Caption: A head-on impactor asteroid aimed at Earth!!!

Features:

1. Newtonian physics dictates that any impactor in orbit near the Earth's orbit must have a comparable orbital velocity to that of the Earth's: both of which are measured relative to the free-fall inertial frame of the Solar System barycenter which is approximately at the Sun with orbital rotation measured relative to the observable universe (which is considered to be absolute rotation).

2. Since the Earth's mean orbital speed = 29.78 km/s, a head-on impactor asteroid would have relative velocity toward the Earth of ∼ 60 kilometers per second (km/s). Pow!!!!

   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.

3. However even at kilometers per second (km/s) or 10s of kilometers per second (km/s) an the impact will be devastating for any impactor asteroid or comet of order 1 km in diameter or larger. All the impactor's kinetic energy gets rapidly converted to heat energy via a deep impact and the heat energy explosively vaporizes surrounding rock creating a massive explosion.

4. The following fiducial-value formula gives the TNT equivalent for impact events:

   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.

5. Cretaceous-Paleogene (K-Pg) extinction event (a mass extinction) of 66 Myr Before Present (BP) (which extincted the dinosaurs, except for the birds) is believed to have been caused by an impactor asteroid or comet of order 10 km in diameter (which cubed gives a factor of 10**3 increase over the fiducial yield) with a relative velocity of order 10 km/s (which squared gives a factor of 10**2 increase over the fiducial yield): there is, in fact, a net factor of 10**5 increase over the fiducial yield.

6. To be concrete, for the estimated values, the fiducial-value formula gives a yield of order 10**8 Megatons TNT. Better estimates also give ∼ 10**8 Megatons TNT (see Wikipedia: Cretaceous-Paleogene (K-Pg) extinction event: Effects of impact).

7. The impactor probably caused vast plumes of hot solid and molten rock to be thrown up and fall back all around the Earth causing global or at least continental firestorms. Then there was probably a multi-year global impact winter caused by aerosols lofted into the Earth's atmosphere and other bad things (see Wikipedia: Cretaceous-Paleogene (K-Pg) extinction event: Effects of impact). But such large impactors are relatively rare and we only have to worry a little about them relative to other things: e.g., pandemics, climate change, nuclear war, etc.