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Gravity Assists

The MESSENGER spacecraft greatly reduces its fuel load requirement by using the gravitational attraction of planets to bend, rotate, and shrink its orbit about the Sun. The more massive the planet and the closer the spacecraft passes above the planet, the greater the change in the spacecraft's orbit. For an August 2004 launch, the journey to Mercury has one Earth flyby, two Venus flybys, three Mercury flybys, and five major course-correction maneuvers prior to Mercury orbit insertion (also called "Mercury arrival"). Given the launch date and spacecraft operational rules (e.g., sunshade must protect the spacecraft from overheating; Earth-based antennas must monitor every course-correction maneuver without interference from the Sun; science goals and other factors limit the shape, orientation, and dimension of the orbit around Mercury; the spacecraft must carry enough fuel for all course-correction maneuvers), the path from Earth to Mercury requires the lowest amount of fuel.

MESSENGER's flight to Mercury moves closer to the Sun as it catches up with the fastest planet in our solar system. Earth orbits the Sun every 365 days (Earth's "orbit period"), travels at an average speed of 29.8 kilometers per second (18.5 miles per second) relative to the Sun, and has an average distance from the Sun of 1 Astronomical Unit (an "AU" is the average Sun-Earth distance, about 150 million kilometers or 93 million miles).

Figure 1 presents the relative dimensions of and orientations of the orbits of Earth and Mercury. Table 1 shows how each gravity-assist flyby changes these orbit characteristics.

 

Figure 1. Relative dimensions and orientations of orbits of Earth and Mercury

 


Figure 2. MESSENGER mission timeline featuring major trajectory adjustments (DSM dates may change)

 

Click here to view a more detailed explanation showing the contribution of each gravity-assist flyby.


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