Achondrites

These are stony meteorites that have undergone a degree of thermal stress resulting in melting. Intriguingly, three of the most interesting types of achondrites – the howardites, eucrites and diogenites, or HEDs as they are collectively known – have been traced back to the second most massive asteroid, (4) Vesta, which was visited in 2011 by NASA’s Dawn spacecraft. The theory is that HEDs are debris from a significant impact that Vesta suffered a few billion years ago, which ripped off about one percent of its mass. Howardites and eucrites are breccias (smashed rocks - howardites even contain bits of eucrites and diogenites), with the eucrites being basaltic and diogenites full of pyroxene that has settled out of cooling lava flows.

The remaining categories of achondrite meteorites are the angrites and the aubrites. Angrites are very special. They have the same mineralogy as basalt (copious amounts of pyroxene and plagioclase), and less than twenty have been picked up worldwide, but that’s not why they’re special. It’s the fact that they are incredibly old, having formed 4.556 billion years ago, which makes them so vital to studies of Solar System formation. Scientists have nailed the flag of the angrites to the masts of two asteroids, (289) Nenetta and (3819) Robinson, which are unprocessed lumps of primordial rock in the Asteroid Belt.