Possibility isn't limited by technology. And it's certainly not limited by human imagination. What makes something impossible is the lack of cold, hard, cash.

Gundam Mecha

Cost: $725M (Estimated parts total)

Someone went ahead and did it: calculate the cost of constructing a military-grade giant robot. The result, when you throw in flexible aluminum alloys, seven engines, thirty helicopter motors and a computer fast-thinking enough to keep it upright, is $750m a piece. That would get you half a dozen Eurofighters, a trio of Raptors, or about 125,000,000 copies of Peggle.

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Moonbase

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*Cost: $105bn (Half of NASA's anticipated $14bn 2019 Exploration Missions budget every year for a decade)

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After mankind's famous leap, it seemed the next step: a permanent manned presence on the moon. But with the Soviets scuppering their own lunar plans and the withering of the space age, it took NASA a generation to finally announce its plans.

The schedule sets completion for 2024, and it must be paid from NASA's fixed budget of $17bn a year. But let's imagine it was happening now. How much would it have cost? Assuming it was the agency's main priority, it would have easily added up to more than $50bn in the first three years, with the lion's share of the work still ahead of us. The real deal could cost more than twice that when it's finally built: don't forget all the regular service missions it'll require until self-sufficiency or (far more likely) abandonment.

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Orbital Hotel__

Cost: $1.2bn (Britain's Proposed ISS Astronaut's Lounge)

The International Space Station, at $20m a trip, isn't exactly a week in Tahiti. Hell, it's not even a dirty weekend at the local Lamplighter, as far as amenities are concerned.

A dedicated hotel? It makes a big difference who builds it: NASA managed to pork away about $50bn not finishing the International Skipping Stone, but Russia will complete its share of the project at a fraction of that cost. Neither seem interested, however, in space hotels.

Leave it to the Brits. After decades of disengagement from human space exploration, it recently reversed its position—to propose a lounge for the station. Either it understands something about space travel NASA doesn't, or it's mad. Can you say Virgin Orbital, Sir Richard?

So, how much would it cost to make a permanent hotel in space? If we don't allow ourselves the luxury of appending it to the ISS, it's going to cost tens of billions of dollars. Britain's supplemental habitation module, used as a hotel, would be far more economical: its proposal is tagged at $1.2bn. At $10m for a week on board, it could pay for itself in a couple of years—if the buyers are out there.

Transatlantic Tunnel

Cost: $1.6 trillion to $10 trillion, depending on how it's cut.

How long is a piece of string? Figure that out, and you have the cost of building an intercontinental bridge or tunnel. Istanbul can sit this one out: it's already got more than one! (And is about to build another, over a nearby 2km span between Europe and Asia, at a cost of a billion dollars.)

One such project is on the cusp of reality, linking Spain to Morocco beneath the Mediterranean sea. In essence, however, it's little different to the existing rail tunnel between England and France, with a similar cost in adjusted terms: about 27 billion euros.

A proposed bridge or tunnel between Alaska and Chukotka, Russia is touted as the "Intercontinental Peace Bridge," and could turn the intermediary Diomede Islands into the world's most remote rest stop. The total length is about fifty-five miles, and the total cost about that in billions. Some claim as little as $15-25bn: one estimate places the cost at $105bn.

Italians plan to build the longest suspension bridge in the world, between the mainland and Sicily, but the latest E4.6bn proposal died in 2006 with the incoming government of Romano Prodi.

But none of this is what we want: a sleeper express between London and New York, right? An immersed tube under the Altantic could cost half a billion dollars a mile, about three times the cost of a modern bridge.

We're already looking at about $1.6 trillion dollars, at that burn rate—and this doesn't account for the precipitous drops at the continental shelves, or, indeed, the engineering problem of deep-ocean tunnel immersion.

If you're thinking of cutting a tunnel the traditional way, under the sea bed, it gets even crazier. Extrapolating the cost of the Channel Tunnel, built in this fashion, and the price lands somewhere in the $10tn region.

Start saving your pennies!

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Supersonic Transport__

Cost: $30m (Repair decaying BA Concorde) to $180m (Inflation-adjusted cost of a new one)

With the retirement of Concorde, the only supersonic commuter airliner, 2-hour trips over the Atlantic are a thing of the past. It's perhaps the first tech to go from sci-fi to reality and right back again, all in the space of a generation: no zombie apocalypse required.

When British Airways killed the beast (now generally thought to be a ploy to move its first-class addicts to more profitable planes), Virgin offered to buy them for the same price BA bought them from the British taxpayer: $1 each. Ultimately offered $5m for each, BA preferred to let them fall into disrepair: to get one of the rotting hulks airworthy again may now cost far more than that.

Experts estimate that it would cost billions to develop a new supersonic airliner, however, so restoration of the existing ones remains a more likely scenario.

New York-L.A. Maglev Express

Cost: $70bn (Based on established construction costs)

At $70bn, it's tantalisingly affordable by the standards of this roundup: a train that could beat airliners from one side of the country to the other.

Many agree that Maglev has enormous potential. Bite-sized examples are in operation all over the world. Birmingham, England, had the first in the 1980s, though the promise of airliner-like speeds on land is still unrealized. The British system sped along at a pathetic 26MPH and was designed to get air travelers to the planes, not to outrun them.

In fact, Maglev carriages, forced along by magnetic fields instead of traditional propulsion systems, still struggle to outpace normal high-speed trains: both forms of transit post speed records at about 360 MPH. In everyday use, modern Maglev systems manage about 260MPH — still ten times as fast as the first one, which ran only 20 years ago.

Nonetheless, they're extravagantly expensive to build, even if maintenance is relatively cheap. Based on current construction estimates of $25m a kilometer, that 400MPH New York-L.A.'s $70bn route would be hard-pressed to find itself funding. Let's get BosWash covered first, shall we?

A Floating City

Cost: $11bn For 18,000 Homes

International waters offer plenty of real estate, but getting foundations to stay put is another matter. Micronations like Sealand don't count: we want the gigantic floating pyramid cities (pictured here) that Future Cities promised us in 1979.

The nearest real-life analogs to that particular wonder — perfectly possible as it is — are found in land reclamation efforts by the Dutch, in Japan and elsewhere. Kansai International Airport cost $20bn, and is a good example of a "city" built where waves once lapped.

As for something on the water itself, however, the best example is probably the Queen Mary 2, Cunard's most expensive and majestic cruise liner. It cost $900m to put into service, and accommodates nearly 4,000 passengers and crew. A rival, the $800m Freedom of the Sea, packs in 5,730: it's practically an ocean-going skyscraper. The next-generation of liners will hold as many as 10,000, and cost $1.2bn to build.

Realistic proposals for more grand sea-cities are thin on the ground. The Freedom Ship, proposed by Norman Nixon, is about four times the size of cruise liners. It would permanently accommodate some 50,000 people and cost more than $11bn to build.

Android Armies

Cost: $1bn a battallion

Seen Asimo? If you want your own, expect to pay Honda about a million dollars. Cheapskates can lease one for $166,000 a year. And he still can't handle stairs.

Still, if you need to defend something very flat with robots (rather than automatic turret guns), simply because that's a totally bad-assed thing to do, we could put out a battalion for about a billion dollars. Not too shabby, eh?

Blasters and Railguns

Cost: $10m for a field-effective weapon



Directed energy weapons are so well-understood that there are dozens of variations on the same basic theme.

Unfortunately, available batteries and coolants are just are not up to snuff.

For a laser gun, for example, we need a luggable assembly that can power a beam strong enough to match a firearm's effectiveness, and a way to siphon away the heat such a gun would leak. Such tech doesn't exist yet, at any cost.

We're making progress: the U.S. now has test laser installations able to take out fighter jets and missiles, including a mobile one stashed aboard a Boeing 747.Railguns are another sci-fi staple finally bearing fruit in portable-ish form. Just this morning, the Naval Surgace Warfare Center in Dahlgren, Virginia, fired a 10 megajoule electromagnetic railgun. They're talking about using these to replace 5-inch guns in the 2020s, though there's no mention of cost.

Finally, no mention of death rays is complete without a tip of the hat to Nikola Tesla. Priceless stuff.

Interstellar Exploration

Cost: Unimaginable.

Scientists think that a probe could be sent to nearby stars, if it was light enough to be accelerated to relativistic speed with a giant laser. Let us not forget this would still be a decades-long mission, with a probe so small as to be measured in grams, not kilos.

Screw that nonsense: I want a warp drive. Pricing up the mathematical exegeses behind such notions, however, is simply beyond calculation.