Even though Mercury is one of our closest celestial neighbors, we know less about it than most other planets. It has inspired legends and mysteries through the ages and is only recently beginning to yield its secrets. Let's look at this strange little world.
The ancients actually thought there were two "Mercuries". They saw one that magically appeared in the sky just before sunset and another that magically arose in front of the sun just before sunrise. It didn't occur to them that they were actually seeing the same planet on both sides of the sun at different times in the year.
Of course, a "planet" to them was nothing more than a point of light in the sky. That's why Mercury was named for the wing-footed Roman messenger god. Most of the other planets simply "wandered" among the stars. By comparison, Mercury "scooted" - appearing here, then there, then not at all, then back again. Surely, Mercury must have seemed to be the swiftest and most perplexing of all the gods.
As planets go, Mercury is kinda an odd duck. A planet's orbit is never exactly circular, but Mercury's is more oblong than most of the inner planets. In its path around the sun, it gets as close as 25.6 million miles and as far away as 43.4 million miles. In the course of a Mercurian "year" - which is really only about 90 earth-days - the sun seems to slowly "pulsate" in the sky.
Such a close yet eccentric orbit causes another weird aberration: Mercury's unusual day. For years, astronomers assumed that Mercury was "tidally locked" into the sun, i.e., the year and the day were the same length with the same side always facing the sun, much as the moon is tidally locked to the Earth. But we now know that Mercury's day and year are in a strange 2:3 resonance. There are exactly three Mercury days for every two Mercury years.
That means that there are some areas on Mercury that would see the sun rise, then set, then rise again before traversing across the sky. This weird orbital and rotational ballet challenges our very thoughts of what is a "night" and a "day".
Mercury is the only major planet without an atmosphere. Just like our moon, it is rocky and barren, pocketed with craters formed by millions of years of meteor hits and occasional collisions with small asteroids. With no protective atmosphere to shield it, it is literally lying naked to space.
Mercury sits up proud in space. Most planets tilt from their orbit somewhat - such a tilt is what gives the Earth our four seasons. But not Mercury. Its axial tilt is less than 0.01 degrees.
That means that Mercury hosts some of the hottest and the coldest surface temperatures in the solar system. At noon on the equator, the sun is directly overhead and stays in one place for so long that the surface is baked to 800 degrees F - hot enough to melt lead. But at the bottom of some craters near the poles of Mercury, there are surfaces that have never seen the light of the sun for millions of years. Temperatures there hover around minus 300 degrees F and never get any warmer.
For all the mysteries of this planet, only one spacecraft has ever seen it up close. Mariner 10 visited it in the mid-1970s. We haven't been back since. Getting to Mercury is no easy task. Even though it's one of the closest planets, it actually takes more rocket fuel to get there than it takes to go to Mars. That's because once you get there, you have to contend with the gravity of the sun to slow the spacecraft into an acceptable path for scientific purposes. Otherwise, the craft would pass by Mercury in the blink of an eye.
The Messenger spacecraft is currently on its way to Mercury to unlock more of its secrets. It solves the whole orbital gravity problem by looping around the sun a few times, passing the Earth and Venus in a round-about path to Mercury to get it into the right orientation. After a couple of quick fly-bys to Mercury in 2008 and 2009, Messenger will settle into an orbit around Mercury in 2011.
When it gets there, who knows what strange stuff we'll learn about this quirky planet?