A total solar eclipse will take place on August 21. It will cover a narrow slice of the United States, from Oregon to South Carolina. Millions of Americans are expected to travel to the eclipse path, making it the most-viewed solar eclipse in history.
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Mercury's surface closely resembles the Moon's. It is covered by impact craters, ancient lava flows, and quake fault lines. Mile-high cliffs stretch for hundreds of miles across the planet's surface. The huge Caloris impact basin, 800 miles (1,300 km) wide, decorates one side of the planet.
Why does the Moon have phases?
For astronomers, though, a comet's beauty is not just in its appearance, but in its content: A comet is an icy time capsule that contains some of the original material from the cloud of gas and dust that gave birth to Earth and the other planets.
Current theory says that the planets formed as small grains of ice and rock stuck together to form larger bodies, called planetesimals, which then merged to form the planets.
Deimos is farther away and moves slowly from east to west. Deimos would look like a small dot of light in the sky. Phobos is slowly moving closer to Mars. In another 50 to 100 million years, it will crash into Mars.
Phobos is small, dark, and airless. And it's one of the driest bodies in the solar system.
Because it is so far from the Sun, astronomers had a hard time measuring Pluto's size. They finally got it right in the 1980s, after James Christy discovered a companion object. By watching Pluto and the companion, named Charon, eclipse each other, they measured Pluto's diameter at about 1,400 miles -- about one-third less than the diameter of Earth's Moon.
Pluto is basically a ball of frozen nitrogen, methane, and carbon dioxide wrapped around a small core of rock. On average, it's farther from the Sun than any other of the major planets, so its surface is bitterly cold.
Triton, the largest satellite of Neptune, orbits in the opposite direction from most moons, suggesting that Neptune captured it in the distant past. Millions of years from now, Triton will move so close to Neptune that tidal forces will rip Triton apart, forming bright new rings around the giant planet.
Most of what we know about Triton came from Voyager 2, which photographed a landscape that is tinted subtle shades of pink, brown, and blue. Much of it resembles a cantaloupe, with ridges thousands of feet tall. Flowing ice or vaporizing gas may have carved this wrinkly terrain.
But the next year, astronomers found a second object in a similar orbit, then another, and another. Instead of a planet, they had found the first of the asteroids -- large chunks of rock that were left over from the formation of the solar system. Ceres is the largest, with a diameter of almost 600 miles (1,000 km).
Astronomers have discovered hundreds of thousands of these boulders, and with automated searches they are discovering thousands more each year. Most orbit the Sun in a broad region known as the asteroid belt, which is between the orbits of Mars and Jupiter.
Io: Fire World
Robotic probes may someday provide close-up views of some of the most remarkable vistas in the solar system, from the canyons of Mars to the ice-geysers of Triton. For a true hot-spot, they might show us the surface of Io, one of the moons of Jupiter. It is an eerie landscape of active volcanoes, tall mountains, and plains covered with frozen sulfur.