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Why haven't some meteoroids gotten caught in Earth's or Venus' orbit?

AFAIK most meteors are tiny fragments from comets. Shouldn't some comet tail sometime have passed Earth orbit at velocities suitable for our planet to capture such fragements? And 100 000s of asteroids have been detected. Why haven't the inner planets gotten asteroid like moons like the outer planets have? The asteroid belt isn't too far away.

Earth's large Luna might clean them away, but that won't explain Venus' lack of tiny moons.

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  • $\begingroup$ This question has been asked (for Earth) on the British television series QI three times, with 4 different answers, one of them being that Earth has 11,000+ moons. Ultimately, it depends on how you define "moon". $\endgroup$
    – user21
    Commented Feb 21, 2015 at 16:03

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The strength of the Earth's gravitational field compared to the Moon and the Sun is not enough to capture and hold satellites - there are too many disruptive forces that would rip them away over time.

However there are some objects at the Lagrangian points - the points where the gravitational fields of the Earth and other objects are equal and so it is possible to have a (likely-meta) stable orbit.

This gives some details about what might be found at the various Lagrangian points: http://en.wikipedia.org/wiki/List_of_objects_at_Lagrangian_points

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    $\begingroup$ But Mars has two (possibly captured asteroid) moons, although Mars has just about 1/10th the mass of Earth. And Venus has no large Luna to distrurb orbits. The sample is of course inconclusive, but your explanation is not immediately convincing. The nearer the Sun, the higher the density of comet debris and eccentric asteroids, I would think, simply because space in the ecliptica is smaller here. Could Venus have a yet undiscovered ring of captured comet dust? Venus has less than half of Earth's eccentricity, 0.0067, Sun gravitational disturbance shouldn't be all that great. $\endgroup$
    – LocalFluff
    Commented Feb 14, 2014 at 16:37
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@adrianmcmenamin Has the right idea here. These objects he is referring to are called Trojans, and are defined to be:

a minor planet or natural satellite (moon) that shares an orbit with a planet or larger moon, but does not collide with it because it orbits around one of the two Lagrangian points of stability (trojan points), L4 and L5, which lie approximately 60° ahead of and behind the larger body, respectively.

Near to each massive body in the solar system (Earth included) are minima, maxima, and saddle points in the potential energy landscape due to the combination of said object and the Sun.

lagrangepoints

It is possible for objects to essentially get caught in the more stable of these extrema. Jupiter, being the most massive, has quite a number of these types of objects.

JupiterTrojans

However, one such object has been discovered orbiting around a stable Lagrange point in 2010, called 2010 TK7.

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  • $\begingroup$ Yes, but why doesn't Venus and Earth have the most common type of moons, Phobos and Deimos types? Is a part of the explanaition that we are inside the frost line so that water sublimates, destroying a common moon material? $\endgroup$
    – LocalFluff
    Commented Feb 16, 2014 at 18:40
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    $\begingroup$ Because we are too close to the Sun (and the Moon). The gravitational forces these bodies produce would make the orbits of micro satellites unstable. Mars is further away (and also closer to a big source of micro moons which may be a factor) $\endgroup$
    – adrianmcmenamin
    Commented Apr 17, 2014 at 11:33
  • $\begingroup$ Venus sphere of influence is about 606,000 km. Earth SOI about 912,000 km. Mars SOI is about 57,000 km. The moon will disrupt a lot of stuff within earth's sphere of influence. But that's not the case with Venus. $\endgroup$
    – HopDavid
    Commented May 19, 2014 at 23:57

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