APOD: Saturns Hyperion: A Moon with Odd... (2011 Feb 27)

[qbragg]

We have picked up corals on beaches in the Caribbean that look like this. Somehow I doubt this one is oganic, in any way.

[alphachap]

Great picture of Hyperion! How can you have an impact crater so large on a moon so small and not have the moon fracture? It's not like you can have a soft impact!

Actually you can have a soft impact.

[neufer]

Great picture of Hyperion!
How can you have an impact crater so large on a moon so small and not have the moon fracture?
It's not like you can have a soft impact!

Actually you can have a soft impact.

[supamario]

I think Chris L Peterson is probably on the money here. My initial thought was an impact, probably low impact which did not totally destroy the body, and the low density body, rather than venting out, created sinkholes which sank into the low density, cavernous body - the rubble pile analogy.

[JeffLawrence]

How do astronomers calculate the density of extraterrestrial objects? I think I understand calculation of speed (red shift) and perhaps distance, but density has me puzzled.
Thanks,
Jeff

[Chris Peterson]

How do astronomers calculate the density of extraterrestrial objects? I think I understand calculation of speed (red shift) and perhaps distance, but density has me puzzled.

Your question is very broad, since it covers a wide range of different objects. But in this case we are talking about a small body in orbit around a much more massive body. Since the mass of Saturn is known, and the orbital period of Hyperion is known, the mass of the Hyperion can be calculated. We've observed this moon closely enough to directly measure its size and shape, and therefore we can calculate its volume. Density is just the mass divided by the volume.

[neufer]

How do astronomers calculate the density of extraterrestrial objects? I think I understand calculation of speed (red shift) and perhaps distance, but density has me puzzled.

Your question is very broad, since it covers a wide range of different objects. But in this case we are talking about a small body in orbit around a much more massive body. Since the mass of Saturn is known, and the orbital period of Hyperion is known, the mass of the Hyperion can be calculated. We've observed this moon closely enough to directly measure its size and shape, and therefore we can calculate its volume. Density is just the mass divided by the volume.

Calculating the mass involves more than knowing the mass of Saturn and Hyperion's orbital period.

Hyperion perturbs it's neighbor moons in subtle ways that indicate it's mass
but mostly our knowledge comes from Hyperion's perturbing effect upon passing spacecraft.

There is tremendous uncertainty about the mass of small isolated moons, asteroids, comets, etc.
that have not been closely visited by spacecraft (e.g., Pluto's Nix & Hydra).

[msmerza]

Are you sure this isn't a Motie beehive? It sure looks like an artifact out of The Mote in God's Eye!