Different River

”You can never step in the same river twice.” –Heraclitus

September 19, 2005

Moonshot Cost Estimate

Filed under: — Different River @ 7:08 pm

Last year, President Bush announced a new mission for NASA — to return to the Moon and set up a permanent base, and eventually go on to Mars. Today, NASA Administrator Michael Griffin announced the first cost estimate for the Moon program: $104 Billion doll.

Now, when scientists and engineers write numbers, they typically use a concept called “significant figures” — that is, they only write as many digits as they have some reasonable confidence of being correct. As described here (boldface in the original):

It is important to be honest when reporting a measurement, so that it does not appear to be more accurate than the equipment used to make the measurement allows. We can achieve this by controlling the number of digits, or significant figures, used to report the measurement.

The number of significant figures in a measurement, such as 2.531, is equal to the number of digits that are known with some degree of confidence (2, 5, and 3) plus the last digit (1), which is an estimate or approximation. As we improve the sensitivity of the equipment used to make a measurement, the number of significant figures increases.

If the people at NASA who came up with that estimate understand the concept of significant figures — and surely Dr. Griffin knows, what with his Ph.D. and four engineering master’s degrees (really!) — the implied precision of the estimate is preposterous. Here they have a proposed 13-year program to do something that’s only been done once before at all and never before on this scale, in a totally different technological environment, and they think they can estimate the cost to within 1-2%? That it makes sense to say “$104 billion, but not $114 billion”?

Frankly, I wouldn’t be surprised if not only the third digit, but even the first digit turns out to be wrong. There is a story (perhaps an urban legend, but I’d love to verify it) that when the second NASA Administrator James E. Webb, Sr. (no relation, as far as I know, to the novelist/historian/Secretary-of-the-Navy James H. Web, Jr.) was asked by President John Kennedy how much it would cost to go to the Moon, he asked his staff for a cost estimate, received it, and then in the car on the wa over to the White House decided to double the estimate before telling the President, just to be on the safe side. This doubled estimate was pretty close.

Based on that, I hereby guess “$200 billion.” I can’t claim it’s accurate enough to say “$208 billion,” but by using only one significant figure, I mean that if you round to the nearest hundred billion, you’ll get my prediction. So it’s really a prediction of “$150-$250 billion.” In constant year-2005 dollars, of course.

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