On September 29, NASA's spacecraft Messenger will use the gravitational pull of the planet mercury to alter its path so it can then orbit that planet in 2011.    

Confused? If so, a free online simulator developed by a team from Montana State University's Burns Technology Center can help explain.

This program was funded through a NASA educational outreach grant. It shows the science behind spacecraft gravity assist maneuvers.

Technology center employee Keri Hallau helped design the simulator. She said these fly-bys are a technique often used by NASA with effective results.    

"It's pretty dramatic. In the case of New Horizons spacecraft, which is on its way to Pluto, it shaved six years off the length of the mission," Hallau said.

The September 29 fly-by of Messenger will be the last of six for the spacecraft. It will be the first craft ever to orbit Mercury.

Click here to check out the simulator.