The initiation of detonation by blunt projectiles traveling at supersonic speeds is experimentally investigated. A blunt projectile traveling through a detonable gas may induce combustion which occurs decoupled from the bow shock or may fully couple to the bow shock, resulting in a free-running detonation. The boundary between these two phenomena is the subject of this work. Spheres were fired from a gas gun into a mixture of stoichiometric hydrogen and oxygen with 70% argon dilution. The diameter of the spheres varied between 4.8 mm and 25 mm, and the Mach number of the spheres ranged from Mach 2 to Mach 6.5. The initial pressure of the mixture varied between 0.4 atm and 10 atm. The results of each experiment were determined from pressure transducers mounted on the walls of the test chamber. It was necessary to isolate the experiment from the influence of the walls of the chamber and from the diaphragm used to contain the gases. Without an inert gas buffer after the diaphragm, initiation occurred at Mach 2 as a result of unsteady shocks generated by the sphere impacting the diaphragm. When a buffer of inert gas was used, an autoignition limit around Mach 3.2-3.4 was observed which was independent of sphere size. Below this Mach number, the sphere was unable to detonate the mixture. When the sphere was traveling at the Chapman Jouguet (CJ) detonation speed of the mixture, the critical mixture pressure required for initiation decreased as the size of the sphere increased. These results are compared to a simple theory for detonation initiation by blunt projectiles due to Lee and Vasiljev which equates the critical energy per unit length required to initiate a cylindrical detonation with the work done by the drag of the projectile. While this theory was formulated for hypersonic speeds, where the projectile is traveling faster than the CJ speed, it agrees well with the results for a sphere traveling at the CJ speed, even as the sphere's size is varied by a factor of 5.