I accept that escape velocity is real, and that unless a body reaches it, it cannot escape the earth’s atmosphere. What I can’t understand is why that is so. Any explanations would be appreciated.
Here’s my problem. It’s obvious that any object propelled (e.g., thrown) from the surface of the earth needs to have enough energy (velocity) to escape the earth’s gravitational pull. I get whole PE=KE thing. What I DON’T get is why an object with an engine (e.g., a rocket) needs to achieve any particular speed if it has enough fuel to keep going. Let’s say I built a rocket with loads of fuel that went up at, say, 1 mph. Assuming it doesn’t run out of fuel, why won’t it eventually escape the earth’s gravity? What happens in the upper atmosphere that all of a sudden won’t let it chug along at it’s speed of 1 mph? Does the atmosphere get so thin that the concept of rocket propulsion no longer works? Or what?
Thanks in advance to all those who take the time to read this and thoughtfully respond.
I think that escape velocity is the speed at which you no longer need any more force to escape earth’s gravity. So for a rocket, as long as it’s still moving up you don’t need to worry about escape velocity until it’s engines are turned off and stop applying force.
Also, the escape velocity becomes lower as you get further from the planet.
So, for your hypothetical rocket, at some point it would be far enough from the earth that it’s escape velocity would be less than 1mph. At that point it could turn off it’s engines and keep drifting into space forever.
Engineers don’t build rockets like this because the speed of a rocket is the largest component in keeping the rocket pointed in the right direction (both it’s momentum and the air hitting it keep it pointing up).
You are correct. If you can acheive an upwards velocity of 1 mph at sea level, and your fuel holds out, you will eventually escape the earth’s gravity.
Escape speed has nothing to do with the atmosphere, nor is it the speed needed to enter orbit. The escape speed is the answer to the question “neglecting atmospheric friction, how fast must I throw a ball so that it will never come back to earth?” It depends on the mass of the planet and on where you are standing (or floating).
Orbital speed of a low Earth orbit is about 7 km/sec. If you were in such an orbit and momentarily fire a big rocket to increase speed by 4km/sec, you will coast into deep space, never to return to Earth. Anything less and you merely enter a higher orbit around the Earth. Or you could turn on a low thrust engine and keep it running for weeks or months - it will gradually raise your orbit. As you get farther away from the Earth, escape speed gets smaller, and eventually you can escape the Earth’s gravity with even the smallest engine.
I thought the OP was asking “why is there a minimum velocity? Why can’t you go into space slowly?” To which the answer is, there is no minimum velocity, you can go into space slowly. It would be stupid to go from ground to orbit slowly though, since you need fuel to hover. It’s like walking up a downward escalator - if you walk just a bit faster than the escalator is moving, you will eventually reach the top, but it takes less energy to run up quickly. In this analogy, the escape speed is how fast I need to kick you so you slide all the way to the top.
Right, and I’m sure there were no classified flighs either. The only benefit to the military would be propaganda, so why keep it a secret? I wonder if the Apollos reached escape velocity though, anybody know? Would have gotten pretty close to it, anyway.
I should add that although there is no minimum speed for escaping the Earth’s gravity, there is a minimum speed for entering orbit - the orbital speed. As I said, it’s about 7km/s, or around Mach 30. (Of course there is no speed of sound in space, but it’s about 30 times the speed of sound on the ground). If you reach an altitude of 300 miles but don’t have this speed, you either have to keep running your engines to hover, or you fall back down. Since it takes less energy to accelerate to 7km/s than to hover for even a few minutes, all satellites and spacecraft go into orbit and shut down the engines.
OK, well, I think I’ve gotten my question answered. I’d submitted it to the regular old Straight Dope address and was told (twice) that my slow, steady spaceship could never leave orbit, but apparently it can.
Thanks to those of you who took the time to reply.
Jeez, dude - it’s known it can be done - just a matter of $ - and if you don’t think “social engineering” isn’t important for that kind of stuff, take some marketing classes.
Say what you will, somebody has to pay for all that H2 and LOX. And that stuff Costs!
rmorgan, that is peculiar. One of my first questions posted here was this very same question. That’s not so peculiar just by itself, but what adds to the peculiarity is that I had used the 1 mph thing in my question too. From where did you get your (mis)understanding of escape velocity? My high school physics teacher did a poor job of explaining escape velocity (right along with poorly explaining just about everything else in that class). The more I look back on what I “learned” in high school, the more I believe that the teachers should be tested in their area of expertise every couple of years.
I’m glad rmorgan asked this question, I had the same one. But the answers are not quite sitting right with me. What about black holes & light? Photons don’t “run out of fuel” (or do they through redshifting?) but they can’t escape the event horizon of a black hole because the escape velocity is greater than the speed of light. And back to rmorgan’s example, if you travel upwards at 1 mph, how does that overcome the 32ft/s/s downward pull? Does the escape velocity also somehow relate to overcoming the inertia of the shared angular momentum with the planet? (i.e., you need to exceed orbital velocity). I guess I need to refer back to my physics textbooks.
Big Joe, you’re partly correct. But, I think you are confusing the OP re: “escape velocity” with a different issue of “orbital velocity”. Once escape velocity has been reached, your destination doesn’t matter…in light of the OP question. However, if you do wish to enter into orbit and late you may wish to break out of an orbit, then you are correct…you’re talking about space dynamics. Basically, there’s an extra step in solving the equations.
OK, Morgan, but consider this… If I were NASA, and I heard this proposal, my first question might be:
Assuming I’m going to leave the earth in the manner you propose, how much fuel will be required? In short, I am asking how much energy will be spent to accomplish this task? Afterall, you hope you don’t run out of fuel using your proposed process, right?
Ergo, fully understanding and applying the true physical meaning of “escape velocity” still enters into your design.
Oh, I understand that a 1-mph spaceship isn’t what one might call practicality. As I’d hoped was clear from the question, the issue isn’t all one of practicality, it’s one of possibility. My point is that, to my feeble mind at least, “escape velocity” might more accurately be termed “escape energy” or “escape work”, although in the real world the best way to achieve the desired result is to make the object go like a bat out of hell as quickly as possible.
I’m not sure what prompted the question in the first place, other than it’s one of those things that’s pops into my mind every now and again (along with some others that I, having found this resource, may well post). So, I posited this admittedly impractical spaceship to see if my intuition was correct.
Not to complicate the matter further (although it may too late), but the entire concept of escaping the earth’s gravity seems a bit simplistic to me. If I remember my physics correctly, you can NEVER actually entirely escape the gravitational influence of the earth or any other body, but you can get to the point at which the earth’s gravity becomes secondary to the combined gravitational pull of all the other bodies in the universe. Actually, it seems that the altitude at which that would take place would actually change slightly over time as the other stars, planets, etc. in the universe change position, and thus the net effect of their gravities changes slightly.
rmorgan, I recall hearing the bit about escape velocity being required for a rocket when I was a kid, and wondering the same thing. You’re right that if you have a really good power source, you could leave the Earth at a snail’s pace. But fuel is the major problem with putting things in orbit, and crawling away would be horribly inefficient (actually, not possible due to the weight of the fuel itself). The most efficient (or least inefficient) way to get something into orbit with a rocket is to blast with as much force as the structure and passengers can withstand. This is very similar then to throwing a ball, and the question becomes one of exceeding the escape velocity.
About your other question, although you never will escape the Earth’s gravity, if you’re going more than the escape velocity, then you’ll never be pulled back to Earth. Ever. Your speed will be decreasing because of the pull of the Earth, but the Earth’s gravity will also be decreasing as you go out, and you’ll never turn around and fall back. This is of course assuming the Earth is the only body in the universe.
Short answer: yes, a continuously-powered vehicle, no matter how slow its speed, could sidle out into space. However, the energy outlay to power such a vehicle continuously is enormous, so it’s easier to give the vehicle a massive initial “push” that’ll allow it to “coast” the rest of the way.
Side note, for which I wish I had something more substantive than my own sketchy memory: my old Differential Equations professor, in order to wake us up one day, showed us how to calculate escape velocities. He went on to demonstrate how, given knowledge of the acceleration of gravity and the approximate mass of the Earth, one could calculate the escape velocity of an infinite mass. And hey presto, the answer was equal to the speed of light.
Wish I could remember how the equations went…anyone here even have an inkling about this?
