The theory of moon creation when a Mars size planet hit Earth As we know the predominant theory where does the moon come from is that a Mars size planet hit the earth and took a chunk out of it which eventually materialized into moon.
My question is that if a Mars size object were to hit Earth, wouldn't it knock it off the orbit all together? What kind of collision is required to knock a planet of its orbit. By 'knock off' I mean it would alter the orbit of Earth and possibly speed so that it will not have stable orbit anymore so it will either (gradually) leave solar system or (gradually) collapse into sun.
 A: The hypothetical collision has been mathematically modeled, and the results of those models are consistent with what see.
The impacting body (referred to as "Theia") would have hit at a low velocity and relatively shallow angle. It would certainly have affected the Earth's orbit, but not enough to knock it out of the Solar System or into the Sun.
Knocking Earth out of the Solar System would require speeding it up by about 40% (escape velocity is sqrt(2) times circular orbital velocity), which means doubling its kinetic energy. This would require a high-velocity impact by a body whose mass is comparable to Earth's -- which would probably be enough to disrupt the planet altogether, creating a new dense asteroid belt.  Theia is believed to have been about the size of Mars, which has only about 10% of Earth's mass.
Ignoring gravitational influences from other bodies, any solar orbit is stable, as long as the velocity is greater than zero (so it doesn't fall into the Sun) and less than solar escape velocity.
A: There are no simple orbits that gradually spiral in or out. In the two body system orbits are either closed ellipses or open (hyperbolic) with the (barely open) parabolic orbit as the dividing case.
The decay of the orbits of artificial satellites is due to interaction with the Earth atmosphere. 

There is an interesting questions here: how do we come to have a near circular orbit in this epoch given that such an event would likely have left the Earth/Moon system with a substantial eccentricity in the immediate aftermath?
Perhaps one of our astronomers can provide some insight.
A: 
What kind of collision is required to knock a planet of its orbit.

It actually wouldn't require a collision.  If a sufficiently massive object passed very close to Earth, the gravitational interaction (no collision) might eject Earth out of the solar system.
Here's an article about a conjecture that a 5th gas giant was ejected from our solar system.
