Editing Nuclear pulse propulsion (section)

===Project Orion===
{{Main|Project Orion (nuclear propulsion)}}
[[File:Orion pulse unit.png|thumb|A nuclear pulse propulsion unit. The explosive charge [[radiation implosion|ablatively vaporizes]] the propellant, propelling it away from the charge, and simultaneously creating a plasma out of the propellant. The propellant then goes on to impact the pusher plate at the bottom of the Orion spacecraft, imparting a pulse of 'pushing' energy.]]

Project Orion was the first serious attempt to design a nuclear pulse rocket. A design was formed at [[General Atomics]] during the late 1950s and early 1960s, with the idea of reacting small directional nuclear explosives utilizing a variant of the [[Teller–Ulam]] two-stage bomb design against a large steel pusher plate attached to the spacecraft with shock absorbers. Efficient directional explosives maximized the momentum transfer, leading to [[specific impulse]]s in the range of {{convert|6,000|isp|abbr=on}} seconds, or about thirteen times that of the [[Space Shuttle main engine]]. With refinements a theoretical maximum specific impulse of {{convert|100,000|isp|abbr=on}} (1 MN·s/kg) might be possible. Thrusts were in the millions of [[ton]]s, allowing spacecraft larger than 8{{X10^|6}} tons to be built with 1958 materials.<ref name="GeneralDynamics1964NuclearPulseVehicleCondensed">{{cite web |title=Nuclear Pulse Vehicle Study Condensed Summary Report (General Dynamics Corp.) |date=January 1964 |author=[[General Dynamics]] Corp. |publisher=U.S. Department of Commerce National Technical Information Service |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760065935_1976065935.pdf |access-date=December 24, 2008}}</ref>

The [[reference design]] was to be constructed of steel using submarine-style construction with a crew of more than 200 and a vehicle takeoff weight of several thousand [[ton]]s. This single-stage reference design would reach Mars and return in four weeks from the Earth's surface (compared to 12 months for NASA's current chemically powered reference mission). The same craft could visit Saturn's moons in a seven-month mission (compared to chemically powered missions of about nine years). Notable engineering problems that occurred were related to crew shielding and pusher-plate lifetime.

Although the system appeared to be workable, the project was shut down in 1965, primarily because the [[Partial Test Ban Treaty]] made it illegal; in fact, before the treaty, the US and Soviet Union had already separately detonated a combined number of at least nine nuclear bombs, including thermonuclear, in space, i.e., at altitudes of over 100&nbsp;km (see [[high-altitude nuclear explosion]]s). [[Nuclear ethics|Ethical]] issues complicated the launch of such a vehicle within the Earth's [[magnetosphere]]: calculations using the (disputed) [[linear no-threshold model]] of radiation damage showed that the [[fallout]] from each takeoff would cause the death of approximately 1 to 10 individuals.<ref>{{cite book |last=Dyson |first=George |title=Project Orion: the atomic spaceship, 1957–1965 |date=2003 |publisher=Penguin |isbn=0-14-027732-3 |location=London |oclc=51109229 |url=https://www.worldcat.org/oclc/51109229}}</ref> In a threshold model, such extremely low levels of thinly distributed radiation would have no associated ill-effects, while under [[hormesis]] models, such tiny doses would be negligibly beneficial.<ref>{{cite journal |author=Heyes |display-authors=etal |title=Authors' reply |date=1 October 2006 |journal=British Journal of Radiology |volume=79 |issue=946 |pages=855–857 |doi=10.1259/bjr/52126615 |url=http://bjr.birjournals.org/cgi/content/citation/79/946/855 |access-date=27 March 2008|url-access=subscription }}</ref><ref>{{cite report |author=Aurengo |display-authors=etal |title=Dose-effect relationships and estimation of the carcinogenic effects of low doses of ionizing radiation |publisher=Académie des Sciences & Académie nationale de Médecine |date=30 March 2005 |citeseerx=10.1.1.126.1681 |type=PDF |url=https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.126.1681&rep=rep1&type=pdf |access-date=27 March 2008 |url-status=live |archive-url=https://web.archive.org/web/20220205043705/http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.126.1681&rep=rep1&type=pdf |archive-date=5 February 2022}}</ref> The use of less efficient [[nuclear weapon design#Clean bombs|clean nuclear bombs]] for achieving orbit and then more efficient, higher yield [[dirty bomb#Other uses of the term|dirtier bombs]] for travel would significantly reduce the amount of fallout caused from an Earth-based launch.

{{Anchor|deflect2016-01-30}}One useful mission would be to deflect an asteroid or comet on collision course with the Earth, depicted dramatically in the 1998 film ''[[Deep Impact (film)|Deep Impact]]''. The 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.<ref>{{cite journal |last=Solem |first=J. C. |year=1994 |title=Nuclear explosive propelled interceptor for deflecting objects on collision course with Earth |journal=Journal of Spacecraft and Rockets |volume=31 |issue=4 |pages=707–709 |bibcode=1994JSpRo..31..707S |doi=10.2514/3.26501}}</ref> The prospect of an imminent asteroid impact would obviate concerns over the few predicted deaths from fallout. An automated mission would remove the challenge of designing a shock absorber that would protect the crew.

Orion is one of very few interstellar space drives that could theoretically be constructed with available technology, as discussed in a 1968 paper, "Interstellar Transport" by [[Freeman Dyson]].