A bow shock, also called a detached shock, is a curved, stationary shock wave

Shock wave

A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

 that is found in supersonic

Supersonic

Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...

 flow past a finite body. Unlike an oblique shock

Oblique shock

An oblique shock wave, unlike a normal shock, is inclined with respect to the incident upstream flow direction. It will occur when a supersonic flow encounters a corner that effectively turns the flow into itself and compresses. The upstream streamlines are uniformly deflected after the shock wave...

, the bow shock is not necessarily attached to the tip of the body. Oblique shock angles are limited in formation and are based on the flow deflection angle, upstream Mach number

Mach number

Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...

.
When these limitations are exceeded (greater deflection angle or lower Mach number), a detached bow shock forms instead of an oblique shock. As bow shocks form for high flow deflection angles, they are often seen forming around blunt objects.
In other words, when the needed rotation of the fluid exceeds the maximum achievable rotation angle for an oblique attached shock, the shock detaches from the body. Downstream of the shock, the flow-field is subsonic

Speed of sound

The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....

, and the boundary condition can be respected at the stagnation point

Stagnation point

In fluid dynamics, a stagnation point is a point in a flow field where the local velocity of the fluid is zero. Stagnation points exist at the surface of objects in the flow field, where the fluid is brought to rest by the object...

.

The maximum flow deflection angle for a given Mach number can be found using shock-expansion theory.

The bow shock significantly increases the drag

Drag (physics)

In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

 in a vehicle traveling at a supersonic speed. This property was utilized in the design of the return capsules during space missions such as the Apollo program, which need a high amount of drag in order to slow down during atmospheric reentry

Atmospheric reentry

Atmospheric entry is the movement of human-made or natural objects as they enter the atmosphere of a celestial body from outer space—in the case of Earth from an altitude above the Kármán Line,...

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