August 17, 1877. On this date, American astronomer Asaph Hall discovered one of two small Martian moons. He found the other moon, even smaller than the first, later that year. Both Martian moons are more like asteroids than they are like Earth’s large companion moon, and it’s thought they might have been asteroids that were captured by Mars’ gravity. Astronomers named the two moons Phobos and Deimos – Fear and Terror – for the horses that pulled the chariot of the Greek war god Ares, counterpart to the Roman war god Mars.
Phobos is tiny, with a mean radius of 6.9 miles (11.1 km). But it’s more than 7 times as massive than the second moon, Deimos, which is just 5.5 miles (9 kilometers) across.
Scientists got the first good look at Phobos in 1971 and 1972, during Mariner 9’s mission to the planet. They discovered a large crater that later received the name Stickney Crater, after Chloe Angeline Stickney Hall, wife of Phobos’ discoverer.
Asaph Hall probably couldn’t have imagined the video below, which was acquired by NASA’s Mars rover Curiosity on August 1, 2013. The rover had been taking a series of shots of the sky above, from Mars’ surface. This video shows both moons, Phobos and Deimos, as you might see them while standing on the surface of Mars. Large craters on Phobos are clearly visible in these images from the surface of Mars. No previous images from missions on the surface caught one moon eclipsing the other.
Bottom line: ON August 17, 1877, American astronomer Asaph Hall discovered Phobos, one of two Martian moons. He also discovered Deimos, the other known Martian moon, later that year. Both moons are believed to be captured asteroids.
Elizabeth Howell is an award-winning Canadian journalist who can't stop talking about space and science. As a teenager, she saw the movie Apollo 13 and wanted to be an astronaut. That hasn't happened - yet - but at least she gets to write about them. Elizabeth's favourite career moments so far include attending three shuttle launches, and legitimately writing the word "snot" into a Mars Curiosity story. Besides EarthSky, you can read Elizabeth's work in SPACE.com, Universe Today, SEN.com, All About Space and other fun places. Elizabeth's space obsession extends to her hobbies; she's a big fan of Battlestar: Galactica and has met all five TV Star Trek captains. She even visited Captain Kirk's future birthplace in small-town Iowa.