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Tom Siegfried

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Visits to Proxima Centauri’s planet are probably millennia away

Even alpha particle propulsion is too slow for trip to new world

illustration of the distance to Proxima Centauri

Proxima Centauri, the nearest member of the Alpha Centauri triple star system, is just a little more than 4 light-years away from the sun. With a proposed propulsion scheme using alpha particle decay, travel time to the star's newly discovered planet would be no shorter than 4,000 years.

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If you’d like to vacation at the newly found planet orbiting Proxima Centauri, you might want to reconsider. It’s nearby astronomically — a mere 4.2 light-years away — but still too far away for any plausible transportation technology to reach within the current millennium.

In fact, it’s a pretty safe bet the Chicago Cubs will win World Series before any human steps foot on Earth’s nearest exoplanetary neighbor (known as Proxima b). Unless P. Centaurian aliens arrive soon with a “To Serve Man” cookbook, your chances of visiting Proxima b before you die are about the same as sainthood for Ted Bundy. By the time anybody from here goes there, years will have five digits.   

It took NASA’s New Horizons probe — the fastest spacecraft humans have ever launched — over nine years just to get to Pluto. At its top speed of 16 kilometers per second, New Horizons would need almost 80,000 years to get to Proxima Centauri.

Solar sail propulsion — in which lightweight craft could be accelerated by pressure from sunlight — would be a little be faster, but not by much, taking (by one estimate) 66,000 years to make the Proxima Centauri run.

Novel propulsion schemes have been proposed that could reduce that time substantially. A sail driven by alpha particle recoil, for instance, provides some serious advantages over solar sails, as Wenwu Zhang and colleagues point out in the August issue of Applied Radiation and Isotopes.

Ordinary rocket speed is limited by how fast the combusted fuel can eject exhaust; NASA has investigated a plasma engine design that can attain exhaust speeds of 50 km/s. But that approach requires huge energy input and high voltage, Zhang and colleagues point out (and so would be prohibitively expensive). Alpha particles emitted by radioactive substances, on the other hand, can speed away about 300 times faster. Therefore, Zhang and coauthors assert, “alpha decay particles … may be a potential solution for long-time acceleration in space.”

Usually, of course, a chunk of radioactive matter would emit alpha particles in all directions. So your craft would need a shield on one side to absorb the particles before they got very far. The rest would stream away in the opposite direction, pushing the craft forward (by virtue of the law of conservation of momentum). True, alpha particles are tiny and the effect of their recoil would be small. But it would add up. Shot into space with standard technology (thereby achieving a 16 km/s start-up speed), an alpha recoil spacecraft could eventually reach a speed in the range of 200–300 km/s or so.

It helps to choose the right alpha-emitting material. Uranium-232 would be ideal. It has a long enough half-life (almost 70 years) to last for an extended voyage, but it also decays into daughter nuclei that emit alpha particles more frequently, boosting the recoil effect. (You won’t find any U-232 in uranium mines, though — it would need to be produced in nuclear transmutation factories.)

Assuming a suitably light and thin absorption material, Zhang and colleagues envision an alpha-powered interstellar sail about 24 meters across. They calculate a travel time to Proxima Centauri between about 4,000 and 9,000 years (depending on the ratio of fuel mass to total spacecraft mass). That would easily win the race against a solar sail, but would far exceed most people’s available vacation time. “Interstellar travel definitely asks for even better propulsion technologies,” Zhang and colleagues understate. And surely within 4,000 years somebody will invent a faster technology that could pass the alpha-decay craft and get to Proxima b first.  

Other people already have ideas, as Science News astronomy writer Christopher Crockett noted in his story on the discovery of Proxima b. Philanthropist Yuri Milner recently announced a research project to explore the prospects of sending numerous nanocraft to Proxima Centauri’s neighborhood — the Alpha Centauri triple star system. (Proxima is the third star, presumably in orbit around Alpha Centauri A and B.) That plan envisions wafers weighing about a gram or so carried along by similar-mass light sails propelled by a powerful laser beam. If current technological dreams come true, tiny cameras and lasers on the wafer could capture and transmit information about Proxima b back to Earth.

Supposedly such nanocraft could reach 20 percent of the speed of light, allowing them to reach Proxima Centauri by maybe 20 years after launch. So there’s an outside chance of getting a message back from Proxima b before the Cubs win a World Series. But there’s no hope of hitching a ride on such a wafer, unless, perhaps, you’re a tardigrade.

Even if some futuristic technology permitted building a real ship, say the size of the space shuttle, that could fly 20 percent of the speed of light, it might not be a good idea. Such a ship could, in the wrong hands, become the most devastating weapon ever imagined. Flying 20 percent of light speed, a space shuttle would possess a kinetic energy roughly the equivalent of 1,000 hydrogen bombs (or millions of Hiroshima-sized bombs). Of course, it would be an expensive ship and probably nobody would want to crash it. Unless the people who took it to Proxima Centauri got really mad at the people back on Earth.

Follow me on Twitter: @tom_siegfried

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