NASA file

Astronaut Rex Walheim mugs for the camera during a flight on the shuttle Atlantis in 2008, with a dried rose floating in zero gravity inside a case.

One of the last astronauts to ride on a space shuttle will be riding a totally different vehicle on Monday: a flower-bedecked float in the 2012 Rose Parade.

Say what?

The 5.5-mile journey down the parade route in Pasadena, Calif., doesn't hold a candle in distance or danger to the 5.3 million-mile journey that NASA astronaut Rex Walheim made in July during STS-135, Atlantis' program-ending mission. But it's a perfect follow-up for several reasons:

• Walheim will be riding on the "Garden of Imagination" float with his brother, Lance, a horticulturist who wrote the book "Roses for Dummies." Lance is the go-to garden guy for Bayer Advanced, an arm of the chemical conglomerate that makes lawn and garden prducts.
• With Lance's encouragement, Rex carried up a dried rose as one of his personal items during a 2008 mission on Atlantis. That rose was featured on Bayer Advanced's trophy-winning Rose Parade float in 2009.
• This Rose Parade appearance gives Rex a chance to smell the roses, literally, after a high-pressure spaceflight. It also gives him an opportunity to reflect on how a little gardening could come in handy for future generations of space explorers.

"If you think about it, that's one of the things we need to develop on our long-term vehicles," he told me this week. "We need to have a self-supporting ecosystem, environmental control systems for recycling air and water, and you have to grow your own food. We're doing that in space. ... These skills that any astronaut has as a young child when they work in the garden and help their parents, well, those turn out to be important on the most advanced vehicles ever made, and on the most complex exploration missions."

It's not clear whether Walheim will have a chance to do some of that space gardening himself. The 49-year-old already has three shuttle flights under his belt, and he's not currently on the schedule for a future spaceflight. With the shuttle program finished, journeys to the International Space Station are the only game in town for NASA astronauts. By the time NASA is ready for trips beyond Earth orbit, Walheim could well be pushing 60.

"I'm not giving up on ever flying again, but it's a litt more difficult to do at this point," Walheim acknowledged.

My interview with Rex and his brother Lance touched on life after the shuttle, the story behind the Rose Parade ride and the future of spaceflight. Here's an edited transcript of the Q&A:

Cosmic Log: So, Rex, what have you been up to since the last shuttle mission?

Rex Walheim: Well, immediately after the mission we did a number of debriefs ... the training folks and the flight controllers. We told them how the mission went, and then we started doing debriefs at the different NASA centers. Then we took our post-flight presentation out farther around, to different organizations and places. We had a chance to go overseas to Italy and Turkey, and talked to some of the military people and their families. That was very rewarding.

Then I went by myself to Australia, to Melbourne, to a museum down there that was doing a space exhibit, and that was my chance to see Australia for the first time. We topped it off with a trip to the White House on Nov. 1. We got to meet President Obama in the Oval Office, and that was exciting. The next day we had a picture taken with John Young and Bob Crippen of STS-1.

Pete Souza / White House

President Barack Obama checks out his astronaut flight jacket in the Oval Office on Nov. 1 as Janet Kavandi, director of flight crew operations at NASA's Johnson Space Center, stands alongside. In the background are STS-135 shuttle crew members Doug Hurley, Sandy Magnus, Chris Ferguson and Rex Walheim.

NASA / Houston Chronicle

The astronauts who formed the crews of STS-1, the first space shuttle mission, and STS-135, the final shuttle mission, pose for a group photo at the Johnson Space Center on Nov. 2 in Houston. They are, from left, STS-135 pilot Doug Hurley, STS-1 pilot Robert Crippen, STS-1 commander John Young, STS-135 commander Chris Ferguson and his crewmates Sandy Magnus and Rex Walheim.

Now it's time to transition to a ground job. I'll be working in the exploration branch, on some of the Orion systems, mostly the launch abort system, and also some of the ground launch systems. And I'll keep my hand in the EVA [spacewalk] business. I'll be working on the early flight suit — kind of a modified ACES [Advanced Crew Escape Suit, popularly known as "the pumpkin suit"], and then work on the next-generation suit for going to an asteroid or to Mars.

Q: Are you interested in a rotation on the space station?

Rex: It's a possibility, but I wouldn't do that for a few years. I'm not sure if I'll get a chance to do that or not, since I've flown three times already. But we'll see. I'm not giving up on ever flying again, but it's a little bit more difficult to do at this point.

Q: If it turns out that you're not in the space station rotation, how do you see the next few years working out? Would astronauts be involved in flight tests?

Rex: Yeah, we'll be involved in the flight tests and in the development of the vehicle, just like we were with the space station. When I first came here in '96, I was involved in the project even before the early assembly flights, developing the procedures, monitoring systems development, things like that.

We already are involved in the same way with Orion, with the crew displays and development of the flight systems. I love working down here at Johnson Space Center. I want to stay here for a while, and be in the Astronaut Office and provide our experience from having flown on the shuttle and having been on the space station to design the next generation of vehicles — whether they be commercial crew or the Space Launch System / Orion exploration system.

Q: Some of your colleagues have gone over to the commercial side. Do you feel as if there are some significant losses in expertise, or is it good to spread that expertise around?

Rex: I think it's really good to spread that expertise around. We'd love to have a lot of these folks stay with NASA, but I think it's great that they're going to the commercial providers and the other contractors, because we need to keep that experience around. The hard part would be to see people going to different industries, and I know that's a possibility for some people. It's hard-won experience, and it's nice to retain that.

The people we've had working on the shuttle program and the space station program are some of the best people I've come in contact with. I enjoy working with them and hate to see them go. But the fact that some are working with the commercial providers will give those companies a realistic appreciation for the difficulties and dangers of spaceflight. That experience will help those companies come along and develop the vehicles they're working on.

Q: I suppose it's a challenge for anyone who's involved in exploration to go through a transition like this. Are there things that you've found are helpful? How do you keep your focus on what's ahead?

Rex: Two ways. One is getting these commercial providers to give us American launches from American soil as soon as possible. Obviously I have a special connection with that, since I was one of the last ones launched. I really have a visceral desire to see people launched from the United States again, so to be able to help out with that ... that hopefully will be a short-term goal, three to five years. There's a lot of work being done on that.

And then there's the chance to dream a little bit, too. Let's get out of Earth orbit. Let's go to these places we've never been to before. That's always exciting to think about, and it's need to be a part of that. That's more long-term, but if you concentrate a little bit on both, the short term and the long term, that gives you a good balance.

You've got to keep in mind that we've been through this kind of transition before, between the Apollo program and the shuttle program. It's not easy, there's no question about that. A lot of people lost their jobs, and they're great people, but we'll gear up again and do some new and exciting programs.

Q: Do you and the other astronauts think about those future trips? What will it take for trips to a near-Earth asteroid, or to Mars?

Rex: I think it'll take the kind of people who left Europe and headed for America and didn't know what they were going to find. They left their old life completely behind. That's what it's going to take for the early explorers of Mars. They'll have a chance to come back, but it's going to be a three-year voyage. When they leave Earth, Earth becomes not this beautiful planet underneath them, like it is with the space station, but it's going to become like any other star. They're going to find themselves in a void of blackness with stars all around them. I've heard people say it's going to rewrite the definition of loneliness. But we have those people. We've had them in the past, and we'll have them in the future: people who will leave their homes and their families for three years on a very risky mission to be the first people to go live on Mars.

That' the kind of person it's going to be. It's going to be the younger folks, because it's going to take some time to get there, but we have those brave explorers coming up. I hope the kids in school now are studying math and science, so they can be among those first explorers who really reach out into the solar system.

Q: How does it feel to be a member of the last crew to fly on a shuttle?

Rex: You know, it's interesting. You don't really think about that a whole lot during the mission. During the preparation for the mission, you never know whether you're going to be the last one, or whether you're going to get to fly. Remember, there was a funding question about our mission. But it's neat to be a part of the last mission and see the tributes that people have done for the shuttle program.

It does start to hit you a little bit later than other folks. A lot of folks felt it at the launch, or when their part of the mission was done. For us, we were so busy over the last four or five months that it took a while to sink in. But when you see people leaving — for example, when Fergy [STS-135 commander Chris Ferguson] left NASA to go work at Boeing — it does sink in. The band is breaking up, and we're going on to different things.

Q: So how did this whole idea of being with your brother on the Rose Parade float come up?

Rex: I'll let Lance take that one.

Lance Walheim: People always said to me, "Why don't you have your brother take a rose up into space?" They were thinking about a live rose. I kidded him — I said, "Well, Rex can't even keep a rose alive in his backyard, how's he going to keep it alive in outer space?" So we came up with the idea of taking a dried rose up, and Rex did that on his previous flight, before the last one. Then we put it on the Bayer Advanced float in 2009.

NASA file

Astronaut Rex Walheim and his brother, Lance Walheim, collaborated on a mission to take a dried rose into space in 2008.

It just became natural to do something with the theme of the parade this year: "Just Imagine." We thought this'd be the perfect place to pay tribute to Rex and the whole shuttle program. We weren't sure until fairly recently that we were going to be able to do it, but it all worked out. I'm the Bayer Advanced expert, of course, so we're very proud to have him on the float.

Q: How does space tie in with the theme of the parade?

Lance: Well, the title of the float is "Garden of Imagination," and it's the story of two brothers who worked in the gardens when they were young and imagined bigger and better things. Rex was imagining going to space, and I was really into plants and wanted to work with plants. There will be a rocket on the float, and there's a trail going up. We'll be at the base of that trail.

Bayer Advanced via Luckie & Co.

Bayer Advanced's Garden of Imagination 2012 Rose Parade float has a flowery trail leading to a rocket ship.

Rex: The tie-in with the "Garden of Imagination" is that it's a place where you have a chance to dream. When you're gardening, you have a chance to let your mind wander. I think it's a place where you can do a lot of good thinking. When I was in our backyard, in our garden or where we were landscaping, it was in San Carlos, which is near San Francisco. You can see down into the Bay Area, and the neat thing about it was the airplanes that were landing at San Francisco International Airport. I could watch the 747s fly over and dream of flying one day. That's where my dreams of flying really took flight, in my backyard in California.

Q: Do you feel as if you have a little more time to spend in the garden, spend with your family, or are there more things that you have to keep your eye on?

Rex: There's always more to accomplish, and a lot of hard work ahead, but the neat thing is that the pressure is off now. It's always tense when you're preparing for a mission and getting ready to fly. The time you have with your family is somewhat limited, and there's always a little pressure there, realizing that you're going to be doing something dangerous. So when you come back, there's almost a joy of life you get. It turned out well, and everybody's safe. It adds a little bit of freshness to all the things that you do. Unfortunately, it doesn't always last that long, but for me, it's lasted a really long time. I just went skiing with my boys in New Mexico, and we spent some time outside and really enjoyed ourselves.

Q: So, Lance, would you say Rex's gardening has improved?

Lance: It's been a little while since I've been to Rex's garden, but hopefully I'll find out soon. A couple of years ago, I sent him some rose bushes as a Christmas present. So the first thing I'm going to do is look for those roses and see how they're doing.

Rex: Well, to be honest, he sent me three rose bushes. Two of the three still survive, and the first one never made it into the ground. I just didn't get around to planting it. But Lance is my consultant, whenever I find some new disease or pestilence out in my backyard here in Texas, I call Lance and ask "what the heck is that" and send him a picture. And he gives me the advice on what to do about it.

Q: Sounds like you better get that garden spruced up, because the boss is coming.

Rex: That's right.

More on the future of spaceflight:

• Next steps in the new space race
• From July: Meet the last shuttle crew
• Cosmic Log archive on the new space race

NBC and other networks are airing television coverage of the 123rd Annual Tournament of Roses Parade at 11 a.m. ET Monday (check your local TV listings). The "Garden of Imagination" float will be the fifth float and the 10th unit in the parade lineup. Look for it on TV between 11:10 and 11:20 a.m. ET (8:10 to 8:20 a.m. PT) Monday.  

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.