Antimatter engines rely on the extraordinary amount of energy released when antimatter and matter annihilate each other, and two physicists formerly of Fermilab and Los Alamos National Laboratory are planning a Kickstarter to build a prototype antimatter drive. However, all current methods of manufacturing antimatter only produce infinitesimal amounts of it — it could cost $100 billion or more to produce a gram of the stuff.Even if humanity could generate enough antimatter for interstellar travel, it remains unknown whether one could store and manipulate it safely — so far, scientists have managed to contain anti-hydrogen for roughly 16 minutes at most.
Fusion rockets would use nuclear fusion reactors to blast super-heated propellant out the backs of the rockets. However, it may take decades before scientists can build a fusion reactor that puts out more energy than it receives, and then researchers still would have to figure out how to make a fusion reactor small enough to fit on a rocket.
In science fiction, ramscoops, also known as Bussard ramjets, cross interstellar space by using electromagnetic fields to suck fuel up from their surroundings instead of carrying fuel with them. However, Landis noted that interstellar space is not as dense with hydrogen fuel as the designer of the ramscoop thought it was. Moreover, further research suggested that ramscoops had the unfortunate effect of producing more drag than thrust, acting much like parachutes. Furthermore, Landis said it remains uncertain how exactly one might use the ordinary hydrogen found in interstellar space as fusion fuel — proposals for artificial fusion instead generally involve heavy isotopes of hydrogen, such as deuterium.
Nuclear pulse propulsion would hurl small nuclear bombs behind a ship that is shielded with a giant plate. The explosions would push against the plate, and the ship would ride these explosions through space. Famed physicist Freeman Dyson calculated that such engines could reach Alpha Centauri within roughly a century, and Carl Sagan suggested that this would be an excellent use for current stockpiles of nuclear weapons. However, treaties strictly regulating nuclear weapons make these difficult to build.
Generation ships accept the fact that a flight to Alpha Centauri might take millennia and so are giant spacecraft that can house generations of astronauts, but much remains unknown about whether people can face the physical and psychological challenges of spaceflights lasting that long.
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