As a side effect of forming the thermo-chemical energies over the surface of his body, Cannonball is rendered virtually invulnerable while in flight.
Accompanying the release of energy is a half-inch thick energy field that channels the explosion and protects his skin from the direct effects of the blast.
It also negates momentum and related effects, thereby cushioning his body from any impact up to a half-minute from the depletion of his energy. Cannonball's "blast field" extends to any person or object with which he is in physical contact, and he can extend it further by concentrating to form a protective shield, even while standing still.
Cannonball is also able to absorb kinetic energy through the field he generates and use it to either strengthen his field or redirect it as concussive force.
Cannonball once recovered from mortal injuries that left him seemingly dead.
It has been theorized that Cannonball's recovery was an indication that he was an External, one of a group of immortal mutants. However, since it has subsequently been demonstrated that even Externals can be permanently killed, doubt has been cast on his alleged immortality.
Cannonball | Characters | Marvel.com