I thought astronomers noticed a twisting in the rings of Saturn and tracked it to their big ship. Or maybe I'm thinking Clark's Rama. It's been a while.
"though I recall the world came out better from the alien invasion. "
Yeah, giant ice cream sundays from space pack a wallop.
It was the comet-watching bit that reminded me of Lucifer's Hammer. I think you're right about Footfall - at least, they noticed something weird while looking at Saturn. The twisty rings may have been natural?