As a kid, when my parents would buy a washing machine or a fridge, I'd take the cardboard box it came in and turn it into an imaginary spacecraft, complete with buttons, knobs and dials. That's part of being a kid - dreaming about the possibility to explore.

For me, exploration represents a quest for knowledge and the innate curiosity we all have. Whether it's working in research laboratories trying to understand how the human nervous system works, as in my case, or trying to unlock the cure for cancer, it's that inner drive to understand and seek solutions. There's always that sense of wondering what's on top of the mountain or across the valley at the end of the road. That sense of exploration is really what takes us to space or to the other final frontier, which is the ocean.

When I was an undergraduate at McGill University, I really enjoyed comparative physiology, comparing the physiology of different species, but also, the physiological adaptations that take place in humans working in extreme harsh environments - flying at high altitudes in an aircraft, living and working under water, scaling Mount Everest or flying into space.

I remember reading Arthur Guyton's textbook of physiology, and there was this chapter about the physiology of space flight. So, I was reading this in the 70s thinking, "Wow, wouldn't it be incredible to by a physician physiologist going into space to study how the body adapts and to be part of the research enabling humans to go farther into space and allow them to stay longer!"

I was one of four out of 5,300 applicants accepted into the astronaut training program back in 1992. The funny part about that is there were over 600 applications from kids less than 10 years old, which is just great to see. There's nothing wrong with getting those applications in early!

To actually be selected was amazing. I was always surprised every time I made it through to the next step. I remember looking at all the other candidates thinking, "These are amazing people; they're all more talented than I am. Any one of them would do a great job representing Canada." Of course I was thrilled to be chosen.

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I experienced my first space flight in April, 1998 as Mission Specialist 3 aboard Space Shuttle Columbia STS-90. The first time in space is pretty dramatic, because it's the big unknown. Of course, you can't help but remember Challenger [1] and the tremendous tragedy of that loss. It probably took me two hours to fall asleep on the eve of the launch.

The next day I awoke, and it was a beautiful sunny day - the perfect day for a launch. We were going for a run and one of the security vehicles stopped us and asked us if we were the astronauts. We said, "Yeah, we are."

They said, "Well, you scrubbed for today, but you're not going to launch." They told us there was a problem with the space shuttle's NSP - the Network Signal Processor communication system. So, we ended up working out and then going to the beach and relaxing for the day.

The next day, as I was lying there going to sleep, I was thinking, 'We're scheduled to launch, but who knows - maybe we will, maybe we won't.' I wasn't worried about it at all and I fell asleep right away. This time, it was a picture perfect launch, right on time.

The last thing you do on your checklist before lift off two minutes prior to launch is close the visor on your helmet and turn on your oxygen. People are really curious as to what goes through one's mind in those last two minutes. Some astronauts will tell you it's the fastest two minutes of their lives; others said that it seemed to take forever.

For me, I was just looking out the window of the flight deck thinking about how incredible it was. At about 15 seconds to launch, I had this huge reality check that there were seven million pounds of thrust beneath me. By the time my brains registered that little bit of information, the main engines had already started to ignite, and I just focused on what I needed to do.

This last time around, aboard the space shuttle Endeavor STS-118 (August 8 to 21, 2007), it was totally different. I was on the mid-deck getting ready to take a picture of the external tank once we got to main engine cut off. I went through the checklist, put my visor down, and turned on my oxygen, waiting for ignition. It felt just like the first time in space, and I wasn't particularly concerned one way or another. I was thinking much more about what we had to do once we got to space than I was about the lift off.

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It's a very unique sensation once you get close to the cut off of the main engines. The force of gravity triples. I'm 180 pounds, so multiply that by three and that's what I would weigh in that last phase of flight. It feels like there's an elephant sitting on your chest; it's hard to take a deep breath.

All of a sudden, the main engines stop, you're thrown forward against your harness and your arms are floating freely in front of you. It's just this tremendous feeling of freedom as you release your harness and you float out of the seat and take your helmet off. It's a very short-lived sense of freedom because there is so much to do once you arrive in space.

I had to go up to the flight deck and take pictures of the external tank and bring the camera back down to the mid-deck, start taking the seats apart, getting everybody out of their space suits, getting shuttle ready for its role and documenting with the space station.

The first space walk is out-of-this-world. Again, it's the sense of unknown. You wonder, 'What am I going to do when I look down a few hundred kilometers toward Earth below me? What will it be like? Will I be able to move around in the space suit?'

Once we got out of the air lock, it was unbelievable. It's really hard to describe what it's like to hold onto a handrail and turn your body so that you can see half the planet, the curvature of the Earth and the thin layer of atmosphere that keeps all of us alive on the surface of our planet.

Being in space is actually quite comfortable. I'm always amazed by what the engineers have done in building that space suit. Basically you're living inside your own spacecraft while you're inside of it.

There's a thermostat you can adjust called the Temperature Control Valve. There are adjustments you can make on the front of the space suit, but once you get used to it, by the second or third spacewalk, there's really no need to adjust it, depending on the path that you're doing. If you think about the complexity of the tasks we're doing, it's amazing the dexterity you have in the gloves and the space suit.

One of the experiments I carried out in space, which was a continuation of research I was doing at York University, looked into how hand-eye coordination changes when you get to space. What was really interesting about that was being able to compare data from my first space flight with that of the second one.

When the body is in space, it changes in ways that mimic what happens to the human body as we get older. Our muscles become weaker, our bone density is less. In fact, if you stay in space for long periods of time, you get osteoporosis. When you first arrive in space, it takes a while for your organs to readjust, and every time you move your head, it feels like things are spinning around.

What's really remarkable about all this is that when we come back to earth and readapt, all these changes are reversible. That's quite surprising when you think that as we get older on earth, the same things are taking place in our body, but they tend to be irreversible. The question is, "What can we learn from space flight that will help us maintain physiological function here on earth?"

This is not to say there exists some fountain of youth, but what we'll likely find is that some things that we develop for space may help delay the inevitable. A great analogy to that is that as you get older, if you're active, that actually helps keep you young.

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While I'm honored to hold the Canadian space record for space walks and being the first Canadian to live and work under water, for me, it's not about setting records; it's about making dreams come true. I remember vividly wondering as a kid if I would ever succeed at my goals. I was just an average kid who had big dreams. What allowed me to make my dreams come true was really just having a tremendous passion.

I was always very excited about science and I worked very hard. There are a lot of times when life throws you challenges, and it's really what you do with those times that determines whether or not you'll be able to succeed.

What I like to share with kids now is that it's not so much about becoming an astronaut, because not everybody wants to be an astronaut, but more about whether you're willing to risk in order to achieve your dreams and make that personal commitment. It's not easy, and sometimes it's much safer to do the easy things in life and never strive for what we really want, whether that is to become an artist, engineer, doctor, musician or astronaut. It takes a lot of patience, persistence, believing in yourself and refusing to take 'No' for an answer.

Whether you've been living and working underwater or in space, returning to the surface of the Earth always brings a new perspective. Underwater, we'd go diving before dawn, with total blackness surrounding us and then watch the sun come up through the water column. It turns from black to dark blue and then just gets lighter and lighter, until finally it just this brilliant turquoise. It's spectacular.

In space it's equally impressive and inspiring. For me, it generates a sense of planetary stewardship. I am truly in awe of the beauty of the planet, and it's something I've been able to experience in so many different environments, whether in space, underwater, camping, hiking, climbing mountains or whatever. It makes me want to share how impressed and excited I am about the beauty of the Earth, and we can do that of course with the amazing slides we're able to bring back.

I think when we show those images, everyone feels the same way. Wouldn't it be incredible if we could stimulate everyone to develop this sense of planetary stewardship and realize that we all can play a role in protecting our planet for the future? So often we think that we're powerless as individuals to effect any change. I tend to disagree; I think each one of us can act in our own way and make a big difference.

The best way to see evidence of planetary change is to compare photography from space taken by astronauts 10, 15, 20 years ago with the imagery coming back today. There's no question that you can see the effects we're having on the planet. You can see pollution in the ocean, air pollution, fires that are burning on the surface of the Earth...

That's really what drives it home - when you're flying over the beautiful, pristine water of the Great Barrier Reefs of Australia, and then you go to some other part of the world, which is polluted, you say, "We really have to do something about this."

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We've spent the last 30 or 40 years or so exploring the earth's orbit and trying to understand how humans can adapt to microgravity, and now the major space agencies are working on sending humans back to the moon and onto Mars. At the same time as that's happening, we have commercial groups working on providing access for more people to be able to experience space travel. Ultimately, the space exploring nations enable others to follow. Certainly it's exciting to look at the possibility of sending humans to Mars and having them live and work on another planet.

Of course, in the 60s, it was an incredibly competitive program, and it's hard to know if it will be competitive again. Right now what we're seeing is a collaborative program, where we're all working together on the international space station. Of course the focus right now is the peaceful utilization of space.

As we go forward, it's very critical for us to sit back and reflect on how the human species explores other environments-How will we deal with the potential for multiple nations to have habitats on the surface of the moon? Will it be like the South Pole for example, where we have nations all working together? We have a lot of terrestrial models on how we can do this already.

Is space infinite? Is there more life out there? I get asked to speculate on that a lot, and those are great questions. I was in Long Beach on Vancouver Island, looking up at the heavens wondering if there is life out there somewhere. When you think about what is required for life on Earth, it narrows the possibilities.

We happen to have a planet that is sufficiently close to and far away from the sun that life can flourish. The presence of water is very critical for life as we know it. Whether life exists somewhere else, we don't know, but of course, that's one of the big challenges in our quest for exploration. We can't help but wonder, 'Are we alone?' Seeking the answers is what I find so exciting.