The unmanned U.S. space probe Mariner 10, launched by NASA in November 1973, becomes the first spacecraft to visit the planet Mercury, sending back close-up images of a celestial body usually obscured because of its proximity to the sun. Mariner 10 had visited the planet Venus eight weeks before but only for the purpose of using Venus’ gravity to whip it toward the closest planet to the sun. In three flybys of Mercury between 1974 and 1975, the NASA spacecraft took detailed images of the planet and succeeded in mapping about 35 percent of its heavily cratered, moonlike surface. Mercury is the smallest planet in the solar system and completes its solar orbit in only 88 earth days. Data sent back by Mariner 10 discounted a previously held theory that the planet does not spin on its axis; in fact, the planet has a very slow rotational period that stretches over 58 earth days. Mercury is a waterless, airless world that alternately bakes and freezes as it slowly rotates. Highly inhospitable, Mercury’s surface temperature varies from 800 degrees Fahrenheit when facing the sun to -279 degrees when facing away. The planet has no known satellites.