why not just coast down as slow as possible?
It was going a lot faster than that before entering the atmosphere… in fact, it was being slowed down a lot by the atmosphere itself.
(Sorry, I don’t have figures off hand.)
why so fast? wouldnt slower mean less temperature, less wear on the spacecraft?
i know that this shuttle didnt stop at the spacestation, but sometime they do. I assume the spacestation is not going very fast. ( in other words, it is possible to go very slow or stop in space) when they leave the station and go back to earth, why go so fast?
To maintain an orbit you have to be going at a certain speed. Orbital velocity is about Mach 20. If you’re going any slower than that, you’re not in orbit, you’re falling.
Most of the energy spent to get the shuttle into orbit is not for altitude, but to accelerate it to orbital speed. When coming back, the shuttle has to slow to a reasonable speed to land. It doesn’t have enough fuel to do this, so it uses the atmospheric drag to slow itself down.
They have to reach a certain speed to achieve orbit, around 25,000mph. When they decide to come down, they have to slow down somehow. The gradual descent into the atmosphere uses air friction to slow them down to land. They could use a rocket motor to slow themselves down to reenter, but to do that would take a motor nearly the size of what got them up there in the first place.
They can’t just come to a dead stop and let themselves fall. All that speed has to go somewhere.
OK, thanks andrew and fabris.
how far up there would a spacecraft have to be when it reaches the point that it wouldnt fall back to earth?
The ISS is going very, very, fast. The shuttle doesn’t stop to meet the station, it simply matches its enormous speed. Something that close to Earth can’t just hover there, it takes a lot of speed to maintain orbit.
I believe the minimum velocity needed to stay in orbit (and not be sucked down by gravity) is around 17,500 mph. I’m sure this would depend on how high you are, but they absolotuely need to maintain a good rate of orbital velocity to stay up there.
The space station also has a high rate of true speed. The key to docking is matching relative speeds. Both vehicles are really travelling fast, but relatively to eachother, not at all (think about driving next to someone else at 70 mph on the highway).
Shuttle orbital velocity is typically about 17,200 MPH, given an orbital period of about 90 minutes. See this site for the math.
Well, technically speaking, gravity has unlimited range, although it falls off with the square of distance. A spaceship will actually never completely escape the earth’s gravity, although it can get to the point where the gravitational effect from earth is too small to have any effect.
Orbital speed depends on altitude. The closer to the earth you are, the faster you have to be going to maintain orbit. It takes more energy to get into a higher orbit however, since you have to climb against gravity to get there. The shuttle orbits just outside the atmosphere, in aboutthe lowest orbit you can be in and not have atmosphereic drag be a problem.
very interesting, thanks everyone.
dp, the answer is there is no distance it could go to to escape the gravitational pull of the Earth as their is no limit (except infinity) to the distance over which gravity can act. However, the escape velocity at the Earth’s surface is about 25,000 mph, ignoring other factors such as friction, once an object flys away from the Earth’s surface, heading directly away from the Earth’s centre at a speed exceeding this speed it will of ‘escaped to infinity’ (i.e. unles another force is applied to it, it will keep heading away from the Earth).
Of course as long as you have some source of energy you can always maintain an orbit.
[nitpick]
I think the important aspect here isn’t so much the relative speed as much as the acceleration. The earth’s gravity accelerates at 9.8 m/s/s, so to escape that, the acceleration much be greater than 9.8 m/s/s. This will logically translate to a certain specific speed at a given altitude.
[/nitpick]
IT’S A MATTER OF FUEL!
Suppose that a spacecraft had an unlimited amount of fuel and reaction mass. (Suppose it had fusion power, and a water tank for reaction mass.)
In that case a craft orbiting at 15Kmph could brake to a halt, then fall straight down. And rather than speeding up to a dangerous velocity as it fell, it could point its engines downwards and run them just enough so atmospheric friction didn’t heat up it’s skin much.
Well, it’s more complex than that, ffabris. The acceleration diminishes as the altitude increases. And if you could fire a projectile at escape velocity (well, a little more to compensate for air resistance) from a giant cannon on the ground, it would indeed not return to Earth even though you would not be applying any force to it once it left the gun.
[nitpick of a nitpick]
Escape velocity refers to the initial velocity of an object in ballistic flight. The only acceleration involved is the initial one, which can be considerable.
[/nitpick of a nitpick]
Airplanes can fly at a much slower velocity than 17,200 MPH and they are closer to the earth’s center of gravity, and they don’t fall, so why does the shuttle have to go so fast?
do sattellites and the spacestation have to run engines to keep themselves in orbit?
Sort of. If they were in a perfect vacuum, they wouldn’t. As it is, atmospheric drag gradually slows the station and shuttle, and they occasionally have to use their manuevering rockets to regain speed. Not a continous thing - more like an occasional nudge every few days or weeks.
Two things. First, they’re using air for lift, while the Shuttle is in a near vacuum. Second, airplanes are continuously running their engines to keep their speed up, which they have to to keep moving against atmospheric drag.
The direct answer to the OP was done very well, so I’ll not try to answer that. But I thought I’d throw in a visualization I was taught about orbits:
Picture throwing a rock parallel to the Earth’s surface. What happens?
The rock travels for a bit and falls back to the surface. It stops going forward because of drag. It moves back to Earth because of gravity.
Throw the rock harder. What happens?
The rock goes farther, then hits the Earth. Remember, the rock is always falling towards the Earth’s surface because of gravity.
Throw it REALLY, REALLY, REALLY hard. What happens?
By the time it will fall to the Earth, it has gone past the Earth.
If you can throw the rock so that the fall exactly matches how fast it is going, you’ve achieved orbit. Orbit means something is always falling towards the Earth, but because it is going forward, it keeps missing the Earth and round and round it goes.
If you are really close to the Earth, you have to throw that thing REALLY fast, because there just isn’t a lot of time between when it you throw it and when it will hit. The higher you are, obviously, the smaller your forward speed will need to be.
Now, because of drag, sometimes things need to be pushed along (a force applied) to maintain orbital speed. Near the surface, this obviously makes it impossible to keep a rock in orbit, no matter how hard you throw it. The higher you go, the less air there is, and the less thrust it takes. But then you need to find a way to get up there…
DaveRaver - Airplanes aren’t in orbit. See my little explanation above for that. They are balancing the downward force of gravity with an upward force produced by their angle of attack. It is completely different.
Throw a ball straight up. Is it falling? Not for the first few moments. It isn’t in orbit, either. You’ve just momentarily applied a force greater than gravity.
dp - do they need to run engines to keep themselves in orbit? Depends on the height of the sattelite. The moon, obviously, doesn’t. Others, like the ISS, which are dragged by the atmosphere, do.