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There are two schools of thought on what exactly the outer Solar System is. One group believes that the outer Solar System is everything beyond the main asteroid belt that orbits between Mars and Jupiter. That means that the terrestrial planets makeup the inner Solar System and Jupiter, Saturn, Uranus, and Neptune make up the outer. The second school of thinkers believe that this is the middle Solar System and the outer is everything that is outside of Neptune’s orbit.
No matter how you define it, the outer Solar System is an area that humans have not explored in any depth. Pioneer 10 in 1972 was the first spacecraft to visit a planet in the outer areas. It used the gravity of Jupiter to sort of slingshot itself so that it could flyby Mercury. Voyager 1 and 2 visited the outer system in more detail after they were launched in 1977. Each of those probes is still sending data back to Earth. They are now leaving our Solar System and sending back the first information about the heliopause and beyond. Pioneer 11 also made a brief visit to the vicinity after its launch in 1979.
There are currently several probes exploring the outer Solar System. Cassini-Huygens is now encountering Saturn and many of its moons. The Cassini orbiter section is sending unexpected data nearly every day. The Huygens lander was deposited on Saturn’s moon Titan and returned a great deal of data before battery power was depleted. The data being sent back has lead to many new discoveries about the Saturnian system. Future missions are being planned to explore Uranus and Neptune in more detail and to hopefully flyby a few of the dwarf planets and into the Kuiper Belt.
With current technology, exploring the outer Solar System is difficult. Because of that, other missions have a greater priority. In the last 30 years technology has made many advances that should make more distant exploration a viable possibility. It is the hope of many space agencies to visit a great number of outer system bodies within the next three decades and on into the future.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.