Credit: NASA/Johns Hopkins APL/Carnegie Institution
Let's turn our attention once again to the night sky by considering its wandering points of light, the planets. We begin this tour of the solar system with Mercury, the planet closest to the sun. Named for the messenger to the Roman gods, Mercury moves quickly in its orbit around the sun, but spins very slowly on its axis. As a result, a Mercurian year is equal to 88 Earth days, while a Mercurian day is 58 Earth days. With days that last so long, Mercury has extreme temperatures, with high temperatures up to 752 degrees F (400 C) and lows down to -292 F (-180 C).
Because of its proximity to the sun, Mercury has always been difficult to observe. The Hubble space telescope is unable to take images of Mercury due to safety precautions that prevent it from pointing too close to the sun. Humans got their first close-up look of Mercury in 1974, when the spacecraft Mariner 10 sent back images surveying about half of the planet's surface. The image below is a composite of data from its fly-bys:
Credit: Mariner 10/US Geological Survey
When I first saw this image, I thought, wow, Mercury is one beat-up little planet, all pock-marked with craters. But scientists are looking to Mercury for help in understanding how the solar system formed and changed over time. The surface of Mercury is less changeable than Earth's, and so might be able to reveal information on early planetary formation. To further explore Mercury's secrets, the space probe Messenger was launched in August 2004. Its first fly-by produced this image, of the "hidden" side of Mercury that Mariner 10 missed:
Credit: NASA/Johns Hopkins APL/Carnegie Institution
This image was sent back to Earth just last week, along with the image at the top of this post. It doesn't get more current than that. Swift in its orbit, both fiery and frigid, Mercury keeps close company with the sun while potentially holding the key to understanding the young solar system. Messenger is scheduled to enter its orbit in 2011, while the European Space Agency and Japan plan to jointly launch of pair of probes to Mercury in 2013.
If you want to take a look at Mercury yourself, it's best viewed at dusk or dawn. Check out this link for the best viewing dates for Mercury (and all the planets) in 2008.
