The sun heats the object and causes it to emit vapor, it emits more vapor on the side facing the sun, these emissions have a measurable affect on the orbit. Se

The sun heats the object and causes it to emit vapor, it emits more vapor on the side facing the sun, these emissions have a measurable affect on the orbit. See the section on 2l/Borisov https://en.wikipedia.org/wiki/Comet#Long_period https://en.wikipedia.org/wiki/Comet#Long_period