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Time to Check Your Stocks?
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Travelers
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<bold><fontfamily><param>arial</param><smaller>P A G E 2
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Nuclear fusion, which happens in the hearts of stars like our sun, requires tremendous pressures and temperatures. "Low" temperature fusion starts at 1.5 million degrees Celsius. The antimatter plasma gun is intended to create temperatures hot enough to trigger fusion. To date, experiments with fusion have managed to maintain the reaction for only a few seconds before it burns out.=20
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That's where the third part of Penn State's efforts, developing engine concepts, comes in. Smith's Aimstar engine concept offers a way to maintain the fusion reaction. It starts with a small amount of antimatter, which comes into contact with lead or uranium, causing it to fission. The energies released by the fission help drive the fusion reaction of deuterium, or "heavy" hydrogen, and a form of helium. The high temperatures create a plasma of negatively charged electrons and the positive nuclei of atoms. Magnetic nozzles direct the charged particles for propulsion.=20
But with such hot temperatures, wouldn't the nozzles be destroyed? "I'm working on that," promises Smith.=20
Another Smith concept gets around the problem of melting by, in effect, melting away only a little at a time. "We're working our heads off to move forward as fast as we can," he says. "Because if there's a technology that's capable of going to the stars, it's antimatter."=20
Another fusion approach, called the Bussard Ramjet, attempts to solve the weight problem by not taking much fuel at all. Instead, the concept calls for a gigantic magnetic field "scoop" that would harvest what it needs from interstellar space. The scoop is, of course, far bigger than any that has ever been generated on Earth =97 about the distance from Earth one-third the way to the moon, at a strength hundreds of times what is possible in labs today. Leifer of JPL describes the system: "You need to pick up your fuel, compress it and heat it, and expel it out the back without slowing down the spacecraft," she says, "and we don't know how to do that either."=20
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Solar sails may be initially more promising than antimatter or fusion, says Frisbee. "In terms of a closest and cleanest development program, solar sails may be the first step." The idea was first conceived decades ago by Robert Forward, a retired Hughes physicist who now consults for NASA; heads Forward Unlimited, a Clinton, Washington, company that is developing space tethers; and writes science fiction novels.=20
In Forward's basic concept, a laser beam would "push" a solar sail. That is, particles of light, called photons, have momentum that is transferred to the sail. The idea requires a very large transmitter lens, a sail some tens of kilometers in diameter to travel 4.5 light-years, and power generation in gigawatts. A multistage mission would enable the sail to slow down when it reaches its target.=20
Henry Harris, a JPL physicist, is another proponent of light sails. "You need a large amount of power for a long time, and that's no problem when you're getting the energy from the sun," he says. In a report to NASA this spring, he and his team outlined a multifunction mission with solar sails. Multiple lasers would be built and placed in orbit around the sun; they would be phase-locked to create a powerful beam that could alternatively be used for pushing a spacecraft and imaging the target star system. The laser beam might also be used to remove Earth-orbiting space junk, provide power for our planet, and maybe even destroy threatening comets and asteroids. "The ultimate instrument for interstellar travel will not be merely for travel," says Harris.=20
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While military research into space-based lasers has laid some of the groundwork for accurate pointing of the devices over great distances, a solar sail's beam requirements would have to be many times more stringent. Steering is also an issue. And to save weight, the sail might be only a few atoms thick. Furthermore, current models of photon propulsion need to be confirmed with actual experiments. So Harris is calling for tests this year of thin sail materials that would be zapped with light photons by an instrument that has been nicknamed a Photon Flinger.=20
Researchers like Harold Puthoff of the Institute for Advanced Studies at Austin in Texas have another idea concerning propulsion: They propose to "engineer" the vacuum of space itself. The concept is based on the prediction of quantum physics that empty space contains an enormous residual background energy known as zero-point energy. Tiny particles flit in and out of existence in the vacuum, perhaps causing inertia. The ability to control inertia, or "warp" space, the theory goes, could help solve problems with interstellar flight. How to do that remains an open question.=20
While such exotic technologies are decades away from implementation, NASA is also working on interstellar science that can be done near term. In March, a team of scientists met at JPL to discuss options for a mission past our solar system's heliosphere. That is the point at which the stream of charged particles from our sun, called the solar wind, is overwhelmed by the interstellar medium. The team would like to see the mission launch within the next 10 years.=20
What would such a mission do? Lots of science, says Richard Mewaldt of
the California Institute of Technology in Pasadena, California, who
chairs the science and technology definition team. He describes the
heliosphere as a bubble, 25 billion miles in diameter, that has to be
pierced.=20
"To measure the properties of the interstellar medium directly, you have
to break through that bubble," says Mewaldt. One instrument might detect
low-energy cosmic rays, which cannot penetrate the heliosphere but which
are important for astronomers to understand galactic dynamics.=20
Comparing the composition of the solar system and the interstellar region
just beyond it could also provide clues about the evolution of our
galaxy. Astronomers would also welcome a survey of the Kuiper Belt, where
perhaps 100 small bodies from tens to hundreds of kilometers in size have
been found, many by the Keck telescope in Hawaii. Farther out is the Oort
Cloud, likely the source of long-period comets like Halley. Another
mission could place a telescope outside the heliosphere to measure star
positions more accurately, by comparing them with Earth measurements.=20
And indeed, NASA will eventually go to the stars for no less reason than
because it must, says Gerald Smith of Penn State. "Clearly, NASA's future
is to go to the boundaries of the solar system and beyond. Whether it's
solar sails, antimatter, fission =97 they have to be dealt with. We can
talk about <italic>Star Wars</italic> all we want, but if we don't have
the engines it'll never happen. It'll just be entertainment."=20
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