There was a talk at the University of Maryland today by Daniel Kleppner, one of the co-chairs of the American Physical Society's Boost Phase Missile Defence Study Group. The report is summarized here, and the whole thing is available here.

Anyway, the talk was very well presented, and it's clear that if you accept the initial assumptions the conclusions follow logically. It's the input assumptions that are somewhat problematic. I've seen people complain that the choice of initial assumptions is due to liberal bias, but Kleppner defended them quite well on the basis of the National Intelligence Estimate and the systems actually under consideration. Some of the parameters considered, such as the burn time, were skewed in favor of the defender, and they considered zero decision time cases, which also favor the defender. The minimum kill vehicle mass considered (90 lbs, including sensors, thrusters and fuel) seemed to me a little large, but I don't have a basis to dispute it. This is a critically important parameter, since it scales all the other masses in the system.

Kleppner stated at a number of points that there is room for reasonable people to disagree on the assumptions, and that judgement calls needed to be made in order to choose concrete numbers to put into the models. Given the credentials of the people on the study team it's unlikely that the numbers are off by much, though. It is clear, however, that under only slightly more optimistic assumptions (or under the assumption of an aggressive research program), the system can be made to work, at least against limited attack by relatively unsophisticated opponents. The technology horizon for consideration in the study was only ten years, and there was no consideration of potential breakthrough technologies.

The study also comes down firmly in support of the effectiveness of boost phase intercept against missiles launched from ships close to the borders of the US. This is significant for two reasons. First of all, this is the scenario considered most likely by the National Intelligence Estimate (which makes sense - it's an easier attack, and it makes tracking the attacker harder). The second reason is that high tech systems tend to be most effective when developed incrementally. By starting with short range boost phase interceptors the system can be debugged before extending capabilities to intermediate range and finally to ICBM ranges.

Kleppner dismissed Brilliant Pebbles as not credible, despite considering much larger space based systems. The conclusions on the large space based systems were negative due to the need for greatly increased launch capacity. Launch is the long pole in the tent - all the other technological components needed were considered viable. Yet another reason to work to increase launch rates. The increase needed was cited as being 5-10 times current rates, which ought to be well within reach. If Uncle Sam was to decide to just go ahead and deploy space based BPIs it would be a massive boon for the launch industry, but I think that's a faint hope at best. The dismissal of Brilliant Pebbles is a little more puzzling to me, and I can't help wondering if maybe the study group simply didn't look hard enough at the possibility of dramatically reducing sensor size, which would also help on the kill vehicle mass.

Overall it was a good talk. If you get a chance to see Kleppner talk, take it. He's very clear, gives complete answers to questions and admits the limits of the study and his own knowledge. The study obviously has considerable limitations, but it's a good starting point for policy discussions, which is pretty much the point. It's a start, not an attempt to have the final word. A lot of the reporting on this topic in the press has suggested that the study's conclusions are much firmer than they in fact are. It's a nuanced study, as appropriate for a problem of this complexity. Trying to shoehorn it into either the pro or anti missile defense camp does a disservice to the study group and to the seriousness of the issue.

I've seen the old solid-propellent vesus liquid-propellent thrown up by earlier critics of ballistic missile defense. But what about throw weight?

Any switch to solid propellents will greatly reduce the potential payload that can reach the target. That grants the defender virtual kills on the enemy missile force. Without firing a single shot the defense has much less to deal with at the mid-course phase of interception.

I'm not a missile expert by any means, but at least for land based missiles I see no reason throw weight should be a showstopper for the bad guys. To throw more weight, scale up the booster. If I recall correctly Aerojet cast and fired a 260 inch diameter solid, which is plenty big enough for a really large ICBM.

The virtual kill issue is one the study rightly didn't consider. I'm taking the term in a different way than you intended, to include systems never developed because they are unable to defeat the NMD system. I think the importance of planting FUD in the mind of a potential attacker is far more important than opponents of NMD accept.

The most effective way to stop an enemy missile is to keep it from ever leaving the ground by disabling or destroying it prior to launch.

It might be a better use of resources to devote our anti-missile efforts to improving military intelligence capability and conventional ground strike technology.

At the same time, it is frightening to contemplate the fact that several potentially hostile and domestically tyrannical nations are quickly developing ICBM and spaceflight technology.

The 1967 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies may become irrelevant in a world where despotic regimes have the ability to swing the space hammer around. I have seen one pro-Al Queada commentator in Pakistan suggest implementation of a space technology alliance between radical Islam and China. The overt militarization of space is a sad but inevitable certainty.

Interesting and valid for the assumptions in the study. Even so, I find it interesting that a space based system of only 2000 tons using kinetic interceptors is shown to be feasible.

A couple of factors that favor a space based solution that were not considered in the study:

* a space based laser would have zero flight time, and could cover a considerable area.

* any space based system could also serve as a mid-course attack system as well.

* A space based system would have global coverage

Too bad the DC-X program was killed. That began specifically as a near-term system for lower cost/more reliable launches by the SDI office.

It is important to remember that a missile defense system doesn't need to be perfect, only credible. If an attacker thinks there is a good chance their attack won't work, they will have to think twice about it. Mind you, given the potential damage and deaths, I think it is insane not to pursue space based systems. Look at what 9/11 cost. Not counting the human cost, a nuclear attack would, if we were lucky, only start a depression and cost many trillions of dollars.

Fred: A laser doesn't have zero flight time. It is fast, yes, but there are significant velocities and distances involved. Lasersats would also need far more development and would likely be very heavy. I'm not against it, but I'd put that out at 20 years, IF we had a major development program.

VR:

Granted, the speed of light isn't infinite, but the distances involved in missle interception would be on the order of 1000 K or less, while c is approx 300,000 K/s which gives 1/300 of a second travel time. One of the limiting factors of the study was the mass and range of the interceptor, which was strongly influenced by the distance that could be covered in the short time involved in the boost phase (100s of secs). In that context, lasers are practically instantaneous.

A laser solution would most likely be limited by beam focus diffraction limits (as discussed in the article for ground based lasers). Thus you could imagine a laser interceptor constellation of relatively few but heavier sats in relatively higher orbits covering a wider area than kinetic interceptors.

Your point is well taken that nothing remotely resembling a sufficiently powerful space based laser has yet been developed/tested/deployed. At the level of the article, which was to discuss the basic physical contraints of the problem in light of our present technology, I think it is fair to look at the cost/benefits of a space based laser system vs space base kinetics vs ground based laser/kinetics.

Considering the damage that could be wrought by a such an attack, or the diplomatic leverage a nuke missile in the hands of a rouge nation would imply, (witness N Korea today) a global space based deterent would be worth a great deal. In light of the current tens of billions of dollar yearly expenditures on ground/air/sea based mid course interceptors it would seem that a few billion dollar a year devoted to development of space based kinetic and laser systems would be a good side bet that would lower development risk for an eventual full deployment.

There are two broad classes attacker that a useful missile defense system needs to defend against -- the terrorist and the blackmailer. The assumptions you describe, of a missile launched from ship, is characteristic of the terrorist, who just wishes to inflict harm. However, I consider the terrorist to be a somewhat less likely case, since someone attacking in that fashion can most likely just carry the weapon into the country.

The blackmailer is a rogue dictator or the like who wishes to force the US or other major countries to either take or refrain from some course of action, like, for instance, intervening in a regional conflict. He doesn't actually want to drop WMDs on the US; he just wants to use the threat of them for manipulation. This is a much stickier wicket with the proposed rocket-based boost-phase interceptors, because you need to neutralize missiles launched from home soil.

A dictator with the sophistication to own and use ballistic missiles is also almost certainly aware that ship launched missiles are ineffective for blackmail, due to the US's powerful Navy and excellent ocean surveillance. They'd get caught, and even if they popped off as soon as they saw a cruiser, a missile in early boost phase is probably an easy target for existing SAMs such as the Standard.

Pierce - I agree with that the most plausible NMD use scenario is counterdeterrence. IMO this favors solutions like the airborne laser. Certainly it suggests deployability is an important consideration.

More on solid propellent ICBM's

The point I first made about the critics of ballistic missile defense (BMD) and thier claims about solid propellent ICBM's, boils down to one thing. You can't get something for nothing. Yet something for nothing is exactly the way the critics usually present their case against BMD.

The critics present a switch from liquid to solid propellent as all benefit and no cost to the offense, when in reality it is a tradeoff. Solid propellent missiles have shorter burn times and are tougher, but at the cost of reduced payload.

One could try to build an even bigger solid propellent booster to compensate for the smaller payload. But that means fewer missiles could be built for the same effort and therefore fewer targets for the defense to cope with. It's just another tradeoff, not something for nothing. Then there is the size issue problem with solid propellent rockets.

The enemies of America have always had a problem with building large solid propellent rockets, they just keep blowing up or malfunctioning. Even where the advantages of a solid rocket is almost mandatory, such as submarine launched ballistic missiles, the Soviets relied instead on liquid propellent rockets.

It's only in very recent years that Russia and China began to field solid propellent rockets. And even those are small rockets that require miniature warhead technology. Neither nation has even tried to build a solid propellent rocket with a payload equal to their older liquid propellent rockets.

That the states of North Korea or Iran might catch up in solid technology without an equal lag time is doubtful. North Korea is barely able to make a liquid propellent rocket with the range to hit America, let alone a solid propellent rocket.

I don't trust these new critics of BMD because they seem to be aping the old critics. The language they choose seem aimed at the technologically illilterate audience of the news media who will eat it up without question and regurgitate a headline, "Scientists prove missile defense will not work."