Written by Ivan Oelrich
According to the North Korean statements, the Kwangmyongsong-2, or Bright Star Light, satellite is a communications satellite. This is transparently nonsensical, of course. Most communication satellites are in geosynchronous orbit, far beyond the reach of the North Koreans. A single satellite in low Earth orbit will not be a useful communications satellite and I do not believe anyone is expecting the North Koreans to launch a whole constellation of satellites. Perhaps what they mean by a "communications satellite" is that the satellite will be communicating to us, not being used by people to communicate among themselves. According to North Korea, their last satellite, launched in August 1998, orbited the Earth continuously broadcasting the "immortal revolutionary" tune, "Song of General Kim Jung Il." (This is consistent with the name, Kwangmyongsong or Bright Star Light or Bright Lode Star, is one of the innumerable sobriquets of Kim Jung Il.) Such a satellite never existed in fact. All reports from outside North Korea state that the last stage of the rocket exploded, destroying the satellite. United States Space Command has never tracked any object that could be the North Korean satellite and has detected no such transmission from space. (There is a musical precedent: The first Chinese satellite, which had the notable distinction of actually going into orbit, transmitted the tune, "The East Is Red.")
The 1998 launch used a Taepodong-1 missile as the space launch vehicle (SLV). The Taepodong-1 is made up of a Nodong missile as a first stage with a Hwasong-6 missile as a second stage. (We should keep in mind that some North Korean missiles, such as the Nodong and Hwasong-6, have been produced in number and even exported so they are well characterized. The Taepodongs are different, they have never been successfully test flow and they are put together from other components. I am somewhat uncomfortable assigning them names as though they are production missiles; at this stage, each one might me a one-off. We shall see.) In the 1998 flight, a third stage was added to boost the small satellite into orbit. It was this additional third stage that apparently failed so the North Koreas could have got substantial and important data on the performance of the first two stages that would have made up the two stages of a military ballistic missile.
The Taepodong-2 was tested only once in 2006 and exploded about 40 seconds into its flight and it seems this upcoming launch is a retry of that failed test. The first stage of the Taepodong-2 appears similar to the Chinese CSS-2 missile. David Wright speculates that the first stage will have four Nodong engines operating together. A single Nodong serves as the second stage. The North Koreans have declared warning zones where the first and second stages are expected to impact. David Wright and Geoffrey Forden have worked backward from the announced splash down zones to see whether they are consistent with the presumed configuration of the Taepodong-2 and they check out.
It would be better for the world if the North Koreans did not go through with this test (perhaps the best outcome would be for the rocket to blow up a few seconds into its flight test-we can always hope) but if they conduct the test, the rest of the world might as well learn as much as we can from it and we can learn a lot.
The missile is being launched from the sort of launch pad that one would expect for a SLV. It is being assembled out in the open and the North Koreans are making no effort to hide the missile. The press has released some low resolution images but the United States, and others, have photo-satellites that can take much higher resolution images, perhaps seeing detail down to several centimeters. It is conceivable that the United States and perhaps others are bold enough to fly unmanned drones nearby to take even more detailed photographs. But unless a drone gets shot down, do not expect any public announcement from either side. So while we on the outside speculate about what the size and shape of the missile is, national intelligence services already know that before the missile even flies.
The missile will be tracked by radar-the Americans and Japanese have radar ships in the area and South Korean and Japanese will have ground-based radar-and this will provide a detailed, instant-by-instant record of the trajectory of the missile. That allows a calculation of the acceleration, which, in turn, allows us to calculate the ratio of the thrust to the weight of the rocket. If we knew one of those, we could then calculate the other and we will get to ways to determine the weight.
The rocket will accelerate and the rate of acceleration will increase because the thrust of the engines remains constant (some more advanced rockets do fancy things with throttling their engines but the North Koreans are probably not there yet) but the rocket is always getting lighter because it is burning up fuel all the time. So we do not know the weight of the rocket, or the thrust, or the fuel flow, but we can figure out all the ratios and some unknowns cancel out in the equations. By seeing how fast the acceleration changes, we can figure out one of the most important measures of rocket technology, the specific impulse. Specific impulse is the amount of "impulse" or total push (technically, momentum change) provided by a given amount of fuel. It is measured in newton-seconds/kilogram or, in English units, pound-seconds/pound. (Some American engineers cancel the pounds of force in the numerator and the pounds of mass in the denominator and report specific impulse in units of seconds, which makes any good physicist weep. And trust me, I will get tons of letters explaining how I am totally wrong and don't understand specific impulse.) The specific impulse depends on the type of fuel, the efficiency of the combustion, and the maximum temperatures and pressures that the rocket engine can stand, all things that are technical challenges, making specific impulse a good measure of overall technical sophistication of a rocket builder.
Keep in mind that, for long-range rockets, the initial weight of the fuel is about 90% of the total weight, the structure-the tanks, engines, and so forth-are most of the remaining 10% and only a couple of percent of the total weight is payload. So when the rocket first takes off, the fuel is mostly lifting itself. So the efficiency of converting the fuel into thrust is critical; small changes in efficiency translate into large changes in payload that can be delivered to great distances.
As the first two stages separate, they will fall back to Earth. Radar will be able to track their trajectories as well and measure how they are decelerated by falling through the atmosphere. If we knew the drag coefficient of the stage, then, in principle, we could figure out the weight of the empty stage. The problem is that the stages will be tumbling in some complex way as they go down. Even so, by measuring the radar cross section at each instant, particularly at a variety of radar frequencies, and comparing measurements from more than one radar, a computer could develop a picture of how the stage tumbles and then calculate the air resistance and, from that, the weight of the empty stage. I do not know whether the accuracy is great enough to add to information that we would have from other sources, for example, based on knowledge of the Chinese CSS-2 missile. When the stage breaks up in the atmosphere, all bets are off but the first stage at least might hit the water intact. That raises the interesting possibility that pieces could be recovered but the predicted impact area is over very deep water.
Based on past North Korean practice, we know the general category of propellant the rocket uses but not the precise type. The oxidizer will be nitrogen tetroxide or nitric acid or some mixture of the two. The fuel could be kerosene or something more energetic, like dimethyl hydrazine. The two stages could use different propellants. I wondered whether, by observing the plume from the rocket, perhaps from space, and analysis of the spectra, it would be possible to determine the type of propellant. I discussed the idea with a couple of people and the consensus is that you could identify atomic species but not ratios. So the spectrum would reveal nitrogen, but not enough information to know that it came from nitric acid or hydrazine. In fact, the specific impulse will be a better indicator of the propellant type.
Remember that radar tracking gives us ratios, of weight to thrust, for example. If we had one, we could calculate the other but we cannot calculate payload mass directly. If this were a ballistic missile test, then the weight of the reentry vehicle could be determined by watching how it decelerated in the atmosphere. Then we could make a guess as to whether they could build a nuclear bomb within that size and weight. But this looks to be a satellite launch. Instead of a reentry vehicle, the rocket will have a small third stage that will push a small satellite into orbit. The payload ends up in a ballistic orbit in space where there is no air resistance (or very little). So a one kilogram satellite will follow the same trajectory as a thousand kilogram satellite. That is unfortunate, because if we knew how much the third stage weighed, or how much the rocket could launch into space, then we could calculate how far the rocket could throw a payload of any chosen weight. Because the payload ends up in space, we have to make some guesses about the weight.
Telemetry offers the potential for a great deal of information. By intercepting telemetry, we could get direct information on fuel flows and the like. Since the Taepodong-2 is still being developed-they have never had a successful launch-one would expect the North Koreans to have the rocket heavily instrumented and to transmit all those data back. They might do that but, in past flights, their telemetry has been quite limited.
Could the United States shoot the rocket down? Well, sort of. We do not have the capability to intercept the boosting rocket. But a satellite has to be boosted up to the point where it enters its orbit. (In fact, that is sort of the definition of the "orbit," the point where the boosting stops and the object goes into an unpowered ballistic trajectory; if it is a ballistic trajectory that doesn't later intercept the atmosphere, we call it an orbit.) The United States has already demonstrated that it can intercept low altitude satellites; lat year the Navy intercepted an old U.S. spy satellite in a decaying orbit. That was in some ways an easier target because the path could be calculated days in advance. While the North Korean satellite is still under power and being boosted up to orbit, it will not have a perfectly predictable path, making intercept complex but not impossible especially because, based on the predicted first and second stage impact areas, we can make a good guess about the flight path and the Aegis missile could be positioned to make an intercept of the third stage or the satellite before it reached final orbit. Note that this intercept would destroy the satellite, which is a stunt which is just a North Korean stunt anyway, but does not deny any information to the North Koreans about what they presumably really care about, a two-stage ballistic missile with military applications. So, intercept of the third stage might give the U.S. some macho pleasure but would not accomplish any military goal. (It would have political implications that I won't even try to guess at.)
Does this mean that Aegis would work as an intercontinental ballistic missile defense? No, for several reasons, for example, a ballistic missile does not have to power the reentry vehicle up to orbit, the rockets burn for three to five minutes and the RV is on its way and the trajectory is pointing up rather than horizontally, quickly getting out of range of the Aegis.
Overall, the outside world will gather a lot of information about the Taepodong-2 as a ballistic missile based on this satellite test. We will not know the exact payload and range of the ballistic missile version but will certainly know a great deal more than we do now. Unfortunately, so will the North Koreans. Especially since their test of a nuclear explosive, this is a dangerous development.
written by Ivan Oelrich