The United States is planning to intercept a dying reconnaissance satellite with a missile launched from a Navy ship. The administration justifies the intercept on the basis of public safety. That is a long stretch, indeed, and thus far in the news coverage that I have seen there is virtually no mention of the political consequences of the United States’ conducting its first anti-satellite test in over two decades.
The United States, along with China, Russia, and other space-faring nations, should be working to ban anti-satellite weapons. Such a ban would work strongly in the best interests of the United States because we depend more, by far, than any other nation on access to space for our economy and security. Any measure that reduces the threats to satellites will enhance American security. The proposed test is a potential public relations bonanza, showing the public how a defensive missile can protect us from a—largely imaginary—danger from above. What follows is a simple analysis of what some of these dangers might be and a description of what might happen. These are questions that should have been asked of the administration.
In December 2006, the United States launched a 5000 pound spy satellite from the military space center at Vandenberg, California. Virtually everything about these satellites is highly classified. The government has declined to even say who manufactured it, much less give details about what it does. Most accounts seem to assume it is a photoreconnaissance satellite. It is designated as US193.
The satellite was successfully placed into a 220 mile high orbit but radio communication with the satellite was soon lost. At 220 miles, the Earth’s atmosphere, while tenuous, still exerts some drag on a satellite the size of a small school bus that is pushing through it at 17,000 miles per hour. Reconnaissance satellites are normally placed in low orbits; they are just big cameras after all and you get better resolution in the pictures if the camera is closer to the thing it is photographing. Reconnaissance satellites usually carry some propellant and small thrusting rockets, for three reasons. First, if there is a particular place on the Earth that the spy satellite needs to photograph, the orbit of the satellite will—eventually—naturally bring the satellite right overhead but the intelligence analysts, the military, or the president might not be able to wait. So the satellite can use its propellant and rockets to nudge it a little one way or the other to shift its orbit enough to bring it over the desired spot on the Earth sooner rather than later. Second, because the satellite is in such a low orbit, it will lose energy to air resistance and slowly come closer to the Earth. The rockets can be used as a booster to occasionally nudge the satellite a little higher to keep it in orbit. Third, when the propellant is almost gone and the satellite is doomed to reenter the atmosphere, the last bit of propellant can be used to intentionally slow the satellite and force it down, usually aiming for the Pacific Ocean where debris will fall harmlessly.
The propellant is hydrazine, a compound of hydrogen and nitrogen. It is not the most efficient propellant but it is extremely simple to use. It is a monopropellant, which means it can be used just by itself, there is no need for separate oxidizer and fuel. Just squirt some onto a catalyst in a reaction chamber and it decomposes, forming hot hydrogen and nitrogen gas that rush out a rocket nozzle, creating thrust. The problem with US193 is that radio control has been lost, the propellant is not being used up, the satellite is about to reenter the atmosphere, and the propellant tank is still filled with a thousand pounds of hydrazine. There is some chance that the tank will not burn up on reentry, some chance it could land on a populated area, and some chance the hydrazine could injure someone. Hence the plan to intercept the satellite.
You Can’t “Shoot Down” a Satellite.
Almost all press reports include some statement about how the Navy is going to “shoot down” the satellite. The image suggests a hunter with a shotgun shooting down a duck. Bang! The duck gets hit, its wings fold, and it falls to Earth. Not the way it works with a satellite. This is a satellite; it is in orbit. It stays in orbit because of its momentum and the balance between the centrifugal force and the Earth’s gravity. It is not being “held up” the way an airplane’s wings hold it up in the air.
When the interceptor hits the satellite, it is not like a bullet hitting a car, punching a hole in the side and coming out the other side. The interceptor will hit the satellite at about 18,000 miles per hour and the energy of the interceptor is far more than needed to melt and even vaporize the material of the interceptor. This happens so quickly it is as though it were an explosion. Shock waves will travel through the structure of the satellite and break it into pieces, some large, some as small as dust. The hope is, apparently, that the interceptor will break open the hydrazine tank so it will leak out before the tank reaches the ground.
But the interceptor is not “shooting down” the satellite. The satellite weights 5000 pounds and the interceptor weights 20 pounds. Even if the satellite breaks up into pieces those pieces are going to be moving in roughly the same direction as the satellite was moving, that is, in the same orbit. Some smaller pieces will encounter proportionately more air resistance and will come down sooner than the satellite would have. But if the propellant tank breaks free, the density of the propellant tank is higher than the average density of the satellite so the propellant tank by itself might actually stay up longer than the satellite by itself would have, had it remained whole.
The satellite is not being controlled but this intercept is going to trade one big uncontrolled satellite for several uncontrolled pieces of a satellite. If that helped it burn up in the atmosphere, that might be useful but the first thing that happens when a large satellite enters the atmosphere is that the structure fails and it breaks into pieces anyway. Recall the sad photos of the Columbia reentering the atmosphere: it was not a single hot streak across the sky but several huge pieces moving together; and that was a vehicle that was specifically designed to survive reentry.
The Pentagon tells us that the hydrazine tank will survive reentry. I remain unconvinced. Deorbiting something is not easy. The tank may not burn up entirely from the heat of reentry but the heat should be enough to vaporize the hydrazine, creating a high pressure in the tank that would rupture the tank, spilling the hydrazine harmlessly at very high altitude. Also keep in mind that this is not a sealed tank, there will be pipes going in and out and these will absolutely be sheered off by the reentry, allowing the hydrazine to vent. Finally, intercepting the satellite will not necessarily destroy the tank. Most likely the interceptor will break the satellite into pieces, one of which will be the hydrazine tank, which will reenter the atmosphere independently but keep in mind that the satellite would have broken up quickly upon reentry anyway.
And if the tank makes it to the surface? Well, we are told it might cover an area the size of two football fields with hydrazine and if someone remained in the area they could get a fatal dose. (If I were outside and a large tank of strange material fell from outer space, I confess, it would never occur to me to leave the area.) Well, if “two football fields” is as large at 100 meters by 100 meters, that is 10,000 square meters or just less than one ten billionth of the surface of the Earth. That makes winning the lottery seem like very good odds, indeed. To put this in perspective, the United States produces 36 million pounds of hydrazine every year, the world produces 130 million pounds of it. (It is used in, among other things, the production of plastic.) Most of this is transported around the industrial world by trucks and rail. At any given time, vastly more hydrazine is in transit around the world than is in this satellite. If the government were interested in public safety it would be better to take the $3 million cost of the Standard-3 missile and pay for a traffic light at a bad intersection or pay for children’s vaccinations.
So what is going on? When control of the satellite was first lost, the risk from the satellite was dismissed as trivial, not worth any real concern. Now we need to “shoot it down.” I cannot attribute motives without being able to read minds but a normally skeptical person could be forgiven for at least suspecting that this satellite is offering a chance for the Navy to test its missiles in an anti-satellite mode for the first time since the end of the Cold War. I have seen virtually no discussion of the arms control implications of this. Are we fueling an anti-satellite arms race? Who knows, but I don’t think anyone in this administration cares.