A light sail spacecraft uses light itself as a source of thrust. Photons have no rest mass, but they carry momentum, so reflecting them from a shiny sail gives the sail a tiny push. If the sail is extremely light and the laser power is enormous, that tiny push can build up to a very high speed.
Concepts such as Breakthrough Starshot explore whether gram-scale probes could be sent toward nearby stars within a human lifetime.
In a laser-pushed sail mission, a phased array of lasers on Earth or in space aims a narrow beam at a reflective sail attached to a small probe. The sail must stay stable in the beam, reflect most of the light, and survive intense heating. After the laser acceleration phase, the probe coasts through interstellar space at a large fraction of the speed of light.
On arrival near a target system, it could take images and measurements, then send data back across many light-years.
Key Facts
- Photon momentum is p = E/c for a photon with energy E.
- For a perfectly absorbing sail, radiation pressure is P_rad = I/c, where I is light intensity.
- For a perfectly reflecting sail, radiation pressure is P_rad = 2I/c.
- Force from a reflected laser beam is approximately F = 2P/c, where P is laser power.
- Acceleration is a = F/m, so low mass is essential for high acceleration.
- Travel time at constant coasting speed is t = d/v, so a probe at 0.2c takes about 21 years to cross 4.24 light-years.
Vocabulary
- Light sail
- A light sail is a thin reflective surface that gains momentum from photons striking or reflecting from it.
- Photon
- A photon is a particle of light that carries energy and momentum.
- Radiation pressure
- Radiation pressure is the force per unit area exerted by light on a surface.
- Phased laser array
- A phased laser array is a group of lasers whose beams are timed and aligned to act like one powerful, steerable beam.
- Interstellar probe
- An interstellar probe is a spacecraft designed to travel between stars and collect data beyond the Solar System.
Common Mistakes to Avoid
- Thinking light cannot push objects because it has no rest mass is wrong because photons carry momentum and can transfer it to a sail.
- Using F = P/c for a perfectly reflecting sail is wrong because reflection reverses photon momentum and gives about twice the force, F = 2P/c.
- Ignoring spacecraft mass is wrong because the same laser force gives much larger acceleration to a gram-scale probe than to a heavy spacecraft.
- Assuming the laser pushes the probe all the way to the star is wrong because practical beam spreading and pointing limits mean acceleration happens near the launch system, followed by a long coast.
Practice Questions
- 1 A 100 GW laser beam reflects from a light sail. Using F = 2P/c with c = 3.0 x 10^8 m/s, what force acts on the sail?
- 2 A 2.0 g probe experiences a force of 600 N during laser acceleration. What is its acceleration in m/s^2, and how many g's is this if 1 g = 9.8 m/s^2?
- 3 Explain why a light sail must be both highly reflective and extremely low in mass to make laser-pushed interstellar travel practical.