

The weapon that the Pentagon wants to use to shoot down a failing satellite is well-tested. But its one weakness could pose a problem, as the military tries to take out that satellite before it hits the atmosphere. Center for Defense Information missile specialist Victoria Samson explains...

Today, the U.S. Aegis Ballistic Missile Defense system uses the Standard Missile (SM)-3 to make what are known as "direct intercepts." That means there's no explosion in space. Instead, the SM-3 destroys its target by ramming into with lots and lots of sheer kinetic energy – more than 130 megajoules worth, or "the equivalent of a 10 ton truck traveling at 600 miles per hour," it maker says.

The missile uses two stages to get into near-space, and a third, kinetic warhead to strike. The interceptor finds its targets largely because its seeker has been programmed to look for a certain type of object. (The SM-3 Block1B, the upgrade to the current model, is supposed to have a two-color infrared seeker which presumably would increase its discrimination capabilities.)

Right now, the SM-3 has tested against short- and medium-range ballistic missiles, and has made twelve intercepts out of fourteen attempts during testing. These intercepts have been made while the target is in space (tests have been at altitudes of over 100 miles), which provides it with a hot target against the cold environs of space.

The most recent flight test intercept attempt by a U.S. ship occurred on Nov.6, 2007 and was a success.

Raytheon, which makes the interceptor, explains how it's supposed to work:

As a ballistic missile threat rises above the horizon, [the] ship’s radar acquires, begins tracking, and the weapon system begins calculating the engagement solution. Upon command from the ship’s weapon system, the SM-3 boosts out of the launcher and establishes radio communication with the ship. After MK 72 booster burnout, the MK

104 Dual Thrust Rocket Motor (DTRM) ignites. In-flight communications from the ship guide the missile toward the predicted intercept point.

After MK 104 burnout and separation, the MK 136 Third Stage Rocket

Motor (TSRM) ignites, propelling the third stage out of the atmosphere.

Throughout its flight, the missile continues to receive in-flight target updates from the ship to refine the intercept guidance solution.

The TSRM contains two separate pulses that can be initiated to optimize the engagement timeline. During flyout, the third stage pitches over and ejects the nosecone, exposing the SM-3 Kinetic Warhead (KW).

Following TSRM burnout roughly 30 seconds before intercept, the SM-3 KW

separates from the third stage and immediately searches for the target based on pointing data received from the ship. The KW acquires the ballistic missile warhead with its long-wavelength imaging infrared seeker. The KW’s Solid Divert and Attitude Control System precisely maneuvers the KW to enable a hit-to-kill intercept. As the KW

closes on the target, it will identify the lethal payload area and shift its guidance aimpoint to ensure a lethal hit, destroying the target with more than 130 megajoules of kinetic energy, or the equivalent of a 10 ton truck traveling at 600 miles per hour.

But things have gone wrong. The primary cause of one of the few U.S. flight test failures is the new guidance control system, the Divert and Attitude Control System, whose ceramic components cracked during a test. This problem has not been remedied and is not being used in its most advanced mode. Which means that the SM-3's maneuverability against more demanding targets may be affected. Given how all of the tests so far have dealt with targets that we knew down to the smallest detail how they would behave in-flight, this means that it has not been tested against unknown targets that may require last-minute changes in its trajectory.

The Pentagon has, as of the end of 2007, 21 SM-3 interceptors. It's not like they have a huge inventory to draw upon. For this proposed shoot-down, the Pentagon is claiming that it will make modifications to three SM-3 interceptors so that they can look for the malfunctioning satellite. The modifications involve changing the software to target a satellite rather than a missile, David Wright of the Union of

Concerned Scientists tells New Scientist. "This interceptor is really intended for missiles traveling at 3 to 4 kilometers per second; the satellite they're going to be shooting at has a speed of 7 to 8 kilometers per second."

But the change draws attention to one of the program's biggest flaws: if it doesn't know what to look for, it is unlikely to make an intercept (situational awareness is pretty slim). And since the tests to date have all been highly scripted, the system has not had any practice of on the fly (if you'll excuse the phrase) intercept attempts.

– Victoria Samson

(Image: Raytheon)

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