Instead of using expensive, fragile heat shielding tiles on the
leading edges of the space shuttle, why doesn't NASA just have the
shuttle decend in gradual spiral loops under full control of the
astronauts while they monitor heat sensors at criticaL areas on the
ship?  Temperatures on the outside surfaces of the shuttle could be
decreased by decreasing the rate of decent in these loops.  The
shuttle is already sealed for the vacuum of space so water would not
be able to enter if a water landing is chosen and air bags could keep
it afloat.  Why don't they just re-enter the atmosphere far more
gently than a "plasma hot" fast re-entry?  Why not include the
possibility of off-shore ocean landings as an extra safety option?

Because the space shuttle would burn up. Do a google on Aurora (blimps
in space). The space shuttle is obsolete. The Air Force is being
persistently asked about these possibly profitable projects.

If there's enough air to allow the shuttle to be flown in loops,
there'll be enough friction for things to get very hot.

Ian G.

It's not as simple as just "going more slowly". If the spacecraft is
traveling through the atmosphere slowly enough to avoid high
temperature, this means it's also traveling too slowly to generate
aerodynamic lift. Therefore, it'll drop vertically very fast, and
crash like a meteorite. It has to plow through the air in pretty much
the way it's now configured to do.

   As for landing in the ocean, that's also deadly. The orbiter's
airframe is fine for a controlled runway landing, but water is another
game altogether. It wouldn't be like the vertical splashdowns of the
Mercury, Gemini & Apollo spacecraft. They were more compact & robust.
It was feasible to land them on water safely. A craft as large as the
shuttle orbiter, on the other hand, would be subjected to severe shear
& torsion, and would likely be torn to shreds.

As pointed out by qed100-ga the shuttle has insuficient lift to fly
slowly in the upper atmosphere and avoid stalling. The lift could be
substantially increased by increasing the size and profile of the
shuttle wings but it would then be impossible to get the thing into
space.

Landing in water as a extra safety option does not exist because
procedures already allow for an emergency bail out over water - this
would result in the loss  of the craft but then so would, almost
certainly, landing in water but only at much higher risk.

You also have to remember that the Shuttle is meant to be reused,
while a lot of the Mercury/Gemini/Apollo  craft were not.  While the
water itself wouldn't pose much of a problem for the astronauts, since
the shuttle is sealed, the ensuing corrosion would over time destroy
the shuttle.

Excluding human error and using the present shuttle's the NASA considerations are :

The shuttle would have to be re-designed and modified to accomodate a
larger internal fuel storage system, new control surfaces and
associated controls, also huge floatation devices.

( Time-consuming and very costly to modify and test, systems
maintainance involved, the extra components add more possible failures
)

So the reason is Safety, Interference with planned missions, Cost and
the designs in progress for next generation reusable transports.

Spud

Ps "sealed" submarines can reach great depths