Ballistic missiles are means to rapidly and accurately deliver a lethal payload to a target. The lethal payload can include conventional explosives, biological, chemical or nuclear warhead. Ballistic missiles are very cheap, which makes their proliferation more likely and ensure that their numbers will be rising in the coming future.
Once its fuel has been consumed, the ballistic missile follows an elliptical orbit around the center of the Earth, defined strictly by the combination of velocity/flight angle at burnout and the Earth's gravity. Ballistic missiles can be solid or liquid propelled. Liquid propellants are relatively cheaper, but they are less stable (and so more difficult to store) and more toxic. Solid propellants are more expensive, but more easily maintainable and more stable. Hybrid fuels are under development combining benefits of solid- and liquid-propelled ballistic missiles.
By careful control and maneuvering of the missile during its powered flight, the payload can be very accurately delivered to the desired target point.
Operational ballistic missiles are deployed in silos, on submarines, on ships, on planes and on land-mobile launchers (trucks or rails). Mobile missiles are favored by many nations because they can be hidden, which greatly increases their survivability. They are also very mobile and relatively difficult to defend from.
Longer-range ballistic missiles can carry Multiple Independent Reentry Vehicle (MIRVs) warhead, which hold up to 10 reentry vehicles. Reentry vehicles reenter the Earth's atmosphere at very high velocities, on the order of 4-5 miles per second. Such a warhead is more difficult to intercept since we have 10 targets instead of one. Some countries also develop maneuverable reentry vehicles that would complicate the intercept further. Those are referred to as maneuvering reentry vehicles and have independent maneuvering capability.
Ballistic missiles are composed of one or more stages. Multiple-stage missiles, which are configured so that each stage has its own independent propulsion system, are used for longer range missions. Intercontinental ballistic missiles (ICBMs) typically have two or three stages with powerful liquid-propellant engines or solid propellant motors that propel the payload toward the target, as well as a post-boost vehicle (PBV) with a much smaller propulsion system. The technology needed to separate each of the stages in high velocities and under difficult atmospheric conditions is relatively sophisticated and difficult. That is why there are only few countries with intercontinental-range ballistic missile technology (e.g. Russia, China, and India).