It's different on re-entry: we use the small orbital engines to slow down a tiny bit and thus descend into the atmosphere, and then use friction from the air to slow down.
That saves us having to bring fuel to slow down, but it creates a very high temperature around the vehicle, near 3000 C.
Humera Jabir asks: I would like to ask the astronauts how going to space impacted any religious or personal beliefs they may have? Big question, but I would just like to know what comes to mind for them.
Marc Garneau: This is a personal question and every astronaut will answer it differently.
Like most humans, I have sometimes struggled with my faith. At certain moments, it has been strong and at others, I have experienced doubt.
When you are strapped into the shuttle and waiting for it to launch, you certainly reflect on your faith and as with all intense moments of this kind, you hope that God is with you because you really need him.
I have to say that when I arrived in space after the wild ride that took me there and first looked down on the majesty and the beauty of planet Earth, I experienced a quiet but very strong conviction that God does exist.
Intense experiences can open your mind and make you see things differently. Although I am basically the same person now as I was before my first spaceflight, I have also been altered in important ways by the experience. I see the world and indeed life itself differently.
Corey Richard asks: Mr. Hadfield, you will be doing your third space flight. How do astronauts mentally prepare for their first space flight?
Chris Hadfield: Corey: Through realistic visualization and preparation. We study every detail, mentally work our way through each step and misstep, and build a plan that will work, no matter what goes wrong. Then we practice it over and over in simulators. That gives us the calm and confidence and ability to launch.
Ian Martin of Elora, Ont. asks: What is your view on the costs/benefits of manned space flight? Can it be justified? It seems to me that there are very few big science questions that cannot be answered by unmanned space flight, which would be much cheaper in terms of both financial and energy costs as no life support systems would need to be lifted.
Marc Garneau: Thanks Ian. When the answer to a scientific question related to space can be provided by an instrument or a robot, then we should build that instrument or robot because it will cost less and no lives will be at risk.
Once in a while however, we may decide that we need to send humans to space because there is a strong public will to do so, or because a human is more capable than an instrument or robot for certain very complex tasks. If we make that decision, we accept the greater cost and the possibility of loss of life - never an easy decision.
As robots become more sophisticated, there may be fewer arguments for sending humans to space. Having said that, a robot will never be able to express its emotions as it lands on another planet or when it orbits our own planet earth. Machines don't inspire us like humans do (although I have to admit as an engineer that I'm very inspired by those who design and build the machines).
Finally, without trying to sound boastful, I have looked into the eyes of too many young people not to know that astronauts capture their imaginations as very little else does. A complicated answer to a complicated question.
Kenyon Lee, age 5, asks: Mr. Hadfield, my Daddy met you once and bought you lunch in the Vancouver International Airport. We have the picture you gave him framed on our wall and we love to go to the Space Center in Vancouver. There is so much cool stuff in space. What is your favourite cool space thing? My favourite is the pictures we see of the earth from space. I would like to met you one day like my Daddy did.
Chris Hadfield: Thank you. Kenyon, my favourite cool space thing is doing a spacewalk. To be floating free in your spacesuit, alone in the universe, holding on with one hand watching the world roll by in all its glory is a wonderful human experience. I would like to meet you too!
Send your questions to firstname.lastname@example.org with "Ask an astronaut" as the subject. A selection will be answered by Canadian space pioneers during the final mission of the space shuttle program.