Do asteroids float?

[Aethon]

NASA is searching for a suitable asteroid for the real life ARM, and has this to say about the latest candidate (2011MD) :

Analysis of Spitzer’s infrared data show 2011 MD is roughly 20 feet (6 meters) in size and has a remarkably low density -- about the same as water, which supports the analysis of observations taken in 2011.

http://www.nasa.gov/press/2014/june/nasa-announces-latest-progress-upcoming-milestones-in-hunt-for-asteroids/index.html#.U6OMq_ldUTphttp

[Brotoro]

If an asteroid has a density about the same as water, it probably has a lot of empty space in it (like a rubble pile). Not the easiest kind of asteroid to move. And if it is a rubble pile with empty spaces, it would not float (since the density of the separate solid chunks would be greater than the density of water).

[Excalibur]

Think of a piece of pumice, generally it has a porosity of ~90% (90% of the pumice's volume is void space) so despite it being composed of dense volcanic glass, it floats extremely well.

So if you could find an asteroid with high enough porosity (and therefore low enough density) it would theoretically float as well. Other things to consider are the structural strength of the asteroid (it is believed that some of these highly porous 'rubble pile' asteroids are barely held together under the extremely weak gravity of it's constituent mass) and whether the void spaces are connected to each other and the surface (thus able to allow water to enter and increase density).

Edited June 20, 2014 by Excalibur

[softweir]

Pumice can float because it is rock with gas-bubbles in that formed while it was solidifying after melting. The gas bubbles don't connect well, so only a few can get flooded when pumice is immersed in water.

Low-density asteroids are not thought to have bubbles in them - they are thought to be aggregations of small pieces of material fused together by impacts or held together by electrostatic forces. Even if one were solid enough to be moved and entered into our atmosphere, the spaces between granules would be highly interconnected and would flood, allowing the asteroid to sink.

And as has been pointed out before, it is quite likely that a low-density asteroid would fall apart long before it reached any water worth floating in!

[Dedjal]

Then we should bring the water to it.

Because science!

[vger]

Another thing about an asteroid of such low density though. If it's accurate, an asteroid 6 cubic meters in volume would weigh about 13000lbs. That's only about half a ton heavier than the Apollo command module. So, put some heat shielding on it, and yay, we can bring it back home the Kerbal way.

[Excalibur]

Pumice can float because it is rock with gas-bubbles in that formed while it was solidifying after melting. The gas bubbles don't connect well, so only a few can get flooded when pumice is immersed in water.

Low-density asteroids are not thought to have bubbles in them - they are thought to be aggregations of small pieces of material fused together by impacts or held together by electrostatic forces. Even if one were solid enough to be moved and entered into our atmosphere, the spaces between granules would be highly interconnected and would flood, allowing the asteroid to sink.

And as has been pointed out before, it is quite likely that a low-density asteroid would fall apart long before it reached any water worth floating in!

I did point out that these things need considering (in particular structural strength and whether or not internal voids are connected). Currently we don't know enough about low-density asteroids to really answer the OP's question, I was merely stating that theoretically there may be an asteroid out there that potentially may be able to float in water.

To be honest you're probably right though - it is more probable that any low-density asteroid is likely so fragile that any serious attempts to move it would result it it's disintegration.

Reminds me of reading an astronomy book as a kid and finding out that Saturn should float due to it's low density. I used to imagine it sitting in the Pacific and wondered what it would be like to climb it's rings. I'm certainly glad my understanding of physics has advanced since then.