Comets & Asteroids

Comets and asteroids are the leftovers — the material that never assembled into a planet — and they are scientifically valuable for exactly that reason: they preserve the composition of the early solar system, which the planets have long since cooked and reprocessed. They are also the only astronomical objects that pose a direct physical hazard, which is why more than 35,000 near-Earth objects are catalogued and tracked, and why DART deliberately hit one in 2022 to prove that its orbit could be nudged.

DART worked considerably better than the requirement. Hitting Dimorphos at six kilometres a second was expected to shorten its orbit around its parent asteroid by around seven minutes; it shortened it by thirty-two. The difference is momentum carried away by the debris thrown off the far side, which multiplies the effect of the impactor by a factor that depends on what the surface is made of — and Dimorphos turned out to be a loose pile of rubble rather than a rock, which is why so much came off. That is the useful finding and also the awkward one: deflection works, and how well it works depends on a property you cannot measure from Earth. Hera arrives in 2026 to survey what is left of the crater and turn the number into a model.

The other current era is sample return. Hayabusa2 brought back material from Ryugu and OSIRIS-REx from Bennu, and both carried organic molecules and hydrated minerals — amino acids among them, in a sample sealed in space rather than picked up off Antarctic ice, which removes the objection that has hung over every meteorite result for fifty years. Meanwhile the surveys have begun finding objects from other stars passing through: two confirmed so far, one of them behaving like a comet and one that has still not been satisfactorily explained. Vera Rubin Observatory is expected to make them routine rather than remarkable, which will turn a curiosity into a population with statistics attached, and that is usually where the interesting part starts.