American Scientist

To the Editors:

In the otherwise excellent article, "The Voyagers’ Odyssey" (July–August), the description of the gravity assist technique includes the statement, “A retrograde flyby, where the craft travels in the opposite direction of the planet’s spin, can be used to subtract momentum and decrease energy, a move that can be used to change a craft’s direction.” The authors must have meant to refer to the planet’s orbital direction. The gravity effect due to the spin of a massive body is frame dragging, a relativistic effect, and far too small to apply here.

Walter Woolfolk
Cincinnati, OH

Drs. Krimigis and Decker respond:

Mr. Woolfolk is absolutely correct. The cited statement was not present in the original manuscript but was introduced during the editorial process to explain how a retrograde flyby works. This is an obvious typo that we did not catch in the proofs. We are sorry about this oversight. Thus, the sentence should have read “A retrograde flyby, where the craft travels in the opposite direction of the planet’s orbital motion about the Sun, can be used to subtract momentum and decrease energy, a move that can be used to change a craft’s direction and slow it down.”