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Why are long-period comets harder to detect before a possible Earth impact?

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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.

Long-period comets are significantly harder to detect before a potential Earth impact compared to other near-Earth objects because they are typically discovered only as they enter the inner solar system for the first time in millennia.1

Unlike near-Earth asteroids, which often make frequent passages near Earth—allowing for multiple opportunities to track and compute their orbits—long-period comets originate from the distant Oort cloud. Because they move on highly eccentric and randomly inclined orbits, they approach the planetary system from all directions and are generally identified only a few months or, at best, a few years before a potential collision.213

This limited window of observation makes it difficult to acquire an orbital "arc" of sufficient length to accurately predict their future path. Consequently, while search programs are effective at identifying larger near-Earth objects that have been in the vicinity for longer periods, the outlook for detecting a long-period comet on a collision course remains poor.1