July 20, 2017 – While photographing Mars, NASA’s Hubble Space Telescope captured a cameo appearance of the tiny moon Phobos on its trek around the Red Planet. Over the course of 22 minutes, Hubble took 13 separate exposures, allowing astronomers to create a time-lapse video showing the moon’s orbital path. The Hubble observations were intended to photograph Mars, and the moon’s cameo appearance was a bonus.
Discovered by Asaph Hall in 1877, the potato-shaped moon is so small that it appears star-like in the Hubble pictures. Phobos is only 16.5 miles by 13.5 miles by 11 miles, making it one of the tiniest moons in the solar system. An observer on Mars would see the moon at only one-third the width of the full moon we see on Earth.
The little moon completes an orbit around Mars in 7 hours and 39 minutes, which is faster than the Red Planet rotates. Rising in the Martian west, it orbits the Red Planet three times in the course of one Martian day, which is about 24 hours and 40 minutes. It is the only natural satellite in the solar system that circles its planet in a time shorter than the parent planet’s day.
Phobos currently orbits approximately 3,700 miles above the Martian surface, closer to its parent planet than any other moon in the solar system, but its orbit is slowly shrinking. Phobos draws nearer to Mars by about 6.5 feet every hundred years. Scientists predict that within 30 to 50 million years, it either will crash into the Red Planet or be torn to pieces and scattered as a ring around Mars.
About two weeks after the Apollo 11 manned lunar landing on July 20, 1969, NASA’s Mariner 7 flew by the Red Planet and took the first crude close-up snapshot of Phobos. On July 20, 1976, NASA’s Viking 1 lander touched down on the Martian surface. A year later, its parent craft, the Viking 1 orbiter, took the first detailed photograph of Phobos, revealing a gaping crater from an impact that nearly shattered the moon.
Hubble took the images of Phobos orbiting the Red Planet on May 12, 2016, when Mars was 50 million miles from Earth. This was just a few days before the planet passed closer to Earth in its orbit than it had in the past 11 years.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C. The camera that took these images was built by Ball Aerospace in Boulder, Colorado.