|Issue 14.12 - December 2006
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The Sound of Silence
IT'S 1:45 AM ON A WARM SUMMER NIGHT at the White Sands Missile Range in New Mexico. The huge protective enclosure at Launch Complex 36 has been rolled back, exposing an Orion missile, primed and ready to shriek into the sky. The 15-foot projectile is equipped with a 50-pound explosive payload. After takeoff, it will veer north and self-destruct roughly 30 miles above the desert floor.
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Traffic has been stopped along a nearby stretch of Highway 70 as a safety precaution. The last call has gone out for personnel to take shelter in the blockhouse, a Cold War-era cement structure 100 yards from the launch ramp. Inside, 30 or so Navy and civilian technicians sit under fluorescent lights, running final system checks on vintage control panels. A battery of TV cameras survey the area to make sure no one is outside during the launch. But someone is outside.
Milton Garces is on the roof of the blockhouse, sitting at a card table and nursing a venti-size coffee. He stares intently at the screen of a souped-up Dell Latitude laptop linked to a $60,000 infrared camera. If the missile explodes on the launch ramp, the University of Hawaii will lose one of its most promising – and certainly most adventurous – scientists. If it goes off as planned, the information he's gathering could prove critical to the emerging science of infrasound.
The airborne detonation will send powerful waves of infrasound – ultralow-frequency acoustic energy – shuddering across the southwestern US. The audible sound of all but the biggest explosions diminishes over a short distance, a few miles at most. But infrasound, pitched well below the range of human hearing, can travel far – even crossing oceans – and 20 microphone arrays scattered throughout six states are poised to record the blast's subsonic rumble. Capturing such signals allows scientists to keep tabs on a host of frightening phenomena, particularly nuclear explosions, a concern that has become all the more urgent in the wake of North Korea's recent test. These waves can also help scientists detect and measure natural threats like volcanic eruptions, stray hurricanes, and apocalyptic meteors.
Tonight's exploding-rocket experiment will generate data that promises to make the science of infrasound more precise. With a better understanding of how infrasound behaves, scientists will be able to use it not only to monitor cataclysmic events but possibly to predict them as well.
But the information collected by distant listening stations isn't enough for Garces, a trim 39-year-old with glowing hazel eyes. He wants to videotape the launch itself. "The ignition stage is a source of infrasound," he says, "and it's useful to have a time-stamped record of that sequence. Plus, if the rocket blows on takeoff, it will make for some killer footage."
The White Sands experiments – this is the third in a series of four – are rooted in a national tragedy. When the space shuttle Columbia disintegrated over Texas in February 2003, US infrasound stations were listening. First, scientists heard a massive sonic boom trailing behind the craft. Then came a succession of smaller impulses, some arising from turbulence, others from changes in the ship's spatial orientation. The final signals, picked up by mics in Texas, coincided with the Columbia's "unscheduled disassembly," in the impassive parlance of rocket engineers.
Garces and his colleagues wrote reports for the subsequent NASA investigation, interpreting the infrasound signals generated by the incident, whose cause was not yet clear. They were able to rule out some possibilities, such as a meteor collision and high-altitude electrical discharges known as sprites. But they were frustrated by their limited understanding of how infrasound moves through the upper atmosphere. Unlike seismic signals – low-frequency audio waves reverberating through Earth's relatively stable crust – infrasound moves through an environment that changes constantly. Without knowing how factors like air currents and pressure systems interact with sound waves at the bottom of the frequency spectrum, scientists couldn't explain much of the shuttle disaster's infrasound signature.
The explosion tonight will give the researchers a rare opportunity to correlate effect and cause. Usually, infrasound scientists wait for signals to show up and then try to determine their source, a difficult and sometimes fruitless task. But when the missile detonates, the data record will come from an event whose parameters, from the force of the explosion to the surrounding weather conditions, are fully known. It's the closest thing to a lab experiment that can be conducted in the upper atmosphere.
Garces describes the goal of the test with a term borrowed from satellite research: ground truth. Ground truth is the context infrasound scientists need to decipher the signals they collect. The data generated tonight will help them model the myriad pathways that infrasound waves take as they move through the sky. With an accurate representation of infrasound's behavior in the upper atmosphere, the next time an unexpected disaster occurs in the sky – whether it's a disintegrating spacecraft, the skyward blast of a volcanic eruption, or an exploding meteor – Garces and his colleagues will be able to figure out exactly what happened.
If he makes it off the blockhouse roof alive.
THE COLUMBIA DISASTER wasn't the first unexpected event to produce a notable infrasound signature. On August 27, 1883, the volcano Krakatoa blew its top near the Indonesian island of Java, creating atmospheric ripples so intense they circled the planet several times. Barometers worldwide recorded wild fluctuations that resembled, as science writer Simon Winchester wrote in his 2003 account, Krakatoa: The Day the World Exploded, "an earthquake in the air." Researchers who gathered the paper records compared the waveforms to determine the magnitude of the explosion. The science of infrasonics was born.
The technology quickly proved a boon to the US military. During World War I, primitive microphones were placed on the battlefield to pick up infrasound signals generated by enemy artillery, allowing technicians to triangulate the cannons' positions. Cold war fears prompted further research into the technology. In 1954, US Atomic Energy Commission chair Lewis Strauss recorded signals produced by a US nuclear test, playing the recording for President Eisenhower at high speed to make the slowly unfolding sounds audible. Soon infrasound was being used to detect, locate, and estimate the yield of Soviet nuclear tests half a world away.