QUESTION:
I read that the probe will pass through liquid hydrogen on Jupiter.
Would it be possible for the probe to ignite the hydrogen causing a
massive explosion that could detroy the entire solar system? 

ANSWER from Leo Cheng on Dec. 21, 1995:  
Before I answer your question, let me make a correction.  The Galileo
probe will not pass through the liquid hydrogen layer on Jupiter.  In
fact, the probe won't get anywhere close.  When the probe is destroyed
by Jupiter's pressure and temperature, it'll still be about 7,103,300
kilometers away from that stuff!  Also, when you said Jupiter "takes up"
more than half of the solar system, you are correct if you mean it's
*mass* takes up more than half of the *mass* of the solar system.

So why didn't Jupiter explode?  Well, let's think about explosions here
on the earth...no no I'm not going to tell you how to build a bomb.
Believe it or not, there are lots of little explosions going off inside
the engine of your family's car.  These explosions drive "plunger" like
parts inside metal cylinders, which turn gears and shafts in the engine,
which then turns the tires, causing the car to go.

Let's get back to the explosions.  To have an explosion you need 4
things:

    1) Fuel: something that burns easily.  In the car, it's gasoline.
    2) Oxygen: Yes, you always need oxygen.  The amount of it is
       important too.
    3) An energy source: In the car, it's the spark that comes from the
       spark plugs.
    4) A chain reaction:  All of the fuel must burn at the same time.
       This is really not a thing, but a process.  When this process
       happens, the gasses it produces expand instantly.  (Where do you
       think the "bang" comes from?).  This chain reaction process
       only happens when there's just enough fuel and oxygen  Auto
       mechanics talk about "fuel to air mixture", which is a ratio.

Now my question to you is: "Which of the things above was *missing* on
Jupiter?"  On Jupiter, we've got lots of fuel (hydrogen), which burns
very easily on the earth.  We also know that it can explode on the
earth, so that's not it.

What about oxygen?  Your dad was right when he said you needed oxygen.
Does Jupiter have oxygen?  If so, does it have enough of it?  I'll let
you figure this out.

Next comes the energy source.  This is a trickier one.  The friction
between the probe and the atmospher does supply energy, but I'm not sure
if it's enough.  Remember last July when pieces of the Shoemaker-Levy 9
comet crashed into Jupiter?  The energy of each piece of the comet was
thousands of times more than the Galileo probe.  I'll let you find out
what happened.

Finally, we need a chain reaction.  Was this the missing ingredient?
Could something like the probe cause all the hydrogen on Jupiter burn at
the same time?

One more thing, you asked if there could be any other elements unknown
to us that could cause an explosion.  I'd like you to look up the
Periodic table of elements and find out why the elements are numbered
that way (hint: it has to do with electrons, protons and neutrons).
This will help you answer the question of unknown elements.

There are a few web sites that you may be interested in.

Galileo Probe FAQ
    http://ccf.arc.nasa.gov/galileo_probe/htmls/jupiter_faq.html

A nice article on the Fate of the Galileo Probe
    http://planetary.org/tps/galprobe.html