As seen from Earth's surface, Mercury hides in dusk and twilight, never getting more than about 28° in angular distance from the Sun. It takes about 116 days for successive elongations—i.e., for Mercury to return to the same point relative to the Sun—in the morning or evening sky; this is called Mercury's synodic period. Its nearness to the horizon also means that Mercury is always seen through more of Earth's turbulent atmosphere, which blurs the view. Even above the atmosphere, orbiting observatories such as the Hubble Space Telescope are restricted by the high sensitivity of their instruments from pointing as close to the Sun as would be required for observing Mercury. Because Mercury's orbit lies within Earth's, it occasionally
Transit of Mercury across the face of the Sun, a composite of five separate images in ultraviolet light taken by the Transition Region and Coronal Explorer (TRACE) satellite in Earth orbit, November 15, 1999. The time interval between successive images is about seven minutes. NASA/TRACE/SMEX
passes directly between Earth and the Sun. This event, in which the planet can be observed telescopically or by spacecraft instruments as a small black dot crossing the bright solar disk, is called a transit, and it occurs about a dozen times in a century.
Mercury also presents difficulties to study by space probe. Because the planet is located deep in the Sun's gravity field, a great deal of energy is needed to shape the trajectory of a spacecraft to get it from Earth's orbit to Mercury's in such a way that it can go into orbit around the planet or land on it. The first spacecraft to visit Mercury, Mariner 10, was in orbit around the Sun when it made three brief flybys of the planet in 1974-75. In developing subsequent missions to Mercury, such as the U.S. Messenger spacecraft launched in 2004, spaceflight engineers calculated complex routes, making use of gravity assists from repeated flybys of Venus and Mercury over the course of several years. In the Messenger mission design, after conducting observations from moderate distances during planetary flybys in 2008 and 2009, the spacecraft will enter into an elongated orbit around Mercury for close-up investigations in 2011. In addition, the extreme heat, not only from the Sun but also reradiated from Mercury itself, has challenged spacecraft designers to keep instruments cool enough to operate.
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