Happiness often sneaks in through a door you didn't know you left open. -John Barrymore
Question #26443 posted on 07/24/2006 3:01 a.m.

Dear 100 Hour Board,

Re: Board Question #25833

I may not have asked my question very clearly, since the response I received did not contain anything that even nearly represented an answer to what I thought I had asked.

I do realize that it is the low mass/surface area ratio that allows these objects to float, and not the materials that they are made of. My question was more aimed toward what the effect of a low density atmosphere would be on objects with a low mass/surface area ratio, of which dandelion seeds and feathers are purely examples.

For instance, if a (small) meteorite enters earths atmosphere it will burn up. However, a meteorite has a relatively high mass/surface area ratio. If a feather were to enter earth's atmosphere, would its low mass/surface area ratio change the result, or would it also burn up?

Perhaps another way to ask this question would be this: Could an object be found or made that would have a low enough mass/surface area ratio that it could safely avoid heating up when entering earths atmosphere?

The only reason that I mentioned the material of the parachute was because of the huge amount of problems that would arise from using a typical parachute fabric in such a situation.

- Fredjikrang

A: Dear soft landing,

To answer the last part of your question first: yes, an object could be found or made which would have a mass/surface area ratio low enough that it wouldn't be burned up. Take, for instance, cosmic dust (dust from outer space); according to this Wikipedia article, "At the Earth, generally, an average of 40 tons per day of extraterrestrial material falls to the Earth." So, it certainly seems that dust fits your criteria. It also states definitely that some of this dust reaches the surface, and has even been collected at places like the polar ice caps.

However, this is not to say that it's trivial to find something that won't burn up. An astronomy website says "Most shooting stars that you see are about the size of a grain of sand or even smaller when they enter the atmosphere from space, and they burn up completely in the atmosphere when they are still very high above the ground."

To get an idea of what, exactly, the mass/surface area ratio would have to be, let's look at an average speck of space dust. The average grain of space dust ranges from 50-500 micrometers in diameter (1 micrometer = 1 millionth of a meter, or 1 thousandth of a millimeter). For an idea of size, the average human hair has a diameter of about 50 micrometers. It has an average density of 2 g/cm^3. A particle with a diameter of 500 micrometers would have a much higher mass/surface area ratio than one with 50 micrometers, so we'll choose 500 micrometers for our calculations (to get an optimistic calculation). Using the volume of a sphere (4/3*pi*r^3), we get a volume of 5E-10 m^3. Multiply that by 2 gm/cm^3, and we get a mass of 1E-6 kg. From this website, it appears that your average dust particles (the Earth type, anyway) could be approximated at 1 m^2/g (and that's a "low" surface area—it could be more per gram). Extending this to our space dust, we get a surface area of 1E-3 m^2. So, the mass/surface area ratio is 1E-3 kg/m^2, which could be rewritten as 1 gm/m^2. That's tiny. To compare a dust particle to a dandelion seed, check this out: the terminal velocity (in normal air pressure) of a dandelion seedthis website is .5 m/s, or about 1 mph. For dust, the terminal velocity is .02 m/s, about .05 mph, or about 8 inches per second. That's 25 times slower. I don't think a dandelion seed or a feather would stand a chance—but dust would make it.

-Laser Jock