Mercury

Size: Mercury is about 3,032 miles (4,880 kilometers) across. That makes it the smallest planet in the solar system. In fact, Mercury is just a little larger than Earth's Moon.

Distance from the Sun: Mercury is the closest planet to our star, with its average distance from the Sun being 36 million miles (58 million km).

Orbit around the Sun: Because Mercury is so close to the Sun, it has the smallest orbit of all the planets. Mercury's year (the time it takes to orbit the Sun one time) is 88 Earth days long.

Rotation: Although Mercury goes around the Sun quickly, it spins very slowly on its axis — about 59 Earth days for every rotation.

Surface: Scientists believe that Mercury has a thin, rocky crust, with a large metallic core, probably made of iron, at its center. Mercury is covered with craters and has ice at its poles.

Atmosphere: Mercury has an extremely thin atmosphere of helium and hydrogen captured from the solar wind.

Temperature: On Mercury, you would either freeze or roast. The highest surface temperature is 870° F (466° C), while the lowest temperature is –300 °F (–184 °C).

Escape velocity: To escape Mercury's gravity, you have to travel 9,600 miles (15,500 km) per hour, compared to 25,000 miles (40,200 km) per hour necessary to escape Earth's gravity.

Other information: The Mariner 10 spacecraft flybys of Mercury in the 1974-1975 greatly enhanced our knowledge of the planet.

Because it moves so fast around the Sun, early Roman skywatchers named Mercury after their speedy messenger god. To the ancient Greeks, Mercury is identified with the god Hermes.