MESSENGER uses gravity assists from Earth,
Venus and Mercury to lower its speed relative to Mercury at
orbit insertion. In a gravity assist, a spacecraft flies close
by a planet and picks up (or loses) a tiny amount of the planet's
angular momentum around the Sun. The planet is so massive
(compared with the spacecraft) that its orbit does not change.
But each gravity assist changes the shape, size and tilt of
MESSENGER's orbit until the propellant onboard is sufficient
to insert the spacecraft into its planned scientific orbit
around Mercury. "Mercury orbit insertion" is the mission planners'
term for the maneuver that will move MESSENGER from an orbit
around the Sun to an orbit around Mercury.
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MESSENGER launched from Cape Canaveral
Air Force Station, Fla., on August 3, 2004. It returned to
Earth for a gravity boost on August 2, 2005, then it will
fly past Venus twice, in October 2006 and June 2007. The
spacecraft uses the tug of Venus’ gravity
to resize and rotate its trajectory closer to Mercury’s
orbit.
Three Mercury flybys, each followed about two months later
by a course correction maneuver, put MESSENGER in position
to enter Mercury orbit in March 2011. During the flybys –
set for January 2008, October 2008 and September 2009 –
MESSENGER will map nearly the entire planet in color, image
most of the areas unseen by Mariner 10, and measure the composition
of the surface, atmosphere and magnetosphere. It will be the
first new data from Mercury in more than 30 years –
and invaluable for planning MESSENGER’s yearlong orbital
mission.
During the 4.9-billion (4,900,000,000) mile (about 7.9-billion kilometers) journey from launch to Mercury orbit, the spacecraft will experience a range of extreme conditions. One such extreme includes repeated passes at a distance 70% closer to the Sun than the Earth's average distance from the Sun. At this distance, the spacecraft receives about 11 times more of the Sun's thermal energy than Earth-orbiting satellites. Shortly after each of the first two Mercury flybys, the spacecraft will approach the ultimate speed record for all spacecraft by achieving speeds of over 140,000 miles per hour (over 225,300 kilometers per hour) relative to the Sun - about eight times faster than the speed of NASA's Space Shuttle in low-Earth orbit. The Helios-2 spacecraft achieved at least 150,000 miles per hour relative to the Sun on April 17, 1976.
![](images/traj_helioecl_earth2moi_07122008_sm.jpg) |
The
image above shows MESSENGER's entire trajectory, looking down
on Earth's orbit plane. Click on it for a detailed explanation
of MESSENGER's path. |
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How much fuel does MESSENGER use in each propulsive event? |
How much fuel remains on board? |
How far was MESSENGER from Earth and the Sun when the maneuver started? |
When did each propulsive event occur? |
How close to the intended goal did MESSENGER perform? |
What information (orbital elements) best describes the orbit between each propulsive event? |
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