On Mars, Fear Overtakes Terror

The entire universe in blog form
May 26 2014 7:30 AM

Martians Moons Jockey for Position

The Martian moons Phobos (larger) and Deimos (smaller) as seen by Curiosity.

Photo (and animation below) by Mark Lemmon/NASA/JPL/MSSS/Texas A&M

Mars has two moons, named Phobos and Deimos. Phobos is a lumpy potato about 27 kilometers (17 miles) long, and Deimos is more of a charcoal briquette 12.6 kilometers (8 miles) across.

Both orbit Mars very close to directly above its equator, so, as seen from the surface, Phobos (which is closer to Mars) will sometimes pass directly in front of Deimos. This is called a mutual event (a kind of eclipse), and if you happen to have a nuclear-powered laser-eyed mobile chemistry lab on the surface you can point the camera upward and actually capture images that can be strung together in an animation.


Like this, say:

How cool is that? I could go into great detail about these observations by the Curiosity rover, but my friend Emily Lakdawalla has done all the work for me. Go over and read her blog, which has all the fun bits.

It would be pretty cool to have two moons, especially ones as weird as Mars’. Phobos orbits so closely to the surface that it goes around Mars faster than the planet rotates, so it rises in the west and sets in the east (much like the International Space Station does over Earth). Deimos is near the distance where it goes around Mars once per day (the geosynchronous point), so it moves around the sky very slowly, taking over 2.5 days to rise and set!

Like I said: weird. Don’t get me wrong; I’m pretty happy with what we have. Our Moon is great. But there is something romantic and wistful about the idea of two moons gracing a pink and red sunset sky …

Correction, May 26, 2014: I originally wrote that Deimos moves against the stars very slowly, but that's not quite right. Mars rotates at about the same rate the Earth does, though a little slower (once every 24 hours and 30 minutes or so). The stars therefore rise and set pretty much like they do on Earth, but from the surface of Mars, Deimos would appear to rise and set much more slowly due to its orbital motion nearly matching the rotation of Mars. In other words, it moves slowly relative to its position in the sky, not the stars. Sorry about the confusion.

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


Medical Examiner

Here’s Where We Stand With Ebola

Even experienced international disaster responders are shocked at how bad it’s gotten.

Why Are Lighter-Skinned Latinos and Asians More Likely to Vote Republican?

A Woman Who Escaped the Extreme Babymaking Christian Fundamentalism of Quiverfull

The XX Factor
Sept. 22 2014 12:29 PM A Woman Who Escaped the Extreme Babymaking Christian Fundamentalism of Quiverfull

Subprime Loans Are Back

And believe it or not, that’s a good thing.

It Is Very Stupid to Compare Hope Solo to Ray Rice

Building a Better Workplace

In Defense of HR

Startups and small businesses shouldn’t skip over a human resources department.

How Ted Cruz and Scott Brown Misunderstand What It Means to Be an American Citizen

Divestment Is Fine but Mostly Symbolic. There’s a Better Way for Universities to Fight Climate Change.

  News & Politics
Sept. 22 2014 6:30 PM What Does It Mean to Be an American? Ted Cruz and Scott Brown think it’s about ideology. It’s really about culture.
Sept. 22 2014 5:38 PM Apple Won't Shut Down Beats Music After All (But Will Probably Rename It)
Sept. 22 2014 4:45 PM Why Can’t the Census Count Gay Couples Accurately?
  Double X
The XX Factor
Sept. 22 2014 7:43 PM Emma Watson Threatened With Nude Photo Leak for Speaking Out About Women's Equality
  Slate Plus
Slate Plus
Sept. 22 2014 1:52 PM Tell Us What You Think About Slate Plus Help us improve our new membership program.
Brow Beat
Sept. 22 2014 9:17 PM Trent Reznor’s Gone Girl Soundtrack Sounds Like an Eerie, Innovative Success
Future Tense
Sept. 22 2014 6:27 PM Should We All Be Learning How to Type in Virtual Reality?
  Health & Science
Medical Examiner
Sept. 22 2014 4:34 PM Here’s Where We Stand With Ebola Even experienced international disaster responders are shocked at how bad it’s gotten.
Sports Nut
Sept. 18 2014 11:42 AM Grandmaster Clash One of the most amazing feats in chess history just happened, and no one noticed.