# Could a “bouncy planet” work in principle?

Would it be possible to have a planet created by a God - but still obeying the known laws of physics - to have a bouncy surface that would be strong enough to bounce any large incoming matter, like asteroids, back into space?

• The small problem is that you must arrange for the "large incoming" object to be made from the same kind of magical material, or else it will shatter and most likely vaporize; and, of course, there cannot be any atmosphere so that the incoming object does not lose energy coming down and going up. – AlexP Nov 9 '19 at 1:33
• The Earth's magnetic field deflects ( or bounces ) charged particles from the solar wind back into space. Would that qualify for what you're asking for? Otherwise, the best you can perhaps get is something like a smooth steel surface and an incoming object a sphere of steel, and at much, much lower velocity than an asteroid. A steel ball bearing on a steel plate is about as close as any natural material can get to perfect elasticity. – Nolo Nov 9 '19 at 6:53
• And in fact, without theoretically perfect elasticity ( not to mention much less than typical impact velocities of meteoroids ), the object would not stay in space after the bounce... Anything less than perfect bounce, and each bounce would degrade until the object finally came to rest on the surface. – Nolo Nov 9 '19 at 6:56

Hardly, for a very simple reason which I am going to explain.

The bounciness of a substance is due to the interaction of the atomic bonds within it, which allow to accumulate and release the energy due to the interaction with an outer body, without breaking those bonds.

Interaction with an outer body propagates in any material at the speed of sound. Any object moving faster than that speed will see the material as a non responsive wall and break through it.

Have a guess? Spatial objects travel at several km/s, speed of sound is at best a couple km/s.

Therefore for any impact with a space object the bouncy material would not notice the object until it has already pierced through it.

The only condition under this could work would be:

• planet with no atmosphere
• temperature of the two bodies in a range where they are elastic and not fragile
• planet and impactor made of the same material, or materials with high speed of sound
• impact happening at a velocity lower than the speed of sound in those material

For example, a typical steel alloy has a speed of sound of 6 km/s, if the bodies where both made of steel and were impacting at say 1 km/s with respect to each other they could bounce.

Additionally, the kinetic energy at stake in the impact has to be less than the energy needed to melt the smallest of the two bodies, else the work done with its deformation will obviously melt it.

First, the object hits:

# The Atmosphere.

Earth's gasses and most other like-fluids as we know them would exert pressure and friction on any incoming objects, the objects would heat-up on the surface ablating in the process, as the heat penetrates, the objects (say take a typical comet - part water-ice and part rock) - parts of them would heat faster than others and expand, breaking the object apart.

One of the "tricky bits" is objects starting to vaporize and massively heat-up in the atmosphere before they even hit a surface - if they are able to bounce, then they'll suffer the same process of heat-ablation and breakup on the way out.

Unless:

Unless the atmosphere consists of a friction-less super-fluid, which exist in labs - if not on Earth otherwise. Liquid helium is one possibility, if god is involved, then I'm sure they know more than I about possible alternative liquids/gasses or mixtures of arcane compounds which would behave the same way.

Then the object hits:

# The Surface:

Most objects (rock, wood, even water) have elastic properties with a defined limit - if you throw a glass marble against concrete, if it doesn't shatter, then it will bounce off at the opposite angle from which it was thrown. But the marble shattering is what happens when an object accelerates even just by gravity into a planet's surface, churning-up the ground, throwing material upwards to be scattered for miles creating a massive impact crater for us to remember it by.

Unless:

Perfect elasticity. The Planet's surface and some distance beneath is made of a material that exhibits this property. Ie. any mechanical energy directed into it is resisted with equal force in the direction it is applied with no energy being transformed into heat your object is simply reflected off the surface as if it were a child on a trampoline.

Alex.P is quite correct, an object would need to be made of similar material or be otherwise very resilient. We simple humans don't have such a substance at present (but we're looking into it: meta-materials), but then again what are we compared with a deity with knowledge of all possible materials.