The United States' Mariner 4 probe was launched in 1964 and took the first ever close-up images of the Mars' surface. A few years later the then Soviet Union launched a series of modules that orbited Mars. In 1975, the United States launched the Viking probes, which both landed on Mars' surface and transmitted color images. The U.S. also launched the Mars Observer probe, which lost contact with Earth before it reached Mars in 1993. The next U.S. mission to Mars was the Mars Global Surveyor, which was launched in 1996 and transmitted massive amounts of data before it stopped communicating with Earth in 2006. NASA currrently has one other rover on Mars, Opportunity, which has been exploring the planet since 2004.