AS ON A DARKLING PLAIN

Various permutations and combinations of the points of a romantic triangle travel from Titan to Earth to Jupiter to Sirius and back to Titan again to learn the purpose of the inimical alien machines on that planet — and to find the builders. The love interest lacks interest but Bova's first-rate at establishing and unravelling elaborate scientific puzzles and there are some fine minor mysteries here in addition to the central one.