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Why an Earth Flyby?

The MESSENGER spacecraft swooped around Earth on August 2, 2005, with a closest approach point of about 1,458 miles (2,347 kilometers) over central Mongolia at 3:13 p.m. EDT. MESSENGER is heading toward a rendezvous with Mercury, so why did it come back to Earth a year after launch (which occurred on August 3, 2004)? In 1974, the Mariner 10 spacecraft made it to Mercury in about 5 months, so what is MESSENGER doing? The answer is very simple: Mariner 10 was a flyby mission, not an orbital mission. A flyby mission is just what it sounds like, the spacecraft "flies by" the target and collects data for only a brief time. To understand Mercury, you need a spacecraft that orbits for an extended period of time, allowing numerous detailed measurements over the whole planet. And that is exactly what MESSENGER will do - orbit Mercury for an entire Earth year.

Mariner 10's trip to Mercury was a downhill slide. Mercury is the closest planet to the Sun, so a spacecraft traveling there from Earth can simply "fall" into the Sun's gravity well - a lot like skiing down a hill. Gravity does the work; the skier just has to maintain balance and steer. If you head straight down the hill, you accelerate, but you manage your speed if you zigzag from side-to-side. By the time Mariner 10 reached Mercury, its speed relative to the planet was about 11 km/s - it had no chance of slowing down! Since MESSENGER will go into orbit about Mercury it can't just slide downhill - its speed and direction have to be very carefully controlled through a series of six close planetary encounters. The Earth gravity assist helped to aim MESSENGER toward a precise flyby of Venus for its second gravity assist.

Why Take Images of the Earth?

The planetary flybys, while required for MESSENGER to settle eventually into its mapping orbit around Mercury, are also critical opportunities for the mission team to "test drive" the spacecraft and its scientific instruments. Operating a spacecraft is a complicated balancing act - scientists want to point the spacecraft in certain directions to gather data, while flight personnel must make sure MESSENGER's solar panels get enough light and that sensitive components don't accidentally point at the Sun. The Earth and Venus flybys allow the Mission Operations team to practice for the three critical Mercury flybys.

The Mercury Dual Imaging System (MDIS for short) acquired spectacular images of the Earth during the flyby, including a "film" of Earth as it receded in the distance. These images also served important engineering functions: the camera's automatic exposure function was checked on a target with contrast greater than what scientists expect to find on Mercury; pre-launch calibration values were verified, the camera's gimbal pointing was exercised, and stray light tests were performed. All told, the Earth flyby was a fantastic boon to the Mission Operations and Science Teams.

Beyond the critical operations and engineering and calibration value, the MDIS Earth flyby images also reminded us of the spectacular beauty of our own planet.

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