Space junk is dangerous because the objects tend to collide and generate a lot more junk. This is already happening.

There is a danger that the process will eventually cascade and make it too dangerous to put anything into low Earth orbit, or maybe even to cross it. The movie Gravity depicted this disaster, in somewhat accelerated form. The disaster itself would be so harmful to humanity, and so permanent, that the survival of some of those who were in orbit hardly reduces it.

In the quotation from the ESA, does "mitigating the impact of existing objects" refer to "impact" in a physical sense, or in a metaphorical sense? That's amusing as a double entendre. I wonder if it was intentional.

Assuming it means the metaphorical sense, that sentence illustrates the way "mitigate" introduces vagueness. I think we should avoid that word.