October 7, 2008 11:52 AM PDT
Since its launch in 2004, Messenger has completed six orbits of the sun and has swung around the Earth once and Venus twice to prepare for its second encounter with Mercury on October 6. Messenger will pass by Mercury again in September 2009 before its final insertion into orbit in 2011.
Why the long way? Packing enough fuel to run a direct route and brake when the spacecraft reaches Mercury would require a much larger rocket for launch and greatly increase the cost of the project. To slow down enough to orbit Mercury, Messenger is using a technique called "gravity assist." Instead of flying around a body or planet, which boosts the speed of a spacecraft, Messenger bounces off the front of the body to slow down.
Photo by NASA
Caption by Andy Smith
