Mercury is a hot, rocky, heavily cratered planet named for the winged messenger of the gods because it moves quickly across the night sky. It orbits the Sun in 88 Earth days, takes 59 days to spin on its axis, and because it travels two thirds of the way around the Sun during one rotation, the time between two sunsets is 176 Earth days, making its day twice as long as its year. Only one spacecraft, Mariner 10 in 1974, visited Mercury, completing three orbits and leaving over half the planet unphotographed, while the Hubble Space Telescope cannot observe it because Mercury is too close to the Sun. Mercury is covered with thousands of craters because it has no atmosphere to burn up meteorites and no weather to disturb the dust and sand around impacts for billions of years. Caloris Planitia, 875 miles across, was formed by an asteroid impact 3.6 billion years ago that was so powerful it created hills and fractures on the planet’s opposite side. By the early 1900s, Mercury’s orbit was known to differ from that predicted by Newtonian physics, leading some astronomers to propose an undiscovered planet called Vulcan, until Albert Einstein’s Theory of Relativity explained the effect as the result of the Sun warping nearby space-time. Mercury is the first planet from the Sun, with a diameter of 3,032 miles, an average distance from the Sun of 36 million miles, a speed of 107,000 miles per hour around the Sun, temperatures from -292 to 806 degrees Fahrenheit, a mass 5.5 percent of Earth’s, gravity 38 percent of Earth’s, and no moons.