Why must astronomers observe a near-Earth object for several days to calculate its orbit accurately?

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The Chicxulub asteroid — Earth impact hazard An artist's rendering of the impact of a near-Earth object 66 million years ago in what is today the Caribbean region. Many scientists think that the collision of a large asteroid or comet nucleus with Earth triggered the mass extinction of dinosaurs and many other species near the end of the Cretaceous Period.

To calculate an accurate orbit for a near-Earth object (NEO), astronomers must observe the object over a period of several days to acquire an "arc" of sufficient length.1

While initial detection involves comparing images taken over tens of minutes to identify moving objects, these short-term observations are insufficient for precise trajectory modeling. A definitive orbit is essential to make an accurate prediction of an object's true distance and its future path through space. Without this extended observation arc, the uncertainty in the object's position remains too high to determine whether it poses a legitimate impact hazard to Earth.1