The flight took off around 6:30 pm, just a little behind schedule. About half an hour later, a bomb exploded onboard. The plane broke apart. Chunks of metal showered a small Scottish town called Lockerbie. The accident, now known as the Lockerbie Bombing, killed all 243 passengers, all 16 crewmembers, and 11 people on the ground.

It wasn’t the first attack on an airplane, and it certainly wasn’t the last. Perhaps the worst, and most famous, attack happened on September 11, 2001. That day, terrorists flew two airplanes into skyscrapers in New York City. Nearly 3,000 people died.

But Pan Am flight 103 also stands out because it was one of the first times that terrorists carried out a major attack on a non-military airplane. The bombing also started a new era in science, says Doug McMakin, an electrical engineer at the Pacific Northwest National Laboratory in Richland, Wash. Suddenly, airport security was a big deal.

“It kicked up research dollars on explosive detection technologies,” McMakin says. “This is one way [terrorists] try to put fear in us — by blowing up airplanes. What we’re trying to do is counter that threat.”

In the race to stay one step ahead of terrorists, researchers are working on a new generation of machines that can peer through fabric and see what people are carrying inside their clothes. Some airports are already starting to use these devices, called scanners.

Even as technology makes skies safer, however, worries have popped up. Some people wonder if airport scanners will threaten their privacy by showing every curve of their bodies and what’s in their pockets. People also worry about how the machines might affect their health.

Experts are doing their best to assure people that the new equipment is safe. More than that, it’s necessary. Our lives, they say, depend on it.

The long wait

If you’ve ever been on an airplane, you know the drill. After arriving at the airport, you wait in one snaking line to check in your luggage. Then you move to an even longer line, where you wait to get into the main terminal.

Security gates are a staple of modern airline travel. New work would add new detail to security screening.

fenlan1976/iStockphoto

After throwing away your water bottle, you show your I.D. and boarding pass to an official. You take off your jacket, belt and shoes. You put them on a conveyor belt, along with your carry-on bag. As X-rays zap your belongings, you walk through a doorframe that has no door. If it beeps, you start over.

As annoying as the whole process may seem, each step is designed to keep travelers safe. And behind the scenes, a whole lot of technology — and history — is involved.

When everyday people started traveling by air in the 1930s, they simply walked into the airport and onto their planes. In response to hijacking threats in the 1960s and 1970s, American airports began to require that passengers and their bags be screened. Airports added metal detectors to prevent weapons and explosives from getting onto planes.

Since then, airport security agencies have been pursuing the latest technologies. Every time an attack happens, new security rules follow. In December 2001, for example, the “shoe bomber” boarded a plane with explosives hidden in the soles of his shoes. After that, airports started to require that passengers send their shoes through X-ray machines.

A spectrum of strategies

Among the security technologies you may have already seen at airports are chemical-detecting swabs, explosive-detecting puffer machines, even drug-sniffing dogs.

Coming next are full-body scanners that look through clothes to detect what people might be trying to sneak through the gate. Two types of scanning technologies are in the works. One is called a backscatter machine. The other is a millimeter-wave system.

This illustration gives the general idea: New scanners in the works would quickly reveal what’s in your pockets.

cirnishman/iStockphoto

Both are already starting to appear in airports around the world. Still, experts continue to debate which one is better.

Behind both machines is the same science: the electromagnetic spectrum. The spectrum describes a range of energy forms, which travel and behave as waves. At one end of the spectrum are low-energy radio waves and microwaves. At the other end are high-energy gamma rays, X-rays and ultraviolet radiation. Visible light — what we can see — lies somewhere in the middle.

A millimeter-wave imaging scanner looks like a round phone booth. It uses a part of the electromagnetic spectrum called millimeter waves. These waves lie just to the right of microwaves, which means they carry a little more energy than microwaves do. But they have less energy than infrared waves.

Millimeter-wave machines would scan passengers using a part of the electromagnetic spectrum (illustrated) that sits between microwaves and infrared. See the full video.

NASA, Teachers’ Domain

When a person steps inside a millimeter-wave imaging scanner, two 7-foot long beams of millimeter waves scan his body from head to toe. These waves pass through fabric, but they bounce off the water in our skin and in liquids that people might be trying to sneak onboard. (Some explosives start out as liquids). Millimeter waves also ping off plastic, paper, ceramics, even little nuggets of gum.

A computer inside the scanner detects the reflections and turns them into a three-dimensional image that pops up on a screen. The picture shows the outline of a passenger’s figure and everything he’s carrying under his clothes or in his pockets. Workers survey the image for suspicious objects.

“They’re going to detect threats that metal detectors don’t get,” says McMakin, who researches millimeter-wave machines. “The eyes will go to things that shouldn’t be there.”

The system is quick, and getting quicker. With modern computers, McMakin says, a scan takes about a second and a half. A visible image shows up just two or three seconds after that. As computers get even faster and cheaper, the equipment is becoming increasingly practical.

Millimeter-wave technology is also safe. It uses 10,000 times less power than a cell phone does. And millimeter waves have too little energy to harm human health.

Backscatter

In a sort of technological face-off, millimeter-wave scanners are going head-to-head with backscatter scanners. Instead of using millimeter waves, backscatter devices employ X-rays. These are the same waves that machines at hospitals use to penetrate flesh and look for broken bones.

Because they use the higher-energy X-rays, backscatter scanners produce images that are more detailed than millimeter-wave machines spit out. Some people think the images are clearer and make unusual objects easier to identify.

For now, backscatter scanners are a little slower than millimeter-wave scanners, McMakin says. The technology has also raised some health concerns. X-rays produce a type of radiation that can, in some cases, damage human cells and cause cancer.

Despite the rumors, however, even backscatter machines are safe, says medical physicist James Hevezi, chair of the American College of Radiation Medical Physics Commission. One scan, he says, delivers the same amount of radiation as spending two minutes in an airplane at 30,000 feet.

In other words, the machines are far safer than flying is.

“It really is not a health concern,” Hevezi says. “It is a much lower dose than anything used in medicine.”

Perhaps a bigger concern is privacy, and that applies to both systems. In addition to showing slips of paper in pockets and explosives in underwear, scanners reveal every curve of a passenger’s body.

Some day, computers might be able to read the scans on their own without the help of human eyes, making this worry irrelevant. In the meantime, Hevezi, says, people will have to compare the benefits of scanning technology with the potential risks of flying without its help.

“The benefit is safety in the air when we travel,” he says. “If people want to travel in modes other than air travel, they can do it that way. Or they can ask to be patted down instead. There are still some choices we can make.”

Toward the horizon

As engineers continue to tinker on imaging technologies, their discoveries are fueling industries that have nothing to do with security. A company called Intellifit, for example, is using millimeter-wave scanners to help people buy jeans.

In less than 15 seconds, the machine spits out precise body measurements that can be used to design custom-made clothing. Scanners could also help dieters take body measurements to see whether they are getting slimmer. Scanners are already being used for these reasons in a few places.

As for security, advances in science should make air travel safer and safer, experts say.

“We’re just at the start of where we could go,” McMakin says, “and what it could be.”

TEACHER’S QUESTIONS

Here are questions related to this article.

Originally used to detect elusive particles from space called neutrinos, the four-story detector at the Sudbury Neutrino Observatory could be retrofitted to detect antinutrinos produced by natural radioactivity inside Earth.

Scientists have long known this strange fact: It’s easier to look deep into space than into the center of Earth. Light can pass through most of space, so the light from distant stars can easily be seen with the naked eye. But Earth is opaque, which means that light cannot pass through it.

If light cannot pass through it, then we cannot see what’s on the inside of our planet. So if we can’t use light to see inside our own planet, what can we use?

Recently, some scientists have been trying to use neutrinos — tiny particles smaller than an atom that zip through space. Neutrinos come from the sun or other distant stars, and astronomers have studied them for years. Now, a team of geoscientists — “geo” means Earth — think a kind of neutrino may have something to say about the Earth, too.

Not all neutrinos come from outer space. Special neutrinos called geoneutrinos are generated from within the Earth. (Remember that “geo” means Earth.) Most of these local neutrinos come from either the crust or the mantle. The crust is Earth’s outermost shell, what we stand on, and the mantle is five to 25 miles below the crust. Certain elements within the Earth can send off geoneutrinos when undergoing a process called radioactive decay.

During radioactive decay, a material loses some of its energy by sending out particles and radiation. An element that goes through this process is said to be radioactive, and radioactive elements occur naturally in the Earth. Some radioactive elements produce geoneutrinos.

After they are produced, geoneutrinos pass straight through the solid Earth without being absorbed or bouncing around. If they’re not stopped, they go straight into outer space — and keep going, and going and going. Geoscientists hope to catch a few of these particles on their way out, but it’s not going to be easy.

There are two big problems: There aren’t that many geoneutrinos, and they’re hard to find. To catch these elusive particles, scientists have designed special geoneutrino detectors. These strange-looking scientific instruments are giant, metal spheres buried deep underground.

In an abandoned mine in Canada, for example, scientists are preparing a geoneutrino detector that is four stories tall and more than a mile underground. The detector will be filled with a special liquid that flashes when a geoneutrino passes through. The liquid “produces a lot of light, and it’s very transparent,” says Mark Chen, the director of the project. When it’s up and running, probably in 2010, the detector will find only about 50 geoneutrinos per year. Other detectors are being planned all over Earth — one of them is even supposed to sit on the bottom of the ocean!

The geoscientists who study geoneutrinos hope that the particles will help answer an old question about the Earth. The interior of the Earth is blistering hot, but where does the heat come from? They know that part of the heat — maybe as much as 60 percent — comes from radioactive decay, but researchers want to know for sure. By measuring geoneutrinos, scientists hope to figure out how radioactive decay helps heat Earth.

Going Deeper:

Credit: NASA/Imagine the Universe

Energy travels throughout the universe at the speed of light in the form of electromagnetic radiation. What that radiation is called depends on its energy level.

At the really high-energy end of the spectrum, you’ve got gamma rays. You’re probably familiar with a close cousin to these: X-rays. They’re the ones doctors and dentists use to probe for unusual structures inside your body. Radio waves fall at the extreme other end. Those radio waves are the ones that deliver music and news broadcasts to your home radios.

Credit: NASA/Imagine the Universe

Ultraviolet rays, visible light, infrared radiation, and microwaves fall at energy levels in between.

Together, all of these types of radiation make up one long, continuous electromagnetic spectrum. Its energy travels in what’s usually referred to as waves.

What separates one type of electromagnetic radiation from another is its wavelength. That’s the length of a wave of that type of radiation. To identify the length of a wave of water in the sea, you would measure the distance from the crest (upper part) of one wave to the crest of another. Or you could measure from one trough (bottom part of a wave) to another.

It’s more difficult to do, but scientists measure electromagnetic waves the same way—from crest to crest or from trough to trough. In fact, each segment of the energy spectrum is defined by this wavelength. Even what we refer to as the heat given off by radiators is a type of radiation—one that has wavelengths in the infrared portion of the spectrum.

Sometimes these segments of the electromagnetic spectrum are also described in terms of frequency. A radiation’s frequency will be the inverse of its wavelength. So the shorter the wavelength, the higher its frequency. That frequency is typically measured in hertz, a unit which stands for cycles per second.

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