Revise GCSE Physics

Question: Why is it difficult to observe NEO's?

NEO's do not emit light of their own. We can only see them because light from the Sun is reflected from their surfaces.

NEO's are usually quite small and very far away, so the amount of reflected light is tiny, and they are therefore quite hard to find, until they are close to the Earth or until they pass across another bigger object that we happen to be looking at.