter
noted.
To determine the position of the spacecraft, it is necessary to see
the
target body - in this case, Mercury - in the same field of view as the
background star field, using MDIS. "The stars are far away, so their
positions may be assumed to be fixed in space," Prockter said. "The
position of Mercury along its orbit is also well known from hundreds
of
years of ground-based telescopic observations. Thus, by comparing
where
Mercury is in the field of view to the stars visible behind it, and by
controlling where the camera is pointing, we can estimate the position
of the spacecraft."
The 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.
"Unfortunately, such long exposures tend to saturate bright objects,
such as Mercury, making it difficult to image both the planet and the
stars in the same WAC image," Prockter said. 'The NAC is not sensitive
enough to see stars, but it has a resolution ~7 times better than the
WAC and is excellent for imaging the planet limb."
To carry out optical navigation with MESSENGER, the team uses a
combination of the two imagers, taking a star image with the WAC, then
quickly switching to the NAC and taking 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 team has obtained practice optical navigation images
on
previous flybys of Earth and Venus, but the Mercury encounter is the
first time we have used this method 'for real' to determine the
position
of the MESSENGER spacecraft," Prockter said.
------------------------------------------------------------------------
NASA Teleconference to Preview MESSENGER's Flyby of Mercury
NASA will host a media teleconference at 1 p.m. EST on Thursday,
January
10, to preview MESSENGER's historic January 14 flight past Mercury.
The
briefing participants are:
* Marilyn Lindstrom, MESSENGER program scientist, NASA
Headquarters,
Washington
* Sean Solomon, MESSENGER principal investigator, Carnegie
Institution of Washington
* Eric Finnegan, MESSENGER mission systems engineer, Johns Hopkins
University Applied Physics Laboratory, Laurel, Md.
* Faith Vilas, MESSENGER participating scientist and director, MMT
Observatory at Mt. Hopkins, Ariz.
To participate in the teleconference, reporters should call
1-888-398-6118 and use the pass code "Mercury." International
journalists should call 1-210-234-0013. Audio of the teleconference
also
will be streamed live at: http://www.nasa.gov/newsaudio
------------------------------------------------------------------------
As the flyby continues toward closest approach, additional information
and features will be available online at
http://messenger.jhuapl.edu/mer_flyby1.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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