On Dec 30, 4:37*pm, C J Campbell
wrote:
On 2007-12-28 04:05:46 -0800, "Oz Lander" said:

Just watched a show on the Columbis disaster, and a question came to me.
Why does the shuttle have to be travelling so fast to re-enter the
atmosphere?

The Shuttle's orbital velocity is dependent on the altitude of the
orbit and the mass of the planet that is being orbited. Consider an ice
skater that is spinning in place. If she extends her arms she will spin
much more slowly than if she folds her arms close to her side. This is
the law of conservation of momentum.

A typical Shuttle orbit is at 300 km. The orbital velocity at 300 km is
7.73 km/sec. for an orbital period of 90.52 minutes. At the lowest
possible stable orbit of 185 km the orbital velocity is 7.79 km/sec.
and the orbital period is 88.19 minutes. A lower orbit requires a
higher orbital velocity due to the stronger pull of gravity exerted on
it.

So the thing is, the closer the Space Shuttle is to the earth, the
faster it must travel in order to maintain a stable orbit.

Now, in order for the Shuttle to descend, the most efficient method is
to accelerate the Shuttle in a direction opposite the direction of
orbit (the Shuttle could also be made to descend by accelerating it in
the direction of the orbit, but that would require more fuel).
Accelerating the Shuttle straight down is essentially accelerating it
forward.

if you accelerated in the direction of the orbit , my understanding is
you would just turn a cicular orbit into an elliptical orbit. ie you
would go out further from the atmosphere on the other side of the
orbit and come back to the original ht where you first accelerated.
this would not result in the shuttle descending at all.
terry