The website io9 has come up with an estimate on how much it would cost to build the Death Star, and the answer is: A lot.

To the disappointment of thousands who signed the petition, the Obama administration recently informed us that it has, and will have, no plans to build a Star-Wars-style death star. Now, there may indeed be good reasons to forgo this addition to the nation’s defense, but the first one listed — that it would cost 850 quadrillion dollars — was based on an extremely flawed estimate. Which isn’t surprising, because among the people doing the estimating, only one has any experience in aerospace engineering (and probably none in costing of such projects).

They go off the rails in their estimate right from the beginning, when they assume that a death star would be simply a scaled-up battleship, and built primarily from steel. But a battleship is not actually a good analogue for a death star. First, a death star is not a "floating" weapons platform — it is an orbiting one. Battleships are built of steel because, given sufficient thickness, it has reasonable (though not impervious) resistance to explosive weapons, such as torpedoes or shells from other battleships, and it is possible to build a ship out of that material that will float in the ocean.

But rockets and satellites (a death star would be the latter) have never been built from steel, because its strength-to-weight ratio is far too low — a steel rocket, if it could get into orbit at all, would have very poor payload performance, and a steel satellite would be far too heavy to be able to lift affordably. Traditionally, aluminum was the structural material of choice, though over the past decades, carbon composites have become more popular, because they outperform aluminum. Thus, while it might be that a death star would have steel plating on its hull (that assumes weapons similar to naval ones, when it’s more likely that it will have to defend against high-energy power beams), the vast amount of its structural mass would be a different, much lighter material. Thus, calculating a scale up of the steel would only involve the surface area, not the volume of the death star.

Typical armor thickness for heavy artillery can be up to a hundred millimeters (or about four inches), so for a smallish death star of only 160 km in diameter, the surface area would be about 80,000 square kilometers, or 30,000 square miles, or about 90 billion square feet. Multiply that by four inches (a third of a foot) and you get 30 billion cubic feet of steel. At a density of about 500 pounds per cubic foot, that means we need a little less than two quadrillion pounds, or close to a trillion tons (the estimate scaling up from the battleship was about a quadrillion). Prices currently range from $500-$800 per ton, but I’d go for the lower estimate, given that for a project of this size, we’d likely get a bulk discount. So the cost of the steel for the death star would be about half a quadrillion dollars, far below the original estimate of over eight hundred quadrillion for the steel. That number is only a few times the world gross domestic product.

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