An air-to-surface missile (also, air-to-ground missile, AGM, ASM or ATGM) is a missile designed to be launched from military aircraft (bombers, attack aircraft, fighter aircraft or other kinds) and strike ground targets on land, at sea, or both. They are similar to guided glide bombs but to be considered a missile, they usually contain some form of propulsion system. The two most common propulsion systems for air-to-surface missiles are rocket motors and jet engines. These also tend to correspond to the range of the missiles - short and long, respectively. Some Soviet air-to-surface missiles are powered by ramjets, giving them both long range and high speed.
Guidance for air-to-surface missiles is typically via laser guidance, infrared guidance, optical guidance or via GPS signals. The type of guidance depends on the type of target. Ships, for example, may be detected via passive or active radar, while this wouldn't work very well against land targets which typically don't contain such a large mass of metal surrounded by empty space.
There is some cross-over between air-to-surface missiles and surface-to-surface missiles. For example, there was an air-launched version of the Tomahawk missile, although this has been superseded by the AGM-86 ALCM. Other missiles used in both roles include the Penguin anti-ship missile and AGM-84 Harpoon anti-ship missile. Many air-to-surface missiles can be used against both ships and land targets, although some of them have to be modified to perform both roles effectively. For example, the Standoff Land Attack Missile is a land-attack version of the Harpoon.
One of the major advantages of air-to-surface missiles over other weapons available for aircraft to use to attack ground targets is the standoff distance they provide. This allows them to launch the weapons outside the most intense air defences around the target site. Most air-to-surface missiles are fire-and-forget in order to take most advantage of the standoff distance - they allow the launching platform to turn away after launch. Some missiles have enough range to be launched over the horizon. These missiles (typically either cruise or anti-ship missiles) need to be able to find and home in on the target autonomously.
Sub-categories of air-to-surface missiles include:
Typically, the higher and faster the launching aircraft is flying, the further away the missile's target can be. For long range missiles this difference can be small, but short range missiles (like the AGM-65 Maverick) often dramatically increase in range when launched at altitude.
There have been examples of air-launched ballistic missiles, but they are rare. Sometimes air-to-surface missiles are divided into the categories of tactical and strategic. Typically this indicates conventional explosive or small nuclear warhead (tactical) and large nuclear warhead (strategic).