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Why did the U.S. Congress ask NASA to search for asteroids larger than 1 kilometre?

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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.

The U.S. Congress directed NASA to identify and track near-Earth objects (NEOs) because of the potential for impact events to cause global catastrophes.12

Specifically, Congress focused on asteroids with a diameter of 1 kilometre (0.6 miles) or larger because these objects are considered "planet-killers". Scientists have determined that an impact by an object of this size is the smallest capable of causing a global catastrophe, such as widespread climate disturbances or massive tidal waves, which could potentially result in the death of one-fourth or more of the world's population.345

By mandating that NASA detect 90 percent of these large NEOs, Congress sought to enable early detection, which is a critical component of planetary defense. The goal is to provide sufficient time to analyze and potentially implement mitigation strategies—such as altering an asteroid's orbit—to prevent a collision that could threaten human civilization.324