[LINK] Finally up to date on the Shuttle

James Pearce james.pearce@zdnet.com.au
Fri, 14 Feb 2003 15:40:07 +1100


> A question in return, - I seem to see contradictory information about the
> relationship between shuttle velocity and altitude. On the one hand,
orbital
> calculators make it quite clear that a body at a lower orbit has higher
> velocity than one at higher orbit. Look at
> http://liftoff.msfc.nasa.gov/academy/rocket_sci/orbmech/vel_calc.html
> to check me (the moon's orbital velocity is slow compared to a shuttle for
> eg).
>
> However, in discussing Shuttle flight, re-entry is achieved by slowing the
> shuttle down so its orbit decays.
>
> There seems, to someone whose physics needs work, to be a contradiction
> here. Anyone good enough at aerospace to sort me out?

Richard,

IANAAEOA (I am not an aeronautical engineer or astrophysicist), but as far
as I can tell an object in orbit at a low altitude has to go faster than an
object at higher altitude in order to counteract the stronger effect of
gravity. The physics of this is really weird, since an object in orbit has
an acceleration directly towards the Earth, and a velocity (at a single
point in time) perpendicular to it. So the object is constantly accelerating
while maintaining a constant speed.

When the space shuttle slows down, it is no longer going fast enough to
counteract the effect of gravity, and so it's orbit shrinks. The difference
is it is not trying to go into a lower orbit, but land. If it was trying to
go into a lower orbit, it would have to speed up to get the required
velocity.

I hope that helps,

James