A speed of 7,5 kilometers per second is the threshold beyond which a cosmic collision turns dust grains into a chemical reactor. In the interstellar medium drift tiny cosmic dust grains — the legacy of dying stars. Some are made of silicates (silicon, oxygen, magnesium), others of carbon with hydrogen, fluffy like soot. Usually they keep to themselves. But when a supernova shock wave accelerates them to tens of kilometers per second, a different story unfolds.
Astrochemists recreated this picture on a computer using numerical simulation — molecular dynamics where every atom moves according to Newton's laws. To keep calculations accurate, interatomic forces were described by a Machine Learning potential — an AI approximation trained on millions of quantum calculations. The test subjects were two grains about a nanometer in diameter (80 000 times thinner than a human hair): one silicate, the other carbon-hydrogen. They were smashed head-on at speeds ranging from a gentle 0,8 to a blistering 11,3 km/s. Over a hundred runs.
The picture resembles a mill, where the rotation speed determines the grind: coarse flour or fine powder. At the lowest speeds, grains bounce elastically — the dust remains separate. A bit faster (1,5–3,5 km/s) they start to stick, clinging to each other. In the range 3,5–7,5 km/s, sticking is almost guaranteed: strong chemical bonds form between the silicate and carbon parts. And only when the speed exceeds 7,5 km/s do the millstones fully engage: grains shatter, fragments fly apart. Four regimes — four grinds.
It is shattering that is the main supplier of interstellar organics. On impact, up to six and a half molecules of carbon monoxide CO are born, along with a cascade of hydrocarbons: from simple CH radicals to long carbon chains C₁₀H. Organic molecules in space also appear — ketene, propenal, polyyne aldehydes, exactly the ones astronomers observe in molecular clouds. The silicate part produces silicon oxides SiO and molecular silicates.
Thus the mechanical mill of space explains a long-standing puzzle: why cold clouds are so rich in organics. Collisions of dust nanograins are not just destruction — they are a universal natural reactor. The revised fragmentation threshold (7,5 km/s instead of the previously assumed roughly 3 km/s) means that in many environments dust may not shatter but instead grow. Sticking grains in protoplanetary disk become the seeds of future planets. And the same mill that creates molecules grinds dust into construction material for new worlds. The circle closes.
In the high-speed simulations, exotic pure carbon chains up to eight atoms long sometimes formed — molecules like l‑C₃ and C₅, previously found in the atmospheres of cool stars and in molecular clouds.