# Would it be possible to skip an asteroid off of a planet's atmosphere?

If some super-advanced, spacefaring race could carve an asteroid into the right shape, and launch it at the right angle and velocity, would it be possible to “skip” the asteroid off of a planet’s atmosphere, like you would skip a stone off the surface of a pond? How big or small would it have to be for this to work? What would the effect upon the planet’s surface be? Would it be catastrophic, or would the effects be minimal? Could this be used as a weapon?

To simplify things, I’m going to focus on what would happen if we tried this on earth, so I don’t have to worry about how things might shift with different atmospheres, planet sizes, gravities, and et cetera. That being said, if it wouldn’t work on earth, what qualities would a planet need so this would work?

Also, for the sake of semantics, my definition of “skipping” is the asteroid never actually making contact with the planet’s surface, and not just blazing through the planet’s atmosphere in a straight line without being diverted in any way. The asteroid has to be deflected by the atmosphere itself, not the planet, and skip off at a noticeable angle.

• It's typically called a flyby or gravity assist. You don't even need to enter the atmosphere. – ratchet freak Mar 16 '16 at 15:40
• This seems more like a question for Astronomy or Physics (or English, since there's a word for what you're describing, as ratchet pointed out). What are you trying to build? – Frostfyre Mar 16 '16 at 15:43
• I think that is quite possible and even a risk during re-entry of spaceships. If the angle is too shallow, the spaceship bounces off. – Cyrus Mar 16 '16 at 15:55
• @ratchetfreak I wouldn't call a gravity assist very analogous to skipping a stone on a pond. While there's bounce-like mechanics, there's a gain in kinetic energy and it's a one-time deal. I think InterCity's answer is closer to the spirit of this question, but I might be thrown off by OP's inclusion of that simile. – The Anathema Mar 16 '16 at 16:20
• @Cyrus the "bouncing off" isn't due to the atmosphere but instead due to orbital mechanics. For reentry spaceships put the lowest point of their orbit into the atmosphere and then use the air to slow down further (easier on the fuel that way). If the ship couldn't get slow enough then the orbit simply continues around (slightly decayed). – ratchet freak Mar 16 '16 at 16:46

It is indeed possible. You'd need a very particular shape, though, and you'd need to hit the atmosphere at just the right angle.

Short explanation:
It would look much like a flat stone being thrown at a calm lake surface. It will bounce off.

Long explanation involving physics: (I hope you understand basic orbital mechanics)
It's more complicated than a lake. As the body enters the atmosphere, it starts generating aerodynamical forces - mainly lift and drag. Under normal circumstances, the drag is enough to slow the object so much that it cannot fly on itself. If it enters under a shallow angle and has a good shape, the lift it generates will be enough to pitch it up and gradually push it out of the atmosphere.
It doesn't end there, however: The drag has acted at the object, possibly slowing it down. Which means that it's no longer at an orbital velocity, and returns back to earth. The skipping then repeats itself until it's not flying fast enough to generate enough lift.
If you threw the asteroid hard enough, however, and hit the atmosphere under a shallow angle (Think an highly eccentric elliptical orbit, with its periapsis right at the edge of the atmosphere), it would bounce off and change its trajectory.
The change will not be too big, though. It's not like it would bounce off to outer space, it's more like the periapsis rising (or dropping, depending on the angle) by several kilometres.

• The periapsis would not chang much, since the inpulse comes close to it. Also the astroid would be coming in faster than escape velocity. (otherwise it would be a moon) So its possible that it would bounce off into space. – Taemyr Mar 16 '16 at 22:33

Since no one else has mentioned it, not only can it happen, it did. August 10, 1972. Since there was no inter-webs back then, the news articles about it are not necessarily available, but here are a few reasonable references:

TL;DR: Finally, my years of experience in Kerbal Space Program pay off. It's possible to do, but won't do much compared to just blazing through the atmosphere.

As @TimB mentions, you can't "skip" an asteroid from the atmosphere quite the same way as you would skip something off the surface of a pond. You can, however, make use of the atmosphere to change your trajectory and effect a "bounce".

## Theory

To get an idea of how this effect would work, let's forget about planets and orbits and work for the moment with an infinite flat plane with a homogeneous gravitational field. Even if you fire it of with a lot of horizontal velocity, it will eventually just drop. Add atmosphere and it just gets slightly slowed down towards the end.

If instead of an asteroid you use a glider, however, you have options. The most straightforward one is to pull up, which will under favourable circumstances get you back out of the atmosphere again, but with less velocity. You may be able to repeat this process, but eventually you'll just glide down. We can do something similar with an appropriately shaped asteroid.

To planets now. We won't be doing any "slingshots" or "gravity assists", since those don't actually require an atmosphere and don't work in two-body systems anyhow.

What happens when a spherical asteroid passes sufficiently close to a planet to dip into the atmosphere (but not so close as to hit the planet) is that it slows down some, losing velocity (and thus energy) to aerodynamic drag. This changes its orbit; doing it on purpose is called "aerobraking" and if the orbit goes from hyperbolic (ie. speeding back into space) to elliptic, it's called "aerocapture". It's not, strictly speaking, "skipping off", since it's not the atmosphere bouncing you off, it's orbital mechanics carrying you away.

Now the thing to understand is that your orbit is at any given point fully determined by your position relative to the body you're orbiting, and your velocity relative to that same body. Aerobraking changes your velocity, generally just by braking, which has the effect of shortening your semi-major axis (bringing you to a "lower orbit") and bringing your periapse down some, as both horizontal velocity and vertical velocity are affected equally.

At this point, using wings (and the golden rule of aircraft design tells us that at these velocities, anything is a wing) you can cause the drag to be asymmetric, gaining what we call "lift". Note that it's impossible for you to gain energy this way, the only thing that's happening is that you're trading some of your velocity to change the direction of the rest of it. You can take advantage of this to change the altitude of your periapse (and hence the eccentricity of your orbit) or effect a plane change, but you'll lose energy doing so.

## Applications

If you have a controllable aerodynamic shape, you can take advantage of a planets atmosphere to alter your trajectory at no cost in propellant to you. Apollo capsules (IIRC) took advantage of this; by having a centre of mass slightly offset to the side from the geometric centre of the capsule, they would get carried away slightly to the side in an atmosphere, and could rotate the capsule lengthwise to gain some limited control authority.

I have taken advantage of this trick to keep the periapse of a moon lander in the high atmosphere during multiple aerobraking passes on a return from a moon, allowing me to gently slow down and rendezvous with a space station in orbit of Kerbin at a minimal cost in propellant.

Could you use this as a weapon? Maybe. The energy expended in these maneuvers manifests as shock heating and is usually absorbed by the orbiting body, so the effect on the planet is limited. But what if there was enough of it?

This begs the question of how much energy this maneuvre consumes. We can get the answer from the vis-viva energy equation. Taking the initial orbit and calculating the energy (multiplied by the mass of the asteroid), we get the maximum energy we may deposit by deorbiting (read: crashing into the planet). The difference between the energy of this orbit and the new orbit is how much energy was expended/deposited.

For Earth-like planets, just attaining escape velocity compared to sitting on the surface gives you an energy $-62,6 MJ/kg$ which is about $15x$ the energy of TNT. Impressive at first glance, but not much in the grand scheme of things, especially if you consider that you're only expending a tiny part of this energy (however much you're willing to sacrifice without falling) an most of it will be absorbed by the asteroid.

Perhaps a better use of this capability would be to just drop some kinetic impactors (like tungsten rods) to do some damage and then use the planetary atmosphere to fine-tune your trajectory to your next target.

Oh, and since you were asking about the shape of the asteroid: it would probably end up being vaguely reminiscent of a space shuttle if you wanted to optimize, but unless you dip too deep, any shape with asymmetric drag or control surfaces would do, albeit with less efficiency.

While intercity is not wrong I'm skeptical that you could make it work in reality. There is a big difference between the atmosphere and a lake. In particular the lake has a hard transition surface that you can use surface effects on to skip a stone.

The atmosphere has no such effects, and it is very hard to conceive a situation where you would generate enough lift that it would counteract the drag of passing through the atmosphere. Dipping into the atmosphere is a great way to shed speed and is called Aerobraking but it's going to slow you down by doing so.

The only thing similar is called gravity-assist which is essentially skipping off a planet but it uses the gravity well, not the atmosphere, to do it..