The US Navy's RIM‑161 Standard Missile 3 (SM‑3) serves as a cornerstone of sea-based ballistic missile defense, protecting naval forces and regional allies from modern threats.

What sets the SM-3 apart is its ability to destroy incoming ballistic missiles during the midcourse phase of flight -- outside the Earth's atmosphere -- using sheer kinetic force rather than an explosive warhead.

This approach delivers a direct impact with enough energy to obliterate high-speed targets, including intercontinental ballistic missiles.

With its exoatmospheric reach, advanced guidance systems and integration with Aegis platforms on ships or land, the SM-3 offers a layered, flexible defense against some of the most sophisticated missile threats.

It is one of several Standard Missile variants, enabling multi-tiered naval air defense. The SM-2 is used to engage aircraft and cruise missiles in the terminal phase, while the SM-6 provides over-the-horizon defense.

US Navy cruisers and destroyers use the Aegis Combat System, which employs a complex array of radars and sensors, to integrate the SM-2, SM-3, SM-6, Evolved Sea Sparrow Missile (ESSM) and other defense systems.

Aegis-equipped ships use the AN/SPY‑1 radar to detect incoming missiles before launching interceptors from their Mk 41 Vertical Launching Systems (VLSs).

Three-stage power

Derived from the same booster and control systems used in the SM‑2 Block IV, the SM‑3 adds a new third-stage rocket motor designed for exoatmospheric interception, extending the Aegis system's defensive capabilities.

The missile operates in stages. During the first two stages, a Mk 72 solid-fuel booster and Mk 104 dual-thrust motor launch and guide the missile through the atmosphere with precision.

Once the missile exits the atmosphere, its Mk 136 third stage lights up, propelling the interceptor into space.

There, the Lightweight Exo‑Atmospheric Projectile (LEAP) kinetic warhead separates and homes in on its target, guided by infrared sensors and Aerojet’s Throttleable Divert and Attitude Control System (TDACS).

It also receives continual mid-course updates from the launching ship and GPS.

Delivering over 130 megajoules of destructive energy, SM‑3 is designed to neutralize even maneuverable warheads in midcourse flight. It has evolved through a series of block upgrades, each expanding its capabilities and reach.

Standard Missile 3 variants

The SM-3's Block IA and IB variants were built to counter short- to medium-range ballistic missile threats.

Block IB introduced a more advanced seeker and improved the missile's ability to distinguish between actual warheads and decoys, sharpening its accuracy during high-speed midcourse engagements.

The most significant leap came with the development of Block IIA, a larger and more powerful version co-developed with Japan.

With a wider 21-inch body, an upgraded kinetic warhead and a faster burn motor, the Block IIA can reach speeds exceeding Mach 13 and engage targets at much higher altitudes.

In a November 2020 test, USS John Finn launched an SM‑3 Block IIA that successfully intercepted an ICBM-class target in space northeast of Hawaii -- the first time a sea-based interceptor had ever achieved such a feat.

The SM‑3 is not limited to ships. Land-based Aegis Ashore installations, like one in Romania, employ Block IB interceptors in Mk 41 launchers, protecting Europe from short- to intermediate-range ballistic threats.