ANOMALOUS ACCELERATION UPDATE. Last year a team of scientists published an assessment of the longterm trajectories of certain spacecraft, including Pioneer 10 and 11, showing that even after all known sources of gravity (sun, planets, comets, etc.) and other forces were taken into account, an extra acceleration seemed to be present (Anderson et al., Physical Review Letters, 5 Oct 98; Update 391). Now a series of letters concerning this assessment appears in the same journal (30 August 1999). Edward Murphy of Johns Hopkins (410-516-4521) argues that the explanation lies in the asymmetric radiation given off by the crafts' electronics as waste heat. Jonathan Katz of Washington University (314-935-6202) implicates the recoil of radiation (from radioactive thermal generators, or RTG, powering the craft) off the rear of the high-gain antenna. The authors of the original paper (contact John Anderson, JPL, 818-354-3956) basically assert (Reply 1, Reply 2) that these two particular explanations fall short of accounting for the anomalous acceleration by roughly a factor of five or more. (All PRL papers are available to journalists from AIP Public Information.)