How hot did the insides of asteroids get?
It varied. Some asteroids stayed cool enough for ice to melt but not rock, while others reached 1,000 °C or more and melted completely into metal cores and rocky crusts.
Meteorites record a wide range of temperatures inside their parent asteroids.
Cool bodies. The parent bodies of CI and CM chondrites stayed cool, with temperatures mostly below about 150 °C. That was enough to melt ice and allow water to alter minerals, but not enough to cause significant change to the rock otherwise.
Warm bodies. Ordinary chondrites record heating in their parent bodies. Petrologic type 3 chondrites reached perhaps 400 to 600 °C, while type 6 chondrites reached about 800 to 950 °C. At these temperatures, minerals recrystallised and chondrules became blurred, but the rock did not melt. Some scientists think these bodies were layered like onions, with the hottest material at the centre.
Hot bodies. Primitive achondrites, such as acapulcoites and lodranites, record temperatures where melting began, around 1,000 to 1,200 °C. Fully melted bodies, which produced iron meteorites, pallasites and eucrites, must have exceeded about 1,200 to 1,500 °C, allowing metal and rock to separate into core and mantle.
The main heat source was radioactive aluminium-26, with a half-life of about 0.72 million years. Bodies that formed earliest contained more of it and became hottest, while bodies that formed later, after much aluminium-26 had decayed, stayed cooler. This explains why iron meteorites, from bodies that formed within about a million years, are older than many chondrites, whose parent bodies formed perhaps 2 to 4 million years after CAIs. Impacts and other isotopes contributed smaller amounts of heat.