The number of independent warheads capable of deployment from ballistic missiles depends on the weapons platform the missile is launched from. For example, one D5 Trident missile carried by an is capable of launching eight independent warheads, while a has missiles capable of deploying 10 warheads at a time. MIRV has a number of advantages over a missile with a single warhead. With small additional costs, it allows a single missile to strike multiple targets, or to inflict maximum damage on a single target by attacking it with multiple warheads. It makes anti-ballistic missile defense even more difficult, and even less economically viable, than before. Missile warheads in the American arsenal are indicated by the letter "W"; for example, the W61 missile warhead would have the same physics package as the B61 gravity bomb described above, but it would have different environmental requirements, and different safety requirements since it would not be crew-tended after launch and remain atop a missile for a great length of time. Cruise missile A cruise missile is a jet engine or rocket-propelled missile that flies at low altitude using an automated guidance system (usually inertial navigation, sometimes supplemented by either GPS or mid-course updates from friendly forces) to make them harder to detect or intercept. Cruise missiles can carry a nuclear warhead. They have a shorter range and smaller payloads than ballistic missiles, so their warheads are smaller and less powerful. The AGM-86 ALCM is the US Air Force's current nuclear-armed air-launched cruise missile. The ALCM is only carried on the B-52 Stratofortress which can carry 20 missiles. Thus the cruise missiles themselves can be compared with MIRV warheads. The BGM/UGM-109 Tomahawk submarine-launched cruise missile is capable of carrying nuclear warheads, but all nuclear warheads were removed. Cruise missiles may also be launched from mobile launchers on the ground, and from naval ships. There is no letter change in the US arsenal to distinguish the warheads of cruise missiles from those for ballistic missiles. Cruise missiles, even with their lower payload, have a number of advantages over ballistic missiles for the purposes of delivering nuclear strikes: Launch of a cruise missile is difficult to detect early from satellites and other long-range means, contributing to a surprise factor of attack. That, coupled with the ability to actively maneuver in flight, allows for penetration of strategic anti-missile systems aimed at intercepting ballistic missiles on calculated trajectory of