JUDY WOODRUFF: For the first time in more than four decades, NASA is set to launch a space capsule tomorrow that has grander plans of human exploration into deep space. This time, the Orion spacecraft will be unmanned. But it is an important test flight, and the first of many, as NASA tries to chart a longer-term vision for human flight.

Science correspondent Miles O'Brien has our report.

MILES O'BRIEN: At a 50-year-old facility that tested the heat shields for NASA's Mercury and Apollo capsules, engineers are working on ways to protect a crew of astronauts returning to Earth from a voyage to Mars.

JEREMY VANDER KAM, NASA Engineer: It's really a whole new ball game in terms of the mission requirements and what we have done before.

MILES O'BRIEN: Aerospace engineer Jeremy Vander Kam is working on the thermal protection system for NASA's Orion spacecraft, a capsule that has been described as Apollo on steroids.

JEREMY VANDER KAM: So what we use is a material called Avcoat. It's actually a derivation of the same material used in the Apollo program for the Apollo heat shield. So, on the Orion heat shield, there's over 300,000 of these individual cells that are all filled by hand.

MILES O'BRIEN: Vander Kam is using the venerable Art Jet facility at NASA's Ames Research Center to torch small samples at the heat shield with blistering hot gases moving at hypersonic speeds in a vacuum. It's as close to a real reentry from space as you can get on the ground.

JEREMY VANDER KAM: Well, this is a four-inch diameter puck. And the Orion capsule is five meters in diameter. So, we obviously have some challenges in scale. And so we really rely at the end of the day on a flight test like — if you want to tell us how those parts of the system are going to work.

MILES O'BRIEN: EFT1, or Exploration Flight Test 1, will subject an uncrewed Orion capsule to a real-world trial by fire on its maiden voyage, giving Vander Kam the data he needs and NASA a big milestone.

Bill Hill is a NASA associate administrator.

BILL HILL, NASA: EFT1 is absolutely the biggest thing that this agency is going to do this year.

MILES O'BRIEN: Orion is slated to orbit the Earth twice, once at an altitude of about 500 miles. And then it will get a lift from a second-stage booster to 3,600 miles, high enough for the capsule to be exposed to a big dose of space radiation and to create enough speed on reentry to generate 80 percent of the heat it would encounter on a return from the moon.

BILL HILL: This is really our first step in our journey to Mars.

MILES O'BRIEN: NASA envisions a human presence on Mars in the mid-2030s.

Charlie Bolden is the agency's administrator.

CHARLES BOLDEN, NASA: I use the term pioneer instead of explore. Exploring implies we're going to go out and come back, like Lewis and Clark. We're intending to pioneer Mars, which means we are going to put people on that planet to be there permanently.

MILES O'BRIEN: But NASA is a long way from that.

WILLIAM GERSTENMAIER, NASA: If you ask us to go to Mars today, we don't think we're in the right risk posture.

MILES O'BRIEN: Bill Gerstenmaier is the man in charge of human space exploration at NASA. The current plan calls for an unpiloted Orion capsule to orbit the moon in 2018 and in 2021 or 2022 carry two astronauts on a short visit to small asteroid or a piece of a larger one that would be robotically grabbed and nudged into lunar orbit. There are no firm plans for what happened after that.

Engineers are dealing with some big technical hurdles, how to protect the crew from radiation, how to land something much bigger than a compact-car-sized rover on Mars, and how humans can safely operate independent of support from Earth.

WILLIAM GERSTENMAIER: The basic strategy that we're trying to do is, we do a series of test, each one of more and more complexity and more and more challenge, that we continue to add, until eventually we build the capabilities and the skills and the operational techniques and the risk management philosophy that allows us to go to Mars.

MILES O'BRIEN: There are many seasoned hands in the space world who wonder if the agency's big plans to visit the Red Planet may become lost in space.

Tom Young is among them.

THOMAS YOUNG, Former NASA Executive: And I think a fundamental problem that we have with today's Mars strategy is that it's not consistent with the available budget, and we don't have the funds to really make it an executable plan.

MILES O'BRIEN: Young is a veteran aerospace executive who knows a little something about getting to the Red Planet.

A. THOMAS YOUNG: I'm assuming we must be sitting right on the X. So that's the smooth area. So, everybody just did fabulous. And couldn't be more pleased. Thank you.

MILES O'BRIEN: He was NASA's program manager for the Viking missions in the mid-1970s, which accomplished the first and second successful landings on Mars.

A. THOMAS YOUNG: We have either got to augment the resources to make the goal achievable, or we have got to adjust the goal to be something that's consistent with the available resources, because, if we don't, what we're funding is going to do is, we're going to — we're going to waste a lot of money.

Ignition and liftoff of Ares I-X.

MILES O'BRIEN: In 2010, President Obama canceled the Bush administration's Constellation program, which envisioned a return to the moon.

PRESIDENT BARACK OBAMA: I understand that some believe that we should attempt a return to the surface of the moon first, as previously planned. But I just have to say, pretty bluntly here, we have been there before.

MILES O'BRIEN: Obama wanted NASA to use the money Constellation would have spent on a capsule and rocket made of Apollo and shuttle legacy hardware to push the development of new propulsion technology and seed the private sector to build new vehicles.

But the cancellation of Constellation ruffled the feathers of some heavyweights in the aerospace world and on Capitol Hill. Senator Richard Shelby of Alabama represents NASA's primary rocket-building facility, Huntsville's Marshall Space Flight Center. He helped force the administration to spend less on new technology and instead design a rocket called the Space Launch System, or SLS.

Led by Shelby's constituents in Huntsville, SLS is being built with beefed-up, shuttle-style solid boosters and surplus shuttle main engines.

SEN. RICHARD SHELBY, (R) Alabama: These were very successful engines. And we build on what we have. That's why we learn. And this is tomorrow's technology. We will learn from this. And there will be other things that will come out of it that will be positive. But you just don't reinvent the wheel. You build on the wheel. And these were good wheels.

MILES O'BRIEN: But, as it is, SLS doesn't have enough thrust to go any farther than lunar orbit. That's what prompted the idea of bringing an asteroid to the moon. Otherwise, SLS is a rocket without a destination.

For a mission to Mars, it will need a redesign with more powerful boosters and a new second-stage motor.

Former NASA deputy administrator Lori Garver was one of the leading proponents of the original Obama space plan.

LORI GARVER, Former Deputy Administrator, NASA: If you were driving to Mars in particular, there's a number of things you would be doing that we're not doing now that are the difficult things. You wouldn't be building a spacecraft now based on technology from 40 years ago, engines from 40 years ago to go somewhere in 20 years, and spending $3 billion to $4 billion a year on that.

MILES O'BRIEN: Garver believes NASA's current path to Mars is a hybrid of ideas born not of engineering elegance, but, rather, political compromise.

LORI GARVER: The purpose has become political and jobs. And I think we're — we have lost the sort of unifying view that exploration is something that we do as a species.

We should have that broader purpose, rather than just the political needs of a few members of Congress with jobs in their district.

MILES O'BRIEN: Senator Shelby rejects the notion that space exploration has become a jobs program.

SEN. RICHARD SHELBY: Well, I would be against a jobs program. I'm for the cutting edge of space. Jobs come with it if you have got a good system that you're building. And I believe this will be a good system and be good for the space program. Otherwise, I wouldn't support it.

MILES O'BRIEN: But will a half-a-loaf with a side of bacon ever get NASA to Mars? Is there enough money in NASA's budget to pay for the compromise and still reach the stars?

John Holdren is President Obama's science adviser.

JOHN HOLDREN, Director, Office of Science and Technology Policy: So, I don't think the current budgets amount to kicking the can down the road. They amount to, within reasonable limits, getting done the steps that we need to achieve in order ultimately to get to Mars.

Eventually, yes, between now and the 2030s, we would need to ramp up the budget. At the current budgets, we would not get to Mars; that's correct.

MILES O'BRIEN: During Orion's first flight, NASA engineers aim to test the riskiest events, things that have to work right the first time when astronauts are on board. But the biggest risk to the overarching goal may have more to do with political science than rocket science.

Miles O'Brien, the "PBS NewsHour," Washington.

GWEN IFILL: Online, we have a slide show of Orion's journey to the launchpad. And you can watch the launch live tomorrow morning at 7:00 a.m. Eastern on our home page.