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NAUTILUS EDUCATION | BETA PRODUCT

[Image of a square solar sail spacecraft in orbit above Earth]

a negatively charged grid draws the atoms toward the back of the ship. They overshoot the grid and stream off into space at speeds 10 times faster than chemical rocket exhaust (and 100 times faster than a bullet). For a post-Voyager probe, ion engines would fire for 15 years or so and hurl the craft to several times the Voyagers’ speed, so that it could reach a couple of hundred AU before the people who built it died.

Star flight enthusiasts are also pondering ion drives for a truly interstellar mission, aiming for Alpha Centauri, the nearest star system some 300,000 AU away. Icarus Interstellar, a nonprofit foundation with a mission to achieve interstellar travel by the end of the century, has dreamed up Project Tin Tin—a tiny probe weighing less than 10 kilograms, equipped with a miniaturized high-performance ion drive. The trip would still take tens of thousands of years, but the group sees Tin Tin less as a realistic science mission than as a technology demonstration.

Going Light: Solar Sails
A solar sail, such as the one used by the Japanese IKAROS probe to Venus, does away with propellant and engines altogether. It exploits the physics of light. Like anything else in motion, a light wave has momentum and pushes on whatever surface it strikes. The force is feeble, but becomes noticeable if you have a large enough surface, a low mass, and a lot of time. Sunlight can accelerate a large sheet of lightweight material, such as Kapton, to an impressive speed. To reach the velocity needed to escape the solar system, the craft would first swoop toward the sun, as close as it dared—inside the orbit of Mercury—to fill its sails with lusty sunlight.

Such sail craft could conceivably make the crossing to Alpha Centauri in a thousand years. Sails are limited in speed by how close they can get to the sun, which, in turn, is limited by the sail material’s durability. Gregory Matloff, a City University of New York professor and longtime interstellar travel proponent, says the most promising potential material is graphene—ultrathin layers of carbon graphite.

A laser or microwave beam could provide an even more muscular push. In the mid-1980s, the doyen of interstellar travel, Robert Forward, suggested piggybacking on an idea popular at the time: solar-power satellites, which would collect solar energy in orbit and beam it down to Earth by means of microwaves. Before commencing operation, an orbital power station could pivot and beam its power up rather than down. A 10-gigawatt station could accelerate an ultra-light sail—a mere 16 grams—to one-fifth the speed of light within a week. Two decades later, we’d start seeing live video from Alpha Centauri.

This “Starwisp” scheme has its dubious features—it would require an enormous lens, and the sail is so fragile that the beam would be as likely to fry it as to push it—but it showed that we could reach the stars within a human lifetime.

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