By the time of the impact of comet P/Shoemaker--Levy 9 with Jupiter, the freshly-broken surfaces of the accompanying rubble will have been outgassing for about two years, and will have produced an expanding and co-moving cloud of gas hundreds of RJ across. Much of this gas, escaping from the cometary fragments at low (≤ 1 km/s) speed, will arrive in the Jovian magnetosphere contemporaneously with the comet and drift through the magnetosphere. This gas, as it is photoionized, will be picked up primarily in the outer magnetosphere and the resulting high-energy ions should intensify magnetospheric processes, such as Io plasma torus and auroral emissions, that are thought to be powered by outer magnetospheric mass loading. If the composition of the comet is similar to that of P/Halley, the power available from mass loading should be comparable to that driving the aurora (1014 W) and at least an order of magnitude larger than that exciting the plasma torus for several weeks or months. Measurement of these emissions during and after the cometary encounter may constrain the mechanisms for energization of magnetospheric charged particle populations and magnetospheric transport processes. ¿ American Geophysical Union 1994