Learn — AsteroidWatch

A near-Earth object (NEO) is an asteroid or comet whose orbit brings it into Earth’s neighborhood as it loops around the Sun. The vast majority are near-Earth asteroids (NEAs) — rocky leftovers from the formation of the solar system. When one passes relatively near our planet, astronomers call it a “close approach,” a flyby on the object’s own orbit. Despite the name, “close” here is astronomical: most close approaches happen millions of kilometres away.

• ☄️Near-Earth objects and close approachesWhat makes an object “near-Earth,” what a close approach really is, and why “close” usually means millions of kilometres away.
• 📏How asteroid size and orbits are measuredBrightness and albedo give a size estimate; tracking an object across the sky pins down its orbit, MOID and velocity.
• 🛰️Impact-hazard scales and monitoringWhat “potentially hazardous” actually means, how the Torino and Palermo scales communicate risk, and how Sentry watches automatically.
• 🌍Putting impact risk in contextHow often objects of each size actually arrive, and how planetary defense — surveys and missions like DART — turns a passive watch into an active one.