There's no argument in the astronomical community—rocket-propelled spacecraft can take us only so far. The SLS will likely take us to Mars, and future rockets might be able to help us reach even more distant points in the solar system. But Voyager 1 only just left the solar system, and it was launched in 1977. The problem is clear: we cannot reach other stars with rocket fuel. We need something new.

"We will never reach even the nearest stars with our current propulsion technology in even 10 millennium," writes Physics Professor Philip Lubin of the University of California Santa Barbara in a research paper titled A Roadmap to Interstellar Flight. "We have to radically rethink our strategy or give up our dreams of reaching the stars, or wait for technology that does not exist."

Lubin received funding from NASA last year to study the possibility of using photonic laser thrust, a technology that does exist, as a new system to propel spacecraft to relativistic speeds, allowing them to travel farther than ever before. The project is called DEEP IN, or Directed Propulsion for Interstellar Exploration, and the technology could send a 100-kg (220-pound) probe to Mars in just three days, if research models are correct. A much heavier, crewed spacecraft could reach the red planet in a month—about a fifth of the time predicted for the SLS.

Photonic propulsion works by firing lasers at a reflective material like a solar sail. Though the photons in the laser do not have mass, they do have energy and momentum, and they transfer a small amount of kinetic energy to the reflective surface when they bounce off. In the frictionless vacuum of space, the continued acceleration could hypothetically push a spacecraft to around 30 percent of the speed of light, the types of speeds we have achieved in partial accelerators. After launching a spacecraft into orbit with a conventional rocket, the probe could unfurl a light sail to be hit by powerful lasers on Earth.

In a video released this month by NASA 360, Lubin discusses using photonic propulsion technology to travel not only to Mars, but also to other stars such as Alpha Centauri. If the technology is actually "completely scalable" as Lubin claims in the video, then it could be the key to sending spacecraft to distant exoplanets to discover whether they are habitable.

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Lubin's narration in the video was pulled from a longer talk that he gave at a NASA Institute for Advanced Concepts (NIAC) symposium in fall 2015.

Of course, there are a number of hurdles to be cleared before a photonic propulsion system can actually be used on a spacecraft. It's hard to even imagine the kind of infrastructure that would be required to shine a powerful enough laser from Earth to propel a 100 kg spacecraft, but Lubin estimates that a similar amount of power to a Space Shuttle launch, 50 to 100 gigawatts, would be plenty. Slowing the spacecraft down after it reaches relativistic speeds also seems like a major challenge, and avoiding collisions with debris could prove tricky.

It will likely be decades before a photonic propulsion could actually be used for space travel, but the fact that the technology seems to work is exciting enough to start paying attention to it.

h/t Science Alert

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