I’ve greatly enjoyed Mr. Shepherd’s column in the past, but this week, he implied something that was way off base [News of the Weird, March 3]. In a section entitled “Nothing Can Go Wrong,” after mentioning the plummeting Mars orbiter and the dead batteries in some mock nuclear warheads, he mentioned the return of the Cassini probe to the vicinity of Earth, and its payload of 72 pounds of plutonium. In context, this clearly suggests a potentially dangerous mishap. Except that the return was not a mishap.

The probe was scheduled to return to the Earth’s vicinity at about the time it did before it was ever launched, and such was announced publicly well before launch, a year before the pass-by occurred. If airlines could plan their mishaps that well, we’d never see a plane crash again.

Probes are not launched toward their destinations in a straight line, for the excellent reason that they couldn’t carry enough fuel to maintain a high acceleration all of the way to the end. Instead, they are put into orbits that will carry them there as quickly as is feasible. In this case, feasibility suggested using the Earth’s gravitation to slingshot the probe outward, just as Jupiter was used in the past. (Remember those four probes that have now left the orbit of Pluto well behind?)

It would have been nice if Mr. Shepherd could have done his homework before rushing into print and adding, however slightly, to the public hysteria that arose over this mission.

Joseph Dunphy

Chuck Shepherd replies:

That it was part of the mission to fly around earth again to get better traction (of which I was perfectly well aware; the mission was intensively reported in 1997) is utterly irrelevant to my point, which was that 72 pounds of plutonium took two swipes at us, not just one.