Mercury A to Z: Orbiting Mercury

Hello, friends!  Welcome back to this year’s A to Z Challenge.  For this year’s challenge, my theme is the planet Mercury, and in today’s post, O is for:


On April 1, 2012 (note that date), NASA announced the discovery of a moon orbiting Mercury.  NASA went on to propose naming this newly discovered moon Caduceus, after the coiled-snake-shaped staff that Mercury carried in ancient Roman mythology.  This would have been a very exciting discovery except, of course, this was announced on April 1st.  Maintaining orbit around Mercury is hard… so hard it’s basically impossible.  The idea of a moon maintaining orbit around Mercury is so absurdly impossible that NASA thought it would make a good April Fool’s Day joke.

But for the sake of argument, let’s pretend that Caduceus is real.  Let’s pretend that Mercury does have a little, tiny moon, similar to the asteroid-like moons of Mars.  What would happen to Mercury’s moon?  Well, very rapidly, she’d find herself caught in a gravitational tug-of-war between Mercury and the Sun—and sadly, this is a tug-of-war that Mercury could never, ever hope to win.

With each successive orbit around Mercury, Caduceus would be feel the increasing and decreasing gravitational force of the Sun.  When she circles around to the dayside of Mercury, the Sun’s pull would be stronger; when Caduceus circles around the Mercury’s nightside, the Sun’s pull would be weaker.  A little stronger, a little weaker, a little stronger, a little weaker, over and over again.  If Caduceus’s orbit started off as near circular, that orbit would gradually stretch into a wider and and wider oval shape.  Eventually, inevitably, that oval would become so stretched out that it would extend beyond the reach of Mercury’s gravity.

Caduceus would not necessarily crash into the Sun after that.  Remember that every action has an equal and opposite reaction.  Every time the Sun’s gravity pulled Caduceus hard one way, she would then swing just as hard in the opposite direction.  So when the moment came and Caduceus finally broke free of Mercury’s gravity, there’s a very good chance that she would launch herself off into space like a child leaping from a swing set.

But regardless of Caduceus’s ultimate fate (crashing into the Sun or flinging herself off into space), the outcome for Mercury is the same.  He loses his moon.  Mercury will always lose his moon, no matter what.  Even artificial satellites, like MESSENGER or BepiColombo, cannot maintain orbit around Mercury for long without their thrusters.  Orbiting Mercury is really, really hard work for a spacecraft, and for a small, asteroid-like moon?  It’s basically impossible.

So if you have ever wondered why Mercury doesn’t have a moon, now you know why Mercury doesn’t have a moon.


Here’s NASA’s April Fool’s Day announcement about the discovery of Caduceus.

And here’s an article from Universe Today entitled “How Many Moons Does Mercury Have?” written by a good friend of this blog, Matt Williams.

9 thoughts on “Mercury A to Z: Orbiting Mercury

    1. Interesting thought. Maybe an object at Mercury’s L2 Lagrange point would be safe from the Sun’s gravity. I guess that wouldn’t be a moon, strictly speaking, but it would be a constant companion for Mercury.

      Liked by 1 person

  1. This is new for me. I never realized orbiting Mercury was that unstable. But it makes sense.

    Somewhat counter-intuitively, it’s extremely difficult to crash into the sun, at least for anything inheriting the orbital momentum of a planet. You have to cancel out most of that momentum (~47 km/s in Mercury’s case).

    Liked by 1 person

    1. I’ve heard about that. Actually, I think I first heard about that from your blog. And after playing Kerbal Space Program, I think I have some understanding of the delta-v involved.

      Though I imagine if a moon of Mercury escaped Mercury’s gravity at exactly the right (or wrong) point in its orbit, it could find itself aimed directly at the Sun. That’s probably not the most likely scenario, but I feel like it’s a possibility.

      Liked by 1 person

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