Planets

Mercury is the closest planet to the sun at a distance of 57,910,000 km (0.38 AU). The velocity of Mercury is faster than any other planet. The temperature on Mercury ranges from 90 K to 700 K, which is the most intense variation in the solar system. Mercury is often visible with binoculars or even the unaided eye, but it is always so close to the sun making the planet hard to see in the twilight sky. Since it is closer to the sun than we are, the illumination of Mercury's disk varies when viewed from Earth. Mercury’s orbit has strange effects for an observer because of its high eccentricity.

In 1974 and 1975 the Mariner 10 spacecraft flew by Mercury 3 times, mapping 45% of its surface. In 2008, The MESSANGER spacecraft took much better quality images of the terrain on Mercury. The terrain appears to have some smooth plains and large escarpments hundreds of kilometers in length and possibly three kilometers high. The escarpments were possibly created from large impacts in the past.

Mercury's interior is mainly a large iron core whose radius is 1800 to 1900 km and some of the core may be molten. The silicate is 500 to 600km thick. The planet’s atmosphere is unstable and it is constantly being replenished because of solar wind blasting atoms off the surface. The atoms escape very quickly into space because of the planets extremely hot temperatures. As far as we know Mercury has no satellites.

The planet Neptune is furthest planet from the sun at the moment, making information more difficult to obtain about the planet. Only one spacecraft, the Voyager 2, has encountered Neptune. On Aug 25th in 1989 the Voyager 2 obtained much of the planets details on its single encounter. HST observations that are ground based have added a plethora of details also concerning distant planets that are difficult to reach. We have far less factual information the further away from the sun a planet is.