We can hit a 23,000 MPH comet with a probe but we couldn't pull off Star Wars?

"Tempel 1 is traveling through space at about 23,000 mph (37,100 km/h) – the equivalent of traveling from New York to Los Angeles, California, in less than 6.5 minutes.

At those speeds, impactor had to be in the right place at the right time to intercept the speeding snowball.

“It’s a bullet trying to hit a second bullet with the third bullet,” Rick Grammier, Deep Impact project manager at the Jet Propulsion Laboratory, said in June."

With a probe the size of a washing machine (lead by the primary machine w/ the cameras, the size of a small car) we hit a comet going 23,000 mph, but Reagan’s Star Wars plan didn’t work b/c the idea was that it was like hitting a bullet w/ another bullet.

Either are technology is much better (prob.), or it’s the fact that that comet is the size of Manhattan (I heard? :confused:) Must be the latter. Discuss?

Also, don’t forget that we were able to design the probe already knowing the comet’s trajectory, whereas we don’t have that luxury with missiles.

The critical difference is that in thecase of the comet, we had several years to work out exactly where it’s going and aim for that spot. In the BMD scenario, you have a couple of minutes, tops.

And you may have a lot more than one target, and they’re near the earth and in amongst all the space junk, and they’re trying not to be seen, and …

There is simply almost nothing in common between the two problems except they both involve an impact if they work.

That fact that we have had many years to compute the proper trajectory, which is completely known (as pasunejen says), and that we only have to do it once, rather than hundreds of times, and that the comet isn’t taking any counter measures, like signal jamming or decoys, has more than a little to do with it also.

How fast do missiles travel? Just a roundabout figure?

ICBM’s are almost as fast as the speed you quoted.

The Deep Impact only had to hit somewhere within a 3.7-mile target zone. An ICBM is, what, a few feet across?

After the Deep Impact impactor separated from the parent spacecraft, it had 24 hours to adjust course and zero in on the target. And the target isn’t actively trying to avoid the impactor.

When I opened this thread, I thought the comparison was going to be, not about missile defense systems, but about how badly George Lucas had flubbed the recent ‘star wars’ franchise prequels. (Though consensus seems to be that revenge of the sith is, at least, not as bad as the first two.)


We will figure out the starwars thing, it will just take some time and better tech.

Let us say that there was a 90% chance Deep Impact would succeed. That’s pretty good.

Let us say that Star Wars would stop 90% of 300 incoming nukes. That’s unacceptable.

That’s the sort of thinking that got us the prequels and the remakes to begin with.

Why is a 90% success rate unacceptable? Isn’t some defense better than no defense?

I think you’d be looking in the neighborhood of about a half hour from launch to detonation.

If you stop 90% of the 300 missiles launched against you, that means that 30 missiles will reach their target. That’s more than enough to wipe out most of the major population centers. In this case, “some defense” just ain’t good enough.

I think that was the sound of a “whoosh” flying about 23,000mph over your head.

Missile Defense Tests May Resume in Fall

The Independent Review Team’s Report on the GMD Program: A Rush to Failure, Redux

ICBM’s travel at 6 to 7 km/sec. Somewhere between 21600 to 25200 kph. A Soviet Topol ICBM is about 21 meters long and 2 meters in diameter.

Also, a ICBM might be (probably is?) a MIRV – Multiple Independently-targeted Re-entry Vehicle. So it would have to be intercepted during the ascent stage before the multiple warheads separate.

Yet, the fact remains that some defense is better than no defense, and 30 destroyed cities is better than 300 destroyed cities.

Further testing and improved technology will increase the effectiveness of the system over time. With luck (and it may take a lot of luck), when the day arrives when we need this system, the effectiveness will be 100%. Still, I’d rather have a 10-year-old boy with a slingshot, than no defense at all.

Interestingly, the Chinese have plans for their own Deep Impact mission, where they will try to soft-land a probe on a comet, and then try to push the comet with engines, hoping to alter its trajectory. A goal of the mission is to test this strategy’s effectivness in protecting the Earth from a catastrophic comet or asteroid impact.

Actually, no it’s not.

Well, let me step back for a minute and qualify that answer. Some fractional effectiveness of defense (since the arbitrary figure of 90% has been tossed out, we’ll use that) does provide for the ability to make a response strike against an attempt at a disarming first strike. If your goal is to maintain a certain percentage of your arsenal against such an effort, then an ABM system provides for an effective strategy. The mentality here is to avoid any stategic confrontation by making the risk to the opponent to great for them to consider launching such a strike.

However, their response to such a system, whatever it’s effectiveness, is going to be to double down their bets; instead of sending 300 missiles, they’ll send 600. Increase your ABM arsenal by a factor of two, they’ll increase the LVs by a factor of four, et cetera ad nausum, until even a 10% leagage is sufficient, if not to prevent a counterstrike, then to totally vanquish their opponent. The resulting arms race is both extremely costly and results in a game that is worse in every result. This was the conclusion that Secretary of Defense Robert McNamara and his theorists came to, and was his reason for opposing the instantiation of of Nike-X/Sentinel. His argument was that no cost was to great to bear in defense against nuclear attack, but that the Nike-X ABM system would make the damages (measured “megadeaths” in the curious and detached jargon of strategic assement of nuclear Armegeddon), and thus was not worth any price. Eventually, he relented to political pressure to agree to some kind of a limited system, and nearly a decade later, under the Nixon Administration, Safeguard was deployed at one site (the Mickelson Ballistic Missile Complex near Grand Forks, ND) as a point defense system and then quickly shut down.

Although the reasons for the shutdown were largely political, the system itself was obsolete by the time it was deployed. By 1975 our launch discernment and tracking systems were sufficiently advanced that a disarming first strike was effectively impossible. By the time any enemy RVs could come sailing into the complex, our birds would already have flown; indeed, while in the days of the liquid fueled Atlas a signficant delay existed between detection of a launch and getting an Atlas prepared for launch, by 1975 the fleet largely consisted of solid fuel Minuteman II and III boosters which could launch within seconds of an order, each carrying a payload of multiple RVs. Trying to destroy the silo would be like figuratively trying to close the barn door after the cows had not only left the barnyard but were in fact running through the neighbor’s pasture. Similarly, our ballistic missile submarines (then) provided a nearly unstoppable threat, further oblivating the effectivness of striking at land-based missiles.

As for the engineering problem of trying to physically intercept an incoming RV with another missile, it is certainly difficult though not impossible. The short-range Sprint missile, while nuclear tipped, was actually accurate enough to act as a hit-to-kill vehicle. The Sprint was, of course, defending an established point, so any incoming booster would be on a converging path, whereas an attempt to hit an incoming RV at a higher altitude would have a larger envelope. However, just by overwhelming the system, i.e. sending in more RVs than there were interceptors to reliably intercept them, the system could be breached. With large MIRV-based boosters like the American Peacekeeper and the Soviet SS-24 (RT-23), both of which can carry up to 10 RVs (some of them decoys, perhaps), overwhelming a point defense system becomes a game that the defender, like a slot machine addict, can’t help but lose.

And of course, no missile defense system is going to stop some lunatic from sailing a yacht into Boston Harbor or Port of Los Angeles and, if not destroying the country, creating considerable havoc and destruction. I’ve said more in this [post=5765955]thread[/post] on the topic. Suffice to say, no ABM system can legitimately make the claim of being able to defend the population, and any system developed against a strategic opponent is going to lead to a race to see who can afford more boosters, one that is far more costly for the defender than the assailant.

As for the current GMD program–it is, IMHO, a joke, a political monstrosity which exists soley for the sake of letting…um…certain individuals lay claim to be “doing something to make the nation safer” while spending less than a minimum of funds to develop and test a technically feasable system. It’s a shame, really; lacking sufficient backing and inability to set reasonable timelines means that billions of dollars spent upon the system are a waste, while deploying any system still encourages opponents to do all they can to defeat or overwhelm it. So, it doesn’t work AND it makes the situation worse. Good call.


There was an article a few months ago that stated that the first successful test of a Star Wars system had been completed. It didn’t give the impression that it was anything near deployment–but that we are at least getting to the point where it is technically feasible.

Er, please cite. To my knowledge, no full-up test of the GMD (Ground-based Midcourse Defense) system has even been conducted, much less successfully. Some individual components–specifically the EKV (exo-atmospheric kill vehicle developed by Raytheon) have been tested with limited success under controlled conditions, but the development program has been rife with problems. In any case, the C[sup]3[/sup]I (Command, Control, Communications, and Intelligence) system which controls the battery is still not in place; a jury-rigged, half-manual launch and guidence system, untried and unvalidated, remains in place on the existing deployment.

[/walter sobchak]Also, dude, Star Wars is not the preferred nomenclature. National Missile Defense, please.[/walter sobchak] :smiley: The NMD people get seriously torqued if you say “Star Wars”, and more than slightly irritated by “SDI”. The SDI office was renamed to the Ballistic Missile Defense Organization (BDMO) under Papa Bush, and the Missile Defense Agency (MDA) under Bush The Younger; the project umbrella itself is now National Missile Defense (NMD), of which the most promenent aspects are the GMD and KEI (Kinetic Energy Interceptor) programs. This is probably attempt to get away from snarky comments about “Ronnie’s Ray-Gun” in reference to science-fiction-as-strategy enthusiast, somnambulist extraordinare, and the 40th (constitutional) President Ronald Reagan. They don’t like to be mocked.