The planet Mars was named after the Roman God Mars. Mars is the fourth planet from the sun. Mercury, Venus, and Earth are closer to the Sun than Mars. Jupiter, Saturn, Uranus, Neptune, and Pluto are farther from the sun in that order. Two moons (Phobos and Deimos) circle the planet. They were discovered in 1877 by Asaph Hall.
The diameter of Mars is about 4,218 miles. The time it takes to rotate on its axis is just 37 minutes longer than that of the Earth's. It takes Mars about 687 days to orbit the sun, compared to about 365 days for Earth. The aphelion, or farthest Mars is from the Sun is about 249,228,730 kilometers. The perihelion, or closest Mars comes to the Sun is about 206,644,545 kilometers. Mars is a little over half the size of the Earth. The red hue seen from the Earth is due to the composition of the abundance of sand on the surface of Mars. The temperature ranges from a very cold minus 207 degrees Fahrenheit at the pole in the winter to plus 80 degrees at the equator in the summer. The average temperature is minus 67 degrees Fahrenheit, or minus 55 degrees Celsius. The escape velocity for Mars is 5.027 kilometers/second, about half that of Earth's. Mars has two polar ice caps, one in the north and one in the south. The composition of Mars is thought to be 2.7% nitrogen, 1.6% argon, 0.2% oxygen, 0.07% carbon monoxide, 0.03% water vapor, 0.01% nitric oxide , neon 2.5 ppm, krypton 300 ppb, formaldehyde 130 ppb, xenon 80 ppb, ozone 30 ppb, and methane 10 ppb, where ppm is parts per million and ppb is parts per billion.
A few billion years ago, it is believed that Mars was abundant with water. There are geological signs that attest to this hypothesis. One theory is that although water might not be able to exist in liquid form for more than a few seconds due to the below zero temperatures. But another theory claims it might be able to exist as salt water in below freezing temperatures.
Percival Lowell is one of the famous astronomers who thought there were canals on Mars. Many lines visible on Mars have been interpreted as irrigation channels for water in past times, but have always been thought to be optical illusions. The first flyby Mars by Mariner 4 in 1965 found that the lines are not canals. The largest object on Mars is Olympus Mons (Mount Olympus). It is 26 kilometers high, which is over 3 times the height of Mount Everest. Olympus Mons is an extinct volcano. About 43,000 craters with a diameter of 5 kilometers or greater have been found on the surface of Mars.
There have been many satellite missions to the planet Mars. The first one, although unsuccessful, was in 1960. After several failures, Mariner 4 was the first successful flyby of Mars, sending back several pictures of the surface on November 28, 1964. Mars 2 and Mars 3 were took several pictures of Mars in May 1971. The Viking 1 and Viking 2 were very successful in 1975. The information they sent back to Earth is useful even today. Mars Pathfinder was a successful and one of the cheaper missions to Mars. It sent good pictures back from July 4, 1997 to March 10, 1998. It landed on the surface of Mars and took several pictures. Mars Odyssey took pictures without landing on the surface in October, 2001. It will be a communications relay for future missions. The last four missions were Spirit ( June 10, 2003), Opportunity (July 7, 2003), Mars Reconnaissance Orbiter (August 10, 2005), and Phoenix (August 4, 2007) . They are very successful missions. Some of them are still taking pictures of Mars.