Mariner 10 may have been the last of the Mariner spacecraft series, but it was the first to complete many new tasks. It was the first U.S. spacecraft to visit two planets - Venus and Mercury - in one mission. It was also the first to broadcast images of Venus back to Earth. The greatest achievement of the spacecraft was its use of a gravitational assist: Mariner 10 used Venus’ gravitational field to slingshot itself towards Mercury, saving fuel and picking up speed.