why does an object (ship, person as in Mission to Mars movie) burn up when entering the atmosphere of a planet. i have heard the standard answer that the speed is so great to maintain an orbit that when you hit the atmosphere the friction gets you. but if that is the case then why dont you just slow down and match the speed of the atmosphere spinning with the planet?.
secondly , how did they ever figure this out? did they shoot rockets higher and higher until one burnt up?
Reentry means you aren’t maintaining orbit but even if you started out on an imaginary platform and just jumped off you’d fall straight down. With no atmophere (at first) to slow your fall, you’d reach an incredibly high velocity. As you get into the atmophere. Toast.
As for matching your speed to the rotation of earth that’s a very high orbit. 22-23,000 miles but I’ll have to look it up.
I’m no rocket scientist; one will probably be along shortly…
I assume you haven’t priced shipping rates for space travel; it costs a lot. Why carry the extra fuel to slow down when the atmosphere will do the job for less?
Well, I’ve seen lots of meteors (as in shooting stars), but I’ve never seen a meteorite. They probably calculated the friction of a ship going through the atmosphere and went from there.
Yes, you could slow down enough that you wouldn’t burn up on re-entry.
However, it would use lots and lots of fuel, which our spacecraft don’t have when they want to come back home. They use the friction of the atmosphere to slow the craft down so they can land it rather than make a crater when they hit the surface.
Maybe when we figure out fusion drive, or some other type of high energy drive we’ll be able to enter the atmosphere slowly. Until then not likely.
Kevin
The relevant speed (re: the Earth) is not the rotational velocity of the atmosphere, but the terminal velocity of gravitic attraction. Even if you enter the toposphere at a “relative standstill”, you will be travelling much faster very soon.*
*Bonus quiz: how long to reach terminal velocity from rest when falling from orbit? (assume no significant braking effect due to friction.)
The Clark orbit is about 22,300 miles
You can’t just slow down without consequences. If you slow down, you’ll go into a lower orbit. If you start to slow down, you’ll start to go into a lower orbit. If the Shuttle, say, tried to match velocity with the atmosphere before hitting the top of it, they’d have to accelerate so hard that they’d pulp all of the astronauts.
It’s purely a matter of economy. Given enough fuel, you *could come to a dead stop relative to the earth’s rotation and slowly descend on a rocket exhaust to a soft touchdown. But it would be hugely inefficient.
On a planet without an atmosphere, this is essentially what you have to do (it’s what the LEM did when landing on the moon). But when you have an atmosphere, it makes much more sense to use the friction from the atmosphere to brake you.
Padeye - I don’t believe you’re correct. From Low Earth Orbit, a plunge straight down through the atmosphere wouldn’t build up all *that much speed. I seem to recall that the shuttle orbits at around 16,000 MPH. But after it finishes the aero-braking phase of flight it only reaches a speed of Mach 5 or so. And as the atmosphere gets thicker (and therefore exerts more friction on the craft), it slows down.
Seems to me if there is no braking effect due to friction then there is no terminal velocity…unless you measure terminal velocity as when you make a crater in the ground (giving even more meaning to the term ‘terminal’).
Otherwise the trick lies not in how an orbit you are but how fast you are going when you hit the atmosphere (wherever scientists consider it to technically start). After that your mass and shape will have a lot to say about your terminal velocity.
At a guess I’d say you reach terminal velocity the moment you strike the atmosphere. After that you’re slowing down the rest of the way in (but that assumes frictional braking).
If I understand the OP, you’re asking why atmospheric friction doesn’t slow down an object before it burns up, with no assistance from the object. There are three major forces here: the original velocity, gravity, and friction. The first two (ok mostly gravity) are FAR stronger than the friction slowing it down. You’ll note that there is frictional braking. That’s what burns it up in the first place, it just burns up before it can slow down.
Also, if it slows down to the speed of the air, it would be going lots slower than an orbit at that altitude, 24hrs vs. ~90min to go around the earth. The laws of physics say that the object should be in orbit, so the object will want to go that fast, which gets you back to the friction thing.
They could have figured it out by going through the force balance in the first paragraph in lots more detail, or they just noticed that things burn up in the atmosphere all the time - meteorites. I don’t know what came first.
As for why the space shuttle doesn’t burn up, it comes in at a very specific angle, using its lifting body to brake. It’s a crappy lifting body, but a lifting body nonetheless. But the key is the tiles, which transmit very little heat. It can be red-hot at one end, and cool enough to hold bare-handed at the other end, 6 inches away. BTW, they have about the density of styrofoam, and are very cool (in a hip way, not a temperature way). Keep in mind, spent rocket stages burn up in the atmosphere all the time, and they don’t even go to LEO.