Mercury (Planet)
Mercury is one of the five planets visible to the naked eye. Because it is so close to the Sun however, it is only visible for about one hour after sunset or for the same length of time prior to sunrise. Even though it can become quite bright, it never looks as impressive as some of the stars blazing forth in a darker sky later at night. It is also difficult to observe since it lies so close to the horizon; any object can block it from view. For that reason, most observers, including Nicholas Copernicus, have never seen it. In addition, it is difficult to observe through a telescope.
The first observations after the invention of the telescope were mainly attempts to see if the planet showed phases like the moon, as Copernicus predicted. Johannes Hevelius (1611-1687) saw the phases in 1644, but it was not until the early 1800s that serious observation of Mercury took place by Johann Schroeter. In looking at the terminator, the border between the light and dark hemispheres, Schroeter thought he saw a rugged line, indicating high mountains. He believed the planet's rotation was about the same as Earth's. Giovanni Schiaparelli (1835-1910) challenged these findings in 1889 when he announced that Mercury rotated once on its axis every 88 days, the same length of time it took the planet to circle the Sun once. Accordingly, Mercury was thought to be like our moon, forever keeping one face turned toward the object it circled. He also believed a thick atmosphere on Mercury would circulate and cause a surprisingly even temperature over the whole planet.
Irregularities in Mercury's orbit gave rise to an interesting episode in astronomy. The French mathematician and astronomer Jean Urbain Le Verrier (1811-1877) checked Mercury's orbit and found it did not match mathematical predictions. He felt there were only two explanations: Venus was more massive than believed, or there was an unknown mass inside Mercury's orbit. Since no one had ever seen any unknown planet crossing the Sun, he thought a second asteroid belt might be located inside Mercury's orbit. Swiss astronomer Rudolf Wolf (1816-1893) combed through sunspot drawings of years past and came up with a list of possible candidates for these asteroids, which he sent to Le Verrier. Le Verrier also received a letter from an amateur astronomer who thought he saw a small planet move across the Sun's face. Le Verrier decided this must be the largest of the asteroids and named it Vulcan. Of course, no new planet or asteroid was ever found. It was not until Albert Einstein's developed his theory of relativity that the irregularities in Mercury's orbit were resolved.
It was not until 1965 that Mercury's true period of rotation was discovered, when a team of astronomers at the Arecibo Observatory in Puerto Rico beamed radar impulses at the planet, measured the returning impulses, and determined that Mercury rotates on its axis once every 59 days rather than every 88 days as Schiaparelli had thought. This disproved the belief that Mercury's day was the same length as its year.
In the mid-1970s, astronomers made a great leap in their understanding of Mercury. On November 3, 1973, the United States launched Mariner 10, the first spacecraft to use the gravitational attraction of a planet to change its direction and speed. Mariner 10 was lauched on a trajectory whereby the gravitational field of Venus would slow the spacecraft down and alter its course toward Mercury. The spacecraft approached the planet within 3,585 miles (5,768 km) on March 29, 1974; it took two other looks at Mercury in September 1974 and March 1975 by looping around the Sun twice.
One of Mariner 10's main objectives was to examine Mercury's surface features. Mariner's television cameras photographed about half the planet; the photos revealed a surface covered with craters probably made early in the planet's history when rocky meteorites crashed into the young planet. Between many of the craters are smooth plains and huge slopes that formed after the meteorites bombarded Mercury.
In addition, the spacecraft measured the surface temperature of Mercury and searched for an atmosphere, finding temperatures varying from 800° F (426° C) at noon on the equator to-280° F (-137° C) at midnight. These temperature extremes, the greatest in the solar system, are caused by a lack of atmosphere, the intense solar radiation, and the lengthy days: from sunrise to sunset each one lasts nearly three Earth months. The planet comes close to the way Percival Lowell (1855-1916) described it nearly a century ago: "the bleached bones of a world."
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