Keith Cooper is a freelance science journalist and editor in the United Kingdom, and has a degree in physics and astrophysics from the University of Manchester. He's the author of 'The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence' (Bloomsbury Sigma, 2020) and has written articles on astronomy, space, physics and astrobiology for a multitude of magazines and websites.
A still image from footage of Deimos passing in front of Jupiter, as seen by the Mars Express spacecraft.
At the time, the Jupiter system was about 463 million miles from Deimos, which is about 7.7 miles wide., passed in front of Deimos as seen by Mars Express. For scientists studying Mars' moons, all of these occultation events are important for nailing down the orbits of Deimos and Phobos more precisely. It's perhaps surprising that astronomers don't know the precise shape and size of Phobos and Deimos' orbits. But scientists do know that gravitational forces between the moons and Mars are causing the orbits of the moons to change.
Phobos, which is 14 miles wide, orbits just 3,721 miles above the Red Planet — close enough that it moves around Mars faster than Mars rotates. The super-speedy orbit makes the tidal bulge in Mars caused by Phobos' gravity lag behind the moon. The bulge's gravity then pulls back at Phobos, causing Phobos to gradually decelerate and slowly inch closer to Mars.
Deimos, on the other hand, has a wider orbit of 14,580 miles , resulting in its orbital period being slower than Mars' rotation. This means that the tidal bulge in Mars created by Deimos' gravity gets carried ahead of Deimos, causing the moon's orbital speed to accelerate as the bulge's gravity pulls on Deimos. Consequently, its orbit grows wider over time.
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