On Sun, 16 Apr 2006 09:23:22 GMT, in uk.sci.astronomy , Peter Lynch
wrote:

On 2006-04-15, wrote:

Do you know the escape velocity for the solar system? How much
propellant you'd have to burn to attain it with chemical rockets? How
long it would take to reach it with ion engines? There's a *reason*
that this is strictly fiction.

My understanding of escape velocity is that it assumes that it is
reached (relatively) quickly) and once the "burn phase" is over there
is no more significant force applied in direction of travel.

Indeed. Escape velocity is the velocitry an _unpowered_ object needs
to be travelling at to escape the gravitational field it is in.
Provided you pick the right course /and/ have power to maintain it,
you can escape at any speed you like.

The reason escape velocity is loved by 1960s genre sci-fi (and therefore
most films) is that when you have to carry your chemical propellant, there
are huge advantages to high acceleration for a short time.

Well, the reason sci-fi films love it is because it makes for good
spectacle... :-)

The point is that you don't need any particular velocity to escape,
just to provide enough extra energy. EV as a concept assumes you need
to provide that energy in short order at lift-off, you can just as
easily supply it in tiny amounts.

Mark McIntyre
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