About AsteroidWatch

What this is

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.

AsteroidWatch tracks these close approaches using NASA’s Near-Earth Object Web Service (NeoWs), showing each object’s estimated size, how fast it’s travelling, and how far it will miss Earth — measured in lunar distances and astronomical units. Seeing the real numbers is reassuring: a steady stream of harmless flybys is exactly what a well-monitored sky looks like.

• ☄️ 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.

Understanding impact risk in context

Asteroid close approaches are routine and almost always harmless — virtually every one passes Earth safely. This section is here to put the numbers in perspective, not to raise alarm. There is no personal action to take for a flyby; the value is in understanding what the figures mean.

Data sources

• NASA NeoWs (Near-Earth Object Web Service) — Close-approach feed providing estimated diameters, miss distances, velocities and hazard flags for near-Earth objects.
• NASA/JPL CNEOS — The Center for Near-Earth Object Studies, home of the Sentry impact-monitoring system and the close-approach database.
• ESA NEOCC — The European Space Agency’s Near-Earth Object Coordination Centre, which independently tracks and risk-lists NEOs.

Frequently asked

Is a “close approach” dangerous?

Almost never. A close approach is just an object’s nearest point on a flyby, and the great majority pass millions of kilometres away — tens to hundreds of times farther than the Moon. A predicted close approach is the result of orbital mechanics working normally, not a warning.

What does “potentially hazardous” mean?

It is a definition about orbit geometry, not a forecast of impact. A Potentially Hazardous Asteroid is one roughly 140 m or larger whose orbit can pass within 0.05 AU of Earth’s. The label means “worth monitoring,” and most such objects will never come near us.

How do we know an asteroid won’t hit Earth?

Once an object is tracked across the sky, its orbit can be projected years or decades ahead. Automated systems like NASA’s Sentry and ESA’s NEOCC recompute the odds as new observations arrive; the usual outcome is that an object’s impact chance shrinks to zero as its path is pinned down.

How is an asteroid’s size estimated?

Mostly from its brightness. Astronomers measure absolute magnitude (H) and assume a reflectivity (albedo) to convert it into a diameter. Because albedo is uncertain, sizes come as a range; we report the estimated maximum diameter in metres.

How often do asteroids actually hit Earth?

Tiny meter-scale objects enter the atmosphere and burn up regularly and harmlessly. City-threatening impacts are rare — centuries to many thousands of years apart — and civilization-scale impacts are extremely rare, separated by millions of years. Surveys have already found most of the largest near-Earth asteroids.