Laser Propulsion
Ground-based or space-based laser arrays that push light sails to interstellar speeds. The key insight: leave the energy source at home.
How It Works
- A lightweight sail (meters wide, nanometers thick) is deployed in space
- A massive laser array on Earth or in orbit fires a coherent beam at the sail
- Photon pressure accelerates the sail — no onboard fuel needed
- Acceleration phase lasts minutes to hours; the sail then coasts for years/decades
Key Advantage
Traditional rockets carry fuel, which adds mass, which requires more fuel (the tyranny of the rocket equation — see concept-relativistic-travel). Laser propulsion breaks this cycle entirely. The sail carries zero propellant. All energy comes from the laser array.
This makes 0.2c achievable for gram-scale payloads with known physics.
The mission-breakthrough-starshot Concept
The most developed laser propulsion proposal:
- Laser array: 100 GW, ~1 km2 phased array (Earth-based or orbital)
- Sail: ~4m x 4m, ~1 gram, highly reflective
- Payload: ~1 gram (camera, communication laser, basic instruments)
- Acceleration: ~60,000 g for ~10 minutes
- Terminal velocity: 0.2c (60,000 km/s)
- Time to dest-proxima-centauri: ~21 years
- Cost estimate: $5-10 billion for the array, pennies per sail
Engineering Challenges
| Challenge | Difficulty | Status |
|---|---|---|
| 100 GW coherent laser array | Extreme | No current array exceeds ~MW scale |
| Sail material surviving 60,000 g + intense laser | Very hard | Candidate materials being researched |
| Beam pointing accuracy over millions of km | Very hard | Adaptive optics needed |
| Communication from gram-scale probe at 4+ ly | Hard | Onboard laser communicating back to Earth |
| Surviving interstellar medium at 0.2c | Hard | See concept-interstellar-medium |
| Atmospheric distortion of ground-based laser | Moderate | Space-based array solves this at higher cost |
The Deceleration Problem
The sail cannot slow down at the destination. There’s no laser array at Proxima Centauri to brake against. The probe flies through the target system at 0.2c — it has seconds in the Alpha Centauri system to take measurements and beam them back.
Proposed solutions (all add complexity/mass):
- Magnetic sail braking: Use a magnetic field to drag against the interstellar medium — very slow deceleration
- Photon braking: Reflect starlight from the destination star — only works for the final approach and only decelerates a tiny amount
- Staged sails: A larger sail separates and reflects the laser beam back onto a smaller sail, decelerating it. Robert Forward’s concept — requires extreme precision
For now, Breakthrough Starshot accepts the flyby limitation.
Status: In Development
- Breakthrough Starshot Initiative funded by Yuri Milner ($100M initial, 2016)
- Research ongoing at multiple universities
- No hardware at interstellar scale yet
- Individual components (sails, small lasers) being tested