Mercury

Mercury is the first planet in the solar system and the closest planet to the Sun. It is the smallest planet in the solar system. For example, its diameter at the equator is 4,879.4 km, which is only 38% of the Earth's. The only moons larger than Mercury are Jupiter 's Ganymede and Saturn 's Titan; Mercury itself does not have any moons or rings.

Its apparent brightness in the sky varies from -0.4 magnitude to 5.5 magnitude. Because Mercury is so close to the Sun, it can only be seen for a short time before sunrise and just after sunset. At certain times of the year, it can be difficult to see even with a telescope, because its greatest elongation from the Sun is only 28.3°.

Since the American Mariner 10 (1974-1975) first approached Mercury, no probe had reached the planet until 2008, and only about 40% to 45% of the surface had been mapped. The Mercury photographed by Mariner had prominent craters and was thought to be very similar to the Moon. For this reason, Mercury was given a lower priority for exploration compared to Mars, Venus, and the outer planets. Even now in the 21st century, there is much that is unknown about this planet. However, it is also a planet that is expected to be explored in the future, such as the American MESSENGER, which began exploration in 2008, and the Japan-Europe joint project BepiColombo, which is scheduled to be launched in 2013.