On Dec. 18, 1999, NASA launched its flagship Terra satellite into Earth's orbit to collect data about Earth's changing climate.

Terra contains five different sensors that monitor Earth's atmosphere, lands, oceans, and radiant energy. The satellite takes several different kinds of measurements that reveal how Earth is changing over time. It also tracks the spread of air pollution. It lifted off from Vandenberg Air Force Base in California on a United Launch Alliance Atlas II rocket and is still in operation today.