# The limits of the earth's Escape Velocity

This has been bugging me lately, and google hasn’t been much help…

I know that earth’s escape velocity is ~25,000 MPH, but does this mean that an object traveling slower can never reach space?

Take a toy rocket, for instance, that has a max speed of 100 MPH or so. The rocket can obviously go up against gravity - at least until its engine burns out. If this rocket had an infinite fuel supply, would it be able to get to the moon (very slowly) or would some force stop it?

No, the escape velocity is how much initial speed something needs to have in order to never come back to Earth, assuming that it will have no thrust afterwards. Rockets in theory could just go to the Moon very slowly, but the most efficient fuel use is to burn it as forcefully as possible, without breaking the frame or killing the occupants, then turn off the rockets and let it coast to its destination.

If you had a long enough ladder, you could just climb into outer space. The 25,000 mph escape velocity is the velocity a needed by a rocket, or a basketball that you throw straight up into the air, to escape Eath’s gravity.

Neglecting air resistance, of course – something with a velocity of 25,000 MPH straight up would burst into flames, melt, and/or violently explode due to air resistance. It would also slow down dramatically, falling below escape velocity. 25,000 MPH is the speed needed to eject something into orbit from an airless earth.

Or, you can assume that the object has an arbitrarily high heat capacity and arbitrarily high density, or an arbitrarily low drag coefficent.

A few years back (more than a few, actually), Andy Griffith starred briefly in a TV series called Junkman. The premise was that this junk yard owner (Griffith) and his crew built a rocket to fly to the moon and salvage the stuff left behind by the Apollo astronauts. It was a short-lived show.

He hired a retired astronaut and explosives expert who had developed a continuous-boost fuel (Plausibility wasn’t a big job requirement for the writers). The astronaut took Griffith out to a test track in his Corvette (candy-apple red, of course) to demonstrate his concept.

He first demonstrates the Apollo concept by flooring the 'Vette, taking it up to 100 MPH, shutting off the engine, and coasting to the end of the track.

Then he prepares to demonstrate his concept for reaching the moon. Griffith, terrified by the first demonstration, braces himself for what is to come. The astronaut puts the 'Vette in gear, steps on the gas, and drives to the end of the track at 5 MPH.

A silly show, but the point is valid. If you have enough fuel available, you never need to approach escape speed at all.

No, itdoesn’t mean that. But if you’re traveling more slowly, you’ll need to continuously burn fuel to provide “thrust” to counteract the force of gravity pulling you down.

Once you reach 25,000 mph, assuming that you’ve cleared the atmosphere so that there is no further air resistance, you need no longer burn fuel. You will continue to slow down due to Earth’s gravity, but you will never reach zero and fall back to Earth. You can reach the Moon or any other destination toward which you may be pointed.

But again, you can still get there at a slower speed. You’ll just need to continually burn fuel.
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If the rocket had an infinite supply of fuel, it wouldn’t have a top speed of 100 mph. It could continually accelerate upward. To be sure, at low altitudes air resistance may impose a limit; the rocket can only accelerate so much before it would burn up. But as long as it can keep burning fuel and clear the atmosphere at low speed, it can then accelerate to any desired velociy by using its infinite fuel supply.

[nitpick]The show was called Salvage One, and, iirc, the car Astronaut Skip Carmichael used to demonstrate his moon mission plan was a Ferrari 246 Dino…[/nitpick]

Salvage One Fan Page

It may be a nitpick but, as others have noted, 25kmph won’t let you escape Earth’s gravity – properly speaking, you can never do that. It *will[\i] allow you to escape in the sense that you never fall back to earth.

The space shuttles do not reach the escape velocity.

Just in case you wondered, you want a forward slash there.

OK, so the escape velocity of a black hole is greater than the speed of light, so light can’t escape. Does that mean that it’s impossible to generate enought thrust even to “crawl” out of a black hole, in the manner of the Corvette or the Shuttle?

sturmhauke’s post reminds me of a question I have about black holes thats been bugging me. since potential energy is the ability to do something, an object falling if it falls long enough will get close to the escape volecity of the body it’s falling too (without wind resistance of course), the escape volecity of a black hole is greater than c, it takes infinite engergy to excede c, does that mean since black holes seem to exist that everything has infinite potential energy? or do I have something incorrect?

All paths beyond the event horizon curve towards the black hole, so far this thread has only delat with Newtonian gravity, but Newtonian gravity can’t be used to describe a black hole.

netscape, potential energy is defined as zero at an infinite seperation and negative otherwise i.e. in all realistic situations.

I did want a forward slash there - I swear it. Why my keyboard can’t grasp such simple concepts escapes me.