Thirty-four years ago today, on September 15, 1991 (August 8, 1991 lunar calendar), the largest man-made satellite entered space. On September 15, 1991, five astronauts on the US space shuttle "Discovery" successfully released a high-altitude atmospheric exploration satellite in space. It weighs 6.6 tons and is worth 740 million dollars. It is said that this is the largest satellite released so far. According to foreign reports, the satellite will operate at an altitude of 570 kilometers above the ground for a year and a half to observe the pollution of the earth's atmosphere and the destruction of the ozone layer. It is the first satellite in the United States to study the fragile ozone layer in a systematic and detailed manner. This exploration satellite is the first satellite launched by NASA for the "Planet Earth Program". " The Planet Earth Project "is a long-term study of the Earth's environment, with the aim of studying atmospheric pollution and the impact of human activities on the biosphere.