Why Is There An Angle of Re-entry?

Sure, the angle of re-entry seems logical at first. It is spoken about in Apollo 13 and (IIRC) The Right Stuff. At the wrong angle, you’ll bounce off the atmosphere like a stone skipping across a lake. OTOH, with all of these rogue asteroids and now meteorites as of late…how come they can enter our atmosphere any old way they please? (Why don’t THEY skip like a stone?)

Lots of them do.

Right, asteroids aren’t particularly interested in gently landing several delicate water balloons at a particular location.

Actually, I never was very clear what was meant by “skipping” when talking about reentry. Is there actually some sort of aerodynamic effect other than ordinary air resistance? Or is “skipping” just a fanciful way of describing an orbit that passes through the atmosphere a little bit?

Maybe astronauts who re-enter at too steep an angle will land too hard, like that meteor is Russia last week.

Sort of. It means that you are going too fast for atmospheric braking to reduce your speed below the level at which Earth’s gravity will pull you to the surface.

The angle of reentry for an object that is hitting the atmosphere at a speed greater than orbital speed, like an Apollo capsule returning from the moon or a passing asteroid, is critical for two reasons. If the angle is too shallow, the object will not slow down enough, will pass through the thin upper atmosphere and then head back out into space. This is what is being referred to, somewhat inaccurately, when they speak of the capsule ‘skipping’ off the atmosphere. If the capsule or asteroid comes in too steeply, it will decelerate so quickly that the force of deceleration will crush or shatter it. That is exactly what happened to the meteor in Russia - it came in too steeply to survive and shattered into pieces.

Actually, the meteor didn’t land. It exploded a goodly ways up.

Meteors will skip if they come in too shallow. I believe this is a video of one doing just that:

If not, there have been plenty of documented meteors that did. It's not just astronauts that have this ability.

If an astronaut comes in too steep, they convert their speed into heat via friction at a faster rate. This makes the spacecraft’s heat shield get even hotter. The heat shields in man-made spacecraft have their limits and if you exceed these limits they will burn through and fail. This is generally considered to be a very bad thing for astronauts. Most astronauts prefer to aim for somewhere in between “skip off and miss” and “burn up and die”. Meteors aren’t so picky.

Something that comes in at a shallow angle will build up compressed air on the side closer to earth, and this air will push back, slightly deflecting the trajectory. This is, in fact, somewhat analogous to what happens when a stone “skips” on the water.

A re-entry trajectory for a returning spacecraft is a finely judged balance between gravity, friction, heat and aerodynamics. The capsule itself is designed as a lifting body, to manage the process of descending through the atmosphere, ensuring that the balance of velocity and air density does not exceed the ability of the capsule to shed frictional (yeah, I know, it isn’t strictly friction) heat. Too steep an angle of attack, and there isn’t enough lift, the capsule descends too steeply, and hits the denser air travelling too fast. If the capsule cannot cope with the heat, it will burn up or impact the ground. If the angle of attack is too shallow, there is too much lift and not enough slowing down, and the capsule will exit the atmosphere again with no hope of recovery.

This “skip” is unlikely to occur for a rock from space - it may pass through part of the atmosphere (as noted above) due to angle/velocity/size, but the aerodynamic features of the skip are not going to occur (stable flat surface presentation to the atmosphere). However, it was a plot-point in the novel Moonfall by Jack McDevitt, where the moon is hit by an extrasolar impactor. A large chunk of moon is deflected from a major earth impact by using nuclear rocket engines to rotate and present the rock so that it does skip off the atmosphere, then the rockets are used to stabilise the orbit to prevent subsequent risk.

Question: if the capsule/shuttle etc. “skips” off due to a shallow angle of re-entry, is there any chance of another re-entry attempt?
I’m assuming that the craft is not going fast enough to escape earth orbit completely and so will approach the earth again (though in an altered orbit) So can they burn again and adjust for another entry? Would they not have enough fuel for such jiggery-pokery?

(Please note above my use of the term “burn” which is inserted in order to give the impression that I know what the hell I am talking about, and also the term “jiggery-pokery” which confirms that I don’t)

Ditto.

This seems to be a point that many of the posters here don’t realize, or at least didn’t mention. If one doesn’t like the word “skip”, I’d suggest that a good synonym might be “bounce”. Watch what happens when someone gets thrown out of a boat, or a water skier lets go of his line – he is moving so fast that the water can’t get out of his way, and the water acts more like a solid than a liquid, so he literally bounces off until his speed slows to a point where the water can get out of his way.

That is what happens when something hits the atmosphere at too shallow an angle. It is NOT simply that the surface of the atmosphere is curved, and the object comes out the other side. It bounces off of it.

No. The subsequent orbit would be highly eccentric and may take an extended time to return, and the next approach will be too fast and at the wrong angle. They wont have anywhere enough fuel for the adjustments, needed - they used most of that losing velocity to drop into the atmosphere in the first place.

There is an intermediate approach, that for re-entry to the Earth might be pretty difficult, but real. With exactly the right approach you can enter a very eccentric orbit, one that grazes the atmosphere, and then with each pass into the atmosphere you can lose a bit of orbital energy, and with some additional control, circularise the orbit, or get to a point where you can safely take the dive down to full re-entry. This isn’t the “skip-off” trajectory, which would probably dump you into a solar orbit to quietly die, with no nearby pass to the Earth in eons. Rather the aerobraking entry takes a very measured pass though only the more tenuous atmosphere. It may be the only viable way of getting a manned mission back from Mars.

An Apollo capsule would have been undocked from its service module by then anyway, and would have had no propulsion unit attached to it. Not that, as you point out, it would have been much comfort to them if they had. Fuel reserves tend to be tight on space missions; when Apollo 11 landed on the Moon it had 25 seconds of fuel remaining.

Frankly, I do not understand the question. What do you mean by “any old way they please”? It’s not like they make a conscious choice to enter this way or that way.

Rather, each one enters at a particular angle based mostly on where it is coming from, and partially based on other influences like the earth’s gravity. But the bottom line is that if the angle is too shallow, it will bounce off, and if it comes in too steep it will burn up or explode or whatever. Only in a rather narrow range will it come in for a not-too-disastrous landing.

Thanks for that.

So theoretically they could try again but practically…nope. I suspected fuel margins would be too tight.

I wonder what the options would be for a lightly-fuelled craft that found itself in such an eccentric orbit?
Not enough fuel to correct for re-entry but perhaps enough to ensure a quick burn-up? Time to break out the cyanide pills? enter hypersleep in hope of landing on a monkey planet?

As pointed out above by Malacandra, there was no fuel (or engines, for that matter). Even the shuttle carried limited fuel, and didn’t (couldn’t) use main engines after launch.

I think the consensus is to open the airlock and breathe space - the air will last a bit longer than the food.

Relative to the idea of “Skipping” off the atmosphere, is it then not possible for the proposed Nazi Sanger spaceplane to have “skipped” off the atmosphere all the way around the world back to its base?

I would think no, for the reasons quoted by si_blakely above - namely that this would result in a wild, random orbit after bouncing off and a huge amount of fuel burned at each attempt.