As a duck paddles across a pond, it creates ripples or waves that move out in front of it. If the duck paddles fast enough, the ripples will merge into a V-shaped wall of water called a bow wave. Bow waves are familiar sights in front of boats as well, and can also be formed
in the atmosphere and in space when objects move more rapidly than the speed of waves in their liquid or gas environments. These waves are usually called bow waves when they are produced in water, and bow shocks, or shock waves, in gases.
Bow waves in water create drag force that slows down the object, and in the case of large ships, can pose a risk to smaller boats, especially in a harbor. Streamlined shapes, mimicking those found in nature, are designed to produce as small a bow wave as possible.
An object moving through the air, or the rarified gas in the solar system, a galaxy or intergalactic space, of space, creates a series of pressure waves, or sound waves. When the object travels at the speed equal to or greater than the speed of sound in the gas, these waves merge into a 3-dimensional bow shock, or shock cone. The width of the bow wave or shock cone depends on the speed of the object. The faster the object travels, the narrower the cone becomes.
A nice applet illustrating the formation of bow waves, a.k.a. bow shocks can be found at:
The Mach number M of an object moving at a speed V through a gas with a sound speed Vs is defined as M = V/Vs. A shock wave is formed when M = 1, sometimes referred to as Mach 1, motion with M > 1 is called supersonic motion. For air at sea level at 20 degrees C, an object reaches Mach 1 at 1,225 km/hr (761 mph, or 1,116 feet/s). The shock wave forms a cone, with an opening angle A determined by the Mach number (sin A = 1/M) with higher Mach numbers producing cones with smaller opening angles.
With so many objects out there moving so fast, it is not surprising that bow shocks are common in the cosmos. In general, cosmic bow shocks can be difficult to detect because they are relatively thin, and are usually found in an environment where a lot is happening. In spite of this, astronomers have used the Chandra, Hubble and Spitzer telescopes to collect some spectacular examples of bow shocks. The bow shocks are like weather vanes: they can reveal which way the wind is blowing. They can also tell how strong the wind is.
Image: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
Back to Bow shocks page
"Here, There, & Everywhere" (HTE) is supported by the National Aeronautics and Space Administration under grant NNX11AH28G issued through the Science Mission Directorate.