Definitely isn't something I'd want to defend against unprepared, but it isn't an unmitigable threat.
Any hypersonic projectile has to keep itself from cooking (heat shielding), and keep itself controllable. Compromising any of those attributes with current materials can be done with a bloody foam chunk (see the Columbia disaster). Just about anything that isn't gaseous, and happy with behaving as fluid is problematic at hypersonic speeds. Though I'd probably start at ball bearings or other types of flak just because of ease of manufacture.
If there's anyone bored enough, a frozen chicken impact simulation would be quite amusing to see.
The problem set you have to solve for successful payload delivery is basically the same as a reentering spacecraft. No one has really looked at doing something like this before, because the cost of slapping something like this together just to blow it up in the end as a tactical weapon has always been more on the prohibitive side compared to just finding better ways to get conventional weapons in place.
The only great innovation here is the innovation with which missile fabricators can dupe someone into buying yet more expensive missiles, and maybe some supply chain/manufacturing wins for some of the more exotic materials.
What I'm curious about is how good the navigation capabilities are, and the structural limits of the frame. Drastic course correction at Mach 5÷ has got to be absolute hell on the frame.
However, if you could create some standing network like a mesh of barrage balloons or on-station drones, perhaps you could avoid the need to launch into the path? You could transmit commands to pop a bunch of pre-positioned flak shells (or some kind of chaff with more hang-time?) in a spheroid patch of sky at a certain time. Of course, this means having a clearly defined theater where the targets and potential launch sites are far enough apart to feasibly deploy your virtual wall or dome of protection in the area in between. It also would be a bit like an ablative armor. Someone could probably focus an attack and exhaust the locally available counter-measures.
A cursory search indicates ICBMs have terminal velocity comparable to, or higher than, of hypersonic missiles, at about 16...34'000 km/h. The key difference is that a ballistic missile has recognizable launch signature -you can detect it via satellites- and coasts high above the atmosphere for several minutes, during which it's readily detectable by radar. We already have tech that reliably intercepts & destroys ICBMs - counter-rockets like Sprint/HIBEX [1], lasers, and even small nukes.
A hypersonic missile could fly low-ish [2], perhaps even "on the deck", thus evading long range radar detection. Still we could probably detect, from satellites, the heat signature of super-heated plasma around the missile.
I think the most stealthy -and dangerous- scenario would be a slow (trans-sonic, or maybe Mach 2) low-level cruise to mask the heat signature, followed up by a hypersonic dash to the target. Essentially leaving very little time to make decisions and launch counter-measures.
>If you are talking about putting the debris in its path after observing the incoming missile, how can it be stationary?
Relatively stationary. Dispersing several tens (or perhaps hundreds) of kilograms worth of small metal cubes or balls in area of a few hundred meters ought to be enough to hit a hypersonic missile with reasonably high kill probability. Basically old-fashioned flak, except delivered by a small rocket. The missiles own high speed would be enough to cause destructive damage upon hitting even small piece. This photo [3] shows effects of impact of tiny 7 gram particle at comparable velocities - some 25'000 km/h. A gaping hole like that in the heat shield would quickly lead to further damage in a hypersonic flight regime [4], perhaps also aerodynamic instability. And that's if the initial shock doesn't disable the mechanisms right away.
[1] https://www.youtube.com/watch?v=xZTjA44T6tY&feature=youtu.be...
[2] however low level hypersonic flight is very fuel-hungry, and causes tremendous heating; both significantly limit endurance
[3] https://en.wikipedia.org/wiki/File:SDIO_KEW_Lexan_projectile...
[4] https://en.wikipedia.org/wiki/Space_Shuttle_Columbia_disaste... was triggered by a small-ish hole of some 15...25cm in thermal protection