Spaceflight began on October 5, 1957 when the Soviet Union launched the first artificial satellite into orbit. The satellite was a 23-inch diameter aluminum ball named Sputnik I which weighed 184 pounds. It carried a radio transmitter. Manned missions to Earth orbit were followed by a series of Moon landings beginning in 1969 by the United States. Unmanned scientific space probes have travelled to all the known planets in the Solar System and into deep space. Visits to comets and asteroids have also been made. More and more advanced manned space stations have been placed in orbit. Both government and private plans have been made to make a manned landing on Mars.