Second Impact

Short answer: Ceres is the largest object in the main asteroid belt between Mars and Jupiter. Giuseppe Piazzi discovered it on January 1, 1801, and it was first hailed as a planet, then called an asteroid.

Since August 24, 2006, it has been classed as a dwarf planet, the only one inside Neptune's orbit, though it also keeps its asteroid number, 1 Ceres. NASA's Dawn spacecraft orbited it from March 6, 2015, to the end of the mission on November 1, 2018. Dawn measured a mean diameter of 939.4 km and a mass of about 9.38 × 10^20 kg, about 40% of the whole asteroid belt's mass. It found a crust of ice, salts and hydrated minerals over a muddy interior, with brines that reach the surface in bright spots like those in Occator Crater. Ceres's orbit stays between 2.55 and 2.98 astronomical units from the Sun, well outside Earth's, so the Reel's collision is purely hypothetical. Still, the numbers are extreme. Anything falling to Earth hits at no less than Earth's escape speed, about 11.2 km/s. At 11 to 20 km/s, Ceres would carry about 6 × 10^28 to 2 × 10^29 joules of kinetic energy. That's my own back-of-envelope estimate (½mv²), not a published figure. A 2022 review in Nature Reviews Earth & Environment puts the Chicxulub impact that ended the age of dinosaurs at about 10^23 joules, so Ceres would deliver roughly half a million to two million times as much. Sleep and colleagues estimated in 1989 that an impactor of roughly 350 to 440 km could boil away Earth's oceans, and later models put the threshold nearer 700 km. Ceres is bigger than either. The footage is computer animation of a huge body striking Earth from orbit, with a glowing crater and a fireball sweeping across the planet.