Mercury:

NASA's MESSENGER spacecraft is sending back images of Mercury in unprecedented detail.

Mercury's elliptical orbit takes the small planet as close as 47 million kilometers (29 million miles) and as far as 70 million kilometers (43 million miles) from the Sun. If one could stand on the scorching surface of Mercury when it is at its closest approach, the Sun would appear almost three times as large as it does when viewed from Earth. Temperatures on Mercury's surface can reach 430 degrees Celsius (800 degrees Fahrenheit). Because the planet has no atmosphere to retain that heat, nighttime temperatures on the surface can drop to -170 degrees Celsius (-280 degrees Fahrenheit).

Because Mercury is so close to the Sun, it is hard to directly observe from Earth, except during twilight. Mercury makes an appearance indirectly, however 13 times each century, Earth observers can watch Mercury pass across the face of the Sun, an event called a transit. These rare transits fall within several days of May 8 and November 10. The first two transits of Mercury in the 21st century occured in May 2003 and November 2006. The next transit will occur on May 9, 2016.

Read More About Mercury