QUESTION:
I know that the SRB's are recovered and reused, but what about the
main external fuel tank?  Does it burn up on rentry? Can an
empty tank make it into orbit?

ANSWER from Michael Wilhoit on March 20, 1997:
The External Tank (ET) remains connected to the Orbiter until shortly
after Main Engine Cutoff (MECO), which happens about 8 1/2 minutes after
launch.  About 19 seconds after MECO, the ET is disconnected from the
Orbiter, and the Orbiter uses it's manuvering jets to move away from the
ET.  At MECO for STS-83, the Orbiter and ET will be about 62 nautical
miles above the Earth.  About 40-45 minutes after MECO, the Orbiter's
two manuvering engines (OMS) will fire together for a little longer than
two minutes to raise it's final orbit to about 160 nautical miles or so,
and nearly circular.  

However, the ET has no such manuvering engines and therefore it
continues on a "ballistic" path back into the thicker atmosphere.
Because it is not designed to tolerate this re-entry into the
atmosphere, it is destroyed.  Much of it probably burns up, but some
heavier parts make it all the way back down and splash into the Pacific
Ocean between Hawaii and South America.  I think that would be an
exciting thing to see, but maybe from a distance.

Many space designers have debated whether or not the ET should be
carried into a "good" orbit.  Some think that a large structure like
that could be used as part of a space station.  To do that that with the
Shuttle would require a great deal more energy use by either the Main
Engines or by the OMS engines, or both.  This would significantly reduce
the amount of additional payload mass that could be hauled into orbit.

Related thoughts for younger students:

1)  A thrown baseball follows a ballistic path, shaped like an arc.  How
does the shape change if you throw the ball faster, or slower?  (For
older students:  How fast would you have to throw a ball to get it into
orbit?  The fastest major league pitchers can throw nearly 100 miles per
hour.)

2)  What could you do with an empty External Tank if it was in orbit?
What if there were 3 or 4 or more ET's in orbit together?