Expert: Steve Nelson - 12/4/2011

Question
QUESTION: Steve,
I hope you can help me get a grasp on something rather basic.  
I know that bodies with smaller orbits around a particular object move faster than ones farther out, and that makes sense.
I also know that if I'm an orbiting body, say a rocket ship around the earth, if I speed up, I'll go to a larger orbit, (and vice versa), which is also intuitive, but seemingly the opposite of the previous statement.
What am I missing here?
Thanks,
Paul

ANSWER: As you add energy, instantaneously you speed up.  But them you go to a higher orbit, slowing you down more than you sped up.  In order to keep the orbit circular, you have to fire the rocket engine in a continuous way.

---------- FOLLOW-UP ----------

QUESTION: Steve,
Thanks for the quick answer.  I had thought of that, but it seemed that if I instantaneously speed up, then I'm at a higher orbit (at least briefly) with a higher speed.  I guess that slowing down due to a higher orbit must take place instantaneously also, right?
Does this mean that when a reentering space vehicle hits the retro rockets, it speeds up more and more until it hits the atmosphere?  If so, how many G's do the astronauts experience.
Thanks,
Paul48407

Answer
You'll be at higher speed if you do it instantaneously you'll enter an elliptical orbit which will slow down when you get further out.  It has nothing to do with re-entry.  Re-entry means that astronauts feel the forces of the rockets de-orbiting the spacecraft, then nothing until they hit the atmosphere...but human-safe orbits are just barely above the atmosphere.  Therefore they feel the braking forces pretty fast.