HISTORY

M ars is the fourth planet from the Sun and is known as the red planet. Its red color inspired the Greeks and Romans to name it after their god of war, Ares or Mars. During certain parts of the year it is the third-brightest object in the night sky, surpassed only by the Moon and Venus.

M ars orbits the Sun at a distance about 1.5 times as far as the Earth does. The orbit is somewhat elliptical, so the planet's distance from the Sun varies from a minimum, at perihelion, of 206.7 million km (128.4 million miles) to a maximum, at aphelion, of 249.2 million km (154.8 million miles). Because Mars is farther from the Sun than Earth is, it takes longer to complete a revolution. Its year is 687 Earth days long.

T he Martian day is 24 hours, 37 minutes, and 23 seconds long. The tilt of Mars results in seasonal changes similar to those on Earth. Because of the elliptical orbit of Mars, summer in its southern hemisphere occurs when the planet is nearest the Sun, as does winter in its northern hemisphere.

M ars in half the size of Earth but almost twice the size of the Moon. Mars is also slightly less dense than Earth. In addition, no magnetic field for Mars has been detected, which indicates that the core of Mars is solid. This also explains why Mars has no radiation belt. The total mass of the planet is only one-tenth that of Earth, and thus Martian gravity is only 38% as strong.

M ars is orbited by two irregularly shaped satellites. The larger is named Phobos and the smaller Deimos. Each is only a few kilometers wide. The moons are heavily cratered which suggests that they may be asteroids that were captured by Martian gravity, or possible remnants of the planet itself.