Because of Mercury's proximity to the sun, the planet has been relatively easy to spot with the naked eye. Its place at the front of the line in our solar system has demystified Mercury for those who prefer to gaze at more-distant objects.
In 2004, NASA launched the Mercury Messenger spacecraft to gather data that probes launched in the 1970s couldn't. Messenger finally arrived in the planet's orbit in March. Four months into its-year long mission, it is already disabusing us of many of our assumptions.
Messenger has beamed back evidence of frozen water at Mercury's poles -- a previously unimaginable scenario given the planet's closeness to the sun.
Instead of being a rock without much personality -- like our own moon, only hotter -- Mercury has a surprisingly diverse topography. We'll have to rethink our theories about the planet's formation in light of new discoveries.
Mercury also has an unusually large iron core that extends 75 percent of the way to the planet's surface. Its magnetic field is stronger in the northern hemisphere than in the south.
These are major discoveries relatively early in Messenger's mission. They vindicate NASA's shift to unmanned probes for exploration of nearby worlds, where visits by humans would be impractical.
Thanks to technology such as Messenger, there's more than one way to explore "the final frontier."