If you're familiar with the Larry Niven / Jerry Pournelle novel The Mote in God's Eye, get ready for a dose of Déjà vu: a NASA scientist has posted a video on the agency's NASA 360 YouTube channel describing how lasers could send a spacecraft from Earth to Mars in three days.
He's perfectly serious, it seems: instead of having a light-sail driven by the relatively weak pressure the Sun provides, a bunch of Honkingly-large Earth-bound lasers would give a spacecraft a lot more acceleration.
Unlike the much-hyped but scientifically-dubious “EM Drive”, laser propulsion relies on experimentally-verifiable properties of the universe. Last year, the physicist – Philip Lubin of NASA and UCLA Berkeley – published this paper explaining the mechanics of the photonic drive.
The physics of a photonic drive are simple: photons may lack mass but do possess energy and momentum. When photons are reflected off a surface a little of that momentum is transferred to the reflecting object. With a lot of photons, you get an appreciable acceleration. Especially in space, where you don't have an atmosphere to rob the object of its movement.
Lubin says the 50-100 gigawatts that we expend to lift a rocket from Earth into orbit would, if it could be transferred efficiently to a 100 kg satellite, propel the spacecraft to Mars in three days. Instead of accelerating a rocket to mere escape velocity, he calculates, the same energy transfer in the 10 minutes needed to lift the SLS into orbit would get a spacecraft to 30 per cent of the speed of light.
He's currently working on a proof-of-concept using a NASA-funded grant.
While Lubin reckons there's “no known reason why we cannot do this”, the development challenges are formidable. The world has lasers much more powerful than 100 Gigawatts – last year, Osaka University demonstrated a 2 Petawatt laser – but only for a picosecond.
Instead of one huge laser, Lubin proposes a phased array of smaller lasers. Getting into the gigawatt class is still some way away, since at the moment his team is working with kilowatt systems. If deployed successfully, he reckons those would be able to accelerate a small probe with a “modest (meter class)” lightsail to relativistic speeds.
Generating all of the thrust the 100 kg spacecraft needed in ten minutes or so would mean there's no need to try and keep the laser and spacecraft aligned all the way to Mars, but aim and beam diffusion are still tough nuts to crack.
The NASA 360 video is below. ®