QUESTION:
Since Mars is iron, how come it just doesn't go out of orbit? 

ANSWER from Mary Urquhart on January 30, 1997:
Mars stays in orbit because the gravity of the Sun keeps it there.  
Gravity works the same way on all matter, iron or otherwise.  

Mars isn't made of only iron, though.  In fact, the Earth has more iron in
it than Mars does.  Mars and the Earth are made up of many things
including iron and rock (many rocks have iron in them). The surface
material on Mars (the soil) has some iron in it (and many other things
too), and that iron has "rusted" and turned red.  

More about gravity:
All matter pulls on all other matter. That pull is gravity.  Gravity is
VERY weak though. In fact, gravity is so weak that it take a HUGE amount
of mass for its effects to be noticeable. (The mass of an object is the
measure of how much substance is in that object. Every physical object has
mass: you do, the Earth does, and the Sun does.) Because you have mass,
you pull other masses toward you with your gravity. It doesn't matter how
far away something is, your gravity is still pulling on it. The pull of
your gravity on an object does get weaker the farther away from you
another object is. It doesn't matter, though. You have so little mass that
the pull of your gravity is very weak and won't be noticed by any person
or object no matter how close you are. It takes the mass of something as
big as a moon to be noticeable at all. The Earth has a lot of mass, and
you definitely feel its gravity.  You have very little mass, so the Earth
isn't affected by your gravity even though you are standing on it. The
Moon, which has MUCH more mass than you do doesn't have much of a pull on the
Earth compared to the pull the Earth has on the Moon.  The Sun has much
more mass than any of the planets, including the Earth and Mars; the 
pull of gravity from the mass of the Sun keeps the planets in orbit. 

Mary Urquhart
Laboratory for Atmospheric and Space Physics
University of Colorado at Boulder