Why put a general at helm of US planetary space program?

It sounds a bit like letting the fox watch over the henhouse: appointing a top-ranking military man to guide America's civilian planetary space program - at a time when national concern about the militarization of space is growing.

Don't worry, say those familiar with the search that led to the selection of Gen. Lew Allen Jr., a four-star general, as director of the Jet Propulsion Laboratory at the California Institute of Technology and a vice-president of Caltech. Until June 30, General Allen was chief of staff of the US Air Force and a member of the Joint Chiefs of Staff.

''I think you have to be careful about knee-jerk reactions,'' says Bruce Murray, who resigned his post as JPL director as of June 30. ''There's a valid concern about the militarization of space. There is a trend that way (in national space policy).''

''But the appointment of Lew Allen as director in no way reflects that trend, '' he adds quickly. ''If anything, he may be a useful counter to it.''

Dr. Murray, who calls Allen's appointment a ''superb'' choice, is known for his outspoken - and sometimes critical - comments on the nation's space and planetary program. He contends that Allen, who holds a PhD in physics, will provide an experienced hand at JPL's helm in a decade of declining budgets for planetary programs. It's a time during which up to 30 percent of JPL's budget may be used for classified research for the Department of Defense (DOD), under a recent Caltech agreement.

''Allen will be a very good buttress against the people who might want to push JPL further into DOD work than it should go,'' says Murray. ''He can judge what JPL can do well, and will know what it shouldn't be doing.''

According to a JPL spokesman, Allen told the Caltech search team that he was attracted to the civilian aspects of the JPL post - and that if his future employers thought he would be bringing a large number of defense contracts with him, then he was not interested in the job.

In an interview with the Monitor, Allen said that even though budgetary support for the planetary program has declined and even though planetary exploration in the future will not rival the excitement of the missions of the 1970s, planetary sciences will still ''be the basis of what JPL does.''

Allen also says he will use great care in determining what defense-related work JPL takes on. He says he believes one of JPL's great strengths has been its close relationship with Caltech - and the sharing of science and technology between the two. JPL's defense contracts, he says, should ''as much as possible'' include nonclassified work so that the openness between the university and the laboratory can continue.

''Doing something in support of the country is not objectionable to Caltech, or to JPL,'' says Allen. ''What is objectionable is if too much of that work requires secrecy. I understand that concern and share it.''