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On a related topic. I read this the other day: http://www.bbc.co.uk/news/science-environment-22736709 http://www.bbc.co.uk/news/science-environment-22736709 "

On a related topic. I read this the other day: http://www.bbc.co.uk/news/science-environment-22736709 http://www.bbc.co.uk/news/science-environment-22736709 "The space rock, which is called 1998 QE2, is so large that it is orbited by its own moon." The rock is 1.7 miles across, and is orbited by another 600 metre rock. I find it bizarre that a rock only 1.7 miles wide has enough gravity to be able to have its own "moon".

bennyg 13y ago
It's all about mass. A neutron star's mass ratio would be like jamming 50 million elephants into something the size of a thimble. The article you mentioned says almost 15% of large asteroids are a binary system. That's pretty awesome to me. I had no idea about that, but I guess it's fairly obvious.
- gus_massa 13y ago
  The relation between volume and mass for most solid in asteroids are: * mostly water or ice: 1 g/cm^3 * mostly rock: 5 g/cm^3 * mostly iron or metals 8 g/cm^3 So density range of the usual solid material of asteroids is only ~10. This asteroid has a very low gravity. So maybe jumping from it can put you in orbit. The easy way to understand what is imagining that you can turn off the gravity force between the asteroid and its moon. Now you have two independent asteroids that are orbiting around the sun. Both have almost the same position and velocity, so both have a very similar orbit. They will travel together for a long time, but the differences in the orbit will began to accumulate and they will start to slowly come apart. Now turn on the gravity force between them. It's a small correction to the independent orbits but they are not separating very fast so a small force can keep them together. If you stand up in the bigger asteroid then the bigger asteroid will look like a moon. (A better method to calculate the apparent orbit of the moon is to use a reference system that is fixed to the center of the asteroid. It's accelerating, so most of the gravity force of the sun cancels, but there are some tidal and centrifugal effects.)

bennyg 13y ago

It's all about mass. A neutron star's mass ratio would be like jamming 50 million elephants into something the size of a thimble. The article you mentioned says almost 15% of large asteroids are a binary system. That's pretty awesome to me. I had no idea about that, but I guess it's fairly obvious.

gus_massa 13y ago
The relation between volume and mass for most solid in asteroids are: * mostly water or ice: 1 g/cm^3 * mostly rock: 5 g/cm^3 * mostly iron or metals 8 g/cm^3 So density range of the usual solid material of asteroids is only ~10. This asteroid has a very low gravity. So maybe jumping from it can put you in orbit. The easy way to understand what is imagining that you can turn off the gravity force between the asteroid and its moon. Now you have two independent asteroids that are orbiting around the sun. Both have almost the same position and velocity, so both have a very similar orbit. They will travel together for a long time, but the differences in the orbit will began to accumulate and they will start to slowly come apart. Now turn on the gravity force between them. It's a small correction to the independent orbits but they are not separating very fast so a small force can keep them together. If you stand up in the bigger asteroid then the bigger asteroid will look like a moon. (A better method to calculate the apparent orbit of the moon is to use a reference system that is fixed to the center of the asteroid. It's accelerating, so most of the gravity force of the sun cancels, but there are some tidal and centrifugal effects.)