What are HED meteorites and why are they linked to Vesta?
HED meteorites are the howardites, eucrites and diogenites, a family of igneous achondrites. Their light-reflecting properties, chemistry and orbits match the asteroid Vesta, and the Dawn spacecraft confirmed a close match when it visited.
HED stands for howardite–eucrite–diogenite, three related types of stony meteorite that formed from melted rock. Eucrites are basalts and gabbros made mostly of pyroxene and plagioclase feldspar, like lava flows and shallow intrusions on Earth. Diogenites are made largely of orthopyroxene crystals that settled out of magma deeper down. Howardites are mixtures of the two, broken and welded together by impacts on the surface. Together they form the largest family of achondrites, with well-known examples including the eucrites Juvinas, which fell in France in 1821, and Millbillillie from Australia, and the diogenite Tatahouine from Tunisia.
The link to Vesta began in 1970, when astronomers measured the way Vesta reflects sunlight at different wavelengths and found that its spectrum closely matched that of basaltic meteorites. Vesta is the second most massive body in the asteroid belt, about 525 kilometres across, and its intact basaltic crust makes it unusual. Later, many small asteroids with similar spectra, nicknamed vestoids, were found scattered between Vesta and orbital zones that can deliver fragments towards Earth, outlining a plausible route.
NASA's Dawn mission sealed the case. Orbiting Vesta in 2011 and 2012, it mapped surface minerals that match HED compositions and revealed an enormous impact basin, Rheasilvia, near the south pole. A collision of that scale could have blasted out the vestoids and, ultimately, many of the HED meteorites in our collections.
There are caveats. A few eucrite-like meteorites have oxygen isotope ratios that differ from the main group, suggesting they came from other basaltic asteroids. But for most HEDs, Vesta remains the strongest candidate, making them one of the few meteorite groups tied convincingly to a named parent body.