The 1998 QE2 Asteroid flew past us, winding it's way through our Solar System. This rock is so huge it has enough gravity to retain it's own satellite.
3 Kilometres across, the spherical rock passed just 6 million kilometres away. If you want to know how close that is, well the Moon is 200,000 kilometres away. so around 29 times further out than the moon.
In space terms, that is close, close enough for Earth's Gravity to affect it's future path.
Do we need to worry about 1998 QE2, no not really, it has been known for a while and it's orbit does not put it at risk of hitting us. But the risk is objects of this size we do not know about yet.
An object 3 Kilometres across would be a species ending event. It would vaporise most surface life, kill most plant life. Only burrowing animals would be likely to survive if they had enough food to last the cold barren period.