Echoing what Germany had tried to achieve during the war, the United States urged astrophysicists and ionospheric scientists to devise scientific instruments suitable for piggybacking on the first round of twenty-five US-assembled V-2s, which were to be tested in 1946 at White Sands Proving Ground in New Mexico.55 Members of the V-2 Rocket Panel, charged with shepherding this effort, included the Navy Research Laboratory, the Army Signal Corps, the Applied Physics Laboratory, the National Advisory Committee for Aeronautics (NACA, the wartime forerunner of NASA), General Electric, Princeton, Harvard, and the University of Michigan. Among the instruments were spectrographs, a shielded Geiger counter, a new type of photographic emulsion, temperature sensors, telemetry systems, and a microwave-band radio transmitter that would propagate its signals through the rocket exhaust. At first, military observers at the V-2 Panel’s early meetings assumed it would be necessary to clarify the sorts of data they sought, but soon recognized the almost complete congruence between what they wanted and what the scientists had already pondered. The agendas resonated.
A fall 1946 editorial in Army Ordnance Magazine portrays the endeavor in upbeat terms, as a journey toward knowledge: “To accomplish research objectives, the ‘war head’ of the V-2, with its explosive filling, becomes a ‘peace head’ filled with scientific paraphernalia for exploring the upper atmosphere and evaluating the performance of the . . . rocket.”56 But whenever the War Department supplies the funding, part the curtains and you’ll see the needs of conflict masquerading as the needs of science.
But let’s get back to the invisibility of radio waves and the persistent military goal of stealth.
Earth is one of the noisiest radio sources in the cosmic sky. We broadcast our existence loud and clear. For any aliens who might be skywatching in our direction with a radio telescope, we practice the antithesis of stealth. Terrestrial planets, of which Earth is our best-known example, don’t naturally emit radio waves in copious amounts. But think of all our activities that generate radio waves: your mobile phone, your remote car-door opener, the radar guns that identify you as a candidate for a speeding ticket, broadcast television, your wi-fi and that of all your neighbors, the Deep Space Network that communicates with space probes, and of course radio stations themselves. Our planet blazes with radio waves—the aliens’ best evidence that we have plenty of technology.
Regarding potential surveillance closer to home, Earthlings are more circumspect. We pay attention to defense. Whenever there’s a new threat, we try to come up with a new countermeasure. Radar king Robert Watson-Watt characterized this continual back-and-forth as “the never-ending series of counter-counter-countermeasures in the agelong contest between projectile and armor.”57
One useful radar countermeasure developed during World War II was what the Americans called chaff, the British called Window, and the Germans called Düppel. The US Secretary of the Navy described it as “a unique method of arranging aluminum foil strips of varying lengths into a package which when released in great numbers by our attacking aircraft had practically the same effect on enemy radar directors as a smoke screen would have upon optical directors.”58
Chaff was—and is—a decoy. To a radar-equipped plane or guided missile, it looks like a target. In the 1940s its key attraction was its capacity to reflect the radar beamed in its direction—to mimic the radar echoes that would be created by an airplane caught in that same beam. Its requirements were not complicated: it just had to be highly reflective, not subject to clumping, and of a length appropriate to the wavelength of the radar. You’d spray the sky with floating strips, and the enemy’s radar tracker would be overcome with confusion, unable to tell the difference between target and chaff. If you didn’t know the wavelength of the enemy’s radar-targeting system, you could spray chaff of varying lengths and count on some of it succeeding. If you did know the wavelength, you could spray only chaff of a suitable length, thereby intensifying its reflectivity and maximizing the chances of its masquerading as the target, especially if the radar beam was wide and therefore likelier to intercept more of the chaff.
The first person on the Allied side to officially propose chaff as a viable countermeasure was the Welsh physicist Joan Curran, the sole woman scientist at Britain’s Telecommunications Research Establishment. Telefunken had already tested Germany’s own version two years earlier, in 1940. In retrospect, the concept itself seems fairly self-evident—though, again, as with the resonant cavity magnetron, the decision makers initially resisted authorizing its use for fear that it would soon increase their own side’s vulnerability. In this case, the concern was that, once used, the strips could easily be observed, understood, and copied by the other side. Nevertheless, it was finally deployed in 1943, and by war’s end, three-fourths of US aluminum foil production went toward the making of chaff.59
Chaff wasn’t the only World War II radar countermeasure. Other attempts included jamming, blinding, obfuscation (for instance, changing the pulse rate of one’s own radio navigation system), noise generation, coating U-boat snorkels with rubber, and radar spoofing, which included jiggering one’s technology so that it returned a disproportionately strong echo, causing the other side’s radar operators to think a large number of planes were heading their way. Whatever anyone could dream up was fair game until something better came along or until the enemy either became too familiar with a given technique or temporarily forgot about its existence. There was also an electronic instrument called a search receiver, which, when fitted with a directional antenna, could locate an enemy’s radar station at greater distances and with greater efficiency than radar itself could achieve.60
Then there were the counter-countermeasures. One of these, invented on the German side, was based on the differences in the motion of a bomber plane and a cloud of chaff. In obedience to the Doppler effect, the bomber’s high speed caused a shift in the wavelength of the signal reflected from the bomber’s surface, whereas the almost weightless ribbons of chaff simply drifted under the influence of the wind. As a result, at least sometimes, the Germans were able to distinguish plane from foil and to direct their flak against the plane.61
Chaff is a countermeasure of interest to the astrophysicist because of its reliance on albedo—reflectivity—an attribute almost indispensable to the study of celestial objects in a variety of electromagnetic wavelengths. Biologists and geologists and chemists and physicists do not typically devote themselves to the detection of light; astrophysicists do. The military, too, has an ongoing interest in albedo. Minimizing it is a prime goal in innovative stealth and national-security solutions, except that the military thinks in terms of radar cross-section rather than albedo.
The albedo of an object is the average percentage of light that it reflects compared with the amount of light that hits it. What doesn’t get reflected gets absorbed. The lower the albedo, the more difficult it is to detect the object. Earth’s moon is shockingly dark, with an albedo of 0.12—about the same as the sidewalls of your car tires. Meaning that overall, taking both dark areas and bright areas into account, it reflects 12 percent of the light that hits it and absorbs all the rest. Cloud-shrouded Venus, our nearest planetary neighbor, has an albedo of 0.75, rendering it a fine bright object in twilight skies, where it routinely gets mistaken for a hovering UFO. Saturn’s moon Enceladus, which is mostly covered in freshly deposited, pristine water ice, has an eye-popping albedo of 0.99. An object that appears bright to your detectors is not necessarily nearby. It could be far but have a highly reflective surface, or it could be nearby but have a surface that’s only moderately reflective. So the albedo alone, though containing crucial information, provides only partial data about your target.
The entire industry of stealth is about getting the albedo of an object as close to zero as possible. You want your aircraft to have the radar cross-section of a bumblebee, so that it can disappear from your enemy’s radar and thus prevent a coherent signal from being reflected back to them. If you succeed, they won’t k
now whether their signal was absorbed or just kept sailing through space unimpeded. You can also put radio-wave detectors on your plane so that you know when you’re being “painted” with a radio signal. You’ll then know you’ve been found, and since you’re aware that your adversary might be sending a surface-to-air missile to get you, you can take evasive measures.
But there’s another, better option: you can turn the entire surface of your aircraft into a series of facets, at assorted but specific angles, so that radar bounces off it every which way except back toward you, making your plane almost invisible to radio waves and, as the Air Force phrases it, “restoring the element of surprise.” Voilà, you have now designed the F-117A stealth fighter, a “low-observable,” more or less triangular one-seater aircraft coated with a black, radar-absorbent substance for extra stealthiness. This plane manages simultaneously to resemble an enormous origami crane and an airborne tank. It’s not fabulous on the aerodynamic front, but at least for a while—since surprise is perishable—it put the USAF back in the driver’s seat regarding time and place of attack.62
Developed during the 1970s and early 1980s at the vividly storied, once-secret Nevada salt flat known as Area 51, the F-117A flew hundreds of close-in strikes and bombing missions in Iraq during Operation Desert Storm in 1990–91 and Operation Iraqi Freedom in 2003. Its scientific parent was a monograph written in 1962 by a Soviet theoretical physicist/engineer who established a firm mathematical foundation for calculating “the diffraction of electromagnetic waves by metal bodies of complex shape”—more specifically, “reflecting bodies with abrupt surface discontinuities or with sharp edges (strip, disk, finite cylinder or cone, etc.).” The monograph was translated in 1971 for the US Air Force and studied closely soon thereafter by a radar specialist at Lockheed Aircraft’s secretive, cutting-edge Skunk Works unit, which had earlier produced the U-2 spy plane.63
Although scientists already understood that certain surface characteristics could enable an aircraft to evade easy detection by radar, the mathematics necessary to a workable physical theory of diffraction did not yet exist. That was the contribution of Pyotr Ufimtsev, author of the 1962 monograph. After spending its first Cold War decade in obscurity, his monograph became “the Rosetta Stone breakthrough for stealth technology,” giving rise not only to Lockheed’s F-117A stealth fighter but also, later, to Northrop’s sleek B-2 stealth bomber, which uses continuously curved surfaces rather than facets for its fuselage. The difference arises from the simple matter of differences in calculating power at the time of their design, between computers of the 1970s and those of the 1980s, which were a hundred times more powerful.64 If Batman flew a stealth bomber, the B-2 would be his Batplane.
For more than half a century, warmakers have exploited the fact that most military detection takes place under circumstances beyond the reach or domain of visible light. Astrophysics has long dedicated itself to detecting phenomena in every single wavelength of light—a pastime that takes advantage of every possible advance in science and technology to accomplish its task. As of September 2015, you can add gravitational waves to this observational arsenal. Discovered by the LIGO collaboration (the Laser Interferometer Gravitational-Wave Observatory), these signals are ripples in the fabric of space and time, caused by the exotic doings of gravity rather than by light. Even so, gravitational waves from across the universe are so weak by the time they reach Earth that many years will probably pass, possibly even centuries or millennia, before gravitational astrophysics leads to innovative military tactics.
Nowadays the majority of astrophysical revelations derive from detectors designed for invisible parts of the spectrum: from several-hundred-mile-long, extremely low-frequency radio waves on the low-energy end to quadrillionth-centimeter-long, extremely high-frequency gamma rays on the high-energy end. Want to see a gigantic stream of stars 76,000 light-years from Earth and several million times fainter than the dimmest stars detected with the unaided human eye? See it through NASA’s infrared Spitzer Space Telescope. How about a sudden flare of gamma rays emitted by a galaxy 7.6 billion light years away, far more ancient than Earth itself? See it with VERITAS, the Very Energetic Radiation Imaging Telescope Array System in Arizona, and confirm it with NASA’s Fermi Gamma-ray Space Telescope. And what of a galaxy almost 10 billion light-years from Earth with a mass 400 trillion times that of our Sun? Use data from ESA’s XMM-Newton and NASA’s Chandra X-ray Observatory to determine the mass.
Today astrophysicists see a universe immeasurably more complex than the one conceptualized by Newton or Herschel. Some things, such as stellar nurseries, glow brilliantly in infrared but are almost completely dark in the visible range. So, too, is the cosmic microwave background. Yet in spite of all the mind-blowing discoveries made in invisible wavelengths since the end of World War II, visible-light detectors still yield surprises. In 2016 astrophysicists using the Hubble Space Telescope announced that they had found the most distant galaxy ever seen, gleaming 13.4 billion light-years from Earth. Its stars would have been made solely of hydrogen, helium, and a tad of lithium, because no other atoms yet existed—no carbon, no nitrogen, no iron, no silicon, certainly no silver or gold.
Each band of light presents its own detection challenges. Earth’s atmosphere is transparent to the visible part of the spectrum, which is why we can see the Sun, but it is largely opaque to ultraviolet. Clouds are opaque to visible light but almost transparent to infrared. Brick walls are opaque to our eyes, but to microwaves those walls are transparent, which is why we can talk on our cell phones while indoors. Humans are transparent to radio waves. Glass is transparent to visible light. You may say a brick wall is opaque, but an astrophysicist will ask, Opaque in what wavelength? The astrophysicist will also ask, What’s the transmission curve—what fraction of the light of a given wavelength gets through a given medium without being absorbed?
Take microwaves, living out their rather low-energy lives at the longer-wavelength end of the electromagnetic spectrum, ranging from a millimeter up to about thirty centimeters. Only about half the microwave light from objects beyond Earth’s atmosphere makes it through to terrestrial telescopes. What happens to the other half? It’s absorbed by atmospheric water vapor. That’s why microwave astrophysicists locate their Earth-based telescopes in deserts or, even better, in a high-altitude desert, above most of the cloud cover. One place on our planet where both aridity and altitude serve the astrophysicist is the Atacama Desert, a high plateau in the Andes mountains of northern Chile. With its few millimeters of annual rainfall (until climate change brought flash floods and hot-pink flowers in 2015), Atacama is the driest desert on Earth, and it’s so high that most of the clouds, and therefore most of the water, lie below it. Not surprisingly, the world’s most powerful earthbound microwave telescope, ALMA, the Atacama Large Millimeter/submillimeter Array, has its home there.
When you chart the transmission curve for microwaves through Earth’s atmosphere, you find a sudden, transparent window between the wavelengths of eighteen and twenty-one centimeters. At either end of this little band, radio astronomers can detect distinct emissions from the universe’s ubiquitous hydrogen atoms (H) and their partners in water, hydroxyl molecules (OH). This band has been dubbed the water hole—a phrase more commonly invoked for places on Earth where wild creatures congregate to drink and wallow. We suspect that any aliens who know of our existence and want to communicate with us might also know the absorptive effects of water on various wavelengths. So if they’re clever, they might exploit the dip and try to reach out to us via the microwave frequencies that get through the water hole.65
How about a less pacific result of the astrophysical discovery that water absorbs microwaves? How difficult would it be to design a nonlethal weapon that targets the water content of the human body? Three-fifths of our average body mass is water. Such a weapon could operate on the same principle as the microwave oven.
Ask, and it shall be given. Raytheon’s Active Denial/Silent Guardian System i
s America’s version. Like peaceable ALMA, it operates at millimeter wavelengths, which are a little shorter than those in a standard microwave oven. This limits the depth of their penetration into the human body. You don’t actually want to cook people with nonlethal weaponry. Let’s say your mayor thinks there could be property damage during next Saturday’s climate protest. He may want to get proactive in the war against domestic terrorists like your Aunt Melissa. The army can send one of its trucks equipped with a millimeter-wave generator to a street corner near the crowd, and when the truck beams its electromagnetic radiation at the center of a crowd of protesters, their skin will feel like it’s beginning to fry, even if they’re wearing clothes. To avoid pain, the protesters will willingly and rapidly disperse.66
There are other small-scale, ostensibly nonlethal weapons, security measures, and crowd-control gizmos that utilize other nonvisible wavelengths, notably infrared, and tend to occupy the MOUT (Military Operations on Urban Terrain) portion of the use-of-force spectrum: surface-to-air missiles, airport security systems that disrupt the guidance system of any missile aimed at a plane, weaponized lasers, non-nuclear electromagnetic-pulse generators, pulsed-energy projectiles, PHaSRs. There are battle aids like night vision scopes and goggles capable of image intensification. And of course there are profoundly lethal electromagnetic weapons—armaments capable of massive devastation. The knowledge that underpins these activities and instruments is what interests the astrophysicist; the instruments themselves are what interest both the destroyers and the defenders.
Accessory to War Page 22
