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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.

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.

Launch and Cruise

MESSENGER is scheduled to launch from Cape Canaveral Air Force Station, Fla., during a 13-day period that opens August 2, 2004. It will return to Earth for a gravity boost in August 2005, then 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.

Working from Orbit

MESSENGER's orbit about Mercury is highly elliptical (egg-shaped), 200 kilometers (124 miles) above the surface at the lowest point and more than 15,193 kilometers (9,420 miles) at the highest. The plane of the orbit is inclined 80° to Mercury's equator, and the low point in the orbit is reached at a latitude of 60° north. The low-altitude orbit over the northern hemisphere allows MESSENGER to conduct a detailed investigation of Mercury's geology and composition of the giant Caloris impact basin – the planet’s largest known surface feature.

About 33% of the spacecraft's propellant is required for Mercury orbit insertion – the process of placing the spacecraft into its primary science orbit around Mercury. MESSENGER's thrusters must slow the spacecraft by just over 0.83 kilometers (0.52 miles) per second. As the spacecraft approaches Mercury, the largest thruster must be pointed close to the forward velocity direction of the spacecraft. The first maneuver (lasting about 15 minutes) places the spacecraft into a stable orbit; it also sets up a much shorter cleanup maneuver two to three days later near the orbit's lowest point.

After MESSENGER arrives in the primary science orbit, small forces, such solar radiation pressure – the force exerted by sunlight - slowly change the spacecraft's orbit. Although these small forces have little effect on MESSENGER's 12-hour orbit period, they can increase the spacecraft's minimum altitude, orbit inclination, and latitude of the surface point below MESSENGER's minimum altitude. Left uncorrected, the increase in the spacecraft's minimum altitude would prevent satisfactory completion of certain science goals.

To keep this minimum altitude below 500 kilometers (310 miles), propulsive maneuvers must occur in pairs once every Mercury year - every 88 days. The first maneuver in each pair increases the orbit period to 12 hours, 15 minutes by speeding up the spacecraft at its closest distance from Mercury. Two-and-a-half orbits later a maneuver at the farthest distance from Mercury slows the spacecraft just enough to adjust the orbit period back to 12 hours and return the minimum altitude to 200 kilometers (124 miles). Because the sunshade must protect the main part of the spacecraft from direct sunlight during propulsive maneuvers, the timing of these maneuvers is limited to a few days when Mercury is near the same point in its orbit as it was at Mercury orbit insertion.

MESSENGER's 12-month orbit covers two Mercury solar days; one Mercury solar day, from sunrise to sunrise, is equal to 176 Earth days. The first solar day is focused on obtaining global map products from the different instruments, and the second focuses on targeted science investigations.

Source: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington