| Published: November 12, 2009 – 10:29 am
Among other things, I’m the vice president of a worldwide, medium-sized organization called The Planetary Society. We are launching a series of spacecraft that will be driven through space, not by rocket fuel, but by the pressure of sunlight.
It is surprising at first, at least for most everyone I’ve ever met, that light has momentum. Light has no mass, y’know. Yet it can ever-so-slightly push things.
You probably know or accept that E = mc2. This is to say that if you could convert all the mass in a bit of matter, say the hydrogen in a glowing star, into pure energy, you’d get an enormous amount of it. A kilogram of mass times 300,000 kilometers per second (186,000 miles per hour, as was said back in the 20th Century) is an enormous number of sustained Watts (Watt-seconds or Joules, okay, or foot-pounds).
Well, if you mess around with this equation and insert the idea that moving energy kinetic energy would carry with it a mass and a speed-squared, you can convince yourself that light has momentum. The theoretical limit being established by a physical measurement called Plank’s Constant.
So, we at the Planetary Society are building three very, very low mass sails, solar sails spacecraft. Our solar sail will be pushed through space by the momentum inherent in photons, particles of light, in this case those beaming from our Sun. A photon carries a quantum of energy, the smallest packet of energy we can measure. But with quadrillions of them impacting the sail at every moment, we’ll get a tiny shove. Since the light beams continuously, we hope to build up speed. By the time we build the third sail we hope to have gotten a nice orbit going.
It’s going to be fantastic the first new propulsion system since, well, since the Greek mathematician Heron built his steam jet reaction spinner engine.
We can sail by starlight. How cool is that?