On Tuesday, theoretical physicist Stephen Hawking and Russian billionaire Yuri Milner announced a plan to send interstellar probes to the Alpha Centauri star system. The audacious project would use a giant laser on Earth to accelerate scores of postage-stamp-size spacecraft to nearly the speed of light. They would cross the void in just 20 years — virtually no time on the scale of interstellar travel.The plan for "Breakthrough Starshot" laid out at the news conference looks both ambitious and exciting. But if it's really going to work, there are several down-to-earth problems these would-be star trekkers will have to overcome.Do You Have A Permit For That Planetary Laser?Starshot's tiny probes are supposed to be given a big push by a mammoth 100 gigawatt laser back on Earth.A laser that powerful could also fry anything in its path, including orbiting satellites. "If somebody says, 'Hey look, I'm concerned about you hitting a potential satellite,' " says Pete Worden, the project's
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