A set of robotic twins will measure lunar gravity while chasing one another in circles around the moon. The two spacecraft are each about the size of a washing machine and will be launched together.
They're due to blast off Thursday aboard an unmanned rocket and will separate an hour into the flight. After traveling independently to the moon, they will circle the planet for three months.
By creating the most precise lunar gravity map ever, scientists hope to figure out what's beneath the lunar surface, all the way to the core.
Since the Space Age began in 1957, 109 missions have targeted the moon and 12 men have walked its surface during six landings.