Year-End List—Best Moons: #8 Phobos

Hey, sky fans! Well, here we are, close to the end of another year. I know everyone’s busy these days, so before we go on, I want to thank you for taking the time to read and talk about astronomy with me for all these months. I’m thankful for getting to know all of you, and it all means a lot to me that you’ve been a part of this. I hope you’ve enjoyed reading as much as I’ve enjoyed writing, and I hope you’ve been a little inspired, and learned at least a little something. Above all, thank you and happy holidays from all of us here at Sky Watch HQ.

Last December, we went on a tour of the solar system to celebrate Hanukkah. This week, we’ve got a fairly quiet few days, with not a lot going on in the skies to bring an exhausted end to 2016. I’m sure I’m wrong about that, and am missing something. It always seems like I’m missing something. Things have lined up just right this time around, so we can squeeze Christmas, tons of other holidays, the New Year, and a year-end top… eight list, my Hanukkah gift to you, together to close out the year.

So, let’s get started! It seems like there’s been a lot of talk about our solar system’s natural satellites, the moons, this year. Either that, or I’m just paying more attention to them. How about we go with the top eight moons of 2016! This is a tough list to make because they’re all pretty fantastic, except for Margaret. No one likes Margaret.

Number 8: Phobos

The eighth best moon of 2016 is the bigger of Mars’s two lumpy piles of rocks, Phobos. Mars is one of only two planets in the inner solar system that has moons (can you guess the other?), and it has almost 67% of them all by itself. See how easy it is to tweak statistics?

Phobos, and its twin brother, Diemos are strange little moons. They’re comically shaped, possibly asteroids that wandered a bit too close to Mars, not spheres, and have all kinds of weird and oddly measured dimensions. They’re so little and so hard to see that they weren’t discovered until over 250 years after Jupiter’s four big Galilean satellites, even though Jupiter’s so much farther away. On average, Phobos is about 6.5 miles, 11 km, across. So, it doesn’t have a whole lot of gravity and can’t pull itself into a ball. If you were walking along it you’d find yourself feeling that loose gravity pulling on you differently depending on where you were. A little lighter here, a little stronger there.

Phobos from Mars Express
Phobos from Mars Express Credits:ESA/DLR/FU Berlin
(G. Neukum)

Phobos, which seems like it’s one of the few places left in the solar system without water, made news earlier this year because it’s in serious trouble. It orbits so close to the surface of Mars that before long, astronomically speaking, it’ll get too close and either crash into Mars or cross within Mars’s Roche Limit (next week’s word of the week?) and be torn apart by Mars’s gravity. The good news is if it is torn apart, it’ll turn into a gorgeous ring around the red planet.

For now, Phobos orbits at about 9500 miles above Mars’s surface. At the scale of the size of Mars, that’d be like the Moon, which is about 239,000 miles Earth in real life, orbiting at about 17,000 miles. Of course, our Moon is much bigger than Phobos and has much more gravity, so there would be no end to the mayhem here if it were that close.

What this tiny orbit does, though, is my favorite thing about it and what gets it on the list. In space, distance relates directly to speed. So, something closer to the ground orbits much faster than something farther away. Take the Moon and the International Space Station, for instance. One takes a month to orbit Earth; the other takes an hour and a half. Phobos’s orbit is so small that it makes its way all the way around in just a bit over seven Earth hours. This is faster than Mars rotates.

Imagine lying on your back in a grassy field, with the dark Martian sky overhead. As your sip your Reisling, you’d see Phobos cruise across from west to east and then reappear in the west about four Earth hours later (after all, why would you set your watch to local time?). The idea of seeing that makes me smile.

So, here’s to number 8 on our list of 2016’s best moons, Phobos!

Note: I meant to post this tomorrow, December 24, in time for the first night of Hanukkah and Christmas Eve, but got a little ahead of myself. Maybe I’ll slow things down a day, maybe not. I know; you’re sitting on the edge of your chair in anticipation.


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