Final answer:
As of 2021, nearly a thousand asteroids larger than 1 kilometer in diameter with near-Earth orbits have been identified, and efforts to detect more continue.
Step-by-step explanation:
The Spaceguard Survey initiated by NASA in 1998 has identified a significant number of near-Earth asteroids (NEAs) that are greater than 1 kilometer in diameter, which could potentially cause global damage if they were to impact Earth. By 2012, nearly a thousand of these large NEAs had been discovered. By 2021, NASA's concerted efforts resulted in the detection of more than 25,000 near-Earth objects, and by that time, over 98% of the hazardous NEAs larger than 1 kilometer were identified. However, there is a possibility that some asteroids of this size have not yet been discovered. Meanwhile, NASA's DART mission aims to demonstrate technology that could potentially alter the orbit of a threatening asteroid, showcasing proactive measures to protect Earth from possible impacts.
So, to answer the student's question: As of 2021, there are nearly a thousand asteroids larger than 1 kilometer in diameter with near-Earth orbits that have been identified. The exact number fluctuates as new discoveries are made and as the capabilities for detecting and tracking these objects improve.