When Mercury is on the same side of the sun as the earth, then its dark side faces us. Since Mercury and the earth orbit at different angles, Mercury is usually not visible at this point. However, every three to thirteen years the planet is in transit and can be seen as a black spot against the sun. This generally occurs in November and May, but mostly in November.
While Mercury Mercury has a moonlike surface, its interior is more similar to that of the earth. Scientists generally believe that the interior consists of mainly iron and some other heavy elements. Since Mercury has a magnetic field around it, it is likely that it also has a large core of liquid iron, the same as the earth.
The sun's rays are approximately seven times as strong on Mercury as they are on earth whcih results in the planet being extremely dry, hot and virtually airless. It a person could stand on Mercury's surface, the sun would appear about two and a half times larger than it is viewed on earth. The temperature on the planet's surface may reach 801 degrees F. However, at night, that temperature may drop as low as -279 degrees F. Also, because of Mercury's lack of atmosphere, the sky is black and the stars can probably be seen during the day.
The reason that Mercury has such a weak atmosphere is that it is surrounded by an extremely small amount of helium, hydrogen, oxygen and sodium. This envelope of gases is so thin that the greatest possible atmospheric prassure (force exerted by weight of gases) is about 0.00000000003 lbs. per sq.in as compared to earths 14.7 lbs per sq.in. Scientists doubt that there is any life on Mercury because of the lack of oxygen and great heat.
Mercury's density is only slightly less than the earth's. However, since Mercury is much smaller than the earth, it has a lot less mass. Its smaller mass cuases its force of gravity to only be one third as strong as the earth's. Thus, an object that ways one hundred pounds on earths, would weigh only thirty-eight pounds on Mercury.
The first and only spacecraft to visit Mercury was the United States Mariner 10 . This unmanned spacecraft flew within 460 miles of Mercury on March 29, 1974. It passed by one more time on September 24, 1974 and photographed protions of Mercury's surface. It was the spacecraft that detected Mercury's magnetic field.
Mariner 10 was also the first spacecraft to study two planets. It made scientific measurements of Venus on its voyage to Mercury. Manned expiditions of Mercury will be difficult because of the unfavorable surface conditions, but time will tell.