21 February 2008
Over the course of the day I've heard the buzz of various rumors and theories about the Navy shooting down USA 193 last night. I might as well try to clear some of this up.
I. THE FACTS:
USA 193 was a U.S. National Intelligence Office controlled spy satellite launched aboard a Delta II rocket on December 14, 2006 from Vandenberg Air Force Base in California. Its average orbital height was reported to be in the vicinity of 160 miles above the surface of the Earth.
The satellite's precise configuration and instrumentation were (and remain) classified, so any guesses as to its purpose is little more than idle speculation. All animations and stock photos shown by the news, from what I've seen, depict known declassified satellites, not USA 193.
Shortly after launch the satellite's photovoltaic solar arrays failed to deploy. Incapable of harnessing the sunlight for conversion into electrical energy, the spacecraft's short-term batteries drained, causing the satellite to go dead. Since the satellite remained unpowered, none of the hypergolic fuel in its tanks was ever used. With no ability to control the powerless satellite, it was only a matter of time before its orbit decayed and it reentered the Earth's atmosphere. Ground stations tracked the satellite's decay rate, trying to pinpoint a probable reentry date.
It is impossible to predetermine an object's exact trajectory as it falls through the upper layers of atmosphere during an uncontrolled reentry. Controlled reentries can be tracked, but USA 193 would have no such luck. It's only when the object is approximately fifty miles up that its flight path can be tracked with any accuracy and its landing spot mapped. By the time the news broke with the first stories, USA 193's reentry was targeted for a vague early March date.
The concerns about USA 193's uncontrolled reentry and questionable landing spot made the decision to shoot it down a presumably easy one to make. If the spacecraft could be destroyed before reentry occurred then the remaining debris would burn up much easier than an intact satellite would. Should its destruction fail then the possibility of large pieces of debris making landfall would be very real. There was a risk of its remains striking populated areas and especially the possibility of intact fuel tanks spilling toxic hydrazine and beryllium, those hypergolic fuels in its still-full tanks.
At approximately 10:26 PM EST on February 20, despite earlier concerns about heavy seas, an SM-3 missile was launched from USS Lake Erie. USA 193 was successfully destroyed on the first attempt at 10:50, though two other vessels were standing by just in case they were needed. The missile launch window was a mere ten seconds and everything went according to plan. The explosion was recorded and the wayward satellite broke into smaller, manageable pieces at an altitude of 133 miles. The Department of Defense has released the video of the explosion.
That's what happened, but the exact reason it was shot down seems to be under some debate. Here's the reasons I've heard:
II. REASONS FOR ITS DESTRUCTION.
TOXIC FUEL TANKS. Having been in a powered down state since just after launch, USA 193's fuel tanks have remained in their pre-launch topped-off state. Where a small amount of fuel would have been used for early orbit insertion just before the batteries died, the thrusters and power systems have not been used since. If a full fuel tank were to survive reentry, its hazardous contents had the ability to cover an area equal to two football fields upon impact with the ground. This danger of hydrazine contamination was the number one reason for destroying the satellite while still high up in orbit.
But wouldn't those tanks just burn up in reentry with the rest of the spacecraft? This oft-repeated question, though logical, can be quickly contradicted by two previous cases of less-than-adequate reentry conditions. The first example is the Skylab space station, which was in orbit from 1973 until its unplanned reentry on July 11, 1979. The station, which was awaiting a reboost from a visiting space shuttle, found itself in trouble when the shuttle program was delayed another three years. Skylab slipped from orbit faster than anyone had anticipated and came down partially uncontrolled. While much of the station's remains crashed into the Indian Ocean, several pieces of debris hit throughout the Australian outback. One significant piece was an almost fully intact fuel tank that had survived reentry and a hard landing with only superficial damage. The tank is now on display at the Space & Rocket Center in Huntsville, Alabama.
The second example is space shuttle Columbia, which broke up over Texas while coming in for landing on February 1, 2003. Suffering a catastrophic structural failure, debris was spread over more than one-hundred miles of Texas and Louisiana. Even under these harsh conditions, a good number of fairly intact fuel tanks were recovered. Fuel tanks, engulfed in a disintegrating vehicle traveling over eighteen times the speed of sound at an altitude of 40 miles above the ground, survived the inferno around them and their high-speed fall to the Earth below. From these two examples the possibility of fuel tanks surviving reentry is entirely believable.
Of course, during the controlled reentries frequently carried out on decommissioned spacecraft, information on fuel tank survivability is lost if all goes according to plan. The craft, deorbited under controlled conditions, can be guided to crash into the wide open oceans, far from any landmasses. All surviving debris sinks into the depths never to be seen again. Such was the case when the Mir space station was successfully deorbited on March 23, 2001, its remains coming to rest in the south Pacific Ocean. Owing to the fact that seventy percent of the Earth is covered by water, the chances of an object striking the ocean after falling through through the atmosphere is quite good.
Though not contamination by hydrazine fuel, the deorbit of Cosmos 954 on January 24, 1978 created a massive environmental crisis. The Soviet Radar Ocean Reconnaissance Satellite, which was powered by a nuclear core, failed to reach a stable orbit and reentered prematurely. Cosmos 954 came down over Northern Canada, spreading radioactive material over a 370 mile path. The Canadian-American clean-up effort, Operation Morning Light, braved -40° F winter temperatures, recovering twelve large pieces but only a small portion of the nuclear fuel before operations had to be called off. Luckily, in this instance, Cosmos 954 came down in a sparsely populated area, thus avoiding a possible disaster situation.
One notion that's been thrown out by some conspiracy theorists of the world is that the U.S. hoped to create a massive hydrazine cloud in the upper atmosphere that would fall onto any number of enemy nations. This is absolutely absurd on several levels. Firstly, the fact that the video released by the Pentagon shows what is most likely the onboard hydrazine igniting from the detonation. Secondly, as spoken of earlier, you cannot accurately predict the landing track of anything above fifty feet. This is especially true for a non-ballistic shape, such as the satellite, which would tumble and turn as its blocky form battled wind sheers. An amorphous body, such as a cloud of fuel, would be dispersed and burned up long before it ever made it to the ground. As it heats up in the atmosphere the hydrazine breaks down into nitrogen, ammonia, and water. The ammonia would further break up into its nitrogen and hydrogen bases. The nitrogen would be quickly absorbed into the Earth's nitrogen-oxygen atmosphere while the water would simply evaporate from the heat of reentry, its own constituent gasses (hydrogen and oxygen) dispersing harmlessly as water vapor into the clouds. In short, a cloud of fuel at 133 miles altitude has no chance of reaching the surface of the planet given how turbulent and thick the atmosphere is.
FALLING DEBRIS. In a continuation of the fuel tank problems, just having a satellite falling out of orbit raises its own set of problems. Even without the risk of fuel contamination, the remains of a satellite, which could range in size from an SUV to a city bus, careening into a populated area is something you want to avoid at all costs. Oh, and the bus-sized space debris, traveling several times faster than the speed of sound, is also probably on fire. You don't want that.
However, as General James Cartwright, Vice Chairman of the Joint Chiefs of Staff stated, the mass itself was not enough to warrant shooting the spacecraft down. The onboard fuel stores were the ultimate deciding factor.
NATIONAL SECURITY. Being a classified piece of U.S. intelligence hardware, it would certainly not be in the nation's interests to have the satellite come down just anywhere. USA 193's exact capabilities are classified. Reports suggest it was a spy satellite but that could be more than just an orbiting platform brimming with cameras. Any number of sensors and reconnaissance equipment could have been installed and having the remains of that equipment coming down in hostile territories would be less than desirable. What if the satellite should crash land in Iran, China, or North Korea? Though the Cold War is long over, the high-altitude eves-dropping continues in force to this day.
BALLS-OUT SHOW OF FORCE. It's long been the Defense Department's night-time fantasy to shoot a satellite out of the sky. During the 1980s Ronald Reagan's "Star Wars" defense program called for space-borne missile defense capabilities. Satellites in orbit would be designed to shoot down incoming missiles from the Soviet Union and missiles on the ground would be able to shoot down Russian spy satellites in the skies above. Although this fanciful program never came to fruition as Ronnie dreamed it might, plenty of planning and testing went into it including, you guessed it, shooting down an old satellite.
The P78-1 Solwind satellite was the designated target. The satellite was in no danger of coming down prematurely, despite having failures in five of its science instruments. Against outcry from the scientific community who wished to continue using the existing operational experiments, P78-1 was destroyed September 13, 1985.
Not that this was the U.S.' first time shooting down a satellite. A series of tests made during the sixties and seventies proved that it was possible to down an orbiting spacecraft using ground based weaponry.
Various nations made their own attempts throughout the intervening years with varying degrees of luck. On January 11, 2007, China launched a missile to intercept and destroy an eight-year-old Chinese polar-orbit weather satellite, at an altitude of 537 miles above the Earth. China's test resulted in creating a great deal of space debris that will gradually fall back to Earth over time, posing a threat to spacecraft operating in orbit (including, potentially, the International Space Station) and to people on the ground if any pieces are large enough. Nations around the world expressed their shock and disappointment in China's blatant show of force, seeing the satellite's destruction as an act of militant showmanship rather than an expansion of their own space program.
However, this should not be viewed as a new practice by the U.S. government and military. According to General Cartwright, both the ships and the SM-3 missile required special modifications to carry out this role. As these modifications took away the weapon's normal missile defense capabilities, shooting down this satellite was a one time event, impractical for introduction into the military's standard inventory. This is especially true since both ships and missiles required several weeks of work to be made ready to shoot down the satellite and now that the job has been done, the ships and remaining missiles will need to undergo an additional number of weeks having the modifications removed once they return to port.
That the U.S. so readily agreed to shoot down a satellite of their own only a year after China's incident has cast a possible notion that USA 193 served as a very convenient U.S. reaction. As you might imagine, that's exactly how China saw it too.
III. IN CONCLUSION.
There you have it, four reasons why USA 193 had to be shot down. Which one is right? They probably all are to some extent. Each of them is certainly valid in its own way, though the fuel tank remains the solid deciding factor in it all. The other three are simply additional benefits gained by destroying the satellite but are by no means the primary objectives.
The threat of falling debris, in the form of large pieces of metal and toxic fuels, meant that destroying the satellite before it reached reentry was a perfectly sound and logical decision. As shown, this debris does reach the ground and can very easily cause significant damage. Additionally, it's best that when that debris hits the ground that it is sufficiently destroyed to conceal its original purpose. An enemy nation finding a charred but still intact spy camera in a field would do the U.S. no good at all. Finally, some in the military may have appreciated the offer to down the satellite. This kind of hands-on training doesn't frequently present itself and, if a show of technology against China's recent display can be accomplished at the same time, then some may find that all the better. It should be noted though that, given the special modifications required to ships and missiles, that this system is wholly impractical to be carried under normal circumstances.
USA 193 had to go. Allowing it to reenter on its own could have had potentially disastrous consequences, both in terms of human life and national security. That it came just over a year after China's satellite kill is just coincidence. The hydrazine fuel was the major worry here, not messy international affairs. It seems entirely likely that the Navy would have been called on to destroy USA 193 whether or not China had made any hostile moves. It was merely a perk that USA 193's destruction happened afterwards.
USA 193 explodes