Why Do Some Asteroids Travel Together? Fragments of the Same Collision

Asteroid orbits are described by several quantities including size, shape and tilt, and plotting thousands of asteroids against those quantities reveals clusters rather than a smooth distribution. Those clusters are families, and the interpretation is that each represents fragments of a single body broken apart by a collision, which scatters pieces onto orbits similar to the original with modest differences reflecting the speeds at which they departed. The quantities used are chosen to be stable over long periods rather than the immediately observable ones, since those oscillate. Confirmation comes from the fragments sharing a composition, measured from how they reflect light, which a chance grouping of unrelated bodies would not.

Several things follow from identifying a family:

• •The parent body's approximate size, reconstructed by adding the fragments together
• •The age of the collision, estimated from how far the fragments have spread since
• •The parent's internal structure, since fragments sample its interior
• •Whether the parent was differentiated, meaning separated into core and mantle, which few were
• •A source region for meteorites, which can sometimes be matched to a family by composition
• •The collision history of the belt as a whole, from the number and ages of families

Dating a collision uses a subtle effect on small bodies. Sunlight absorbed and re-emitted by a rotating asteroid produces a tiny asymmetric thrust, which over long periods shifts its orbit slowly, and the size of the effect depends on the body's size, so small fragments drift further and faster than large ones. A young family therefore has its members tightly clustered, while an older one shows a characteristic spreading with the smallest members furthest from the centre, and the pattern can be modelled to give an age. Ages obtained this way range from a few million years for the youngest to billions for the oldest and least distinct. One young family's formation has been linked to a spike in meteorite impacts recorded in ancient sediments.

Popular depiction of the asteroid belt as a crowded hazard is wrong by an enormous margin and the correction is worth stating. The total mass of everything in the belt is well under a thousandth of the earth's, and the largest object accounts for roughly a third of it by itself. Spread across a region hundreds of millions of kilometres across, the average separation between objects large enough to matter is of the order of a million kilometres, which is why spacecraft have crossed it repeatedly without any special precautions. The belt is also not a destroyed planet, since the total mass is far too small and the compositions are too varied, and the current understanding is that Jupiter's gravity prevented material there from ever accumulating into one body.

Families connect several otherwise separate lines of evidence. Meteorites are pieces of asteroids delivered to earth, and matching a meteorite type to a family identifies where in the belt it came from, which turns a rock in a laboratory into a sample from a known location with a known history. The same drifting effect that dates families also delivers fragments to resonances with Jupiter, which throw them onto orbits crossing the inner solar system, so families are the ultimate source of most near-earth objects. Understanding the size distribution within families constrains how asteroids break, which bears directly on what would happen if one had to be deflected. And the oldest families record the early violence of the solar system.

Clusters in orbital properties, confirmed by shared composition, identify fragments of a single shattered parent body. Sunlight produces a tiny thrust that moves small fragments faster than large ones, so the pattern of spreading dates the collision. Matching meteorites to families turns a rock in a laboratory into a sample from a known place with a known history.