Why is impacting a "rubble pile" asteroid with a kinetic impactor risky?

A high-speed impact could risk shattering it into multiple, equally dangerous fragments.

Characterization is vital because the physical makeup of a threatening asteroid dictates the necessary deflection strategy. If an object is determined to be a monolithic piece of solid rock, a kinetic impactor mission—involving striking the asteroid with a high-speed spacecraft—might be an effective mitigation technique. However, if the asteroid is classified as a 'rubble pile,' meaning it is a loosely aggregated collection of boulders held together primarily by weak gravitational forces, a forceful kinetic impact carries the severe risk of shattering the object. This fragmentation would result in multiple pieces, each potentially still posing a dangerous impact threat, complicating hazard mitigation significantly.