Anyone who's ever seen empty snail houses just knooows what that thing over there in space is .... yes ... a huge empty snailhouse of some sort.

If it's only half as dense as water, does that mean if you put, say, a brick on the surface it would sink to the middle? How come the craters keep their shape at all if it's so un-dense (what's the opposite of dense?). I realise it's not as viscous as water, or it'd be a sphere, but...it's counterintuitive.

Just like the vast majority of satellites on our solar system. Even our Moon it's of artificial origin. NASA/JPL and other agencies worldwide keep censoring and altering facts but the data proving that these satellites are far from natural it's there, it's scattered thru years and censoring but you can still grasp it.

Now all we need is a mad scientist hell-bent on destroying the world to go out and "harvest" the biological stuff on the space rock, transforming into a deadly virus that will destroy all of Humanity, or transform us into fritters.

It's much MORE viscous* than water; it's a foamy matrix of solid material interspersed with vacuum, making it less dense than water in the same way as pumice. A brick would not sink in, because Hyperion's gravity is much too weak.

* high viscosity is low fluidity.

Because it's not very dense, it also has very low gravity...this is why the craters can keep their shape, becuase there isn't enough force acting on them to distort them. And why it's not a sphere.

Try thinking of polystyrene - that is much less dense than water yet holds a shape. Remember, water is a liquid, not a solid. Mercury is much denser than polystyrene but won't sink through it because polystyrene is a solid.

Hope it helps.

The opposite of dense is diffuse. A brick probably wouldn't sink because the body is not liquid. If it is mostly ice, then the reduced density just means there are a lot of voids. When a rigid structure is trying to support a mass, it's the *weight* of the mass which is important, rather than its density. Given the very low gravitiy, the weight of a brick would be very small and ice crystals sufficient to support it.

Density/displacement/bouyancy only come into play when you're talking about fluids or fluid-like behaviour. Crystalline substances are the opposite of fluids.

Its made of rock with lots of holes in it, the *average* density is 1/2 that of water....either that or it's hollow :)

Density is a measure of the ratio of Mass to Volume. Being less dense than water means that this moon would float if dropped in the Pacific ocean. Wood for instance is less dense than water hence it floats, it is still a solid however and retains its shape very well. Viscosity is usually used when talking about liquids, not solid bodies like this moon, but I suppose if it were to apply, it would be much MORE viscous than water, not less.

Low density does not make an object viscous or liquid by nature. There are stones here on Earth with a low density, which resemble the surface of Hyperion; http://en.wikipedia.org/wiki/Pumice .

You're confusing density with liquidity.

It's a jersey royal, rich in carbohydrates. If it came from tescos then it will have been frozen for many years too, which explains the water ice.

Just because an object is not dense does not mean it's not solid.

All the comment about density means is that a bit of Hyperion would float on water, just like a bit of expanded polystyrene (also much less dense than water).

Expanded polystyrene, pumice stone (the proper stuff from volcanos) and most woods are all less dense than water, which is why they float on water. If you put a brick on the surface of a plank of wood or on top of a polystyrene block, would you expect the brick to sink through the plank because the plank was less dense?

Density != hardness. (And I'm sure you can think of plenty of people who are as dense as they come, but aren't at all hard. ;-)

I'm no space scientist but i assume thats its low gravitional pull means object on the surface don't have enough downward force exertion to push through the relatively low density material. Plus it will compact the material as it goes making it more dense and so requiring greater force to move further into the body.

Just my thoughts and i could be missing something so please correct me if I'm way off someone.

In answer to 'Question': density and rigidity are in no way linked. (Some) pumice stone, here on Earth, floats in water but you wouldn't want to hit yourself in the face with it.

"This doesn't mean that we have found life" - just like discovering a stray Lego brick doesn't mean you know how to build a replica of Windsor palace.

Just because it's not as dense as water, doesn't mean it's a liquid. The asteroid is made of rigid material--anything placed on its surface will either remain there or drift away in the nearly non-existent gravity. There are plenty of things less dense than water--many plastic materials for example.

The part of this story that I am struggling with is the explanation of the sharp crater edges. The density is low, so the mass is low, so the gravitational field is low. Fine so far...

So, surely, the velocity of any impacting body will be correspondingly low (From force=G*m1*m2/R_squared)

So the energy imparted to ejecta particles will be correspondingly low

So the proportion of ejecta that reaches escape velocity should be about the same as for a more massive body?

Where is the fault in my reasoning?

So if a bit of Hyperion floats on water, that means it's a witch..... burn it!

Hyperion turned me into a newt last week, I got better though.

(don't worry already got me coat on)

It's quite obviously an inter-galactic wasps nest. I'll get some petrol...

Dave, the flaw in your reasoning is here:

"So, surely, the velocity of any impacting body will be correspondingly low (From force=G*m1*m2/R_squared)"

This is incorrect because an impacting body does not get all of its impact velocity from the gravitational force between the two objects (if it did, it would always hit head-on). Some of the velocity will come from the body's inital trajectory. In the case of an initially high-velocity object (like most impacting bodies) and a low-gravity target, most of the velocitiy comes from the initial trajectory.

@Dave:

The gravitational field of the body is generally not the main driving force in cosmic collisions. The impacting body would likely be moving very rapidly to begin with, so it wouldn't need much help from gravity. Well not at the end of its flight, presumably it was affected by gravity during its flight at some point.

If you assume that things in space move very fast, then use the rest of your logic, you essentially prove what you didn't understand.

And to those that were wondering, once you got Hyperion to liquid ice, you'd need an atmosphere, which would fill in many of the voids in the moon. Depending on the rock / ice / void ratios, the added air would probably make it significantly more dense. Then as the ice melted off, it would become even more so. I do still like the idea of planetary bodies that can float though...

Dave -

I think that you're assuming that an object impavting will be ":falling" due to hyperion's gravity.

Consider, instead, a bit of rock that is crossing Saturn's orbit. It will be traveling at planetary-orbital speeds - much higher than Hyperion's escape velocity.

What is the average velocity of an unladen Space Probe?

GD, I'm tempted to do the predictable line.

However in this case we can find out easily - if it's European, the answer is "fast enough to splat all over whatever it's trying to land very carefully on". So if there are bits of broken Beagle scattered over the surface, you've got your answer.

Lucy.

Thanks again for a good story but...

Please. Can. You. Occasionally. Use. Links.

Fks sake

What do you mean, a NASA or ESA Space Probe?

http://photojournal.jpl.nasa.gov/catalog/PIA07740

*hides*

A v fast body travelling tangential to Hyperion's gravity field would not impact Hyperion, merely body's course would be insignificantly bent? So, all H-impacting bodies have been fully captured by H's (low) gravity