In engineering, fiction, and thought experiments, unobtainium [uhn-uhb-tey-nee-uhm] 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 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 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 their experimental lab, ‘Skunk Works,’ used unobtainium as a euphemism 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. 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 (particularlyterbium, 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. When describing materials that are financially out of reach the term ‘unaffordium’ is sometimes used. In maintaining old equipment, unobtainium can refer 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).

There have been repeated attempts to attribute the name to a real material. Because of the long-standing usage of the term within the space elevator research community to describe a material with the necessary characteristics to tether an elevator in orbit, LiftPort Group President Michael Laine has advocated assigning the term as the generic name for cables woven of carbon nanotubefibers. 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.

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