"Most people don't realize how ambitious interstellar flight is," said Raymond Halyard, an engineer with United Space Alliance in Houston, Texas. The 4.3 light-years to Alpha Centauri equate to approximately 25 trillion miles. That's about 100 million times the quarter million miles to the Moon," he noted.

"I've worked on Inertial Confinement Fusion propulsion designs for years," Halyard said. The best Alpha Centauri flyby design has a terminal velocity of 1/10^th the speed of light and a flight time of 50 years. The cost of the effort would be approximately $100 billion, roughly the price tag of the Apollo program corrected for inflation, he said.

"There's the rub. The experimental work on the various breakthrough physics propulsion concepts remains to be funded, let alone accomplished," Halyard said.

"But like the Wright Brothers" Halyard added, "there are several people working on designs in their garages with shoestring budgets and somebody may hit paydirt!"

Although Halyard doesn't personally gauge any breakthrough in propulsion physics as probable, he does see it as certainly possible. "Since this would be like winning the lottery, work in this area is certainly worth the pittance it will cost to investigate," he said.

Is there a way to get past the dead-end rocket technology of today?

Given time and hard work, propellantless space travel, or even more exotic propulsion systems might open up the Universe to human exploration, said Paul March of Lockheed Martin Space Operations in Houston, Texas.

March said that he personally doesn't view human interstellar sojourns as possible using the current stable of chemically fueled rockets. Without the use of antimatter energized vehicles, reasonable trip times to even the closest stars for a crewed vehicle is not feasible, he said.

"Conventional chemical or fission/fusion rockets just won't cut it for crewed interstellar flights! And even antimatter-powered rockets will only be able to take us to the nearest stars, just like our current chemical based rockets can just barely get us to Mars," March said, noting that his views are his own and do not reflect the official positions of his employer.

Given sufficient resources, March said, nuclear rocketry could open up exploration of our own Solar System within 30 years. "But interstellar flight would be a tough sell."

What's needed is rocketry that can recycle its propellant, commonly referred to as propellantless or field-propulsion. "This little trick requires a new understanding of physics that goes beyond standard Newtonian physics," March said. "This new physics requires a new understanding of the Cosmos that will allow us to manipulate spacetime in such a way that field propulsion becomes possible and Star Trek-like 'warp bubbles' become doable."

Most physicists go screaming in the other direction when faced with this problem, March observed. Nevertheless, there are those dabbling in such arenas, "where these types of propulsion concepts won't be science fiction for much longer."

"If we are lucky…very, very lucky, we might have a solution to the propellantless propulsion side of this interstellar propulsion problem within the next ten years," March said.

Astronomer and author, Gregory Matloff, said he suspects that the first human starship will be a private venture. He points to the work of Team Encounter in Houston, Texas now developing a solar sail spacecraft that will carry human genetic material.

Team Encounter's premiere solar sail mission, Humanity’s First Starship, is scheduled for launch in 2007 and is designed to leave the Solar System. Team Encounter’s precursor mission, Flight One, will launch in 2005 on a journey in near Earth space.

"NASA could launch an interstellar sail with a science payload to explore the heliopause before 2020," Matloff told SPACE.com . The heliopause is the boundary that separates Earth's solar system from interstellar space. "This will be faster than the Team encounter effort and could reach the nearest star in 7,000 years or so," he said.

Matloff is an Assistant Professor of Astronomy and Physics at the Department of Biological and Physical Sciences, New York City College of Technology, CUNY. He also consults on solar sail research for the In-space-propulsion team within the Advanced Space Transportation Directorate at NASA's Marshall Space Flight Center in Huntsville, Alabama.

Matloff said that before the beginning of the 22nd century, both nuclear and solar drives should be approaching their interstellar potential. If a craft was launched then using such propulsion, it could reach Centauri within about 1,000 years, he said.

"New forms of interstellar propulsion will hopefully reduce this travel time. Right now we can realistically talk about interstellar transit times approximating the lifetime of a civilization. But it would be very nice if we could reduce this to a human lifetime or less. Even though robots wouldn't necessarily care if their flight time were long, the people back home waiting for data might lose interest. And humans aboard a generation ship might arrive having forgotten their goal," Matloff said.

If we search for and do not discover Earthlike worlds, the goal of interstellar expansion will not be advanced, Matloff reasoned.

Why fly for decades or centuries to colonize a comet, asteroid, or Mars-like world circling Alpha

Centauri A when we can travel for a few years at much lower cost to reach a similar world in our own solar system?

"But if we search for and find Earths on our interstellar doorstep, watch out! Even if we must reach them by worldships, that's what we'll do. And the hunt for a true star drive will be well funded," Matloff predicted.

But advanced space systems designer, Jordin Kare, said that taking a "robots first" approach may be the correct plan of action.

"I suspect the only way we're likely to send humans to the stars is if robots go first and find a planet we can live on," Kare said. "At that point, some group may decide to take the chance and set out on a multi-century trip so their descendants can have their very own world. If we already have people living in space colonies, or even space hotels, with closed ecologies, it's not that big a step to add a nuclear power plant and ion drives…and head for the stars.

Of course, there's another way to send people to the stars, Kare said, not by sending them physically but by sending them as information on a laser beam.

The first stage is downloading your brain into a computer, then have a copy emailed to Alpha Centauri, where pre-positioned nanotech robots build you a new body. "You wouldn't even notice the 4.3 years you spend in transit. Of course, if the email bounces, you're in trouble," Kare cautioned.