Apart from sending astronauts into space, we also use satellites, probes and telescopes to study space. Like moons, man-made satellites revolve around planets and help us learn more about them. A probe is an unmanned spacecraft sent into space to study objects other than the Earth. We have even put huge telescopes into orbit. The Hubble Space Telescope, sent into orbit in 1990, is the best-known among these. It is better to have a telescope in space than on Earth, because then there is no atmosphere to distort the image received by the telescope. However, the biggest telescopes are too big to be sent to space.