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142 million miles from the sun. Diameter: 4,220 miles. Moons: 2. Length of year: 687 Earth days.
Mars has always fascinated people. Its red, fiery appearance was mysterious and intriguing. Mars has only a quarter the surface area of Earth and only 1/10th the mass (though because it lacks oceans the area of Mars's accessible dry land is approximately equal to that of the Earth's dry land). Mars has two small moons, Phobos and Deimos, both small and oddly shaped, possibly captured asteroids. Mars has polar ice caps that contain frozen water and carbon dioxide. An extinct volcano, Olympus Mons, is, at 27 km, the tallest mountain in the solar system. Mars's atmosphere is very thin: the surface air pressure is only 7.5 millibars compared to an average 1013 millibars on Earth. The atmosphere on Mars is 95% carbon dioxide, 3% nitrogen, 1.6% argon, with only a trace of oxygen and water.
Mountains and craters scar the rugged terrain of Mars. The dust, an iron oxide, gives the planet its reddish cast. A thin atmosphere and an elliptical orbit combine to create temperature fluctuations from -207 degrees to a comfortable 80 on summer days. At the top and bottom of the planet are poles just like on Earth. During the Martian winter, ice caps can be seen at the poles.
Mars is much smaller than Earth, but recent research shows that it once had flowing rivers. Mars also has a canyon that stretches over 2000 miles. Mars has many craters which were formed by meteorites or asteroids hitting it. Mars also has some of the tallest volcanoes and some of the deepest valleys in our solar system. Mars has two moons, Phobos and Deimos which have unusual shapes. Scientists think these potato-shaped moons were once asteroids captured by Mars' gravitational pull.
Mars Closest Approach
August 27, 2003, marked the closest that Mars and Earth have been since nearly 60,000 years ago, when the Neanderthals lived! Mars won't be this close again until 2287. Imagine what life on Earth...and maybe Mars...might be like then.
The distance to Mars varies from about 56 million kilometers (about 35 million miles) to 400 million kilometers (about 249 million miles). Why the difference? And why so close now?
Like all the planets in our solar system, Earth and Mars orbit the sun. But Earth is closer to the sun, and therefore races along its orbit more quickly. Earth makes two trips around the sun in about the same amount of time that Mars takes to make one trip. So sometimes the two planets are on opposite sides of the sun, very far apart, and other times, Earth catches up with its neighbor and passes relatively close to it.
What is Opposition?
During opposition, Mars and the sun are on directly opposite sides of Earth. From our perspective on our spinning world, Mars rises in the east just as the sun sets in the west. Then, after staying up in the sky the entire night, Mars sets in the west just as the sun rises in the east. Since Mars and the sun appear on opposite sides of the sky, we say that Mars is in "opposition." If Earth and Mars followed perfectly circular orbits, opposition would be as close as the two planets could get.
Of course, nothing about motion in space is quite that simple! Our orbits are actually elliptical (oval-shaped), and we travel a little closer to the sun at one end of our orbits than at the other end.
The part of the orbit closest to the sun is called "perihelion." The farthest part is called "aphelion." Mars' orbit is more elliptical than Earth's so the difference between perihelion and aphelion is greater.
When Can Opposition Occur?
Mars oppositions happen about every 26 months. Every 15 or 17 years, opposition occurs within a few weeks of Mars' perihelion (the point in its orbit when it is closest to the sun).
An opposition can occur anywhere along Mars' orbit. When it happens while the red planet is closest to the sun (called "perihelic opposition"), Mars is particularly close to Earth. If Earth and Mars both had perfectly stable orbits, then each perihelic opposition would bring the two planets as close as they could be. That's almost the way it is.
But once again, nature throws in a few complications. Gravitational tugging by the other planets constantly changes the shape of our orbits a little bit. Giant Jupiter especially influences the orbit of Mars. Also, the orbits of Earth and Mars don't lie in quite the same plane. The paths the planets take around the sun are slightly tilted with respect to each other.
So, with all these added factors, some perihelic oppositions bring us closer together than others. The one we're having in 2003 is the closest approach in almost 60,000 years!
Mars' orbit is more elliptical than Earth's, so the difference between perihelion and aphelion is greater. Over the past centuries, Mars' orbit has been getting more and more elongated, carrying the planet even nearer to the sun at perihelion and even farther away at aphelion. So future perihelic oppositions will bring Earth and Mars even closer. But we'll still have bragging rights for awhile. Our 2003 record will stand until August 28, 2287!
Some text and graphics courtesy of NASA.