Light sails
A sail a few metres across and far thinner than paper is accelerated by a laser array of the order of 100 gigawatts for minutes. Lubin’s roadmap describes the physics. Without fuel on board, the speed is limited by the laser and the sail material rather than the rocket equation. The price is size: the probe weighs about a gram.
Nuclear pulse and fusion
Project Orion (1958–1965) studied ships pushed by nuclear explosions behind a shock-absorbing plate; interstellar versions reached a few percent of light speed. The British Interplanetary Society’s Project Daedalus (1970s) designed a fusion probe reaching about 12% of light speed with a 450-tonne payload. Both rely on technology that does not yet exist in this form, and nuclear explosions in space are banned by the 1963 Partial Test Ban Treaty.
Antimatter
Antimatter releases the most energy per kilogram of any known fuel. The problem is supply: all antiprotons produced by humanity so far amount to far less than a microgram, and storing them for years is unsolved. Antimatter-catalysed fusion tries to use only tiny amounts as a trigger.