How did nongravitational rocket forces change the orbit of Encke’s Comet?

The orbit of Encke’s Comet was found to be shrinking by approximately 2.5 hours every revolution, a phenomenon that could not be explained by gravitational perturbations from the planets.1

In 1950, astronomer Fred Whipple proposed that this change was caused by "rocket forces" resulting from the sublimation of water ice on the surface of the comet’s nucleus. As the comet approaches the Sun, these ices transform into gas and erupt from the surface, creating jet forces.123

Because the comet’s nucleus rotates, these jets produce tangential forces—acting forward or backward along the direction of the comet's motion—in addition to out-of-plane forces. In the case of Encke’s Comet, these tangential forces are pointed opposite to the comet's direction of travel, which causes the nucleus to slow down and its orbit to gradually shrink. This effect is further influenced by thermal lag, as the Sun continues to heat the nucleus surface for a period after local noontime.2