Final answer:
Most asteroids are discovered when they leave a trail in long-exposure photographs, further enhanced by modern electronic cameras and techniques. The past use of astronomical photography and current wide-field telescopic surveys play key roles in detecting these celestial objects.
Step-by-step explanation:
Most asteroids are discovered C. when they leave a trail in long-exposure photographs. By the late 19th century, astronomers like Max Wolf used astronomical photography, which significantly increased the discovery rate of these faint objects. In the modern era, electronic cameras and space-based surveys such as the NASA Spaceguard Survey have further enhanced our ability to detect and track asteroids.
The long-exposure photographs capture the motion of asteroids as faint streaks against the static background of stars, making them identifiable to observers. Today, the majority of asteroid discoveries are made through automated searches using sophisticated wide-field telescopes and electronic detection methods, rather than by direct visual observations or spacecraft flybys. Satellites and ground-based telescopes continuously watch the skies, and when asteroids pass in front of their field of view, they are captured in these images and can be analyzed for potential threats to Earth.