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So NASA and researchers around the world have been investigating advanced propulsion technologies, including space-bending "warp drives," enormous solar sails and matter-antimatter engines. Nuclear fusion is perhaps the most promising of these possibilities, at least in the relatively near term, proponents say.

Fusion rockets would harness the energy released when the nuclei of two or more atoms combine. Our sun and other stars are fusion-powered, converting this energy to light; the same principle also gives hydrogen bombs their immense destructive power.

NASA has funded several early-stage fusion ideas recently via a program called NIAC (NASA Institute for Advanced Concepts). One of these groups, led by scientists at the University of Washington, recently calculated that a fusion rocket could make it possible to get astronauts to Mars in as little as 30 days.

The shape of the bomb's reaction mass is critical to efficiency. The original project designed bombs with a reaction mass made of tungsten. The bomb's geometry and materials focused the X-rays and plasma from the core of nuclear explosive to hit the reaction mass. In effect each bomb would be a nuclear shaped charge. A bomb with a cylinder of reaction mass expands into a flat, disk-shaped wave of plasma when it explodes. A bomb with a disk-shaped reaction mass expands into a far more efficient cigar-shaped wave of plasma debris. The cigar shape focuses much of the plasma to impinge onto the pusher-plate.

In late 1958 to early 1959, it was realized that the smallest practical vehicle would be determined by the smallest achievable bomb yield. The use of 0.03 kt (sea-level yield) bombs would give vehicle mass of 880 tons. However, this was regarded as too small for anything other than an orbital test vehicle and the team soon focused on a 4,000 ton "base design".

The biggest design above is the "super" Orion design; at 8 million tonnes, it could easily be a city.[11] In interviews, the designers contemplated the large ship as a possible interstellar ark. This extreme design could be built with materials and techniques that could be obtained in 1958 or were anticipated to be available shortly after. The practical upper limit is likely to be higher with modern materials. Most of the three thousand tonnes of each of the "super" Orion's propulsion units would be inert material such as polyethylene, or boron salts, used to transmit the force of the propulsion units detonation to the Orion's pusher plate, and absorb neutrons to minimize fallout. One design proposed by Freeman Dyson for the "Super Orion" called for the pusher plate to be composed primarily of uranium or a transuranic element so that upon reaching a nearby star system the plate could be converted to nuclear fuel.

The Orion nuclear pulse rocket design has extremely high performance. Orion nuclear pulse rockets using nuclear fission type pulse units were originally intended for use on interplanetary space flights. Missions that were designed for an Orion vehicle in the original project included single stage (i.e., directly from Earth's surface) to Mars and back, and a trip to one of the moons of Saturn.[11] One possible modern mission for this near-term technology would be to deflect an asteroid that could collide with Earth. The extremely high performance would permit even a late launch to succeed, and the vehicle could effectively transfer a large amount of kinetic energy to the asteroid by simple impact. Also, such an unmanned mission would eliminate the need for shock absorbers, the most problematic issue of the design. Nuclear fission pulse unit powered Orions could provide fast and economical interplanetary transportation with useful human crewed payloads of several thousand tonnes.

Later studies indicate that the top cruise velocity that can theoretically be achieved by a Teller-Ulam thermonuclear unit powered Orion starship, assuming no fuel is saved for slowing back down, is about 8% to 10% of the speed of light (0.08-0.1c).[2] An atomic (fission) Orion can achieve perhaps 3%-5% of the speed of light. A nuclear pulse drive starship powered by Fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the max. speed. The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow the ship to travel near the maximum theoretical velocity.[16]

At 0.1c, Orion thermonuclear starships would require a flight time of at least 44 years to reach Alpha Centauri, not counting time needed to reach that speed (about 36 days at constant acceleration of 1g or 9.8 m/s2). At 0.1c, an Orion starship would require 100 years to travel 10 light years. The astronomer Carl Sagan suggested that this would be an excellent use for current stockpiles of nuclear weapons.[17]

A concept similar to Orion was designed by the British Interplanetary Society (B.I.S.) in the years 1973–1974. Project Daedalus was to be a robotic interstellar probe to Barnard's Star that would travel at 12% of the speed of light. In 1989, a similar concept was studied by the U.S. Navy and NASA in Project Longshot. Both of these concepts require significant advances in fusion technology, and therefore cannot be built at present, unlike Orion. From 1998 to the present, the nuclear engineering department at Pennsylvania State University has been developing two improved versions of project Orion known as Project ICAN and Project AIMStar using compact antimatter catalyzed nuclear pulse propulsion units,[18] rather than the large inertial confinement fusion ignition systems proposed in Project Daedalus and Longshot.[19]

From 1957 until 1964 this information was used to design a spacecraft propulsion system called "Orion", in which nuclear explosives would be thrown behind a pusher-plate mounted on the bottom of a spacecraft and exploded. The shock wave and radiation from the detonation would impact against the underside of the pusher plate, giving it a powerful "kick". The pusher plate would be mounted on large two-stage shock absorbers that would smoothly transmit acceleration to the rest of the spacecraft.