Re: something solid half the diamater of juptier hitting it halfway

Simulations of giant impacts on planetary surfaces are very interesting 
indeed!  And, impacts of two large planetary bodies have definitely 
occurred in the history of our solar system.  Let me tell you about one 
particular impact of this sort that is well-agreed upon by the majority of 
scientists to have happened as our solar system formed – and this example 
is much closer to home for us here on Earth.

The prevailing theory for how our Moon (yes, the Moon that you see at 
night, the same Moon that orbits Earth) was created is called the “Giant 
Impact Hypothesis”.  This theory states that early in Earth’s history, 
roughly 4.3 Billion years ago, Earth was struck by an object about half of 
the Earth’s size.  This object collided with Earth and released an 
enormous amount of energy which vaporized much of the smaller object as 
well as a large amount of the upper layers of the Earth itself.  Other 
material from both planetary bodies was broken up and ejected into space.  
But, Earth was large enough that some of this material remained in orbit 
around Earth and recondensed into our Moon.  Needless to say, the entire 
details of this theory are very complicated, but there are a number of 
very good websites that contain further discussion:  

1.  The Planetary Science Institute has a page devoted to the “Origin of 
the Moon” (link below) which contains a number of artists renditions of 
what this event would have looked like, as well as the logic and evidence 
behind this theory.  This page was put together by following the two 
scientists who initially proposed the “Giant Impact Hypothesis” in 1975.

2.  NOVA did a program on the Origin of the Moon and they have an 
animation you can view on their online site (link below).  The people at 
NOVA called it “The Big Whack”, but impact cratering scientists 
call it the “Giant Impact Hypothesis”.  You will be able to find much more 
information on it by searching for the correctly used scientific term 
rather than a phrase that sounded good on television.  

3.  Space.com has a nice series of images from different times throughout 
the impact itself.  Although it is not an animation, you can get more 
detailed information about what the impact looked like in viewing these 
time slices.  Go to the Space.com “Making of the Moon” page (link below), 
scroll a third of the way down and click on the image on the right side of 
the page.  

As you can see, the idea of large objects hitting one another in space is 
something that interests many scientists.  Hopefully I’ve given you an 
idea of what that type of impact might look like.  However, let me now 
specifically address your question regarding Jupiter.  While I’m unaware 
of any specific work done on this question, there is some basic 
information about impacts and their results that I can give you and apply 
to a giant impact on Jupiter.  Any impact of one object (let’s call 
it “Big”) with another that is roughly half the size of the first object 
(let’s call it “Medium”) will produce a tremendous amount of energy, 
especially when “Big” is the size of a planet, be it the Earth or 
Jupiter.  This absolutely enormous amount of energy will generate a lot of 
heat and break up a lot of the material that both “Big” and “Medium” are 
made up of.  What happens to that material when it gets broken up mainly 
depends on how “Medium” hit “Big”.  If “Medium” hits “Big” head-on, then 
the most likely scenario is that both objects would cease to exist as they 
did before.  This is to say that the amount of energy would act like a 
huge explosion and blow both “Big” and “Medium” apart.  If, 
however, “Medium” hit “Big” at an angle, say it just grazed the top 
of “Big”, then enough of “Big” might remain to gravitationally attract 
some of the other material that was broken up, which would then either 
crash back into “Big” or maybe recondense to form a smaller object in 
orbit around “Big”.  (This is basically the situation that happened when 
the Moon was formed and you can see in many of the animations how “Medium” 
hit “Big” almost sideways, instead of straight on.)

Now, there is one big difference between the Moon-forming Giant Impact and 
the type of impact that you are asking about with Jupiter.  Jupiter is a 
Gas Giant planet – it does not have a solid surface like we are used to 
here on Earth.  Rather, it is almost entirely an atmosphere that simply 
gets more and more dense as you move into the planet (much like how when 
you walk into a foggy area, the fog gets denser and denser).  Atmospheres 
can affect what happens during an impact but mainly only in very small 
impacts.  As we saw in 1994 when Comet Shoemaker-Levy 9 hit Jupiter (link 
below), there is still an enormous amount of energy generated in Jupiter’s 
atmosphere by even a small body during an impact.  

So, it is my best guess that if a solid body half the size of Jupiter hit 
Jupiter, there would be enough energy generated that the smaller body 
would be completely disrupted or ripped apart and vaporized.  The planet 
Jupiter would lose a lot of material from its atmosphere and probably be 
very different from the planet we know today once (and if) it recovered 
from this giant impact.  If the smaller body hit Jupiter head-on and at a 
fast enough velocity, then it is possible that Jupiter itself would be 
destroyed and its material spread throughout the solar system.  Luckily, 
the only planetary body in our solar system that is half the size of 
Jupiter or larger is Saturn and Saturn’s orbit will never intersect with 
Jupiter’s, so that is good news for Jupiter!

Planetary Science Institute’s “Origin of the Moon” page http://www.psi.edu/projec
ts/moon/moon.html

NOVA online has movie of “The Big Whack”  http://www.pbs.
org/wgbh/nova/tothemoon/origins2.html

Space.com “The Making of the Moon” http://www.space.com/scienceastronomy/solarsystem/moon_making_01081
5-1.html

Lunar and Planetary Laboratory’s “Collision with Jupiter” page http://seds.lpl.arizona.edu
/sl9/sl9.html