Why do spacecraft have to reach escape velocity when leaving earth?

Question

Why do spacecraft have to reach escape velocity to get into space when leaving earth? Some miltary aircraft can fly close to the edge of space without approaching anything near escape velocity - could they not fly just a bit hgher and actually go into space?

Answer 1

They don't. This is a common misconception. Escape velocity is the velocity at which an UNPOWERED object must attain to leave orbit.

Escape velocity is sometimes misunderstood to be the speed a powered vehicle (such as a rocket) must travel to leave orbit, however this is not the case. The escape velocity can be thought of in terms of the speed an artillery shell or bullet fired from the surface would have to travel (ignoring the effects of drag) to leave orbit, but it is not the speed a rocket or other powered object would have to travel. An object under power could leave the Earth's gravity at any speed, assuming enough fuel.

Answer 2

They don't need to reach escape velocity to leave Earth, but if they don't then some time after the engine is turned off they will fall back to Earth. Escape velocity is only the speed at which something has to be going straight up so that with NO ADDITIONAL POWER being used it will never fall back. In theory, if you have some kind of infinite power source, you could leave Earth at any speed you like, as long as that power was on all the time to counteract gravity. But real rockets can only carry enough fuel for a few minutes of power, so they must reach escape velocity quickly so that can travel the rest of the way without any power at all, just coasting, like a rock thrown up at very high speed.

Answer 3

For a start, military aircraft are exactly that - AIRcraft, not SPACEcraft. They need the pressure of air-flows on their wings to stay aloft. Spacecraft rely on sheer forward speed to 'beat' the effects of gravity; they fall towards the Earth, but they fly forward so fast that the curvature of the Earth causes its surface to fall away from the spacecraft. That's what we call being in orbit.

Next point: Military aircraft do not attempt to leave the vicinity of the Earth, whereas some (though not all) spacecraft do this.

The reason spacecraft need to reach escape velocity is related to the reason 'escape velocity' is so named. If you throw something upward, it will fall back down. The harder you throw it, the higher it will go before it starts to fall. If an object could be sent up at a great enough speed, it would never fall back down because the ever-decreasing effect of gravity would be insufficient to curb the (also ever-decreasing) outward speed of the object.

If a spacecraft could carry an infinite fuel supply, it could rise relatively slowly & just keep burning fuel until it got to the required height. Unfortunately, this is not possible; so there is an intense burn of fuel to get up to great speed before the fuel is exhausted.

Excape velocity is defined as the outward velocity required by an object in ballistic motion in order permanently to resist the gravitational pull of the parent body (in this case the Earth). Since spacecraft going into orbit aren't actually intended to escape the Earth completely, escape velocity is not generally required for this kind of spacecraft. Rather than reach nearly 25,000 miles per hour, speeds of 17,000 - 19,000 miles per hour are more the norm. Only those on interplanetary missions (or other deep space probes) are sent to such speeds.

I hope this helps you to understand.

Answer 4

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RE:
Why do spacecraft have to reach escape velocity when leaving earth?
Why do spacecraft have to reach escape velocity to get into space when leaving earth? Some miltary aircraft can fly close to the edge of space without approaching anything near escape velocity - could they not fly just a bit hgher and actually go into space?

Answer 5

Space Shuttle Escape Velocity

Answer 6

The earths gravitational pull exerts an influence on a flying body to an extent that it has to accelerate to escape velocity to leave the earths influence. most space craft are not powered in the same way as a jet, which flies through the atmosphere but does not leave it.For other senses of this term, see escape velocity (disambiguation).

Space Shuttle Atlantis launches on mission STS-71. The need to reach escape velocity does not strictly apply to powered vehicles and those not leaving the Earth's orbit, like the Space Shuttle.In physics, Escape Velocity is the speed where the kinetic energy of an object is equal in magnitude to its potential energy in a gravitational field.

It is commonly described as the speed needed to "break free" from a gravitational field; however, this is not true for objects under their own power.

Answer 7

Spacecraft have to reach orbital velocity to orbit Earth, or escape velocity to leave it and go to the Moon or Mars, because they switch off their engines once they get there and coast the rest of the time. An aircraft has to keep using its engines to maintain its altitude. Were it to shut down its engines, would fall to the ground in a neat parabolic arc. If an object can get up to 17,500mph, however, before shutting down the engines its forward motion is enough that, as it curves down to the Earth, the Earth's surface curves away from it at the same rate, hence it never actually reaches the ground.

Answer 8

For a given gravitational field and a given position, the escape velocity is the minimum speed an object without propulsion needs to have to move away indefinitely from the source of the field, as opposed to falling back or staying in an orbit within a bounded distance from the source.

For this to actually occur the object must be influenced by no significant forces except the gravitational field; in particular there is no propulsion (as by a rocket), there is no significant friction (as between the object and the Earth's atmosphere; these conditions correspond to freefall), and there is no gravitational radiation.

Answer 9

It is because they need to escape earths gravitational pull (if they want to go to other planets). That speed is 11.2 km/second. If the craft isn´t moving at this speed it will remain trapped in an orbit around earth. The orbit will be highly elongated but eventually the craft will reach the orbits highest point and start falling back towards earth.
Satellites don´t have to reach escape velocity. They just need to move fast enough to enter a low earth orbit.

It is one thing to go into space and another entirely to remain there. As long as you are in the atmosphere you need power to keep moving. Or drag will slow you down and you will go down. Spacecraft need to move so fast they not only get above the amosphere entirely but also remain up there without using rockets. They have to get into an orbit. So satellites are in constant freefall always falling but moving so fast the are also flung outward at the same rate they are falling. All this without using any energy. At the moon you can enter an orbit, if you just move fast enough, even if you are only high enough to clear the highest lunar mountains. Here we have to clear the atmosphere which is about 100 km deep.

Going straight up will eventually bring you down unless you are moving fast enough. This is how Spaceship One flew. It just peeped into space and came drifting down again. The lowest speed a spaceship has to have to enter a low earth orbit is 8.8 km /second. If you move faster, at 11.2 km/second then you will have reached escape velocity. This is the lowest speed you need to have to escape earths gravity entirely.

Answer 10

for one moment, imagine that the earth was perfectly flat. if you throw a stone perfectly parallel to the ground, the attraction of gravity would pull it down to the ground, correct? the stone would follow a somewhat circular path down to the ground. keep this circular path in mind.

now, think about the reality that the earth is actually spherical. this means that the ground is not perfectly flat, but a little curved, like a circle.

if we throw the same stone perfectly parallel to the ground, of course, it will eventually fall back to the ground. but think about it: what if you throw it harder? then harder than that? and then harder still?

the harder you throw the stone, the faster it becomes, and the farther it reaches before it hits the ground. what most people don't see is that, the harder you throw the stone, the faster it becomes, and it's path of flight (it's called "trajectory") becomes more and more like the curvature of the earth.

this means then, that there is a velocity wherein something like this stone can travel *and* it's trajectory will be exactly the same curvature as the earth. this means that the stone will *never* fall to the ground, as long as it maintains this velocity. this is the general rule behind orbiting satellites.

this velocity, wherein something follows the earth's curvature exactly, is borderline escape velocity. go slower than that, and it'll fall back to earth. go faster than that, and eventually, it'll spiral it's way out of the earth's gravity.

it's easier to imagine it this way. escape velocity would be drawing a circle; less than that would be drawing a spiral inwards and more than that would be drawing a spiral outward. ^____^