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Brillian Pebbles illustration
Space-based missile defense systems like the old Brilliant Pebbles proposal would face technological challenges, not to mention political concerns, as a tool for boost-phase missile defense. (credit: USAF)

The fallacy of space-based interceptors for boost-phase missile defense

Last week, Taylor Dinerman posited an argument for using space-based missile defense to counter ballistic missile threats in the boost phase (see “Space-based missile defense and the psychology of warfare”, The Space Review, September 8, 2008). While ballistic missiles are indeed very serious threats and can be used as psychological weapons of terror, space-based missile defense is far from being a desirable solution for countering them, especially in the boost phase.

Mr. Dinerman is quite correct in his historical recounting and analysis of the use of ballistic weapons, from the German use of the V-2 during World War 2 to Iraq’s attempts to use Scuds during the first Gulf War. Research and development into methods of countering such potentially indiscriminate weapons is, rightly so, one of the top priorities for the United States and other nations dealing with these threats.

Under closer examination, several major technical issues prevent space-based interceptors from being effective against boost phase scenarios.

His article specifically argued for the use of space-based assets for boost-phase missile defense against “Scud-type” missiles. Boost-phase defense attempts to destroy a ballistic missile while its engines are still firing, before the warheads release and it enters the midcourse glide phase. From a military perspective, this is indeed a desirable phase to attack the missile. In addition to the psychological reasons Mr. Dinerman cites, it’s also the time when the missile is the most vulnerable and easiest to detect, and occurs before the missile can deploy midcourse countermeasures.

But under closer examination, several major technical issues prevent space-based interceptors from being effective against boost phase scenarios. To understand these, we first need to look at some of the parameters of ballistic flight, in particular the distance from the interceptor to the target, the burn time (the time it takes for the rocket engine to complete its burn), and the altitude at which burnout occurs.

What’s important for our discussion here is the relationship between range, burnout time and burnout height. Table 1 shows parameters for some existing one-stage liquid-fueled missiles up to 3,000 kilometers. As you can see, both burnout time and altitude increase with missile range.

Range (km)Burn time (seconds)Burnout alt (km)Burnout speed (km/s)Sample Flight Path
30062251.6North Korea to Seoul
50080352.0North Korea to Pusan
1000100503.0North Korea to Tokyo
30001401004.5Tehran to Warsaw

Table 1: Nominal parameters calculated for liquid-fueled ballistic missiles of various ranges. Technical calculations provided by David Wright of the Union of Concerned Scientists

For boost-phase missile defense, the burn time provides the time boundary of what’s referred to in military circles as “the kill chain”; the series of steps and time it takes for an attack to take place. To successfully intercept a ballistic missile, the defensive system needs to move faster than the attacker’s kill chain. This means the entire process, including detecting a launch, determining that it is a threat, providing initial guidance to the interceptor, launching the interceptor, reaching the intercept point, and terminal guidance all needs to happen within the booster burn time for boost-phase missile defense. A 2003 study by the American Physical Society (APS) determined that state-of-the-art sensors required at least 45 to 60 seconds to detect, track, and provide initial guidance for the launch of a ballistic missile.

The real liabilities of space-based missile defenses for boost phase interceptions are their distance from the target and the dictates of orbital mechanics. The only way to have a satellite “hover” over a specific spot on the Earth is to place it in geostationary orbit, 36,000 kilometers above the Earth. From this altitude, it would take a missile defense interceptor hours to travel to the target and thus it is not feasible. Placing the interceptors in low Earth orbit, say around 500 kilometers, would reduce the travel time greatly. But it would introduce another problem, called the absentee rate: interceptors at this altitude are only over any one point on the surface of the Earth for a very short period of time and thus are “absent” from the threat area for up to 99.5% of their orbit. The same 2003 APS study concluded that for boost phase missile defense, a massive number of interceptors would be required:

With the technology we judge could become available within the next 15 years, defending against a single ICBM would require a thousand or more interceptors for a system having the lowest possible mass and providing realistic decision time. Deploying such a system would require at least a five- to tenfold increase over current U.S. space-launch rates.

If designed to intercept only ICBMs and not shorter range missiles, the number of interceptors required becomes somewhat more palatable due to their longer burn times and higher burnout altitudes. A 2004 study by the Congressional Budget Office found that at an altitude of 500 kilometers, between 156 and 368 interceptors (depending on the velocity of the interceptors) would be needed to provide coverage for just ICBMs launched from Iran and North Korea towards North America, at a cost of between $29 billion and $84 billion. These constellations would only guarantee two interceptors over one point at any given time, meaning if the interceptors worked with 100% reliability they could only protect against two ICBMs launched at once. Fewer interceptors would be needed if they orbited at even lower altitudes, but that would severely impact their ability to stay on orbit and either require the launching of replacement interceptors every few years or the inclusion of maneuvering capabilities to continually re-boost the interceptors.

The real liabilities of space-based missile defenses for boost phase interceptions are their distance from the target and the dictates of orbital mechanics.

Space-based interceptors also have a lower limit on the altitude at which they can intercept a missile. This is due to blinding of their onboard infrared sensors, required for terminal tracking and guidance, caused by the intense atmospheric heating that results when the interceptor enters the atmosphere at high speed. Typically, space-based interceptors cannot intercept targets below 100 kilometers due to this effect. The numbers in Table 1 indicate that space-based interceptors could not engage liquid fueled missiles with ranges less than approximately 3,000 kilometers. Moreover, if countries develop solid-fuel missiles, which have shorter burn times and burn out at lower altitudes than liquid-fueled missiles, and fly their missiles on slightly depressed trajectories, even longer range missiles can underfly space-based interceptors.

Mr. Dinerman’s desire to have ballistic weapons destroyed over their launching state is also much harder than most assume. Hitting the large missile body, which is what most missile defense interceptors target, with an interceptor can leave the warhead intact. Moreover, a ballistic missile destroyed in mid-flight does not simply stop dead. The laws of physics, particularly that of inertia, dictate that any warheads will continue at whatever velocity they had when the missile was destroyed. Unless this happens very quickly after launch, typically seconds, the warheads will still travel quite a bit downrange, possibly impacting a third country outside the original target, as explained in the 2003 APS study:

A key problem inherent in boost-phase defense is munitions shortfall: although a successful intercept would prevent munitions from reaching their target, it could cause live nuclear, chemical, or biological munitions to fall on populated areas short of the target, in the United States or other countries.

Brilliant Pebbles and policy concerns

Mr. Dinerman’s apparent support of the Brilliant Pebbles program is troubling, given the actual programmatic desires and details of that program:

Smart Rocks was upgraded in 1988 and renamed ‘Brilliant Pebbles.’ In addition to eliminating incoming nuclear warheads, each component of the 4,000-satellite constellation was designed to protect U.S. space-based assets, attack its Soviet counterparts, or sacrifice itself in a one-time spy mission. The interceptor satellites would be controlled from the ground, but would also have the ability to communicate among themselves and attack their targets autonomously. At a projected cost of $11 billion [in 1988 dollars] for the first 1,000 interceptors, Brilliant Pebbles presented a cost-efficient means of countering the Soviet menace.

Setting aside the nontrivial issues of cost and technical feasibility, it is not clear that the proponents of Brilliant Pebbles have seriously thought through the implications of increasing the number of current satellites in low Earth orbit by a factor of 10. What pressures would this put on our ability to track and provide command and control for all these satellites? Is having 4,000 (or more) satellites with the capability to autonomously identify and attack missiles launched from the ground as well as other satellites really a good idea? What if they make a mistake?

To highlight just now easy it could be to make such a mistake, and the policy implications, consider a recent example. In July 2006, North Korea unsuccessfully tested its most advanced ballistic missile to date, the Taep’o-dong 2. Leading up to this event, there was a serious policy debate within the White House and the military, centered around whether or not to launch a first strike to destroy the missile before it launched. Ultimately, the Administration decided to accelerate deployment of its ground-based missile defense interceptors in Alaska and have that system on alert during the test.

If the United States had decided to preemptively strike the Taep’o-dong launch pad or destroy the missile in its boost phase, the North Koreans could have easily claimed it was an orbital launch vehicle carrying Korean children's letters to Santa Claus or perhaps a satellite broadcasting their message to the world. Since the same missile could be either a small space launch vehicle or an ICBM, there would be no way for the US to prove otherwise short of actually pulling off the nose cone.

This gets to the root of one of the great unanswered policy issues of missile defense: how do you prove it was actually a missile carrying a deadly payload and thus represented deliberate intent and endangerment to national security? What's the justification in the realm of international law for actively deciding to destroy another nation's property which could have been used for peaceful purposes and which is very hard, indeed almost impossible, to prove was being used otherwise? What if Russia used this same justification to destroy an Atlas launch from Vandenberg because they had classified intelligence that it was a weapon that could be used against them?

This gets to the root of one of the great unanswered policy issues of missile defense: how do you prove it was actually a missile carrying a deadly payload and thrust represented deliberate intent and endangerment to national security?

In missile defense, the only evidence a state has is the heat signature and radar tracks that show that something was launched on a ballistic arc, potentially towards your territory. And that data comes from classified sources that only the US military has access to and is loathe to reveal to the world. What's worse, if you destroy the rocket in the boost phase then you're destroying the rocket while it is still accelerating and therefore do not know what its final range and velocity will be. That means the intercepting State has made the decision to destroy this missile, while probably inside another sovereign state’s airspace, without actually knowing for certain whether it was a threat or whom it was a threat to.

Motivations and intelligent actors

Most missile defense advocates center their arguments and efforts around the need to protect American lives from the evil people in this world hell-bent on doing them harm. And, if true, that would indeed be a noble pursuit worthy of our highest national priority. However, the first fallacy of that argument is the assumption that states and leaders who pursue ballistic missile programs in the modern age are doing it only to obtain the means to kill or terrorize the populations of their enemies. In reality, their motivations for acquiring such weapons are exactly the same as those of the United States and the Soviet Union: security and influence, both regional and on the world stage.

If countries such as North Korea or Iran were able to acquire ballistic weapons and nuclear warheads, this would serve as a deterrent to any outside attack or invasion of their country. And it would also give them meaningful leverage in any diplomatic or political situation, something they and their people feel are very important given the history of American actions in their spheres of influence. There is nothing evil about this desire; they are two of the exact same arguments as given by the United States and Russia in defense of their current nuclear arsenals.

The second fallacy lies in the assumption that a missile defense shield will prevent “bad guys” from being able to threaten the United States. That is decidedly incorrect. The oldest military truism is that no plan survives first contact with the enemy, because the enemy is not a static entity devoid of intelligence or desire for self-preservation. Exactly the opposite is true: any adversary is just as intelligent and driven as any attacker, and will react appropriately to whatever force is applied. If states such as Iran and North Korea truly want to terrorize and cause harm, as missile defense advocates argue, having “perfect” missile defense will only cause them to seek other methods of doing so, and thus in the long run provide no guarantee of safety at all to the free peoples of the world.


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