Caption: A head-on impactor asteroid aimed at Earth!!!
Features:
Most impactors probably hit with a lower relative velocity of order kilometers per second (km/s) or 10s of kilometers per second (km/s) since they should mostly hit us somewhere between from behind and the side. This is because almost all or all significant solar system bodies orbit the Sun counterclockise as view from way above the North Pole.
So head-on collisions are a bit unlikely.
However, a long-period comet might be a head-on impactor. Long-period comets come from the Oort Cloud: a vast spherical shell of rocky-icy bodies in orbit around the Sun. Oort Cloud objects (OCOs) can have any orbital orientation and direction relative to the ecliptic plane: i.e., the plane of the Earth's orbit.
E_TNT = (625.7 ... Megatons TNT)*(density/1 gm/cm**3)*(diameter/1 km)**3*(velocity/1 km/s)**2 ,where we use for fiducial values for density the fiducial value water density = 1 gm/cm**2, for diameter 1 km, and for relative velocity 1 kilometer per second (km/s).
For comparison, the most powerful hydrogen bomb the Tsar Bomb yielded 50 Megatons TNT.