HED meteorite

• 1. Introduction

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  HED meteorites have been falling to Earth for millions of years, silent travelers from a world that formed billions of years before our own. HED stands for howardite-eucrite-diogenite, three distinct rock types that share a single remarkable origin: the crust of the asteroid Vesta. When you hold one of these rocks, you are holding a piece of planetary crust that crystallized between 4.43 and 4.55 billion years ago. They account for about 5% of all meteorite falls observed on Earth, and they make up roughly 60% of all achondrites ever found. What makes them so abundant? How did rocks from an asteroid in the outer solar system find their way to Earth? And what do they tell us about the violent history of a world we can barely see with the naked eye?

• 2. Three Rocks, One Source

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  Howardites, eucrites, and diogenites look different from one another, but all three formed through igneous processes deep within the crust of Vesta. They are achondrites, meaning they lack the small round droplets called chondrules that define the most common class of stony meteorites. Instead, they closely resemble the magmatic rocks that form inside planetary bodies, the kind geologists study on Earth every day. Several distinct subgroups of eucrites and diogenites have been identified, reflecting the varied conditions under which molten rock cools and crystallizes. The differences in composition between the three types are not accidents of chemistry. They reflect when and where those rocks were ejected during Vesta's long geologic history. An impact does not grab just one kind of rock; it pulls from wherever it strikes, and the mix of howardites, eucrites, and diogenites we find on Earth mirrors the layered structure of Vesta's ancient crust.

• 3. The Wound on Vesta

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  Vesta carries a scar that is hard to imagine: an enormous impact crater covering much of the asteroid's southern hemisphere. This crater is considered the most likely site of the collision that sent HED meteorites on their journey toward Earth. The excavation was so thorough that the volume of rock removed is many times greater than the total mass of all known V-type asteroids, the small rocky bodies that share Vesta's spectral fingerprint. That single impact, thought to have occurred less than a billion years ago, was enough to scatter debris across the inner solar system. Some of those chunks stayed close to Vesta, forming the Vesta family of asteroids. Others were thrown farther out, into the unstable zone between Mars and Jupiter known as the 3:1 Kirkwood gap.

• 4. The Long Road to Earth

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  The 3:1 Kirkwood gap is a region of the asteroid belt where Jupiter's gravity exerts powerful, repeated tugs on any object that drifts into it. Asteroids that reach this gap are eventually kicked into very different orbits, on a timescale of around 100 million years. Some of those displaced rocks were pushed into near-Earth orbits, becoming small V-type near-Earth asteroids. Among them are bodies such as 3551 Verenia, 3908 Nyx, and 4055 Magellan. Later collisions, smaller in scale, chipped rock-sized fragments off these near-Earth objects. Some of those fragments eventually struck Earth as meteorites. Cosmic ray exposure measurements on HED meteorites suggest that most of them came from several distinct impact events of this kind, and that the rocks spent somewhere between about 6 million and 73 million years traveling through space before landing here.

• 5. Relics of a Young Solar System

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  Radioisotope measurements have dated the crystallization of HED meteorites to between 4.43 and 4.55 billion years ago, putting their formation in the earliest chapter of solar system history. They are differentiated meteorites, meaning they came from a parent body that melted, separated into layers, and processed its interior the way planets do. That makes Vesta unusual among asteroids: it behaved more like a small planet than a simple pile of rubble. The rocks that fell from its crust carry chemical signatures that researchers use to reconstruct how magma behaved on a tiny world with very different gravity and pressure from Earth. Each HED meteorite in a collection is, in effect, a core sample from an ancient planetary surface that no spacecraft has yet brought back to us directly.