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

The orbit of P/Swift-Tuttle is investigated by way of a long-term integration 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 identified in Chinese records. No other observations of P/Swift-Tuttle have been found. The non-gravitational forces that affect the motion of most active comets appear to be negligible for this comet, suggesting that either the comet outgasses radially toward the Sun and in a symmetric fashion about perihelion and/or it is far more massive than periodic comets of comparable activity (e.g. P/Halley). Our successful integration is consistent with all the observed returns of the comet. The unobserved returns between AD 188 and 1737 are easily explained, as the comet did not approach the Earth closely enough to allow naked-eye visibility. Our integration and the observing conditions at each return suggest that the comet has maintained about the same intrinsic brightness 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 spin state. Our prediction of the comet's return in 2126 places it well away from the Earth's position at the nodal crossing.