Perihelion Summer

"Taraxippus is coming: a black hole one tenth the mass of the sun is about to enter the solar system.

Matt and his friends are taking no chances. They board a mobile aquaculture rig, the Mandjet, self-sustaining in food, power and fresh water, and decide to sit out the encounter off-shore. As Taraxippus draws nearer, new observations throw the original predictions for its trajectory into doubt, and by the time it leaves the solar system, the conditions of life across the globe will be changed forever."

At 41,810 words, this technically falls into the Best Novel category, but could also apply to Best Novella, using the 20% rule.