THE PAST AND FUTURE MOTION OF COMET P SWIFT-TUTTLE/

The orbit of P/Swift-Tuttle is investigated by way of a long-term inte gration forward to AD 2392 and backward to 703 BC. Two of its previous returns prior to the telescopic period, in AD 188 and 69 BC, are iden tified in Chinese records. No other observations of P/Swift-Tuttle hav e been found. The non-gravitational forces that affect the motion of m ost active comets appear to be negligible for this comet, suggesting t hat either the comet outgasses radially toward the Sun and in a symmet ric fashion about perihelion and/or it is far more massive than period ic comets of comparable activity (e.g. P/Halley). Our successful integ ration is consistent with all the observed returns of the comet. The u nobserved returns between AD 188 and 1737 are easily explained, as the comet did not approach the Earth closely enough to allow naked-eye vi sibility. Our integration and the observing conditions at each return suggest that the comet has maintained about the same intrinsic brightn ess for more than two millennia. The lack of non-gravitational effects for this comet and the relative constancy of its intrinsic brightness place constraints upon the lifetime of its active area(s) and its spi n state. Our prediction of the comet's return in 2126 places it well a way from the Earth's position at the nodal crossing.