Aboard Columbia: Richard Linnehan

'The most important thing I can see me doing'

Astronaut Richard Linnehan: "We actually look back in time with Hubble. We can actually see close to the time of the Big Bang. Probably a human mind can't comprehend all that, but when you start thinking about it, you go, 'Wow, that is heavy stuff.'"

(CNN) -- Richard Linnehan has experienced quite a change in his career path. A veterinarian by training, he recently took part in several spacewalks from the space shuttle Columbia to upgrade the Hubble space telescope.

The two-time space veteran and his crewmates are expected to return home Tuesday before dawn.

Before departure, Linnehan spoke with CNN Space Correspondent Miles O'Brien. Here are highlights of the conversation.

CNN: How important is this mission?

Richard Linnehan: To me, getting to fly this Hubble mission is a big deal and the reason is Hubble. If you look at it in terms of what it has done for NASA in general, you can't go anywhere where people don't know about the Hubble telescope.

And seeing all these images. It has opened up all these areas that people never thought about. Remember when you watched "Star Trek?" When I was in high school, and you see all these beautiful weird pictures and go, "It doesn't look like that." And then you get these images from Hubble and go, "It looks like that."

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It's amazing and the ACS (Advanced Camera for Surveys) that we're going to put on Hubble is going to gather 10 times more area and go 10 time farther in distance than the present cameras.

So we don't even know what we're going to see and to me that's amazing. We have this one instrument that has taken society, civilization, so far in terms of science and what we know -- not only our own solar system but the galaxies around us and the creation of the universe.

We actually look back in time with Hubble. We can actually see close to the time of the Big Bang. Probably a human mind can't comprehend all that, but when you start thinking about it, you go, "Wow, that is heavy stuff."

For me to do this? It will be the most important thing I can see me doing, upgrading this instrument and hopefully making it a better instrument -- and allowing us to learn that much more.

CNN: Kind of a tough act to follow in your career, I imagine?

Linnehan: Yeah. I wouldn't mind going back again.

CNN: You've had a lot of time to think about this obviously, in that (practice water) tank. What are the nuances, the tricks that you have to worry about? You're kind of walking on eggshells.

Linnehan: I've done two flights already, so I know what it's like to be in space. You have to move very slowly and very methodically and think about what you do before you do it and then do it slowly.

In the water when you're in the suit, the one difference that I think that I am going to notice is the viscosity because in space you can move rather quickly, rather fast, if you put a lot of force into it. Now in the water you can put that same force in, but you've got that water drag against you.

From what I understand, you fight more in the water and it is actually easier in space. What you don't want to do when you get up there is try to start moving like you do in the water, because you'll very quickly go out of control and get unstable.

Nice, light touch. No death grips. No trying to move fast and move around. Look where you're going and watch your feet and try to keep a nice straight line because it isn't just Hubble that's up there, (it's also) the solar arrays, the cameras and all that, the antennas.

That falling sensation

One way to recognize Linnehan when he's spacewalking is to check his suit -- he wears the all-white suit, no sripes.

CNN: Other astronauts have talked about this feeling of falling that you get. Are you worried about that?

Linnehan: I can see that happening. I mean you get that when you're up there not even being in the spacesuit outside, but just being in the shuttle floating. You can close your eyes.

CNN: Do you feel like you're falling?

Linnehan: Yeah. Yeah. It's exactly like that. The first couple of nights of my first mission -- I'd say the first four nights, actually -- we were in these sleep studies and we had to go into these small sleep stations, which were basically these small little boxes like coffins.

You'd get in and close the door, and you'd have all this instrumentation on your head, looking at your brainwaves.

But when I first tried to sleep? Initially what happens when you close your eyes is that your body wants to think it's lying on something. So you close your eyes and go back and all of a sudden your brain goes, "There is nothing below you" -- and you just go to the normal response (a sensation of falling).

When I talk to kids, I say, "Remember when you're in school in the back row and you put that chair up on its back legs and you're just kind of bouncing ... and you go just a little too far back and -- oops! Well, that's exactly what happened.