Interstellar travel, a timeworn staple of science fiction, can already be science fact if one has cash to spare. For just $100 million or so, a customer could actually purchase a top-of-the-line commercial rocket and ride right out of the solar system. But patience would be key. If launched tomorrow toward the nearest port of call—Proxima b, a potentially habitable Earth-mass planet recently discovered in the triple star system of Alpha Centauri about four light-years away—that rocket would take 80,000 years to arrive.
Instead of spending $100 million on a slow boat to the stars, in April of last year the billionaire entrepreneur Yuri Milner announced he would use that same sum to forge a new path to Alpha Centauri within a human lifetime. Called Breakthrough Starshot, the initiative calls for largely abandoning rockets in favor of “light sails”—gossamer-thin reflective sheets that, once unfolded in space, could be propelled to very high speeds by laser beams. Starshot’s tentative plans involve using conventional rockets to place thousands of one-gram, four-meter-wide light sails in Earth orbit as early as the 2040s. Each sail would be embedded with a one-centimeter-wide chip containing cameras, sensors, thrusters and a battery. From Earth orbit, each featherweight spacecraft would be boosted toward Alpha Centauri at 20 percent light-speed by a minutes-long pulse from a ground-based, 100-gigawatt laser array. The interstellar crossing would take just a little over 20 years, so the probes could reach Alpha Centauri in the 2060s.
But such high speeds come at a high price. Even the most conservative cost estimates for Starshot far exceed Milner’s initial $100-million investment—the multi-decadal project could easily consume $10 billion, and perhaps much more, largely due to the enormous expense of building the ground-based laser array. Government assistance and international collaboration would likely be required. Moreover, the light sails that survive the 20-year voyage would pass through the Centauri system in a flash, moving so fast they would have only seconds to capture high-quality close-up images and other data from Proxima b and any neighboring planets that may be there. As they fall deeper into the dark between the stars, the light sails would attempt to transmit their precious findings back to Earth using laser beams no more powerful than the signal from a typical cell phone.
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