Re: Why do objects burn as they enter a planet's atmosphere?

Terri, this is a great question: You may have seen simulated
video of space capsules or the Space Shuttle "reentering" the Earth's
atmosphere, and leaving a fiery trail behind as they do. You may have
seen scenes like the one in the new movie "Mission to Mars", where an
object (or even a person! :<) "burns up" as soon as it gets too close
to a planet's atmosphere. And you may even be aware that "shooting
stars" (also known as "falling stars") are really tiny specks of dust,
burning up as they strike our atmosphere.

So what exactly IS it that causes this burn-up? The answer may surprise
you: It's the same thing which causes your hands to warm up, when you
rub them together on a cold day - it's "friction". Basically, whenever
two objects strike one another, they convert a certain amount of their
energy of motion into heat... If the two objects are moving fast enough
relative to one another, they can produce a LOT of heat: enough to turn
one of them (the air) from a gas into a weird substance called a plasma,
and maybe to completely disintegrate the other object - say, a speck of
dust from interplanetary space!

To get an idea how this could be, imagine that you are a dustspeck, and
that the earth's atmosphere is a swimming pool... If you belly-flop into
that pool, it will HURT - but you may also happen to notice your skin
warms up just a little bit: some folks even call that "a burn". Now try
to imagine that you're belly-flopping into that swimming pool at 30,000
miles an hour, or even faster! Your dive would produce so MUCH heat, it
would probably vaporize you and every drop of water in the pool in an
instant: All that might be left was a glowing mist, too thin to notice.
Well, that vaporization is what we see as a "shooting star" whenever a
dustspeck hits our air going at 30,000 to 150,000 m.p.h.!

So how does NASA keep Space Shuttles and other reentering craft from
being destroyed every time they reenter Earth's atmosphere? One thing
they do is try to make the reentry happen as SLOWLY and as SHALLOWLY
as possible: that way, the amount of heat produced is much reduced. In
addition, of course, they need to protect the space craft from all the
heat that will STILL be produced anyway, by encasing it in some non-
flammable material, which will dissipate the heat away from the craft's
precious cargo as quickly and efficiently as possible: in the case of
the Space Shuttle, this material is the famous "heat tiles" which were
so hard for NASA to get right when the Shuttle was first built!