APOD: Phobos: Moon over Mars (2024 Mar 22)

[APOD Robot]

Phobos: Moon over Mars

Explanation: A tiny moon with a scary name, Phobos emerges from behind the Red Planet in this timelapse sequence from the Earth-orbiting Hubble Space Telescope. Over 22 minutes the 13 separate exposures were captured near the 2016 closest approach of Mars to planet Earth. Martians have to look to the west to watch Phobos rise, though. The small moon is closer to its parent planet than any other moon in the Solar System, about 3,700 miles (6,000 kilometers) above the Martian surface. It completes one orbit in just 7 hours and 39 minutes. That's faster than a Mars rotation, which corresponds to about 24 hours and 40 minutes. So on Mars, Phobos can be seen to rise above the western horizon 3 times a day. Still, Phobos is doomed.

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[Bartolo]

looks a bit like North America

[ORIENTEER6]

The caption states that Phobos has traveled from the first dot on the right to the first dot on the left in 22 minutes. It has also stated that Mars is rotating faster than Phobos is traveling. How can Hubble keep Mars in focus if it is spinning at that speed?

[Cousin Ricky]

The caption states that Phobos has traveled from the first dot on the right to the first dot on the left in 22 minutes. It has also stated that Mars is rotating faster than Phobos is traveling. How can Hubble keep Mars in focus if it is spinning at that speed?

Read it again: “It completes one orbit in just 7 hours and 39 minutes. That's faster than a Mars rotation, ...”

[Cousin Ricky]

It completes one orbit in just 7 hours and 39 minutes. That's faster than a Mars rotation, which corresponds to about 24 hours and 40 minutes. So on Mars, Phobos can be seen to rise above the western horizon 3 times a day.

This is better expressed “up to 3 times a day.” Since Mars is also rotating, one standing on the surface of Mars would catch up to one of Phobos’ 3 orbits over the course of a day, leaving only 2 Phobos-rises. (There was a famous SAT gaffe over this geometry.) But since Phobos orbits slightly faster than 3 times a day, one can occasionally catch a 3rd Phobos-rise.

[Bellerophon]

Still, Phobos is doomed.

Phobos has one thing going for it that most moons in decaying orbits don't, namely a nearby spacefaring civilization. Let's hope said civilization lasts long enough to boost Phobos' orbit.

[thekelbell]

Are the bluish areas around Mars ice or an atmosphere?

[ORIENTEER6]

Thanks, so mars has only moved one third of the distance Phobos has (in a circular motion) diminishing the fuzziness, not enhancing it.

[Chris Peterson]

Still, Phobos is doomed.

Phobos has one thing going for it that most moons in decaying orbits don't, namely a nearby spacefaring civilization. Let's hope said civilization lasts long enough to boost Phobos' orbit.

Why would it? If we were going to colonize Mars, the better strategy would be to deorbit it into the planet to eliminate a future problem. (Potentially it could be mined down to nothing, but it's likely that there's little of commercial value or use in its composition.)