On April 2, we said goodbye to Tiangong I, China's first space lab. According to the China Manned Space Agency, Tiangong 1 reentered(重返) the Earth's atmosphere(大气层) and some of its debris(碎片) fell into the South Pacific Ocean. There are many spacecraft that are still in orbit(轨道) above the earth. After finishing their trips, they will all reenter the Earth's atmosphere like Tiangong I.
There are two kinds of reentries: controlled reentry and uncontrolled reentry. Some satellites and manned spacecraft come back to the earth in a controlled reentry way. Experts calculate(计算) the path of the falling spacecraft and its speed. They can guide the spacecraft to fall into a chosen area.
Some spacecraft come back in an uncontrolled reentry way. It is hard to tell when and where they will fall until the last hours. The US space station Skylab came back partially uncontrolled in 1979. Parts of the station fell in Western Australia, but no one was hurt.
During the reentry, most of the spacecraft will bum up while passing through the earth atmosphere. Only a small amount of the debris ends up falling into the ocean.
