NASA challenged engineers to design a mission to Mars that would deliver a payload of at least 1,000 kilograms in no more than 45 days, as well as longer trips deep into, and out of, the solar system. The short delivery time is motivated by a desire to ferry shipments and, someday, astronauts to Mars while minimizing their exposure to the damaging effects of galactic cosmic rays and solar storms. McGill's concept, called laser-thermal propulsion, relies on an array of infrared lasers based on Earth, 10 meters in diameter, combining many invisible infrared beams, each with a wavelength of about...