Research satellites have revealed that the Earth's upper atmosphere expands by day and shrinks at night. Without space probes, we would not have this knowledge. Space probes are unmanned spacecraft which are not designed to return to Earth. Much of the information we have about the planets of our solar system has been provided by probes sent into space.
|Probes to Planets||Probes of the Universe|
|The Outer Planets||HEAO|
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