Image copyright Alamy

Nasa is launching a spacecraft that strongly resembles a flying saucer. For more than half a century the distinctive shape has exerted a powerful grip over the popular imagination.

Fasten your space helmet and set your chrome-plated ray gun to "retro-futurist". There's a disc-shaped design classic hovering overhead.

From 1950s B-movies to latter-day pastiches, the flying saucer has served as visual shorthand for the gleaming, jet-propelled, post-war vision of the future.

Now Nasa is preparing to test the distinctly saucer-shaped Low-Density Supersonic Decelerator (LDSD), which the agency hopes could one day land on Mars.

Image copyright Getty Images Image caption Nasa's "saucer" could one day head to Mars

The saucer shape has been a constant in classic sci-fi films like Forbidden Planet and The Day The Earth Stood Still - before evolving into a knowing visual gag in the 1996 spoof Mars Attacks!

The flying saucer is a worthy recipient of that much over-used adjective, "iconic". Designers have borrowed its contours as a template for as long as ufologists have reported sightings.

Image copyright Thinkstock Image caption They fizz on the tongue

Architects in particular have embraced the interplanetary motif. It's not difficult to imagine Brazil's Museum of Contemporary Art, Eindhoven's Evoluon conference centre or Donesk's Donbass Arena all levitating above city streets blasting lasers at screaming bystanders.

Then there were the prefabricated Futuro houses by Finnish designer Matti Suuronen, each of which looked as though it had just touched down on a day trip from Sirius.

Even in the world of homeware there are flying saucer-shaped kettles, phones, lamps and irons - and many will have fond childhood reminiscences of sherbet-filled flying saucer sweets.

"It has become an absolutely universal trope," says Michael Starr, an expert in sci-fi and popular culture at the University of Northampton.

The flying saucer also nostalgically recalls the Eisenhower-Kennedy era's optimistic imagining of tomorrow, from a time when cold war paranoia was tempered by widespread faith in technology, progress and economic growth.

Image copyright Getty Images Image caption The Futuro House was conceived by Matti Suuronen in 1968 as a "portable" holiday home

While disc-shaped objects have been spotted in the sky throughout history, the flying saucer as commonly recognised can essentially be dated back to 24 June 1947, when pilot Kenneth Arnold reported that he had passed nine shiny unidentified flying objects in the skies around Mount Ranier, Washington State.

Arnold described the airborne entities as either crescent-shaped or disc-like, flying with the motion of a saucer skimming on water.

The case attracted huge attention, with newspapers quickly coining the term "flying saucers" to describe what Arnold had seen. In the months that followed there were hundreds of sightings of such aircraft - including at Roswell, New Mexico, a fortnight later.

Saucers and early sci-fi films Image copyright Ron Burton The 1950s is often seen as a golden age of science fiction films. The growing interest in space exploration played a big part - as did Cold War politics. The post-Roswell craze for flying saucer sightings and the fear of Communism came together in alien invasion allegories, such as Invasion of the Body Snatchers, and The Day the Earth Stood Still. UFO hunters: They are still watching

One of the reasons flying saucers caught the Western public's imagination was that they tapped into a widespread fear of attack from communist enemies, says Starr - "the sleek uniformity of the shape, the shell containing something unpleasant".

The Swiss psychiatrist Carl Jung saw flying saucers as a mythical archetype, comparing their shape to that of the mandala, a ritual symbol in Buddhism and Hinduism.

Jung saw them as "technological angels" for a secular age, on to which people could project their fears about nuclear conflict. Just as the alien in the 1951 film The Day The Earth Stood Still emerged from his saucer to warn humankind not to destroy itself, they offered hope that scientific progress would deliver enlightenment and peace, not mutually-assured destruction.

There were other, more prosaic reasons why Hollywood was keen to capitalise on widespread fascination with the flying saucer phenomenon.

"A lot of 50s sci-fi jumped on this because it was very easy to do - you just need a plate and a piece of string," says Starr. "It's wonderful from a purely practical point of view."

Buildings inspired by saucers

Image copyright Paul Hohmann/Flickr Image copyright Getty Images Image caption Seattle's Space Needle was developed in 1962 for the World's Fair Image copyright Getty Images Image caption Beijing's National Grand Theatre resembling a flying saucer which has landed in a lake Image copyright AFP Image caption Perched across the bay from Rio is Oscar Niemeyer's "flying saucer" Museum of Contemporary Art in Brazil previous slide next slide

During the 1950s those interested in the phenomenon of possible extra-terrestrial sightings used the term "flying saucer" quite happily - hence the journal Flying Saucer Review, which reportedly counted Prince Philip among its subscribers.

Today, however, ufologists "think you are taking the mickey out of them if you use that phrase", according to David Clarke of Sheffield Hallam University, who has spent more than three decades studying the culture around UFO sightings. During the 1960s the original expression fell out of fashion in favour of the more official-sounding "unidentified flying object", a phrase borrowed from the US Air Force.

Over time UFO sightings, too, began to involve saucer shapes less frequently, says Clarke. Craft resembling stealth bombers and other angular shapes became more common - a phenomenon that mirrored the tendency over time of sci-fi films to eschew the traditional circular pattern.

"Either the aliens had altered the design of their aircraft to fit in with the films of our world, or something else was going on," says Clarke.

Is it a bird, is it a train?

Image copyright Other

In 1970, the British Railways Board filed a patent for a spacecraft powered by "controlled thermonuclear fusion reaction".

The original patent application said the reaction would be "ignited by one or more pulsed laser beams".

A patent document reads: "The present invention relates to a space vehicle. More particularly it relates to a power supply for a space vehicle which offers a source of sustained thrust for the loss of a very small mass of fuel. Thus it would enable very high velocities to be attained in a space vehicle and in fact the prolonged acceleration of the vehicle may in some circumstances be used to simulate gravity."

Thus flying saucers became a somewhat kitsch symbol of the more whimsical end of the space age.

But the notion of floating disc-shaped aircraft wasn't considered fanciful by governments and militaries around the world. The new LDSD is far from the first attempt by earthlings to construct a flying saucer-like aircraft.

For instance, German engineer Georg Klein told the CIA he worked on a Nazi flying saucer for the Luftwaffe under designers Rudolf Schriever and Richard Miethe - a claim which prompted the Americans to study the possibility of creating one of their own.

Image copyright Getty Images Image caption In the 50s, Frenchman Rene Couzinet saw vertical take-off as the future of aviation. His RC-360 didn't take off.

"Project Y" - a British-Canadian flying saucer programme - was taken over by the US Navy in 1955 and became defence department weapon system 606A.

In theory, as it travelled within the earth's atmosphere, a flying saucer of the classic 1950s design would have quite an aerodynamic shape, according to space scientist Maggie Aderin-Pocock. "If it's travelling horizontally through the air there shouldn't be too much air resistance. It should glide quite easily," she says.

The problem was the propulsion system. "No-one ever mastered the technology," says Clarke. "They just couldn't get it to work."

Now Nasa's engineers are hoping that LDSD will succeed where Project Y failed. Don't adjust your jet-powered tele-screens.

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