Mercury and Venus
Mercury
Mercury is the closest planet to the Sun, it is also the smallest planet in the Solar System and its orbit is the most eccentric, something Newton's theory of gravitation could not explain. Mercury takes over eighty days to orbit the Sun and, only one and a half days go by on Mercury during this time.
Despite the fact that Mercury is bright when viewed from Earth, it is difficult to observe as it is usually hidden in the light of the Sun. It is best viewed early in the morning or at twilight. Before the 4th century BC, the Ancient Greeks considered Mercury to be two objects, Apollo, which was visible at sunrise, and Hermes, which was visible at sunset. Hermes was later named Mercury by the Romans and was regarded as the messenger of the Gods.
The first spacecraft to reach Mercury was NASA's Mariner 10 probe which had mapped almost half of its surface by 1975. NASA's MESSENGER spacecraft mapped another thirty percent in 2008, and will eventually map the whole surface.
Mercury is a rocky planet, similar to the Moon. It has at least as many craters and is covered with dust. It has no satellites of its own and, like the Moon, it is not massive enough to sustain an atmosphere. Unlike the Moon however, it has a massive iron core which generates a magnetic field about one percent as strong as the Earth's. Radar observations conducted by the Goldstone antenna in California and the Very Large Array in New Mexico in the early 1990s suggest that frozen water exists on the floors of deep craters at the poles.
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Mercury
Mercury Venus Earth Mars (to scale)
Venus
Venus is the second closest planet to the Sun. It takes over two hundred Earth days to complete an orbit and just as long for a single day to pass on Venus.
Apart from the Sun and Moon, Venus is the brightest natural object in space which is visible from Earth. Venus is never far from the Sun and so is best observed in the early morning or at twilight. The name Venus is also Roman in origin, referring to the goddess of love and beauty.
The Soviet Union's fourth Venera probe reached Venus in 1967. This was the first man-made device to enter the atmosphere of another planet. Venera 9 returned images of Venus' surface in 1975. Due to the extreme conditions, none of the surface probes lasted more than an hour.
In 1978, NASA's Pioneer 1, the Pioneer Venus Orbiter, and Pioneer 2, the Pioneer Venus Multiprobe, reached Venus. The Pioneer Venus Multiprobe released four probes into the atmosphere and the Pioneer Venus Orbiter spent the next thirteen years in orbit, using radar to map the surface. The remainder of Venus was mapped by NASA's Project Magellan in 1991.
Venus is sometimes called Earth's 'sister planet' because it is a similar size, mass and density to the Earth. It is believed that Venus once contained large oceans but it has undergone an extreme greenhouse effect and the water has evaporated leaving a dry, desert landscape. Venus is now covered in a thick upper atmosphere of sulphuric acid which reflects sunlight back into space before it can reach the planet's surface. The lower atmosphere is mostly composed of carbon dioxide and the atmospheric pressure on Venus is almost one hundred times that of Earth's. Venus is covered in volcanoes but there are few craters, indicating that its surface must be relatively new, at about half a billion years old.