Are Phobos and Deimos tidally locked to Mars?

  • 2
    $\begingroup$ Hello Star girl. There are really three different questions here. It's best to ask one question at a time. So which question do you want answered first? It is also best to tell us what you've done already to answer these questions. For example, have you read the wikipedia page on Phobos and Deimos? Have you tried google "mission to phobos" $\endgroup$
    – James K
    Nov 3, 2021 at 20:13
  • 4
    $\begingroup$ The fact that two of the questions are answered in the wikipedia pages tempts me to downvote the question. $\endgroup$
    – Jim421616
    Nov 3, 2021 at 20:41
  • $\begingroup$ In partial answer to the first part of the question, nobody has landed on Phobos and Deimos yet, but JAXA is sending MMX, the Martian Moons eXplorer to explore the moons and return a sample from Phobos. Launch is due in 2024, with arrival at Mars in 2025 and sample return to Earth in 2029. $\endgroup$ Nov 3, 2021 at 20:43

1 Answer 1

  1. The gravity of those moons is so low that when and if something "lands" on them it will be more like two spacecraft rendezvousing and docking than like a spacecraft landing on a world. And it hasn't been done yet.

  2. According to this list:


Deimos and Phobos are known to be tidally locked to Mars.

  1. Phobos and Deimos both orbit Mars in the same direction, prograde, at different distances from Mars. So it takes Deimos, the farther moon, longer to make one orbit around Mars than it takes Phobos, the closer moon.

So Phobos periodically passes in front of Deimos as seen from some places on the surface of Mars. Here is a link to a video of such an event:


And you could have researched that easily yourself.


  • $\begingroup$ To add some numbers to your response, the surface gravity of Phobos is ~.05% that of the surface gravity of earth, and Deimos is about .02% of the same. Phobos and Deimos are in a 4:1 resonance, so Phobos orbits 4 times for every one time that Deimos orbits $\endgroup$
    – Justin T
    Nov 3, 2021 at 19:45

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .