There are few days that are the same for NASA's Commercial Crew Program astronauts as they train for flight tests aboard the next generation of human-rated spacecraft, astronauts Eric Boe and Suni Williams told an audience at NASA's Kennedy Space Center on Thursday.

"One of the things I like about being an astronaut is that you're always doing different things," Boe said. "I don't think I have a day or week that's been the same since we started this."

Williams said the constant changes involved in training are similar to what happens during a space mission, so the daily differences are valuable for the crews.

"This is not work, this is fun," Williams said. "It's pretty spectacular the stuff we get to do, but it's not all the same. It's not routine and I think that's good training for space. Up there, day-to-day, every day your timeline changes."

Astronauts Boe, Williams, Bob Behnken and Doug Hurley have not been assigned to specific missions or spacecraft, so all four are cross-training and advising on the development of both Boeing's CST-100 Starliner and SpaceX's Crew Dragon. All four flew on space shuttle missions and have visited or lived aboard the International Space Station during their careers. They each also served as test pilots during their military careers.

Boeing and SpaceX are working toward conducting orbital flight tests of their spacecraft without a crew next year, then progressing toward a flight test for each including astronauts before they begin operational missions to take astronauts to the space station. The new launch capability will enhance research opportunities on the orbiting laboratory by doubling the amount of time astronauts working in space have to devote to scientific investigations.

Both companies will launch their spacecraft from Florida's Space Coast using spacecraft, rockets and support networks they own and operate. That approach is novel for NASA's human spaceflight programs which have always been designed, owned and operated by the agency itself in the past.

Taking questions from around the agency during the Kennedy event, Boe and Williams were joined by Bob Cabana, director of Kennedy and a former space shuttle commander, and Kathy Lueders, manager of the Commercial Crew Program.

What is the progress of the Commercial Crew Program?

Lueders: I think people forget about the time frame and how short the time has been that our partners have been working on the final development of their spacecraft. We awarded the contracts in September 2014. Right now, the companies are in the midst of this grueling periods of getting their vehicles together and getting their structural test articles together. We're getting ready for flight tests. Most importantly, we're getting there as fast as we can safely fly.

What opportunities do you have working on a developmental program?

Boe: We haven't worked on a new manned space program in the United States in more than 30 years, so it's a great honor for all of us. We're also working on Orion that will take us to Mars one day. These all work together for what we're doing. We're going to have people on Mars at some point and a big reason we have the station is to do the research that's needed to make that possible. So as you can tell, we're getting closer to flights and we'll probably start picking up the pace of our training efforts as we get into the fall.

Williams: Now is a good time to try new things. We're really taking advantage of 22 years of technological advances. That might take us out of our comfort zone, but we shouldn't be afraid of that.

What's the biggest challenge you've seen so far in CCP?

Williams: We think we have an idea of what's going to happen, and we start to talk about it, then other ideas come in and change starts to happen and that's where you see the advances. It's challenging because you have to change your mindset.

Boe: When you make a new thing, it's hard work and it's an honor to be able to see these spacecraft and systems and to see the people who are making these things and making these changes. When you make something new, you have to be able to balance the work to get the right allocation and use.

How much faith do you put in engineers or do you go through all the engineering yourself? How much do you trust companies?

Williams: There is just too much information to go through every single part of a spacecraft or an aircraft for that matter. We got to be involved almost a year ago so we were able to provide a perspective of what was being designed for. We have to trust them. The safety stuff is going to happen, everyone knows that and everyone works toward that. It's our obligation to make the spacecraft as good as it can be for the future crews as well.

Boe: Spaceflight is about people. When you fly airplanes you have to count on those people. The hardware is cool, but it's really the people that make everything work.

What do you think of the changes you are seeing at Kennedy?

Boe: It's amazing to see Pad 39A getting modified by SpaceX. It's hardware and to me it shows that we are moving right along and 2017 and early 2018 are coming up fast. It's really amazing to see an old building like the Commercial Crew and Cargo Processing Facility completely remade on the inside like Boeing did to make it into a Starliner factory.

Williams: To me this is not a museum, it's an active spaceport. This is big hardware that's getting modified and getting built and I just can't help but tell a lot of people to come out here and see what is going on. It's not just where shuttles used to fly, it's where new vehicles are getting ready to fly. It's happening and it's happening quickly.

What would you say to the children who first walk on Mars?

Williams: Don't forget to turn around and look at your home planet.

Boe: Have fun! We see Mars from Earth and it's a little thing and when they are going to look back on Earth from Mars and see the same thing, that's going to be pretty amazing.