The Curiosity rover caught an unearthly sight in Martian skies.
I’m pretty fond of our planet, but in one respect Mars has Earth beat: it has two moons dancing in its sky.
On August 1st NASA’s Mars rover Curiosity watched the Red Planet’s larger moon, Phobos, pass right in front of the smaller moon, Deimos. The moon’s pockmarks show up clearly in the rover’s images, making the video sequence look a bit like a potato eclipsing a lima bean.
Even though Phobos is less than 1% the size of our Moon, from Mars’s surface it looks about half as big as the Moon appears from Earth. That’s because Phobos orbits about 6,000 km (3,700 miles) above the planet’s surface, while the Moon is more like 380,000 km above Earth’s. Deimos is about half Phobos’s physical size but farther out, roughly 20,000 km.
Hence, a comparison view of the three moons’ apparent sizes from their respective planets’ surfaces would look like the view at right.
Because Phobos orbits so close to Mars’s surface, an observer standing in the Red Planet’s polar regions couldn’t even see it. (That’d be some discovery for a Martian Magellan.) But in 100 million years the view will be even more different: the moon’s orbit is slowly shrinking, meaning one day the satellite might break up into a ring of fragments girding the planet or even crash into it.
. . . Yeah, I prefer living on Earth.