I think that was supposed to be some kind of small black hole passing through the solar system or something more exotic. Maybe I'm misremembering. Great book though and still haunts me sometimes when staring up at the night sky:)

An 8 MT impact on the Moon is what the Moon calls "Tuesday." It has dealt with far, far worse.

The impact probably wouldn't even be visible with the naked eye unless it hits a part of the Moon not then illuminated by the Sun -- in which case one might see a brief flash of light.

SE leaves the question of exactly what impacted the Moon unspecified, referring to it as the "agent". Both narratively and in-story, the question of what the ramifications of the event are is far more significant than its origin. It's not possible to change the past, nor does an understanding (narratively or in-story) have any appreciable impact on what transpires as a consequence.

As others have noted, an 8 MT impactor on the Moon would be a quite minor event. It would likely be visible to terrestrial observers (if on the near-side) and leave a visible crater. Might generate ejecta which itself could enter the Earth's atmosphere over time, though likely with little effect on the ground.

This happens in the YA book Life As We Knew It. A really great book (for a YA audience). I still feel cold remembering it.

it's the agent...

Read that book over a year ago and it's still eating at me.