Peacefully yours

First it was a letter to fifth-grader Samantha Smith. Now to a galaxy of senior scientists. Yuri Andropov is becoming a veritable pen pal of Americans writing to him about peace.

The question is whether the former KGB man in the Kremlin should be taken any more - or less - seriously when he accepts the grownup scientists' appeal against space weapons than when he tells the 10-year-old she reminds him of Becky Thatcher in ''Tom Sawyer.''

Since his government released both letters, Mr. Andropov evidently is speaking beyond his new circle of correspondents.

We suspect Becky or Samantha would let him know if he falls down on his peaceful promises. Will the Western world in general continue by precept and example to encourage Moscow's compliance with such An-dropov assurances as ''never, but never'' being the first to use nuclear weapons?

The scientists, including Nobel prize winners Hans Bethe and Isidor Rabi, will no doubt persist in their efforts to ban weapons from space. But, again, will a broader public keep up the pressure for Soviet restraint on space weapons in line with the Andropov letter calling for a treaty to prohibit them?

After all, as a US State Department spokesman noted, Moscow continues to test the world's only operational antisatellite interceptor. President Reagan's ''star wars'' call for speeded research on defensive weapons was said by officials to mean the possibility of laser weapons in space. But the spokesman this week asserted that the US is not planning any space weapons except to match the Soviets (though the US antisatellite system is reported to be a technological stride beyond the crude Soviet system).

If this is so, Mr. Andropov could have his stated goal of arms-free space, even without a treaty, by ceasing and desisting on his own space-weapons program. That would really be something to write home about.

Meanwhile - and it may be a long meanwhile, Samantha - both East and West ought to respond substantively to the plea of the scientists, and not only scientists, for a treaty to prevent the militarization of space. The US drew criticism for dragging its feet on the subject of space-arms negotiations at the United Nations space conference last year. Now the immediate reply to Mr. Andropov is to stress problems such as verification which the US says have to be resolved before it can address questions of future negotiations.

Surely the problems could be resolved as part of negotiations just as they have been in the case of terrestrial arms-control agreements, not to mention the treaty that bans at least nuclear arms from space already. Why let correspondent Andropov look like the main enthusiast for peace at the mailbox? Why not say, ''Dear Yuri: We think your unexpected views show a certain promise. Peace is so important we'll be glad to sit down with you and overcome any problems together. Keep in touch. . . .''