Former astronaut Charles Precourt. Image source: NASA.

As vice president and general manager of Orbital ATK's (NYSE:OA) Propulsion Systems business, Charlie Precourt oversees such vital national programs as development of the solid rocket booster that will help send NASA's new Space Launch System (SLS) beyond the Moon in 2018, and to Mars perhaps 15 years later. Precourt is also a four-time veteran of NASA's Space Shuttle (flying once as mission specialist, once as pilot, and twice as mission commander), and member of NASA's Astronaut Hall of Fame. As such, he has singular insight into both where NASA is going and where it's coming from.

Earlier this month, I had an opportunity to chat with Col. Precourt by phone, and pose a few key questions about the cost of NASA's planned mission to Mars -- a cost that some critics have said could reach as high as $500 billion.

What follows are a few excerpts from our discussion.

Rich Smith: Col. Precourt, you served aboard the Space Shuttle. You've seen missions sent up that were fully automated, and others in which astronauts played key roles. How important is it that future NASA missions -- to Mars, to the Moon, to the asteroids -- be manned missions? And how much more does it cost to run a manned mission versus a robotic mission?

Charlie Precourt: Robots and astronauts are complementary. A robot is designed to do a set number of experiments within a budget that will permit this. A human can troubleshoot those missions. For example, a human could perform all the experiments that the Curiosity Rover has done over a term of years in just a few days.

The cost is certainly greater for human missions. Yet the value of human missions is significant in the inspiration it provides to the next generation of scientists and engineers, and the motivation to maintain our world leadership position in space.

Smith: As much as $500 billion? How much will a Mars mission cost, and what's your best guess on how long it will take to get us there?

Precourt: NASA has a top-line budget of approximately $20 billion a year. It budgets approximately $9 billion per year of this for human space flight. Funding for Exploration is about $4 billion, and for the International Space Station, about $5 billion

How long will it take? Mars will get extremely close to Earth in 2033 -- close enough to permit a round-trip mission about 18 months in duration, including time on the Martian surface -- so that's the date we're shooting for.

How much it will cost is harder to say, but if we're going to do it, it has to be done within NASA's budget. Nine billion dollars times 18 years gives you an idea of the ceiling on that spending -- about $160 billion. Much of that money, however, is currently tied up in supporting the International Space Station. More of that $9 billion will be available for spending on Mars after NASA wraps up its work on ISS. Participation with the international partners will be expected, and will add both to the importance of the project and to the ability to execute it affordably. In this sense, the operating model of the ISS will be a good one to continue into exploration beyond Earth orbit.

Smith: What do we need to go to Mars?

Precourt: NASA envisions six main elements as being necessary for a manned Mars mission: SLS to reach space, Orion as the command module, a crew Habitat to live in during transit, a tug to transit from cislunar to Mars orbit (powered by an electric propulsion or ion drive), a lander to reach the Mars surface, and a launcher to return from the Mars surface.

That's a lot to buy. But remember that we would invite partners from around the globe to join us on this mission, and share in the cost. Partners could contribute various elements needed for the mission, for example. America would probably own the majority of the mission, but however much Mars ends up costing, the U.S. would not foot the whole bill.

Smith: If we miss the 2033 window, what does that do to the cost of a Mars mission? Is there a "fixed cost" per year that the effort is ongoing such that delays cost money?

Precourt: Delays do cost money. Costs break down into two main parts -- non-recurring development/engineering costs and construction, and annual operations costs. Every extra year added to the timeline adds annual operations cost to the mission. Additionally, missing the 2033 window would add months to the mission, because Mars would have moved farther away. A longer flight to and from Mars would increase mission cost.

We should also consider that a first Mars landing is a step among others in the Exploration missions we will accomplish beyond Earth orbit, and windows for shorter-length missions will recur over time, providing flexibility in how we execute affordably.

Smith: SLS carries only four astronauts in its Orion vehicle. Is that enough? Are there missions that will require more astronauts? And how would NASA go about building a spacecraft for such missions?

Precourt: The early Exploration missions don't envision larger crews. However, if such a need emerged, you could handle that with a bigger Habitat, not a bigger Orion. We would fly Orion multiple times to a rendezvous point in cislunar space, each time offloading more crew to the Habitat until it is fully crewed up for a particular mission design. Then the tug takes it away for the deep space mission.

Smith: What else does Orbital ATK have in the works related to deep-space exploration?

Precourt: Orbital ATK is developing a space tug of its own to service satellites in Earth orbit. This could have deep-space applications. We are also developing a Habitat concept that leverages our Cygnus cargo spacecraft to enable extended missions in deep space. For example, Cygnus and Orion together might initially sustain a crew of four for up to 60 days in cislunar space, and we'll see if that could then be scaled up to what NASA would need for a crew Habitat on future Mars missions.

And that wraps us up for today, folks. In the course of our talk, we had the opportunity to touch on multiple other matters with Col. Precourt. More of those to follow.

Rich Smith does not own shares of, nor is he short, any company named above. You can find him on Motley Fool CAPS, publicly pontificating under the handle TMFDitty, where he's currently ranked No. 278 out of more than 75,000 rated members.

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