20 August 2008

"It's always science fiction until someone goes out and does it"

From: http://www.wired.com/science/space/news/2008/08/space_limits

Rocket Scientists Say We'll Never Reach the Stars

Many believe that humanity's destiny lies with the stars. Sadly for us, rocket propulsion experts now say we may never even get out of the Solar System.
At a recent conference, rocket scientists from NASA, the U.S. Air Force and academia doused humanity's interstellar dreams in cold reality. The scientists, presenting at the Joint Propulsion Conference in Hartford, Connecticut, analyzed many of the designs for advanced propulsion that others have proposed for interstellar travel. The calculations show that, even using the most theoretical of technologies, reaching the nearest star in a human lifetime is nearly impossible.
"In those cases, you are talking about a scale of engineering that you can't even imagine," Paulo Lozano, an assistant professor of aeronautics and astronautics at the Massachusetts Institute of Technology and a conference attendee, said in a recent interview.
The major problem is that propulsion -- shooting mass backwards to go forwards -- requires large amounts of both time and fuel. For instance, using the best rocket engines Earth currently has to offer, it would take 50,000 years to travel the 4.3 light years to Alpha Centauri, our solar system's nearest neighbor. Even the most theoretically efficient type of propulsion, an imaginary engine powered by antimatter, would still require decades to reach Alpha Centauri, according to Robert Frisbee, group leader in the Advanced Propulsion Technology Group within NASA's Jet Propulsion Laboratory.
And then there's the issue of fuel. It would take at least the current energy output of the entire world to send a probe to the nearest star, according to Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute. That's a generous figure: More likely, Cassenti says, it would be as much as 100 times that.
"We just can't extract the resources from the Earth," Cassenti said during his presentation. "They just don't exist. We would need to mine the outer planets."
A 160-Million-Ton Needle
Interstellar propulsion systems are not a new idea. Rocket scientists, aeronautical engineers and science-fiction enthusiasts have proposed such designs for several decades.
In 1958, U.S. scientists explored the possibility of a spaceship propelled by dropping nuclear bombs out the back, a so-called nuclear-pulsed rocket. The research, called Project Orion, was killed by the signing of the Nuclear Test Ban Treaty and the budgetary requirements of the Apollo Project.
In 1978, the British Interplanetary Society designed a mission to Barnard's Star, almost 6 light years away, using a pulsed fusion rocket fueled by deuterium. Building such a spaceship would require mining the outer planets for fuel for at least two decades, scientists said at the Joint Propulsion Conference this year.
But the thought experiments continue. At the conference, Frisbee presented a theoretical design for a ship using antimatter to propel its way to nearby stars.
Frisbee's design calls for a long, needle-like spaceship with each component stacked in line to keep radiation from the engines from harming sensitive equipment or people.
At the rocket end, a large superconducting magnet would direct the stream of particles created by annihilating hydrogen and antihydrogen. A regular nozzle could not be used, even if made of exotic materials, because it could not withstand exposure to the high-energy particles, Frisbee said. A heavy shield would protect the rest of the ship from the radiation produced by the reaction.
A large radiator would be placed next in line to dissipate all the heat produced by the engine, followed by the storage compartments for the hydrogen and antihydrogen. Because antihydrogen would be annihilated if it touched the walls of any vessel, Frisbee's design stores the two components as ice at one degree above absolute zero.
The systems needed to run the spacecraft come after the propellant tanks, followed by the payload. In its entirety, the spaceship would resemble a large needle massing 80 million metric tons with another 40 million metric tons each of hydrogen and antihydrogen. In contrast, the Space Shuttle weighs in at a mere 2,000 metric tons.
"Interstellar missions are big," Frisbee said, in part because of the massive amounts of energy (and hence fuel) required to get moving fast enough to make the trip in anything like a reasonable amount of time. "Any time you try to get something up to the speed of light, Newton is still God."
With that fuel, it would still take nearly 40 years to travel the 4.3 light years to Earth's nearest neighbor, Alpha Centuri, he said.
Down and Out On Earth
Even improving humans' access to near space is not easy.
Scientists have all but discarded ideas for rockets that can reach orbit using a single stage. Instead, private space ventures have focused on lightening the payload and rocket and on increasing reliability. If space tourism comes into vogue, then launch providers could benefit from economies of scale.
But alternative-propulsion systems? They are not in short supply in people's imaginations, but most fail the test of reality, Marcus Young, a researcher at the U.S. Air Force Research Lab's Advanced Project Group, told conference attendees. Young and his team surveyed ideas for launch vehicles that could be accomplished in the next 15 to 50 years and found most to be unworkable.
Space elevator? Even if the engineering made sense, the design requires a breakthrough in materials science to create cables long and strong enough. Rail guns? A vehicle would have to shoot down a 100-kilometer track at 50 times the force of gravity to achieve orbit. Nuclear power? Radioactivity would limit its use to outside Earth's atmosphere, and the politics are positively toxic.
"There are a lot of ideas that initially you say, 'Hey, that might work,'" Young said. "But after a little research, you quickly find that it won't."
Yet, just because science fiction is not yet a reality is not a reason to make science suffer, said MIT's Lozano.
"There is a lot of interesting stuff that you cannot do even in the solar system," he said. "We have the technical means to do it. But some of the most sophisticated technologies ... we have not developed. Not because we can't, but because we have not made it a priority."
As for interstellar travel, even the realists are far from giving up. All it takes is one breakthrough to make the calculations work, Frisbee said.
"It's always science fiction until someone goes out and does it," he said.

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