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THE PERIL TO EARTHBOUND ASTRONOMY

PITY POOR Earthbound astronomers. Their telescopes have to gaze through more than haze. Other perils include L.E.D. street lighting, the Internet and then there is the prospect of autonomous cars.

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Daniel Clery’s “Your Self-driving Car Could Kill Radio Astronomy” appeared in the January 20, 2017, issue of Science, the magazine published weekly by the American Association for the Advancement of Science. Reading it makes for a good accompaniment to Mike Massimino’s description of the Hubble Space Telescope in his book Spaceman.

The Hubble’s view, Massimino writes, is free from interference of the Earth’s atmosphere. Clery amplifies on man-made perils to Earthbound observation, both of the visual and radio variety.

For example, Clery notes that much of public street lighting has depended on high-pressure sodium (HPS) illumination, “which emits light mostly at the red end of the visible spectrum.” HPS illumination has what’s termed a “light temperature” of around 2200 K.

Then along came L.E.D. illumination with its benefits of longer life and lower energy consumption. In the visual spectrum, it isn’t so much the brightness of today’s L.E.D. lighting, but rather its color that bedevils Earthbound astronomy. First-generation L.E.D. street lamps emit illumination with a light temperature of around 5000 K, at the blue end of the visible spectrum.

However, our atmosphere preferentially scatters blue light (this is why the daytime sky is blue). This scattering also causes a nighttime “light haze” that obscures the view of telescopes.

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Above, standard L.E.D. street lighting. Below, more telescope-friendly L.E.D. lighting. Images from Science, January 20, 2017.

Second-generation L.E.D. street lighting is typically less blue than that of the first generation, with light temperature around 4000 K. However, notes Clery, widespread conversion from HPS to even second-generation L.E.D. could brighten skies “for hundreds of kilometers.”

Newer L.E.D. products have light temperatures less than 3000 K. Other countermeasures include installing fixtures that point downward and planting nearby trees that are taller than the street lamps.

Radio astronomers have challenges different from their visually oriented colleagues, under the control of different government organizations. Notes Clery, “Since the advent of radio astronomy in the 1960s, regulators have reserved about two percent of the spectrum for the sole use of astronomy and Earth observation, and designated another four percent for sharing with other users.”

For instance, the 21-centimeter band is important to radio astronomy because it’s the emission of neutral hydrogen. However, as astronomers search for weaker and weaker signals, mankind’s broadcasts from frequencies nearby generate an electromagnetic fog that mars observation.

Clery provides an example in proposed HAPS (high-altitude platforms). These high-flying balloons and low-orbiting satellites are to provide Internet access to remote communities. But they’d use frequencies adjacent to the protected astronomy spectrum.

As another example, Clery notes, “In 2015, astronomers lost a long fight with the automobile industry” when the International Telecommunication Union assigned automaker collision-avoidance radar frequencies that are close to those important to radio astronomers.

Fleets of autonomous vehicles are potentially even more problematic: Clery writes, “If the radar on a single self-driving car is pointed at a sensitive radio telescope… it can cause interference even at 100 kilometers away.” He quotes a specialist: “You don’t know the impact [of such technologies] until they are released into the wild.”

It isn’t just the Internet that’s a wild, woolly world. ds

© Dennis Simanaitis, SimanaitisSays.com, 2017

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