In their recent commentary, O. Glenn Smith and Paul D. Spudis offer a back-of-the-envelope calculation of what a notional manned program to Mars might cost [“Mars for Only $1.5 Trillion,” March 9, page 19]. By providing a calculation with a specific basis, Smith and Spudis have made a valuable contribution to the discussion of whether such a program is something that the United States should undertake.

However, I respectfully suggest that their estimated cost of $1.5 trillion for a nine-mission Mars program, while providing a useful reference point, is unrealistic.

For one example, Smith and Spudis discuss the requirements for a Mars vehicle, which they call the Traveling Space Station, or TSS. The requirements described in general terms for the TSS exceed the capabilities of the International Space Station by at least an order of magnitude. Yet Smith and Spudis estimate the cost of each TSS unit as 1.3 times that of the ISS. This is not realistic, and this is just one example.

We need to very seriously consider our national experience over recent decades with large, complex, technically challenging, manned and unmanned space and defense projects. Our recent experience suggests that if the notional estimate for a program of manned missions to Mars is $1.5 trillion, then as the program progresses the estimated total cost to complete the program (sunk cost plus remaining cost), or to complete a severely descoped version of the program, would almost certainly rise to $5 trillion, would probably rise to $10 trillion, and could even rise to $15 trillion.

For scale, we can compare these numbers with some other numbers. The Federal Reserve’s estimate of the 2014 U.S. gross domestic product is $17.7 trillion. The Congressional Budget Office gives $3.5 trillion as the expenses of the federal government for fiscal year 2014 (of which approximately $500 billion is deficit).

If recent experience is a guide, moreover, a very large, very difficult program like the one proposed could be expected to suffer schedule slip, relative to the notional timeline, by at least a factor of two, probably more.

Similar considerations would also apply to the notional costs and schedule of a manned lunar program.

Important government officials, both elected and appointed, have begun in the last year or so to speak of manned missions to Mars as a “national goal.” It seems unlikely to me that these statements are informed by a serious consideration of the real costs. Certainly the public as a whole is not aware that manned missions to Mars are a national goal, and moreover is unlikely to agree that they ought to be a national goal, if the costs are discussed realistically.

Name withheld upon request