Military Magic Boosts Astronomy
‘Astronomy’ magazine
January 2001

The timeless precept about beating swords into plowshares still rings true. Military innovations have filtered down to civilian astronomy and helped expand our knowledge of the universe.

We’re fast approaching an astronomical renaissance of unprecedented degree where weapons-related technology turns askew to benefit science. Nailing down the exact details of how original military projects evolved takes some detective work, but the resulting story is an exciting complement to a technology once focused solely on destruction.

The list of examples astounds: Precisely ground mirrors built for antiaircraft searchlights and later used in spy satellites instead reflect starlight in telescopes. Radar receivers built to track enemy planes in World War II map the radio universe. Automated telescopes that followed Cold-War satellites in the 1970s now search for comets and near-Earth asteroids. Britain’s massive 76-meter Jodrell Bank Observatory, built in 1957, employs gimbaled radio antennae that mimic turret bearings developed for battleships. Deep-space detectors watching for secret nuclear tests first spotted and identified gamma-ray bursters — thus helping to solve one of the mysteries of modern astrophysics.

An example of a dramatic conversion of weapons technology into astronomical tools occurred in 1946 when captured German V2 missiles — which rained death on London two years earlier — were fired from White Sands, New Mexico, carrying scientific instruments. Experimental payloads measured micrometeoroid density, background sky brightness, and cosmic, solar, and x-ray radiation. About a decade later, Russia’s Proton and R7 rockets, as well as America’s Thor, Atlas, and Titan systems (all originally funded to carry thermonuclear bombs) opened the space frontier.

One of the best aspects of this technological flow is that it doesn’t stop with professional astronomical programs. Image-intensifiers, and later Charge Coupled Devices (CCDs), developed for military applications in the 1960s, entered astronomy the following decade, soon reaching amateurs as well as professionals. The increase in low-light sensitivity revolutionized astonomical imaging, replacing photographic film in many applications. Small, battery-powered image intensifiers now allow for longer exposures with better results in both research and amateur astrophotography.

GPS Aids Astronomers

Paul Maley, an amateur astronomer who works at NASA’s Johnson Space Center and chases short-lived celestial phenomena as a hobby, rarely thinks of the military origins of many of his observational tools. Without them, however, some of Maley’s observing projects would prove impossible. For instance, hand-held global positioning system (GPS) receivers let Maley and other observers take what was developed as a military navigational device and apply it to solar astronomy — a totally unexpected but serendipitous application. “You need to know your location on Earth to better than 10 meters,” Maley says. “There is no substitute for the GPS.”

Amateur astronomers often use GPS technology during eclipses to measure the profile of the lunar polar regions by watching stars graze past the limb. This information allows observation of the very edge of solar eclipses in order to precisely measure the sun’s radius as the altitude of its photosphere — the glowing surface — rises and falls.

“The biggest error, aside from the heights of the features on the moon, was always the location of the observation sites,” Maley explains. Most GPS systems provide a tenfold increase in accuracy when compared to the best survey maps available, he says. And because the lunar profile is mapped to within 10 meters, ground-site locations must meet that accuracy level.

Current and former observations benefit from GPS, Maley says. He is re-surveying older eclipse observation sites, such as the famous July 29, 1878, site of totality at Pikes Peak, Colorado. (Future Smithsonian Institution director Samuel P. Langley was at Pikes Pike while inventor Thomas Alva Edison watched the eclipse from Rawlins, Wyoming. Each man unsuccessfully tested an instrument to measure the corona that day.)

Now that civilian GPS (with resolution improved from several hundred to several meters) is available, Maley expects to determine long-term variations in the solar diameter.

The Secret of “Rubber Mirrors”

And while GPS is coming into common use, another military-to-civilian adaptation that revolutionized astronomy in the 1990s is the development of adaptive optics (AO). AO eliminates the detrimental effects of “seeing” through a randomly refractive atmosphere. No longer limited by the optical size of the telescope, AO technology enhances observing.

Telescopes equipped with AO provide views unaffected by the gaseous soup through which they peer. Before the development of AO, point sources were smeared into flickering, wavering blobs. Astronomers use AO technology to not only measure the flickering of starlight, but also to cancel out that distortion by activating tiny motors behind thin telescope mirrors that twist the mirror surfaces to redirect the light and produce a more useful image.

While the theory of adaptive optics sprang from imaginative astronomers back in the 1950s, the hardware (and the equally crucial software) was built from the military budget three decades later. Observing celestial beauty wasn’t exactly the Pentagon’s main goal, so for a long time, these capabilities were classified as top secret and withheld from astronomers and researchers who could use AO for different purposes.

Nearly Perfect Seeing

“Two of the adaptive optics systems with which I’m involved have military components or origins,” says Sallie Baliunas of the Mount Wilson Observatory near Los Angeles. “The first was mounted at the 60-inch telescope and produced diffraction-limited images in the visible,” she says. “That was an already developed but decommissioned military system. Our goal was to see if it could be retrofitted for astronomical research to a telescope built in 1908.”

It worked, so they tried another approach with an AO mirror on loan from the U.S. Air Force. In use since 1994 at the 100-inch Hooker Telescope at Mount Wilson, this AO mirror achieves nearly perfect seeing in the visible part of the spectrum.

Like vestigial organs, astronomical equipment that evolved from military gear often retains traces of its origins. University of Alabama astronomer Bill Keel explains how the precise source of the military-derived sensors could sometimes be determined: “One of our support astronomers at Kitt Peak enjoyed pointing out how to tell, from detector characteristics like wavelength range and timing, which systems started life in strategic or tactical use,” says Keel.

Manipulation of atmospheric turbulence corresponds to the portion of electromagnetic spectrum being observed. Air turbulence varies on the scale of arc-seconds, so two target points more than a few arc-seconds apart on a visual image require separate compensation. Scientists have found that infrared untwinkles easier than visible wavelength light.

The military used infrared to see in the dark, track enemy missile plumes, or follow “cold” warheads moving through space. Consequently, infrared technology was part of a highly classified, generously funded, and aggressive research program. Astronomers, on the other hand, were interested in detecting and using infrared radiation at a more intellectual level for research purposes.

These efforts paid off. Last February [2000], the Rockwell Science Center in Thousand Oaks, California, proudly announced the unveiling of “the world’s largest astronomical infrared sensor.” In a program funded by the University of Hawaii, the infrared sensor now searches deep space in order to detect very faint galaxies. This mercury-cadmium-telluride sensor measures infrared radiation with wavelengths between 0.9 and 2.5 microns. It uses a complementary metal oxide semiconductor (CMOS) technology that converts incoming observations into a usable digital signal.

“Faulty” Arrays Reused

Kadri Vural of the Rockwell Science Center explains that the technology was funded primarily by the military. “They used those kinds of detectors for missile seekers and surveillance systems,” he says. Initial arrays were about 16x16 or even 32x32 pixels. They were barely good enough to detect and discriminate missile plumes from reflected sunlight. The current instrument is 4cm square with an array of 2,048 pixels on each side.

Other applications attempted to exploit the reflected sunlight, Vural recalls. “When I came to work here in 1980, we had a contract with the Jet Propulsion Laboratory to make devices for remote sensing,” he says. Different reflections off rocks, dirt, plants, and water allowed scientists to chart Earth’s surface. Manufacturing flaws in some of the 64x64 arrays failed acceptance testing because they contained large areas of inoperative pixels.

Stacks of those faulty arrays were sent to astronomers at the University of Arizona in the mid-1980s, according to Vural. “We built these devices to observe Earth,” he says, “but the astronomers wanted to enhance sensitivity. So they built their own cooling system and bled liquid nitrogen past the sensors.” Cooling sensors don’t always improve sensitivity, but in this case, Vural says, it did.

“Infrared astronomers say we’ve revolutionized astronomy,” Vural notes with satisfaction.

And while many of the latest scientific gadgets appear to come straight from a Buck Rogers scenario, not all the innovations used by astronomers today were spawned by high-tech military developments of the last half century: Consider hot air balloons, for example. Used by the military in the Civil War as lookout stations, recent balloon advancements may soon place these simple devices at the cutting edge of space exploration. A full-scale Ultra Long Duration Ballon (ULDB) mission is planned for December 2001 to study the use of balloon platforms in near-space missions.

Seems we’re still beating on swords after all.

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Adaptive optics technology compensates for atmospheric turbulence while observing from Earth’s surface. Weapons engineers developed adaptive optics (AO) to spot targets such as incoming warheads or enemy satellites. Targets tracked accurately through atmospheric disturbances using AO technology could be blinded, disabled, or destroyed with a concentrated burst of light. Originally funded in the 1980s through the Strategic Defense Initiative (SDI) program, AO development was known as “Star Wars Technology.”

“During the Cold War, AO was extremely classified,” explains Paul LeVan, a scientist at Kirtland AFB in New Mexico. “About 1991, a lot of astronomers were making proposals to try it, and this was noticed in the Defense Department.” By then, the military systems were already much more advanced than the civilian systems. “We normally had 250 to 500 actuators on the mirrors, compared to only a handful of actuators on civilian experiments,” LeVan explains.

The collapse of the Soviet Union — and with it a change in strategy in the “Star Wars” program — meant that the U.S. no longer needed AO technology as protection. In 1992, the Air Force hosted a conference in Albuquerque, New Mexico, describing AO to the public.

“A wavefront sensor generates the correction within a reaction time of 10 milliseconds,” says LeVan. As a result, the military AO systems compete with the theoretical resolution of the Hubble Space Telescope — at least over a limited angle of about 10 arc-seconds. “And it doesn’t have to be close to the vertical,” he adds.

Adaptive optics works by untwinkling starlight. The key lies in measuring the amount of twinkle caused by atmospheric turbulence and then untwisting that light by slight bends in special mirrors. But measuring those distortions is challenging because they are highly localized and change very quickly.

One technique is to focus on a bright, nearby star with a known distance from Earth and measure the amount of distortion its light suffers and then compare that distortion rate to the target of interest. However, if the known star is too distant from the target of interest to use as a distortion control, it cannot serve as an accurate model of the turbulence through which the target star’s wavelength travels.

To solve this limitation, the Air Force developed technology to produce laser guide stars — also called “artificial stars.” High-powered laser beams illuminate a section of the atmosphere 12 to 15 miles (20 to 25 km) above Earth’s surface. As this “twinkling” light bounces back, distortions are measured and compared to the target distortions. (See photo, page 50.)

In more highly classified research, the Air Force used a special laser to excite a small area (no larger than a meter in diameter) in the atmosphere’s “sodium layer” at altitudes of 50 to 56 miles (80 to 90 km). Although this technique used a laser of comparable energy to the first experiments, it produced an artificial guide star 10,000 times brighter — even though it was much higher. Light from this higher “star” was measurable day and night and allowed for measurement and compensation of almost the entire turbulent atmospheric path.

Amateur astronomers noticed and photographed these “artificial stars” over San Francisco Bay during secret military testing programs at Lawrence Livermore National Laboratory. These naked-eye “UFOs” were only explained years later.

Lasers of such power weren’t just lying around warehouses and were prohibitively expensive for astronomers to develop from scratch. But government research had already built these devices for other purposes. For example, one laser used for AO research at Lawrence Livermore was originally funded by the Department of Energy for separating uranium isotopes at a commercial nuclear power plant.

By the late 1980s, these experiments were measuring starlight intensity at about 40 percent of the theoretical maximum, compared to between one and five percent for conventional telescopes. In contrast, the Hubble Space Telescope achieves between 60 and 85 percent, according to Robert Fugate, range director of the Starfire optical facility near Albuquerque, New Mexico.

While astronomers consider a resolution of about five microradians “good seeing,” the military AO telescopes regularly produced images with about one-third of a microradian or better resolution. Thanks to AO, researchers use the increased resolution to look deeper into space and explain the mysteries of the universe. And, from all indications, military scientists are as delighted with the technology transfer as are astronomers.