Detection and High Resolution Tracking of Vehicles at Hypersonic Velocities

The Defense Mach number expresses that ratio. 7 kilometers per second — as the lower boundary of the hypersonic regime. Within that regime it distinguishes "low hypersonic" flight between Mach 5 and about Mach 10 from "high hypersonic" flight between approximately Mach 10 and Mach 30 or above.

At Mach 30, near 10 kilometers per second, velocities approach those of Space Shuttle reentry. The central question the report poses is not merely whether such objects can be seen, but what physical signatures they unavoidably generate and which of those signatures existing sensor architectures can exploit. The document notes that few objects travel at hypersonic velocities at all.

The most familiar are meteors, whose entry speeds may reach 30 miles per second — a Mach number exceeding 150 — and which are preceded by a bow shockwave that compresses the air ahead of them. Across that shock, temperatures and pressures rise…