Astronomy & Celestial Mechanics
the sky, computed
Concept

Catastrophic disruption — where it appears

A collision energetic enough to shatter a body and disperse at least half its mass. The energy per unit mass it needs falls with size while material strength holds a body together and rises once self-gravity does, so the weakest bodies are a few hundred metres across.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Debris discs of very different masses, fading to one line. The fractional luminosity of a debris belt at 50 AU — the share of its star's light the dust absorbs and re-radiates — against age, from a million to ten billion years, for belts that start with 1, 10, 100, 1000 Earth masses of planetesimals up to 60 km across. Each stays near its starting brightness until its largest bodies have had time to collide, 16.5 billion years, 1.7 billion years, 165 million years, 17 million years respectively, and then fades as one over the age. A heavier belt starts brighter and grinds itself down faster, and the two effects cancel: after their collisional times all belts lie on the same line, the dashed maximum, whatever their mass. At ten billion years the two heaviest differ by 1 per cent.

A ceiling that forgets how much there was

A belt of planetesimals around a star shows itself only through its dust, and the dust is ground from the belt's largest bodies by collisions. Give the belt more mass and it makes more dust — but it also collides more often, and grinds itself away faster. At late times the two effects cancel exactly, and every belt at a given distance and age lies on the same line of maximum brightness whatever it started with. A disc above that line cannot be a steady cascade, and it is telling of a recent catastrophe.

orbits · Collisional cascade One impact, three ways to fade. The fractional luminosity of the dust from a giant impact at 1 AU against time since the impact, for 0.01 Earth masses of debris whose largest pieces are centimetre spherules condensed from the vapour, kilometre fragments, or hundred-kilometre fragments, each grinding down as a collisional cascade. The condensate is bright at once and gone within 286 years; the fragments keep the dust above the dashed detection threshold of 0.0001 for 8 thousand years and 9.3 million years. One impact carries two clocks: a flash measured in years and a glow measured in millennia.

Two clocks in the dust of one collision

A collision between two young planets fills the inner system with dust that shines in the infrared far above anything a steady belt could sustain. The dust carries two clocks. The spherules that condense from the vapour grind themselves away within a few centuries, so a fresh impact flashes and fades on a timescale an astronomer can watch. The fragments last thousands to millions of years, and how long depends on the size of the largest pieces — not on how much was thrown out. The share of young stars that glow counts impacts times fragment size; only the flashes count impacts.

orbits · Collisional cascade

Named alongside it

The objects these essays reach for when they reach for this one.

Collisional cascadeDebris discGiant impactCollisional lifetimeFractional luminosityPlanet formationPlanetesimalRadiation pressureSteady stateTerrestrial planets

All concepts