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Some time ago I was thinking about this.

We know that space is almost matter-free, and also is plenty of particles and objects orbiting around all planets and sattellites. If objects orbit around planets then particles can do it too, isn't it? When there are solar explosions, the sun launches tons of particles, and also some comets that crosses our path also leaves a trail of particles.

When the Moon crosses those particles, some particles are destroyed because of collision, but some other particles just pass near it and sure starts an orbit around the Moon. I'm talking about the Moon because does not have atmosphere.

That is, with these assumptions, can we say that the Moon have an orbit of particles almost touching the ground at really high speed? If this is correct, why any astronaut wandering around Moon surface or any Moon lander received an impact of those particles? The Moon has been there for about 4.5 billion years, so that's a lot of chances of having particles orbiting around.

Surely I'm wrong at something, please, give me a bit of knowledge.

Keep in mind that I'm talking about bullet sized particles.

Thank you.

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You are describing the solar wind, radiation belts and the very thin lunar atmosphere. Note that most of these particles are electrically charged (ions or electrons) so electrical forces are far more important than the gravitational force. ... and radiation is a major hazard for astronauts, and electric circuits have to be specially radiation hardened! –  Michael Brown Aug 23 '13 at 10:48
    
I know radiation exists but when I talk about particles, I talk about bullet-sized particles so they can impact a spacecraft or an astronaut like a weapon shoot. Going to edit question. –  Jorge Fuentes González Aug 23 '13 at 11:13
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3 Answers

up vote 2 down vote accepted

There are two distinct dangers for spacecraft, astronauts and electronics, both in Earth orbit and on the Moon (and beyond, of course).

  • One is radiation, both in the form of charged subatomic particles and gamma rays. These originate in the solar wind (what you call "solar explosions") and in cosmic rays (or occasionally from gas in cometary tails) and can be very energetic. Being ionizing radiation, they can inflict cell and tissue damage, and increase the risk of cancer the longer humans are in space (though not by very much). Spacecraft themselves are relatively safe, as this radiation will not normally inflict mechanical damage, but electronic components need to be specially insulated in any spacegoing craft. Since these are charged particles, electric and magnetic forces play the dominating role: they do not normally gravitationally orbit, but they can be confined in radiation belts.

  • The other danger is micrometeors, your "bullet sized particles". These are solid particles that can range in size from microns to tens of centimeters, with a corresponding decrease in frequency. These are naturally present in the solar system as remnants of the original dust cloud, though the Earth's orbital neighbourhood has relatively few of them. This also includes "space junk" which can be debris from satellite collisions all the way up to whole satellites. Micrometeors are a danger anywhere, though they rarely orbit the Earth or the Moon for very long; space junk is typically in Earth orbits which one hopes will eventually decay, but is definitely dangerous on a decades timescale. This class of debris can cause mechanical damage, possibly catastrophic, to both spacecraft and astronaut pressure suits. The current philosophy is to provide the best shielding possible, in the understanding that a catastrophic event - should it happen - cannot be defended against.

Having said this, I must also mention that the Moon also has a tenuous atmosphere of dust particles orbiting it; the Lunar Atmosphere and Dust Environment Explorer spacecraft was recently launched to study it. These particles were most likely produced in meteor collisions, so they are relatively low-speed, and are up to a few microns in size. They do not pose a danger to astronauts or spacecraft, though they might degrade spacecraft surfaces over very long times.

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Searching about micrometeors found this: spaceflight.nasa.gov/gallery/images/station/crew-15/html/… Really impressive, thank you! –  Jorge Fuentes González Aug 23 '13 at 14:15
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Below is my edited answer.

  • If there's no atmosphere in space shielding from particles, why is not dangerous for spaceships/humans?

I don't understand why you are assuming this to be the case to start with. I'm just saying shouldn't you be asking like:-

If there's no atmosphere in space shielding from particles, how dangerous is it for spaceships/humans?*

All the answers given addresses the above question which off course is a very important question to deal with.

  • When there are solar explosions, the sun launches tons of particles, and also some comets that crosses our path also leaves a trail of particles.

None of the particles that sun launches is bullet sized.

  • That is, with these assumptions, can we say that the Moon have an orbit of particles almost touching the ground at really high speed?

    The answer to the above point is No you cannot have this scenario,i mean not realistically.This is what i was trying to answer in my first post assuming you meant sub-atomic particles.

  • If this is correct, why any astronaut wandering around Moon surface or any Moon lander received an impact of those particles?

    Even if you consider a classical object like a bullet.Below is the graph showing how much speed it requires to orbit the around moon at close distance to its surface.Assuming the average height of astronaut to be around 1.5m it require a speed of almost 1700m/s to maintain the orbit at that height above moons surface.So no object can to that by its own at that height.

enter image description here

Apollo astronauts described moon dust as gritty, abrasive and clingy. It can wreak havoc on equipment and computers. Moonwalkers were coated in it and their spacesuits were almost threadbare when they returned to Earth

This is about the dust on the surface of the moon. I am simply trying to make a distinction between "particles" and "orbiting particles". Both can cause danger to the person standing on the lunar surface,but its simply that there happens to be none of the later kind.

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Ok. Thanx to @MichaelBrown talking about very thin lunar athmosphere, I found this:

Apollo astronauts described moon dust as gritty, abrasive and clingy. It can wreak havoc on equipment and computers. Moonwalkers were coated in it and their spacesuits were almost threadbare when they returned to Earth

http://www.space.com/18067-moon-atmosphere.html

So, as @Hubble07 says, is difficult to have a particle orbiting the Moon. The worse the moon can have is dust-sized particles and seems that yes, those particles are a problem for spacecrafts and astronauts.

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Please, tell why downvote. –  Jorge Fuentes González Aug 23 '13 at 14:11
    
I didn't downvote, but your answer doesn't seem to address your own question. –  Ben Crowell Sep 8 '13 at 20:31
    
I asked "why particles are not dangerous", just assuming that they were not. But with help I found that yes, they can be dangerous, at least in part. –  Jorge Fuentes González Sep 15 '13 at 17:52
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