A probe is a robotic spacecraft that orbits a planet and sends back information about that planet, such as temperature, what the atmosphere is made from, what the weather is like, what the land below looks like and sends all this information back to Earth.
Famous probes include:
These probes were launched in 1977 to study the giant planets of Jupiter and Saturn. They sent back photographs of these planets that couldn't be seen, even by the biggest telescopes on Earth.
Once they were finished with Jupiter and Saturn, they sped out into space, still transmitting information. Voyager 2 stopped to take images of Uranus and Neptune.
At the moment, Voyager 1 is beginning to exit from our solar system and is currently 18.5 billion km away.
Voyager 2 is 15 billion km from Earth.
Galileo was sent by NASA to study Jupiter and its moons on a 14 year mission. It was launched from the Space Shuttle in 1986, flew by Venus in 1990 and then reached Jupiter in 1995, where it orbited the planet studying it and sending back data. Finally, it was crashed into Jupiter's atmosphere, having been damaged by the moon Io's radiation.
Galileo launched a probe into Jupiter's atmosphere where it lasted for 58 minutes before being overheated by Jupiter's high pressures.
Cassini was a joint NASA-European Space Agency probe sent to study Saturn and its moons.
It was launched in 1997 and reached Saturn in 2004.
In 2005, it launched a probe that landed on the moon Titan, taking photographs as it entered Titan's atmosphere.
The probe will be destroyed by being flown into Jupiter in 2017.
The New Horizons probe sent to observe Pluto. Launched in 2006, it flew by Jupiter in 2007, Saturn in 2008 and Uranus in 2011; Radio signals take 7 hours to travel from the probe to Earth. It should arrive at Pluto in 2015