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The Solar System has been broken down into two major parts by astronomers: the inner Solar System and the outer Solar System. The four terrestrial planets and all material inside of the main asteroid belt comprise the inner Solar System. This article will concentrate on the three terrestrial planets other than our own: Mercury, Jupiter, and Mars.
Mercury is the least explored of the inner Solar System planets. In the past, NASA’s only exploration of the planet was done during three flybys performed by the Mariner 10 mission in 1974 and 1975. Those flybys mapped 45 percent of the planet’s surface. That all changed in January 2008, when the MESSENGER spacecraft flew by Mercury. It will end up in Mercurian orbit soon and, hopefully, will begin to reveal the planet’s composition and history, thus helping scientists understand more about our home planet and its place in the inner solar system.
Venus is very similar to Earth. Some describe it as a sister planet. The two are of similar size and bulk composition. There are no current missions planned for Venus, but drawing board mission concepts include the Venus In Situ Explorer (VISE) and Venus Mobile Explorer (VME) that would investigate the surface of Venus and help understand the climate change processes that led to the extreme conditions of Venus today.
There is, and always has been, an attraction to exploring Mars. It provides an excellent drawing board for studying planetary evolution as related to the Earth and Venus as well as being the most compelling Solar System object on which to search for extraterrestrial life. It is a target for future human exploration since it afford frequent launch opportunities. Because of these things, the Mars Exploration Program is fully integrated and designed to maximize scientific return and public engagement.
The Earth’s moon has been included as part of the inner Solar System. It will always hold an honored place in space history since it was the first object visited by man once he left the planet. There are plans to return to the Moon with the hopes of establishing a manned space station there.
The inner Solar System tests the limits of our current space technology. It is difficult to get a spacecraft in orbit around Mercury and the chances of placing a human in Martian orbit are astronomical. The future bodes well of further exploration and then, the outer Solar System will become the next target.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.