How far in advance can science predict a fatal asteroid impact?

In the case of an asteroid impact on Earth, we would normally know in advance when the collision will occur by calculating the orbit of this body . So what you write is correct: through observation and programs such as “space watch” (http://spacewatch.lpl.arizona.edu/) we will be able to predict this catastrophe years (> 10) before the impact. But the question remains whether we can avoid such an impact.

With a comet this is less easy and we may have a much shorter time to know when the comet will hit (~1 year). Consider, for example, the collision of comet Shoemaker Levy 9 against Jupiter in 1994. The first observation of this comet was only made in the summer of 1993.

regards

Philippe CLAEYS (VUB)