[SiempreViernes]

Impactors are indeed not seen that far ahead in comparison to the Hubble time allocation time scale, so for scheduling purposes they do more or less pop up out of nowhere.

Since its not very likely that you happen to have a scheduled science target lying close to Jupiter around the time of impact, you would have to do a long slew which now takes such a long time you likely have to cancel some other science. But I think this is a general issue that will discard most targets of opportunity that Hubble could otherwise have observed.