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In engineering, fiction, and thought experiments, unobtainium is any fictional, extremely rare, costly, or impossible material, or (less commonly) device needed to fulfill a given design for a given application. The properties of any particular unobtainium depend on the intended use. For example, a pulley made of unobtainium might be massless and frictionless; however, if used in a nuclear rocket, unobtainium would be light, strong at high temperatures, and resistant to radiation damage. The concept of unobtainium is often applied flippantly or humorously.
Since the late 1950s, aerospace engineers have used the term "unobtainium" when referring to unusual or costly materials, or when theoretically considering a material perfect for their needs in all respects, except that it does not exist. By the 1990s, the term was in wide use, even in formal engineering papers such as "Towards unobtainium [new composite materials for space applications]." The word unobtainium may well have been coined in the aerospace industry to refer to materials capable of withstanding the extreme temperatures expected in reentry. Aerospace engineers are frequently tempted to design aircraft which require parts with strength or resilience beyond that of currently available materials.
Later, unobtainium became an engineering term for practical materials that really exist, but are difficult to get. For example, during the development of the SR-71 Blackbird spy plane, Lockheed engineers at the "Skunk Works" under Clarence "Kelly" Johnson used unobtainium as a dysphemism for titanium. Titanium allowed a higher strength-to-weight ratio at the high temperatures the Blackbird would reach, but the Soviet Union controlled its supply and was trying to deprive the US armed forces of this valuable resource.[nb 1]
In the 1970s, bicycle magazines, such as Bike World, sometimes referred to exotic lightweight bicycle parts as being made of unobtanium, although while expensive they were commercially obtainable. In the same period, driver & engineer Mark Donohue claimed unobtainium was used in the construction of Penske race cars.
As of 2010, the term has diffused beyond engineering, and appears in the headlines of mainstream newspapers, especially to describe the commercially useful rare earth elements (particularly terbium, erbium, dysprosium, yttrium, and neodymium). These are essential to the performance of consumer electronics and green technology, but the projected demand for them so outstrips their current supply that they are called "unobtainiums" within the ore industry, and by commentators on the US Congressional hearings into the "supply security" of rare-earths.
"Unobtainium" has come to be used as a synonym for "unobtainable" among people who are neither science fiction fans nor engineers to denote an object that actually exists, but which is very hard to obtain either because of high price (sometimes referred to as "unaffordium") or limited availability. It usually refers to a very high-end and desirable product; for instance, in the mountain biking community, "These titanium hubs are unobtainium, man!" Old-car enthusiasts use "unobtainium" to describe parts that are vanishingly rare or no longer available.
In maintaining old equipment, unobtainium refers to replacement parts that are no longer made, such as parts for reel-to-reel audio-tape recorders, or rare vacuum tubes that cost more than the equipment they are fitted to (especially true of certain vacuum tubes, such as the 1L6, used almost exclusively in American battery-powered shortwave radios or the WD-11 used in certain early 1920s radios).
There have been repeated attempts to attribute the name to a real material. Because of the long-standing usage of the term "unobtainium" within the space elevator research community to describe a material with the necessary characteristics, LiftPort Group President Michael Laine has advocated assigning the term as the generic name for cables woven of carbon nanotube fibers, which seem to satisfy the requirements for this application. Since he claimed that sufficiently long nanotube cables will be prohibitively expensive to develop without inexpensive access to microgravity, these cables would still be close enough to unobtainable to meet the definition. However, this usage does not seem to have become widespread. The eyewear and fashion wear company Oakley, Inc. also frequently denotes the material used for many of their eyeglass nosepieces and earpieces, which has the unusual property of increasing tackiness and thus grip when wet, as unobtanium.
Frequent Sunday night/Monday morning host of Coast to Coast AM George Knapp usually opens his show mentioning unobtainium. As a play on the word, Obtainium is an album by Skeleton Key, released in 2002 by Ipecac Recordings.
Unobtainium can refer to any substance needed to build some device critical to the plot of a science fiction story, but which does not exist in the universe as we know it. A hull material that gets stronger with pressure in the film The Core was nicknamed unobtainium, but the concept under different names can be seen in the anti-gravity material cavorite and the super-strong material scrith from Larry Niven's novel Ringworld, which requires a tensile strength on the order of the forces binding an atomic nucleus together. More recently, it was the ore mined on the moon Pandora in the movie Avatar, where the compound was described as key to human space exploration and survival, though it was spelled "unobtanium" in that setting.
Unobtainium can also refer to any rare but desirable material used to motivate a conflict over its possession, making it a MacGuffin (it appears in the story as something to obtain, not something that is significantly used).
Unobtainium can be used in a disparaging context (e.g., "That idea is silly; you'd need unobtainium wires to hold the planet up!") or a hypothetical one ("If one were to build an unobtainium shell around a black hole's event horizon, what would happen to the material piling up on it?").
The term handwavium (suggesting handwaving) is another term for this hypothetical material, as are buzzwordium, impossibrium, hardtofindium, flangium, and, less commonly, phlebotinum.
The term Eludium (also spelled with variants such as Illudium) has been used to describe a material which has eluded attempts to develop it.
Another largely synonymous term is wishalloy, although the sense is often subtly different in that a wishalloy usually does not exist at all, whereas unobtainium may merely be unavailable.
A similar conceptual material in alchemy is the Philosopher's Stone, a mythical substance with the ability to turn lead into gold or bestow immortality and youth. While the search to find such a substance was not successful, it did lead to discovery of a new substance - Phosphorus.