This article originally published in
Air and Space Magazine

Then thereís the nightmarish "Gravity Probe B Error Tree" to consider. Thatís a poster that shows 177 boxes representing factors that could affect GP-Bís final measurements. On a mission where a random gas molecule in the instrument or too much sloshing of the liquid helium could queer the whole thing, the tolerances for each of those 177 factors must be excruciatingly fine to guarantee valid results. Says science mission project manager Jeremy Kasdin, whose office displays the poster, "The idea is to make sure weíve tracked all the errors, understand them, and can drive them down." But always there is the overarching question: What if the gyroscopes get rocked by a disturbance no one had foreseen?

Everitt has obviously fielded this question before. He fairly leaps to the board in his office and launches into an hour-long lecture on the checks, cross-checks, and redundancies built into GP-B, as well as the years he and his colleagues have spent considering the possible effects of everything from cosmic rays to micrometeoroids--at the end of which he say, "So Iíve really just given you flavors, rather than a detailed argument of why I think the results of this experiment will be very believable."