We rarely see an asteroid as more than a moving dot, so its size is usually estimated rather than measured directly. The key input is absolute magnitude (H) — how bright the object would appear at a standard distance. A given brightness could come from a small, shiny rock or a large, dark one, so astronomers pair H with an assumed reflectivity, or albedo, to convert brightness into a diameter. Because albedo is often uncertain, size estimates come as a range; AsteroidWatch reports the estimated maximum diameter in metres.
An asteroid’s orbit is reconstructed from a series of position measurements as it tracks across the sky. Each observation tightens the orbit; the more observations, and the longer the arc of time they span, the more precise the predicted path becomes. Newly discovered objects can have fuzzy orbits at first, which is why follow-up observations matter so much in the days after a discovery.
Two orbital numbers do most of the work in assessing flybys. The minimum orbit intersection distance (MOID) is the closest that two orbits — the asteroid’s and Earth’s — can ever come to each other, independent of where each body happens to be. Relative velocity is how fast the object moves past Earth, often tens of thousands of kilometres per hour. Together with size, these frame what a given approach means.
All of these figures are estimates with error bars, and they improve over time. An object’s predicted miss distance can shift as new observations refine its orbit, almost always settling toward a safe, well-determined value. Refinement narrowing the uncertainty is the system working as intended, not a sign that danger is rising.