Final answer:
Detection time for an approaching asteroid varies; for large potential impactors over 1 kilometer, the Spaceguard Survey has tracked most, but smaller ones may only be detected upon atmospheric entry. New telescopes and space missions are enhancing our ability to discover and mitigate these threats.
Step-by-step explanation:
The time we would have to detect an approaching asteroid depends on the size and brightness of the asteroid. While none of the Near-Earth Asteroids (NEAs) discovered so far is on a collision course with Earth, there is a concern for those not yet detected. For large asteroids over 1 kilometer, which could have global consequences, the Spaceguard Survey has done a significant job at tracking 90% of these potential impactors. Smaller asteroids that could still destroy a city are more numerous, and only a fraction have been found; for these, detection might only come when they enter the atmosphere, giving us virtually no warning.
With the deployment of sophisticated telescopes like the Vera Rubin Telescope and future projects like a NASA orbiting search telescope, our ability to discover and track asteroids will improve. In the event a comet or asteroid is found on a collision course, there are initiatives like NASA's DART mission to test technology that could potentially divert such threats.