Cartoon showing the shock structure formed by a hypersonic, dense bullet moving through a cold medium. The flow vectors are shown in the rest-frame of the bullet. The forward shock velocity V₀ is nearly identical to the bullet velocity and heats the post-shock ambient medium to at temperature ~10⁶ K. The reverse shock propagated back into the bullet with a speed V_R ~ V₀(n₀/n_B)^{1 / 2}, where n₀ is the density of the ambient medium and n_B is the bullet density. The transverse expansion of the hot, high-pressure, post-shock medium sweeps-up the colder, lower-pressure ambient medium to form the H₂ wake. Only the transverse component of the vector V_side is shown. The fluid velocity of the material swept-up by the expansion of the hot, shocked ambient medium is the vector sum of the pre-shock ambient medium (as seen in the reference frame of the bullet), and the velocity of the sideways splash produced by the expansing hot plasma.