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.

In order for the Shuttle to re-enter the earth's atmosphere at a
relative zero velocity the Shuttle would have to be in a geo-centric
orbit; that is the orbit would have to be of the same period as the
earth's rotation. That would imply an altitude of about 35,786 km and
the orbit would have to be exactly above the equator. But the Shuttle's
orbit is always inclined to the equator by at least 28.5 degrees. So,
even if there were no east/west motion of the Shuttle, there would be
north/south motion. The Shuttle does not carry even a large fraction of
the fuel needed to reach a geo-centric orbit and even if it managed to
get up there we would be left with the problem of getting back down.

Now we begin to see the problem. Low earth orbit demands high orbital
velocity and any deceleration causes us to 'fall' into the atmosphere
at near orbital velocity. The Shuttle actually depends on drag from the
atmosphere to slow it down; it does not have enough fuel to slow it
down significantly relative to the atmosphere. It barely has enough
fuel to accelerate it against the orbit enough to cause it to enter the
atmosphere in the first place. Furthermore, the g forces generated by
decelerating the Shuttle enough to make a slow entry into the
atmosphere would cause the astronauts inside to be squished like bugs
on a windshield.

--
Waddling Eagle
World Famous Flight Instructor