If an approaching asteroid were detected tomorrow, perhaps 20 years would be required to build and launch an impactor, to reach and impact the target, and to nudge the asteroid from Earth’s path.

However, decades or more might be required to deflect larger asteroids (hundreds of kilometers in diameter) that present the most catastrophic threats.

If time allows, a mission to study the asteroid up close and send information back to Earth before sending the impactor could greatly increase the chance of success.

Kinetic impactors may not be effective in changing the orbit of the very largest asteroids.