MESSENGER Sails on Sun's Fire for Second Flyby of Mercury

http://messenger.jhuapl.edu/news_room/details.php?id=102

MESSENGER Mission News
September 5, 2008

MESSENGER Sails on Sun's Fire for Second Flyby of Mercury

On September 4, the MESSENGER team announced that it would not need to
implement a scheduled maneuver to adjust the probe's trajectory. This
is
the fourth time this year that such a maneuver has been called off.
The
reason? A recently implemented navigational technique that makes use
of
solar-radiation pressure (SRP) to guide the probe has been extremely
successful at maintaining MESSENGER on a trajectory that will carry it
over the cratered surface of Mercury for a second time on October 6.

SRP is small and decreases by the square of the distance away from the
Sun. But, unlike rockets, so-called solar sailing requires no fuel.
And
although SRP's thrust is small, it will continue as long as the Sun is
shining and the "sail' is deployed, providing a continuous
acceleration
source for the probe.

MESSENGER's mission designers and its guidance and control team at the
Applied Physics Laboratory in Laurel, Md., along with the navigation
team, at KinetX, Inc., in Simi Valley, Calif., once viewed SRP as
something of a challenge to overcome, particularly for the critical
gravity-assist flybys - one of Earth, two of Venus, and three of
Mercury
- that the spacecraft would be executing to position it for Mercury
orbit insertion in 2011.

"Because of the changing proximity to the Sun during MESSENGER's
cruise
phase, the SRP varies from one to 11 times the value experienced at
Earth," explains APL's Daniel J. O'Shaughnessy, MESSENGER's Guidance
and
Control Lead Engineer. This variation in magnitude, as well as the
attitude-dependent direction of the resulting disturbance force and
torque, presents a significant challenge to mission designers and the
guidance and control team, he says.

"The Mercury flybys are designed to take the probe within
approximately
200 kilometers of the planet, so precision targeting is absolutely
critical," O'Shaughnessy says. Fly too low and the probe could crash
into the planet. Fly too far away and MESSENGER might have to use its
reserve fuel to correct for the acceleration loss. Either way, getting
off target could jeopardize the mission.

SRP was seen as an impediment to precise targeting, until the first
Mercury flyby in January 2008. About 26 days before that historic
event,
MESSENGER fired its thrusters to fine-tune its trajectory and aim for
the 200-kilometer-altitude flyby point. Prior to the maneuver, the
probe
was on a course to miss the flyby aim point by more than 2,000
kilometers.

After the maneuver, the probe was still about 9.5 kilometers off from
its target. "We still had one more opportunity for another
trajectory-correction maneuver four days before the flyby, but we were
able to skip it by solar sailtarget aim point. Ultimately, MESSENGER
missed its target altitude by
only 1.4 kilometers. This targeting was "spectacular," McAdams says.

The MESSENGER team has planned a more extensive use of this technique
for the second Mercury flyby. "We've developed a process to use the
SRP
force as a control for the trajectory," explains O'Shaughnessy. Using
the knowledge developed from the first flyby, the team has developed a
carefully planned sequence of probe-body attitude and solar-array
orientations that, if all goes according to plan, should reduce the
number of trajectory correction maneuvers needed in the future.

According to NASA, the only other visitor to Mercury used solar
sailing.
In 1974, when the Mariner 10 spacecraft ran low on attitude-control
gas,
its engineers angled the spacecraft's solar arrays into the Sun and
used
solar radiation pressure for attitude control, and it worked. But
MESSENGER's use of the technique represents the first time that a
spacecraft has successfully used solar sailing as a propulsion-free
trajectory control method for the targeting of planetary flybys.

------------------------------------------------------------------------

MESSENGER Team Member Highlight

While the scientists on the MESSENGER team decided what features to
image, and the guidance and control team calculated the pointing of
the
instrument, Nori Laslo - at 29 one of the youngest members on the team
-
pieced together the commands to tell the camera precisely what to do.
Read more about Laslo in the latest MESSENGER Team Highlight,
available
online at http://messenger.jhuapl.edu/who_we_are/member_focus.html.
<http://messenger.jhuapl.edu/who_we_are/member_focus.html/>

------------------------------------------------------------------------

Mercury Does a Sunset Tango with Mars and Venus in September

Sky watchers using binoculars and scanning the horizon about 15 to 30
minutes after sunset on September 7 will see a Venus-Mercury-Mars
grouping that looks like an isosceles triangle, with the Mars-Mercury
and Mars-Venus sides measuring about 2.5 degrees in length and the
Mercury-Venus side about 4 degrees. About 10 degrees to the upper left
of the triangle will be Spica, the brightest star of the constellation
Virgo.

On September 18, Venus, Mercury, and Mars will form an equilateral
triangle, whose sides are 4 degrees in length. And Spica will become a
part of this arrangement, sitting a few degrees to the left of the
triangle. Mercury will remain in the evening sky until October 6, and
then reappear on November 25.

------------------------------------------------------------------------

One Month to Go until Second Flyby of Mercury!

With just 30 days until MESSENGER's second encounter with Mercury, the
spacecraft remains safe and healthy, with all systems operating
nominally. All instruments are on except for the Mercury Laser
Altimeter
(MLA), which will be powered on September 25 and configured for the
encounter. "The final command load for the upcoming flyby is now
complete, has been reviewed by the team, and is being tested on the
hardware simulator," says APL's Peter Bedini, MESSENGER's project
manager. "A successful simulation will represent the passing of the
last
major milestone in the encounter preparations."

MESSENGER is about 55.95 million miles (90.04 million kilometers) from
the Sun and 87.55 million miles (140.9 million kilometers) from Earth.
At that distance, a signal from Earth reaches the spacecraft in 7.8
minutes. The spacecraft is moving around the Sun at 77.5 million miles
(124.7 kilometers) per hour.
------------------------------------------------------------------------

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.
ing the spacecraft closer to the intended
aim point," explains APL's Jim McAdams, who designed MESSENGER's
trajectory.

Three days earlier than originally planned, the team tilted
MESSENGER's
solar panels an extra 20 degrees away from the Sun. The resulting
change
in solar-array orientation moved the flyby altitude very close to the
.

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