Animated transit of a Martian moon

By Phil Plait | October 16, 2012 9:00 am

Back in September I posted an image taken from the Curiosity rover showing Phobos, one of the moons of Mars, crossing the face of the Sun.

That was pretty cool. But this is cooler: video of Phobos transiting the face of the Sun seen from the rover Opportunity on September 20, 2012!

Lest it be overshadowed (HAHAHAHA! Get it?) by Curiosity, remember Opportunity is still going strong after more than eight years on the surface of the Red Planet. These shots from the elder rover are really awesome; Phobos is not even close to being a sphere and you can see its potatoey lumpiness in the animation.

Phobos is about 27 km (17 miles) across its long axis, which is small for a moon. It looks big because it orbits Mars so close in; it’s only 6000 km above the surface. It was actually a bit farther away from Opportunity when these images were taken, making it look smaller than it could be.

In fact, given its size and distance, Phobos has a maximum size in the sky of about a quarter degree, or half the size of our full Moon. As seen from Earth, the Sun and the Moon are about the same size in the sky. But Mars is farther from the Sun, so the Sun looks smaller, about 1/3 of a degree. So even at best Phobos can’t completely block the Sun.

But… Phobos isn’t in a stable orbit. Tides from Mars are dropping it down closer to the planet, making it appear bigger. In a few million years it’ll drop low enough to create total eclipses as seen from the surface of Mars. They won’t last long, since the moon is zipping along pretty rapidly in its orbit. Still though, I have to admit to a bit of delight: creationists like to claim the Earth is special, and we’re the only planet that has the right conditions for total solar eclipses. That’s not even really true right now, and it certainly won’t be once Phobos dips down a bit more.

Of course, once Phobos gets too close to Mars a few million years later it’ll crash into the surface, making the sweatiest apocalyptic scenarios dreamed up by humans look like a warm summer’s breeze by comparison. Nature! It has a way of making our fevered imaginations look like pretty small potatoes.

Image credit: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ. Tip o’ the rocket crane to Mars Curiosity on Twitter.

Related Posts:

A solar eclipse… FROM MARS!
Phobos passes Jupiter… as seen from Mars!
Phobos is, like, totally groovy
More *incredible* Phobos imagery

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (16)

  1. Earth IS special … because if you are curious enough there will never be a thing like a boring planet!

  2. Roach

    Regarding that last point: Will Phobos actually continue until it crashes directly into Mars, or should we expect tidal forces to rip it apart and give Mars a short-lived ring system?

    Wikipedia has some numbers for most satellites in the solar system, but I can’t quite figure out what those numbers mean:

  3. Question:

    When Phobos finally crashes on Mars, will the earth see/feel it?

    Will it just look brighter for a moment, and then fade out? Will we get shrapnel?

  4. Russ

    I thought it would break-up into a ring first, thus it wouldn’t be the whole moon all at once that comes crashing down.

  5. Chris

    Phil, I don’t think Phobos will crash on Mars in one piece. Once it reaches the Roche limit, the Martian tidal forces will tear the moon apart forming rings like we see on Saturn, although they won’t be composed of water ice.

  6. Chris

    Whatever it’s going to do, crash or rip apart, it’s going to be spectacular. Here’s hoping that we can watch from our robot hosted bodies in the future. At which point I hope we won’t miss it due to a ‘map’ error in our software. I’m looking at you Apple.

  7. Chris

    @2 Roach
    They have two possibilities if the object is rigid or fluid. Basically two extremes depending on how tough the object is. The object will actually break apart somewhere between those two extremes. As for the percents, that just gives an idea where the satellite is relative to where the Roach limits actually are. Our moon’s orbit is 21 to 41 times the Roach limit, so we are safe, but Phobos is 0.89 to 1.72 times the Roach limit and its days are numbered.

  8. Mike J.

    The phrase “potatoey lumpiness” made me smile.

  9. Hypocee

    That’s “Roche”, and it governs where an object can initially form by gravitational accretion. If Phobos is a monolithic body, it could survive well inside the Roche limit.

  10. Paul Weaver

    Did they plan for this encounter?

    Or was it just a “target of Opportunity”?

    (I’ll get my hat.)

  11. carbonUnit

    Interesting. Phobos is spiraling in, our Moon is spiraling out…

    Mike J: It’s on its way to mashed taters.

  12. Mark Vincent

    Phobos won’t crash into Mars.
    Our great-great-great grandchildren won’t let it!

    First question – how much power does it take to stop its orbital decay?
    Second question – solar sail, ion engines or ???

  13. You say: ” Tides from Mars” – we all know there’s not much free water on Mars so can I suggest a post talking about what you mean by tides on Mars.

  14. Matt B.

    ^Adam – I think he means Mars’s tidal effect on Phobos, not the other way around. (Note: “tides from Mars”, not “tides on Mars”.) Remember, everything in gravity is mutual. And tides affect the solid parts too. I am surprised that the orbit-changing tidal effects can be so strong on such a small body, but I guess the potato shape helps quite a bit.

  15. Mark Vincent

    To answer my first question, I ran the numbers off Wiki. Its orbit decays about 20 m per century. The change in energy is
    dE = -GMm/2 * [1/a – 1/(a-d)].
    Where a is the semi major axis and d is the decay.
    If I did my math right, that’s “just” 16.5 megawatts.
    New York City peak power consumption is about 11,000 megawatts (

    At 1.38 AU, one would require a solar collector with an area of
    16,500,000 W / (1360 W m^-2 / 1.38^2) = 2.3^4 m^2.
    That’s about 2.25 soccer pitches at 100% efficiency.

    Please check my math, but our great-great-great grandchildren should not phear Phobos’s phlop.


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