The official name for this speed is called the "escape velocity". If a
spacecraft is launched from a pad on the surface of the earth with this
speed or greater, it will escape the Earth's gravitational field. The
escape velocity can be calculated from the Earth's mass, its radius,
and Newton's gravitational constant G: v_esc=sqrt(2*G*M/R). It is
assumed for that formula that air resistance doesn't slow down the
spacecraft (a very wrong assumption). For the Earth, this speed is
11200 meters per second, or about 25,000 miles per hour.
No, the spacecraft will not continue at that speed -- gravity will
constantly pull the spacecraft towards the Earth and slow it down. If
the spacecraft is launched going faster than v_esc, it will still
continue at a slower speed long after it has climbed out of the Earth's
gravitational field. If it is launched at v_esc, it will slow down and
just barely not fall back, continuing farther out, but always more
slowly. If the craft is launched at a speed slower than v_esc, it will
Air resistance is important. Dropping an object towards the earth
from far away in space usually results in the object at least partially
burning up due to the heat caused by friction in the atmosphere. By the
same token, spacecraft shouldn't try to go through the atmosphere at
that speed because they may burn up.
You could launch a spacecraft that slowly lifts itself up, always
burning fuel. Then you could get into space without ever getting up to
v_esc in speed. The biggest problem with this is that it takes more
fuel on the entire trip to get into space in this manner. Hovering
means burning lots of fuel while going nowhere. Going slowly just burns
fuel that could otherwise help send the craft into space. And if fuel
is burned higher up along the path of the spacecraft, more fuel would
have to be burned lower just to lift that fuel up. It is more efficient
to boost a lighter spacecraft up that's burned all of its fuel earlier.
And even more efficient to drop the fuel tank once it's empty!
So actual spacecraft get launched as fast as possible without burning up.
(republished on 07/19/06)