Nudol anti-satellite test
Space junk can grow on its own, through collisions which cause more debris which is less predictable. But worse, collisions cause more collisions, and might at some stage cause a "cascading Kessler Syndrome", effectively closing space.
But at this stage it's deliberate collisions that might be the threat to watch. At least four countries are developing kinetic anti-satellite weapons, also known as "hit-to-kill" weapons, either against incoming ICBMs, or other satellites.
Debris from a Chinese test in 2007 damaged a Russian satellite two years later. More debris from the same test passed within 6km of the International Space Station in 2011.
More recently (in March 2019), India successfully demonstrated its own capability. The test was deliberately carried out in a low orbit to limit the spread of debris, but recent reports from the Arms Control Wonk claim that some debris from this test remains a threat more than a year later.
One solution is to use a "virtual target". This is what Russia has recently done, in a test of its Nudol kinetic anti-satellite system. Targets have been a defined, moving, but empty point in space, and the test would be considered a success if the missile flew right through that point.
It's not all great though, Russia is likely to want to test its system against physical targets at some stage.
The alternative solution is that countries begin to clean up increasingly crowded orbits. Controlled craft may be able to either catch threatening junk, or direct it into the atmosphere where it can burn up harmlessly.
There's a lot of work to do. But if humans want to continue to venture into space, we need to prevent the orbits near Earth from becoming a literal minefield. This is going to take agreement, co-operation and effort.
For more, go to the Arms Control Wonk.