When astronomer Johannes Kepler noticed the tail of a comet bending away from the sun like a willow bending in a strong breeze, he saw a force to power starships.
"Ships and sails proper for heavenly breezes should be fashioned," he wrote, making perhaps the first recorded insight on solar wind, a stream of radiation and ionized particles emanating from the sun.
That was 1609.
Nearly four centuries later, NASA and university scientists are trying to turn Kepler's vision into a real means of interplanetary propulsion. These massive, ultralight solar sails could harness the power of innumerable photons zooming away from the sun to propel ships to other planets and, perhaps, to other stars light-years away.
Solar sails represent just one of a handful of new and exotic strategies that may revolutionize space exploration in coming decades. With humans hitting the outer performance limits of traditional chemical explosive rockets, these new methods of space travel will prove critical in future excursions in the solar system and beyond.
The first step, say many experts, is getting a man to Mars.
"The reason why it is important that we go to Mars is the same reason that it was important why people leave the Kenyan Rift Valley. If they had not done so this global civilization would not exist," says Robert Zubrin, president of the Mars Society and a prominent astronautical designer. "If we do go to Mars, we will become a space-faring species.... We will colonize the asteroids and set forth to the stars."
The possibility of humans surmounting gravity and exiting the atmosphere was remote until the 20th century, when scientists first became proficient at controlling the explosive power of burning chemicals to fire projectiles into the heavens.
The landing of Apollo 11 on the moon in 1969 proved that humans could send astronauts outside the atmosphere to another body and return them home to Earth. But the moon is a relatively easy leap.
Because the moon is fairly close to Earth, the propulsion power needed to go there and back is easy to load on a relatively crude space vehicle. A voyage to Mars, by contrast, would span several years with current rocket devices. Farther voyages into the outer planets or to other stars are beyond the reach of rockets or the space shuttle.
"The main parameter you need to improve is the exhaust velocity," says Edgar Choueiri, an aerospace engineer at Princeton University in New Jersey. "The faster you throw stuff out of the rocket, the faster the spacecraft can travel."
Blowing more exhaust out of rockets at higher speeds is no easy task. The system would have to endure much higher temperatures without melting. And today's fuel sources do not provide enough energy.
So scientists have sought other ways to power starships. Some of the concepts that have emerged stretch the imagination.
For one, controlling the fusion of light elements such as hydrogen could, theoretically, drive starships at velocities very close to the speed of light. Controlling thermonuclear explosions might also work. But, despite extensive research, controlled fusion remains elusive, and controlled thermonuclear explosions seem to be an oxymoron.
Thus, the most promising schemes to date involve exotic efforts to harness power from the sun.
Aside from solar sails, other concepts include creating a magnetic bubble filled with plasma that will use charged particles coming from the sun in a similar fashion to the reflective solar sails. It would also provide some radiation shielding for the astronauts. And one spacecraft, NASA's Deep Space 1 probe, uses ion engines fueled by supercharged matter.
NASA is funding research into many of these areas, but the money doesn't compare with the moon effort and other concerted national efforts to achieve space goals. The technology is there, says Dr. Zubrin, but the government needs to take the lead in creating these chariots to the stars.
Beyond voyages to Mars and other planets lies the distant promise of voyages to nearby stars. Unless the brick wall of light speed laid out by Albert Einstein can somehow be toppled, these voyages would require decades to attain - even to the closest stars. The closest star with planets sits 44 light years away.
This means the first astronauts to get there would likely be in their 50s and would spend most of their lives in transit.
"They are not going to travel like Captain Kirk, 'See you on Alpha Centauri on Tuesday,' " says Zubrin of the Mars Society. "It doesn't work like that."
(c) Copyright 1999. The Christian Science Publishing Society