Why are there few atomic/molecular collisions at LEO level?

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In summary, in LEO and GEO orbits, there are very few atomic/molecular collisions between components of the atmosphere and spacecraft, resulting in heat exchange primarily through radiation. This is because the mean free path at these altitudes is larger than most spacecraft, indicating a sparse density of the atmosphere. The pressure and density of the atmosphere decrease significantly at higher altitudes, such as 200 km, making it even emptier than at sea level.
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tomwilliam2
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The book I'm reading (Spacecraft Systems Engineering) says that there are very few atomic/molecular collisions between components of the atmosphere and spacecraft in LEO or GEO, which means that any heat exchange comes from radiation alone, and the path of the orbit can be modeled as free molecular flow.

I've been trying to work out why there aren't many collisions. Apparently, the mean free path at about 200km from the Earth's surface (as an example of a LEO level) is about 240m, which is larger than most spacecraft , so that makes sense.
However, that seems to be a remarkably sparse density at just 200km in height. Is the atmosphere really so empty up there? Or is it because we're talking about the ionosphere, and there are very few molecules or non-ionised atoms left at that height?
Thanks in advance
 
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tomwilliam2 said:
However, that seems to be a remarkably sparse density at just 200km in height. Is the atmosphere really so empty up there?

Yep. I don't know what the density of the atmosphere is at 200 km, but at 100 km the pressure is roughly 1/100,000th of what it is at sea level so I assume the density falls off in about the same amount.
See here: http://www.regentsprep.org/regents/math/algtrig/atp8b/exponentialresource.htm
 
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FAQ: Why are there few atomic/molecular collisions at LEO level?

1. Why are there few atomic/molecular collisions at LEO level?

The main reason for the lack of atomic/molecular collisions at LEO (Low Earth Orbit) level is due to the extremely low atmospheric pressure. At LEO, the atmospheric pressure is significantly lower compared to ground level, making it a near-vacuum environment. This means that there are very few particles present to collide with each other.

2. How does atmospheric pressure affect atomic/molecular collisions at LEO level?

The lower atmospheric pressure at LEO results in a lower number of particles per unit volume. This means that there are fewer opportunities for atomic/molecular collisions to occur. The particles that do exist at LEO are also moving at high speeds due to the low air resistance, making it less likely for them to collide with each other.

3. Are there any other factors that contribute to the lack of collisions at LEO level?

Yes, there are other factors that play a role in the few atomic/molecular collisions at LEO level. One of these factors is the Earth's magnetic field. At LEO, particles are affected by the Earth's magnetic field, causing them to follow curved paths instead of straight lines. This decreases the chances of collisions between particles.

4. How does the altitude of LEO affect atomic/molecular collisions?

The higher the altitude of LEO, the lower the atmospheric pressure and the fewer the particles present. This means that at higher altitudes, there are even fewer opportunities for atomic/molecular collisions to occur. However, the effect of altitude on collisions is not as significant as the effect of atmospheric pressure.

5. Can atomic/molecular collisions occur at LEO level at all?

Yes, while there are fewer atomic/molecular collisions at LEO level compared to ground level, they can still occur. There are still some particles present, and collisions can happen due to random motion and interactions between particles. However, the frequency of collisions is significantly lower compared to other altitudes due to the low atmospheric pressure and other factors mentioned above.

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