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Let's consider Mars' two moons for a moment. They're small, not even spherical, and most likely are captured asteroids. They weren't discovered until the late 1800's. Telescopes are much more powerful today, so you'd think we'd have found any similar moons at Venus or Mercury by now, if they exist.

The trouble is, there are some very dark asteroids out there. And of course, there are some much smaller ones too.

How do we know Venus or Mercury doesn't have a small dark asteroid moon? Did any probes actually try radar to find them?

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    $\begingroup$ I think there have been enough probes and observations of the two inner most planets to deduce that neither host a reasonable sized satellite. That doesn't mean to say they never had one, most astronomers believe that Venus was the victim of at least one, if not two, proto-planet collisions. This is because of the fact that Venus spins clockwise on its axis, the only planet in the Solar System to do so. However the influence of the Sun's gravity coupled with the weaker gravtiational field of Venus (compared to Earth) probably meant that any satellite that formed probably drifted away over time $\endgroup$
    – Dean
    Commented Feb 1, 2016 at 10:59
  • $\begingroup$ @Dean Concerning having HAD moon(let)s, I think it is interesting that Phobos will be destroyed by Mars' tidal forces within a few tens of millions of years, only 1% of the time Mars has existed. What if it is common that terrestrial planets form with (or eject after impact, or capture) small moons which like Phobos crash into them billions of years later? Earth would be an exception because our large moon cleared out such debris quickly. And Mercury because of its low mass, eccentricity, tidal locking and Solar proximity. Venus being hit by its last moon and resurfaced only about 300 Ma ago, $\endgroup$
    – LocalFluff
    Commented Feb 1, 2016 at 12:16
  • $\begingroup$ @LocalFluff This is an interesting thought, as with all Astronomy we are limited by this very short window in time through which we can observe the Universe as it appears now. If we could look back through time I think we would see a very different Solar System. Like you say it isnt by any means stable now, Phobos is getting closer to Mars while the Moon is moving away from Earth. It will look different again in another few hundred million years from now. $\endgroup$
    – Dean
    Commented Feb 1, 2016 at 12:27
  • $\begingroup$ @Dean The imminent Phobos impact on Mars and the recent resurfacing of Venus indicate that it may not be too uncommon that the Solar System('s terrestrial planets) experience impacts within 10% intervals of their ages even after billions of years. Very speculatively though. Maybe research into minor bodies and exoplanets will put some light on this, it certainly is offering other surprises thus far. $\endgroup$
    – LocalFluff
    Commented Feb 1, 2016 at 12:35
  • $\begingroup$ There always being size-limits to what we can "see" even with the best telescopes/cameras, a more exact question might be, "How small would presumable moons/satellites of Mercury or Venus have to be to be undetected today?" $\endgroup$
    – Jeff Y
    Commented Feb 1, 2016 at 16:26

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It's unlikely that either Mercury or Venus could have moons to begin with. Both of these planets are pretty close to the Sun — and in general, this prevents moons from finding stable orbits.

If a moon were too close to the planets, it would fall within the Roche limit and be torn apart by tidal forces. If a moon were too far from the planets, it would fall outside the Hill sphere and be pulled into the Sun.

The zones where moons around these planets could be stable over billions of years is probably so narrow that no body was ever captured into orbit when the planets were first being accreted.

Now, it gets even more complicated in Mercury's case. Its Hill sphere extends about 3000 miles (~4828 km) — that is, any satellite more than 3000 miles from the planet will be pulled away by the Sun's gravity.

However, its Roche limit extends to 3600 miles (~5794 km), so any satellite within this would be ripped apart. Venus lacks this excuse, but the region between where the Sun would pull away a satellite and the Roche limit is probably too small for a satellite to happen to have formed there.

Next, you have to realize that inner planets generally don't have moons. Solar wind pressure usually sends dust and debris flying away from the planets, so they don't have any spare material for moons.

Remember that Earth's Moon is thought to have formed from a collision with the Mars-sized planet Theia. This makes Earth an oddity in itself. Meanwhile, Mars is thought to have captured Phobos and Deimos from the asteroid belt — so Mars and its moons didn't condense from the same dust mass.

With all this, it'd be extremely surprising if we found moons on either Mercury or Venus.

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  • $\begingroup$ I'd like to add that the Roche Limit is not a given distance from a planet, it also depends on the density and the mass of the captured body. See : astronomy.stackexchange.com/questions/12723/… $\endgroup$
    – Nico
    Commented Feb 3, 2016 at 8:48
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    $\begingroup$ @Nico Of course, I'm just giving some estimates considering Mercury's mass and amount of debris that it formed with. $\endgroup$
    – Sir Cumference
    Commented Feb 3, 2016 at 13:45
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    $\begingroup$ "inner planets generally don't have moons. Solar wind pressure usually sends dust and debris flying away from the planets, so they don't have any spare material for moons." Based on this is there any particular distance defined for a planet from the star to have stable moons? $\endgroup$
    – Astroynamicist
    Commented Feb 5, 2016 at 11:03
  • $\begingroup$ @Astroynamicist Not sure of the exact distance or formulae, but usually near the first circumstellar disc. $\endgroup$
    – Sir Cumference
    Commented Feb 5, 2016 at 11:45
  • $\begingroup$ What about a temporarily captured asteroid? The question is not "do you think they would probably find one if they looked?" it just asks if anyone has looked and if so how and would it be a definitive observation or not. For more about "minimoons see: space.stackexchange.com/questions/16738/… $\endgroup$
    – uhoh
    Commented Oct 19, 2016 at 8:09
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Considering we have sent orbiters to both, I find undetected moons to be ludicrous. Even if they didn't search, the gravitational disturbances would have disturbed their mapping enough to tell there were moons if there were any.

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  • $\begingroup$ Forgot about gravitational disturbances. I remember MASCONs on the Moon and read that they disturbed lunar probes of the 1960's. If those little things can do that, then surely a small moon near Venus or Mercury would do even more. $\endgroup$
    – DrZ214
    Commented Apr 26, 2018 at 23:43

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