Despite a series of problems, Mariner 10 accomplished the first two-planet space mission ever flown by a U.S. unmanned space probe. It was launched in November 4, 1973 to fly close to Mercury and Venus. Using ultraviolet filters, the probe revealed a wealth of details in the clouds surrounding Venus, including several distinct layers and a stationery convention cell near the planet's subsolar point. The Mercury flyby, which was a late addition to the project, allowed a unique path for Mariner to take advantage of Mercury's path around the sun and view the planet three times during the mission.