4 ideas
In Jayant Narlikar's children's science-fiction story, an amateur astronomer discovers a comet, and follow-up orbit calculations show it is on a collision course with Earth. Professional scientists then work out a plan to deflect it before impact. The story was made into a film in 1985, making it an early popular depiction of planetary defence built on an ordinary observer's find.
Detecting a rare, unpredictable threat depends on how many eyes are watching the sky, and amateurs who scan it patiently add coverage that small professional teams cannot provide. The first signal can come from outside institutions, and experts then confirm the object, compute its orbit, and act on it.
An incoming comet does not have to be destroyed. Changing its velocity slightly while it is still far from Earth can make it miss.
Presenting science as a group of people working against a hard deadline shows children the real sequence of the work: observe, calculate, predict, then engineer a response. Casting the calculation itself as the heroic act teaches that quantitative prediction is what turns a cosmic danger into a problem people can solve.