Not sure your new twist holds water, BG, on two counts:
1) If someone jumped the thing in and it is 'guided' on a collision course, that suggests the collision is deliberate, and surely a planetoid planet-killer ship would be guided to do its job very quickly - being jumped in quite close and accelerated to target in a matter of hours/days?
2) If the planetoid was close enough to look like a moon, and was travelling at typical planetoid speed, I would imagine (I haven't done the calcs) it would impact in a matter of hours - by the time your heroes had made their decision, readied their ship and flown up there, the planetoid would have covered half its remaining trajectory.
I would suggest that if it's a ship:
a) it's no longer under control, perhaps with a decayed orbit that has sent it on an accidental collision course.
b) it's not visible from the planet - by the time it is, it'll probably be too close for any human-achievable nudge to deflect it away (the closer it is, the greater the deflection angle needed - even a drive thrust vector may not be enough).