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.
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.
In the late '60s, the Pentagon abandoned infrasound in favor of satellites, which provided visual documentation, deemed more reliable, of foreign nuclear programs. Funding for infrasound research all but disappeared, and the technology became a hobby sustained by a handful of meteorologists.
Then, after 20 years as a scientific backwater, infrasound made an unexpected comeback. In September 1996, the UN General Assembly adopted the Comprehensive Nuclear Test Ban Treaty. To ensure compliance, the accord established a rigorous surveillance regime that comprises satellite, seismic, hydroacoustic, and air-sample monitoring, as well as a worldwide network of 60 infrasound stations. Infrasound was included because it works over extremely long distances and, unlike satellites, has no inherent blind spots. The US signed but didn't ratify the treaty; nevertheless, it has implemented the agreement by supporting a number of treaty-mandated facilities.
Usually powered by solar panels, an infrasound station consists of four or more extremely sensitive microphones spaced about a mile apart, ideally arranged to form a triangle. One mic sits in the center. Signals picked up by the mics are amplified and digitized, then transmitted to research facilities. To foil vandalism, arrays are often camouflaged – though that doesn't prevent the odd moose or boar from wreaking havoc. In desert installations, cables are painted with coyote urine to discourage rats and hares from gnawing through them.
Weather noise can mask the infrasound signal, so an ultrasonic wind sensor – a futuristic-looking cylindrical device that gauges wind velocity – is essential equipment. Scientists use the sensor's output to assess the impact of wind noise and figure out how to minimize it.
Until the 1980s, scientists analyzed infrasound signals by poring over rolls of paper covered with pen tracings. "But there's only so much you can do with paper," Garces observes. Today, researchers use computers to separate meaningful information from background noise. Then they plot the direction, speed, and curvature of incoming waves to determine the location of the sound source.
Sometimes it's not easy. Infrasound waves in the upper atmosphere can be trapped or funneled great distances. Picture the atmosphere as a grand, ever-changing network of pneumatic tubes formed by shifting winds and temperature gradients. Winds can propel, bend, or disperse sound waves. Temperature gradients create ducts that can carry the vibrations thousands of miles. A wave may take multiple paths to a given destination, arriving first via a stratospheric route and later via the thermosphere.
This makes it tricky to track down the source of any given infrasound signal. For example, during the January 2005 eruption of the Maman volcano in Papua New Guinea, several nearby arrays detected nothing. Stations as far away as Alaska and Germany picked up the commotion – but didn't necessarily know what they were hearing. That's why ground truth is so important: If nobody had witnessed previous volcanic eruptions and noted the types of signals they create, the Maman event would have been another inexplicable airborne percussion wave – consigned to the file scientists designate LGM, or little green men.
THE QUEST FOR GROUND TRUTH has been a driving force in Garces' career. Born in Cali, Colombia, in 1967 and raised in Puerto Rico, he studied astrophysics at the Florida Institute of Technology but soon became frustrated by the field's inherent abstraction. Stars and nebulae were distant, inaccessible. There was no way to confirm hypotheses up close – no way to obtain ground truth.
So Garces ditched astrophysics for earth sciences. In 1995, he earned a PhD in oceanography at UC San Diego, where he focused on the acoustics of volcanoes. At the time, only a few infrasound studies of volcanic activity existed, and they were mostly decades old. The young scientist recognized not only an intriguing line of inquiry but also an unfilled niche. He landed grants to work at universities in Alaska, Hawaii, and Japan.
A few years later, Garces experienced an infrasound-producing event firsthand. He was driving his Toyota Corolla around the Sakurajima volcano on the southern tip of Japan's Kyushu island, an open vent that's often in a continuous state of low-level eruption. Garces was downloading seismic and infrasound data amid explosions and raining ash. Fatigued, he parked his car, set an alarm, and fell asleep. He awoke gasping for air, his vehicle filled with choking fumes. He couldn't start the engine at first, but eventually the ignition caught and he drove out of the cloud. He continued his rounds in pitch darkness, downloading data while a dense fog of glowing gases wafted down the mountainside.
Garces and his colleagues published their Sakurajima report in a 1999 issue of Geophysical Research Letters. The article contended that infrasound could be used in conjunction with seismic data to "monitor and potentially forecast volcanic eruptions." The team's measurements showed that such prediction was theoretically possible, though whether his approach can be generalized to other volcanoes remains questionable.
Garces was at the University of Hawaii in 1999 when he met Henry Bass. A physicist at the University of Mississippi, Bass had been tapped to lead the US nuke-test infrasound program and was visiting the Big Island to scout the location. He and Garces hit it off, and the young volcanologist soon became an important member of Bass' team. Bass, 63, has since emerged as the nerve center of US infrasound research, coordinating scientists who have found it to be a useful tool in disciplines as diverse as meteorology, geology, biology, and astronomy.
An avid surfer, Garces has since studied beaches in Hawaii and French Polynesia to see what infrasound can tell him about the waves. This winter, he'll be working with the National Weather Service to train a portable array on Oahu's North Shore, a renowned stretch of prime surf. The goal is to evaluate safety by measuring the amplitude, period, and spatial distribution of breakers in real time.
GARCES' HIGH TECH SURF WATCH is an example of what he calls nowcasting – using infrasound to get a more precise sense of prevailing geophysical conditions. Nowcasting is good for more than finding the best surf breaks, however. It can shed light on imminent disasters as they develop, making it possible to respond quickly and perhaps head off catastrophe.
For Garces, nowcasting usually involves volcanoes. About 60 erupt every year, spewing ash into the sky and posing a serious hazard for airplanes. Volcanic ash can buff windshields until they're opaque. Sucked into a jet engine, it melts and then resolidifies in cooler areas. In the past 50 years, more than 80 planes have flown through airborne ash, sometimes with near-fatal results. In 1982, a Boeing 747 carrying 240 passengers entered the plume of Indonesia's Galunggung volcano at 37,000 feet. All four engines shut down, and the plane plummeted 25,000 feet before the pilot managed to restart three of them.
Volcano observatories and ash advisory centers keep tabs on the threat, but the system is inherently flawed. Many volcanoes lie in remote areas, and bad weather and nighttime darkness can make them invisible to satellites. So the infrasound community aims to complement the existing ash-alert system with a network of microphone arrays. Garces is playing a lead role in this effort, especially at Ecuador's Instituto Geofisico, where researchers now factor data from two infrasound arrays into their daily volcano report.
As scientists become adept at using infrasound for nowcasting, they're beginning to apply it in forecasting, listening for signals that might foreshadow a violent upheaval. "It's the holy grail," Garces says. One of the most promising possibilities is predicting the trajectories of hurricanes. Garces spent part of 2003 collecting infrasound data on Pacific storms. When he plotted it on a map, he saw that the most intense signals formed an ovoid shape, with the narrow end pointing in the direction the storm later appeared and continued to move.
Then came Katrina. Hurricanes often change course unexpectedly, and this one was no exception. The storm struck the coast farther east than expected, complicating evacuation in Henry Bass' home state of Mississippi. Afterward, Bass revisited Garces' charts and wondered whether infrasound data would have made it possible to forecast the course of the tempest. When Hurricane Rita tore through the area a few weeks later, he had an infrasound array at the ready. The mics picked up an ovoid pattern similar to the one Garces noted – not enough data points to be sure that infrasound can predict a hurricane's path, but sufficient confirmation to warrant further investigation. For the 2006 season, Bass assembled an army of portable arrays, ready to be deployed to the Gulf or Atlantic coasts when the next hurricane barrels in.
As helpful as infrasound may be for assessing current conditions and predicting the course of future events, it also serves as a purely scientific tool, a window on phenomena otherwise difficult to observe and measure. Consider bolides, meteors that explode as they enter Earth's atmosphere. The Tunguska blast over Siberia in 1908, believed to have been caused by a bolide, detonated with the force of 10 to 15 megatons – up to 1,000 times the explosive force of the bomb that obliterated Hiroshima – and leveled 800 square miles of forest. Tunguska-size bolides are extremely rare, but exploding meteors of 1 kiloton are relatively common.
Peter Brown, an astronomer at the University of Western Ontario, is working with colleagues at Los Alamos National Laboratory to pair infrasound data with satellite imagery. He hopes to track the number of bolides entering the atmosphere over a given time span. Knowing how often these events occur will make it possible to calculate the probability of damage on Earth's surface. It will also help astronomers date terrain on planets, moons, and asteroids based on the number of craters in a particular area.
And there's another reason to study bolides: "You want to make sure you can distinguish them from nuclear explosions," Brown says. He cites a meteor that burst over the Mediterranean Sea in mid-2002, at the zenith of nuclear tension between India and Pakistan. Had it entered the atmosphere a few hours earlier, it would have exploded nearer the Indian subcontinent and might have been mistaken for a first strike, possibly triggering a nuclear exchange.
Infrasound technology is poised to make such nightmare scenarios a lot less likely. In the coming decade, the network of microphone arrays will be folded into an ambitious international initiative involving nearly 70 nations. Established in early 2005, the Global Earth Observing System of Systems will integrate satellite, seismic, hydroacoustic, tidal, and infrasound sensors into an integrated planetary monitoring apparatus.
BACK IN THE BLOCKHOUSE, the countdown is under way. Technicians cluster on the scuffed linoleum floor. The building has thick, blast-proof windows, but they're tinted and thoroughly scratched by blowing sand. Everyone stares at a big videoscreen on the wall.
A heavyset Navy technician counts down: 3 … 2 … 1 … A bright light pierces the windows. The Orion missile shoots from the launch ramp like an overgrown bottle rocket, and the screech of its thrusters reverberates through the concrete walls.
Seconds later, the steel door flies open and people pour out of the blockhouse. Floodlights illuminate the smoke wafting over the launch area. A tarantula stands motionless in the sand. On the roof, Garces is uninjured, his head cocked back in a full-throated laugh. Now he stands and frames a portion of the heavens with his hands. "Look in this part of the sky," he says. "This is where the rocket will explode."
Two minutes pass. A flash appears in the northern sky. Waves of compressed air fan out from the point of detonation at 660 miles per hour, passing over highways, hamlets, and people sleeping in their beds. Like an earthquake in the air.
credit Robyn Twomey
Milton Garces measuring wind speed in preparation for taking infrasound readings at Keahole Point, Hawaii.
credit Kevin Hand
credit Robyn Twomey
Milton Garces measuring wind speed in preparation for taking infrasound readings at Keahole Point, Hawaii.
credit Kevin Hand
credit Robyn Twomey
Milton Garces measuring wind speed in preparation for taking infrasound readings at Keahole Point, Hawaii.
credit Kevin Hand
