A conscience is what hurts when all your other parts feel so good.
Question #25833 posted on 07/03/2006 3:01 a.m.

Dear 100 Hour Board,

If you set a feather (or dandelion seed, et al.) in space close enough to earth that it would be drawn in by earths gravity field, would it burn up on entry or make it to earths surface relatively unharmed?

The reason that I am wondering is because of the very low terminal of these things in the dense lower atmosphere. If they are moving toward earth, would the friction of the less dense atmosphere just slow them down or would it burn them up?

If they could make it through, then doesn't that mean that if a good material could be found that people could parachute from space?


- Fredjikrang

A: Dear Fredjikrang,

After much thought, and helpful consulation with bawb, Curious Physics Minor, and others, I (we) have come to the conclusion that it is not the material from which dandelion seeds and feathers are made that allows them to float through the air, so much as it is the size, shape and density of those objects.

In other words, in order for an astronaut to make use of the floating properties of a dandelion seed, they would need a scaled-up giant dandelion seed with the same density as a small dandelion seed. This requirement, alone, would be enough to make the contraption highly impractical, but there are other issues.

One is that the upper atmosphere is even less dense than the lower atmosphere, meaning that the giant dandelion seed would need to be even lighter, in order to have a density near that of the surrounding gas.

The other is that the physical relationship of mass vs. area doesn't scale very well. Area increases as r2, but mass increases as r3, which means that many structures which are light and strong on a small scale end up being too heavy to support their own weight on a large scale. (This is why only small animals, such as insects have exoskeletons. Larger animals, so endowed, would collapse under their own weight.)

So, given that you'd have to create an enormous feather or dandelion seed, which would then have to be even less dense than a regular feather or dandelion seed, which would probably still collapse under its own weight . . . I'm going to say that your idea isn't practical. But it's still interesting.

- the physics chick