To accelerate a standard chemical rocket to 15 percent the speed of light, you would need an amount of fuel exceeding the mass of the observable universe.
Yet in the 1970s, a group of British scientists and engineers designed a spacecraft intended to reach Barnard's Star, 5.9 light-years away, within a single human lifetime. To make the interstellar math work, they had to design a theoretical engine capable of reaching exactly 12 percent the speed of light using nuclear fusion.
Reaching 10 to 15 percent the speed of light—roughly 30,000 to 45,000 kilometers per second—is completely impossible with standard chemical rockets. Nuclear propulsion is the only understood physical mechanism capable of crossing this threshold without relying on highly theoretical concepts like large-scale antimatter production.
However, not all nuclear propulsion is created equal:
- Nuclear Thermal Fission: Standard nuclear rockets use a fission reactor to heat a propellant like liquid hydrogen. They are highly efficient for moving around the solar system but max out far below 1 percent of light speed.
- Fission Pulse Propulsion: The Cold War-era Project Orion proposed dropping small nuclear bombs behind a pusher plate to ride the shockwaves. While a massive leap in power, theoretical models suggest standard fission pulse propulsion tops out at around 3 to 5 percent the speed of light. Beyond that speed, the sheer mass of the required nuclear bombs makes the ship too heavy to accelerate further.
- Nuclear Fusion: To hit the 10 to 15 percent mark, aerospace engineers look to nuclear fusion. The British Interplanetary Society's Project Daedalus proposed an inertial confinement fusion engine. The design involved injecting pellets of deuterium and helium-3 into a reaction chamber and compressing them with powerful electron beams. The resulting fusion explosions, occurring 250 times per second, would be directed out the back by a massive magnetic nozzle.
Because fusion reactions convert a much larger fraction of their mass directly into kinetic energy compared to fission, the exhaust velocity of a fusion drive is extraordinarily high. This makes the 10 to 15 percent target physically possible.
The primary barriers are engineering and economics, rather than the strict laws of physics. A Daedalus-style probe would weigh approximately 54,000 tons, with 50,000 tons of that being fusion fuel. Gathering the necessary helium-3 would likely require deploying floating atmospheric refineries on gas giants like Jupiter. While currently out of reach, a ship powered by a continuous stream of miniature star-like explosions remains the most credible way to eventually cross the interstellar void.