A river from a burst dam can exactly keep pace with a pickup truck for several minutes. It will then obligingly pause as the pickup truck turns around and goes in another direction. When a raging river washes over a pickup truck on a bridge, the bridge won't be damaged, the truck won't be swept off the bridge, and people in the open back of the truck won't be swept away. But with only two airplane-mounted lasers, it instead instantly explodes into thousands of pieces. Astronomers are very surprised that it wasn't literally destroyed.
Laser beams are easily visible in space.
Incoming asteroids spend several minutes in Earth's atmosphere. Asteroids made of softer or more volatile stuff than nickel will harmlessly burn up in the atmposphere regardless of size. Asteroids that land in the ocean will do no damage regardless of size. Asteroids are discovered by astronomers peering directly through their telescopes in brightly lit observatories. Whatever they see will appear on computer monitors, however.