August 11, 1972


Dr. John A. Simpson
The Enrico Fermi Institute
University of Chicago
933 East 56th Street Chicago, IL 60637

Dear Dr. Simpson:

Your letter of 11 July 1972 to Dr. Fletcher has been referred to me. Your arguments for committing Pioneer H to an out-of-the-ecliptic mission are well taken and very persuasive. The relatively low-cost of such a mission, since the Pioneer H spacecraft is a spare for Pioneers 10 and G, and the considerable scientific value to be derived, argue very strongly in favor of the mission. In fact, this kind of mission was described to the Space Science Board 1971 Woods Hole Summer Study by our Science Advisory Group. Moreover, on 15 June 1972 our Outer Planets Science Advisory Group presented to NASA management their recommended strategy for exploring the outer planets, in which the Pioneer H out-of-the-ecliptic mission was an important item.

On the other hand, there are other considerations that argue against committing to this mission at the present time. Quoting from the Woods Hole Summer Study referred to earlier:

"The radiation belt of Jupiter constitutes a hazard of undetermined magnitude for close-in Jupiter flybys, orbiters, and entry probes. We recommend that Pioneers F and G be utilized to evaluate the radiation environment of Jupiter as fully as possible, even at the risk of possible disablement of the spacecraft, and that Pioneer H be held in readiness for use as a Jupiter magnetosphere mission for further evaluation of the radiation hazard if it has not been clarified by Pioneers F and G. This will permit the choice of safe trajectories for both Grand Tour missions and those for the more intensive study of Jupiter. Studies of instrument design for Pioneer H to operate in a high-intensity radiation environment should also be started soon in case such hardened instrumentation should turn out to be the only solution for Jupiter exploration conducted within its radiation belt."

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