Why is it necessary to travel 11km per second to escape the earth's gravity (escape velocity), when

Why is it necessary to travel 11km per second to escape the earth's gravity (escape velocity), when surely any speed will do, as long as you keep going up?

Luke, Wellington NZ

• In a way, you are right. if you went straight up at any speed, eventually you would get so far from the earth that the effect of the Earth's gravity was negligable. However, where the 11km per hour comes in is that it is the speed at which an object, rather than being held in orbit around the earth or falling back to it due to gravity, will be thrown outwards by sufficient centrifugal force to describe an open curve away from the planet.

  John Ramsey, Hackney UK

• Escape velocity is calculated on the assumption that the only force applying is gravity, that is, only after the rocket engines (or whatever accelerated you to 11km/s) have stopped firing. Imagine yourself above the Earth’s atmosphere travelling away from the Earth at high speed. Earth’s gravitational pull is slowing you down, but as your distance from Earth increases, that pull weakens. If you were initially travelling at less than the escape velocity then eventually Earth’s pull would be enough to bring you to a stop and from then you on you would fall back to Earth. The escape velocity is the speed you need to avoid this fate and keep travelling forever, although you would need to travel at a much higher speed if you wanted to reach the stars in your lifetime.

  M Baillie, Sawston UK

• If you throw something up in the air, it will carry on rising until the force of gravity overcomes the velocity of the object and causes it to keep slowing until it stops and then starts falling back to earth. You are kind of correct in saying that you just need to "keep going up", but in order to keep going up you need a sufficient amount of velocity to overcome the force of gravity. The amount of velocity needed to escape our planets gravity is approx 11km per second, which is known as escape velocity.

  Steve, London UK

• Escape velocity is the speed required to escape gravity in the absence of any force being applied. To keep going up requires the continual application of force. So yes, of course it is possible to keep going upwards and further from the earth, but without reaching escape velocity you would eventually be pulled back to earth when the fuel runs out.

  Clive Gordon, Ruislip UK

• The escape velocity refers to an the speed an object needs to achieve to move from its current point in a gravitational field to infinity(as the gravitational field has no distinct end point), it is equal to the speed that the object would be traveling at if it was pulled by the same gravitational field from infinity to that same point. However this only applies for an unpowered mass that would be constantly decelerating due to the gravitational pull of the plant and you are quite right in saying a powered mass would be able to escape no matter what its speed.

  Steven, Glasgow

• In theory, you're right, and if only Everest was taller, you could climb it all the way up to orbit as slowly as you damn pleased. Let's assume there's 2 ways into space, the slow way and the quick way. The slow way is to have some sort of propulsion engine, at least big enough to lift the vehicle's weight (e.g. a modern jet fighter can easily climb vertically, at speeds of 500MPH). The problem is that the jet engine can't get the oxygen it needs after about 5 miles up. The answer is to use a rocket motor, which carries all the fuel & oxygen it needs. But that turns out to be a lot of fuel (a HUGE lot of fuel) that you would be carrying, all the way up to 25,000 miles high or so where gravity becomes negligible and where, ideally, the last drop of fuel runs out. That brings us to the quick way: use that fuel in one quick burst, accelerate to 11km per second (25,000 MPH) as fast as possible, then coast the rest of the way with empty tanks and a dead engine. That way, you're carrying spare fuel for the shortest possible time.

  Paul Reilly, London UK

• True, in theory any speed would do. But in practice the difficulty comes from the trade-off between the rate at which you escape the Earth's gravity and the rate at which you consume the fuel needed to do so - and hence the fuel needed at take-off. If you ascended at 5mph, but had the typical fuel capacity of the shuttle and would be largely used up in 10 to 15 minutes, you'd only get up a couple of miles.

  Martin, Manchester UK

• An unpowered projectile needs escape velocity to travel arbitrarily high but a continually powered vehicle does not, of course. When the vehicle is in orbit above the atmosphere almost any force will do to push it further out - hence the ability of puny ion drives to push craft into higher orbits. It's getting into orbit that takes all the power. A grand space elevator is proposed to take the craft out to where the orbital speed is the rotation speed of the Earth's surface, so doing away with the flashy rocketry. Don't hold your breath.

  miles.felton, berkshire

• A stone thrown upwards would need to achieve this speed, however the space shuttle coule go up as slow as required (Assuming enough fuel reserves). The distinction is whether the flight is powered or not.

  Lee, Leeds UK

• The term escape velocity presupposes that the object being considered is not subject to any external acceleration/force other than gravity. As such an object travels upwards it will of course be slowed by gravity, but at the same time an object that moves upwards from the earth the effect of gravity gradually dimishes. If you begin travelling upwards too slowly gravity will bring you back down to earth. If you start out travelling fast enough, whilst gravity will slow you down it will not be sufficient to bring you back down to earth. The escape velocity is the break point between these two alternatives.
  The escape velocity is of course dependent upon the distance from the earth (or indeed any large body), diminshing as you travel away. Therefore if you started the earth's surface and travelled upwards at the escape velocity, although your velocity would diminish due to gravity it would still remain at what would be defined as the escape velocity.
  You could of course escape from earth's gravity if you could continuously move at even a very low speed. The problem is maintaining this speed against the pull of gravity. To do so you would need to introduce some other force, at which point the concept of escape velocity is no longer applicable.

  Paul Hoad, Sutton, UK

• At 11km/s you can successfully break orbit and escape the gravitational pull of the Earth. At 10km/s the Earth will eventually slow down your ascent till you begin falling back towards the ground. These values are at ground level. Once in orbit the escape velocity is lower than 11km/s. The greater the planet mass the greater the gravitational pull. To take off from the surface of MASSIVE Jupiter a rocket would need to be travelling at around 40 miles per second!! To escape the sun it is around 400 miles per second! To escape a black hole you would have to travel beyond 300,000km/s - faster than the speed of light - which is impossible. That is why light cannot even escape from the surface!

  Paul Steele, Marske, UK

• Escape velocity is an energy thing. If you're going at escape velocity you don't need any more energy to escape from the Earth, because your kinetic energy is already enough (assuming you don't lose it; for example air drag). But you can go more slowly if you either spend more energy, or if you go ballistic from higher altitude.

  Ian Woollard, UK

• So how do you explain a helium filled balloon with no thrust and no persistent velocity, travelling at minimal speeds and still being able to leave Earth gravitational pull, assuming no gas is lost and the balloon remains in tact? 'Floating' into space would not require great thrust or huge amount of fuel but requires a helium to rocket ratio of at least 1:000000000001 recurring

  Thomas, Croydon England