MESSENGER's journey will include 1 Earth flyby, 2 Venus flybys and 3 Mercury flybys before it enters orbit in 2011. The flybys will help focus the science mission when MESSENGER enters orbit. It is expected to orbit Mercury for one year.
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Fast Facts
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry & Ranging) is only the 2nd spacecraft sent to Mercury. Mariner 10 completed its mission in 1975.
Mercury is the least explored of our solar system's inner planets.
Mission Design
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, FL, during a 15-day period that opens August 2, '04. It will return to Earth for a gravity boost in July '05, then fly past Venus twice, in October '06 & June '07. The spacecraft uses the tug of Venus’ gravity to resize and rotate its trajectory closer to Mercury’s orbit.
3 Mercury flybys, each followed about 2 months later by a course correction maneuver, put MESSENGER in position to enter Mercury orbit in March '11. During the flybys – set for January '08, October '08 and September '09 – MESSENGER will map nearly the entire planet in color, image most of the areas unseen by Mariner 10, & measure the composition of the surface, atmosphere & magnetosphere. It will be the 1st new data from Mercury in more than 30 years – & 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 & more than 15,193 kilometers (9,420 miles) at the highest. The plane of the orbit is inclined 80° to Mercury's equator, & 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)/ second. As the spacecraft approaches Mercury, the largest thruster must be pointed close to the forward velocity direction of the spacecraft. The 1st 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, & 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 1st maneuver in each pair increases the orbit period to 12 hours, 15 minutes by speeding up the spacecraft at its closest distance from Mercury. 2 1/2 orbits later a maneuver at the farthest distance from Mercury slows the spacecraft just enough to adjust the orbit period back to 12 hours & 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 2 Mercury solar days; 1 Mercury solar day, from sunrise to sunrise, is equal to 176 Earth days. The 1st solar day is focused on obtaining global map products from the different instruments, and the 2nd focuses on targeted science investigations.
