From our vantage on Earth, Mercury has always been a hazy, fleeting visitor. It's proximity to the sun, and rapid orbit around it, are such that the tiny planet can only be seen at dusk and dawn. It's appearances last for just a few days at a time. Ancient civilizations believed that Mercury was two different planets and had a name for each. Until recently, very little was known about the innermost planet.

It doesn't say much for Mercury's status among the planets that it is most often compared to our moon. In size and appearance, the two are quite similar. Both are littered with craters that are billions of years old, and both have remained essentially unchanged. Neither possesses any real atmosphere, nor gives any indication of substantial internal activity. They spin quietly through space, dead testaments to the earliest stages of planetary development.

As it happens, Mercury's orbit around the Sun is one of the most irregular in our solar system. Its long elliptic path is more exaggerated than any planet but Pluto. Mercury swings within 46 million kilometers away at its farthest point.

Mercury orbits the sun in a complicated rosette pattern that perplexed astronomers and physicists for a long time. During the nineteenth century, astronomers tracking the orbit of recently discovered Uranus came to believe that a farther planet was responsible for the eccentricities in its orbit. Employing the same logic that had led them to the discovery of Neptune, they looked for a similar explanation of Mercury's elliptic irregularities. Urbain Leverrier, a French astronomer, correctly predicted Neptune's position in the sky. Soon thereafter he put forth the idea that a small planet even closer to the Sun was responsible for the otherwise inexplicable nature of Mercury's orbit.

Proponents of this theory were so confident of the planet's existence that they christened it Vulcan after the roman god of fire. A search was begun for the hypothetical world, and in 1859 an unknown amateur claimed to have tracked Vulcan in transit across the sun. Leverrier was convinced of the amateur's success and celebrated his newest discovery, but others were increasingly skeptical. A total eclipse in 1878 provided the perfect opportunity to confirm Vulcan's existence, but proved fruitless. By the turn of the century astronomers no longer believed in Vulcan's existence, but they were no closer to explaining Mercury's orbit.

In 1915 a new set of physical laws being formulated by Albert Einstein. His General Theory of Relativity was actually a theory of gravitation, introducing the idea that enormous gravitational forces could warp time and bend light. According to these laws, Mercury's orbit would accelerate as it came closer to the sun and slow down as it moved farther away. While each revolution around the sun took 88 days, Mercury's progress was not fluid. Instead, according to Einstein, it lurched and lagged during the extremes of its eccentric orbit, creating a pattern that defied standard Newtonian gravitational theory.

An interesting result of this complex orbit is that at perihelion, the point at which it is closest to the sun, Mercury rotates on its axis more slowly than it moves in its orbit.

A few years ago scientists were completely unaware of Mercury's sodium emissions. Today we believe that sodium is the most prevalent element in the Mercurian atmosphere. To state anything with great certainty about Mercury, however, can be dangerous. As in past centuries, today's scientists continue to find themselves fooled by the mysterious planet. Ever the elusive messenger, Mercury brings us tales of the solar system's past before spinning away quickly. At this time there are no missions planned to the barren world hurtling through the solar system. Recent conferences have shown that many scientists still keep an eye on tiny Mercury, but the general public remains unenthused about the Moonlike little rock when confronted with the more colorful worlds seen in recent expeditions. Return to Journey Through the Solar System Main Menu