Bits of rock and dirt collected and returned to Earth by Apollo astronauts and robotic missions by the Soviet Union and China. To date, roughly 850 pounds (385 kg) of samples have been returned to Earth, with more scheduled for return in the next few years. In the United States, most of the samples are stored in an environmentally controlled facility at Johnson Space Center in Houston. They are parcelled out (usually a few grams at a time) to researchers around the world. They have helped scientists piece together the formation history and evolution of the Moon, as well as the history of asteroid impacts on the Moon and Earth. Some samples have been held unopened since the 1970s, although a few were opened for the first time in 2021 and 2022.