Pale Blue Dot ( PDFDrive.com ) (1)

higher resolution and sensitivity, and at wavelengths a little further into the infrared than we were
able to reach with our crude glass emulsions. Galileo photographed great mountain ranges. We
already knew of their existence, though; a much more powerful technique had earlier been employed:
radar. Radio waves effortlessly penetrate the clouds and thick atmosphere of Venus, bounce off the
surface, and return to Earth, where they are gathered in and used to make a picture. The first work had
been done, chiefly, by. American ground-based radar at JPL's Goldstone tracking station in the
Mojave Desert and at the Arecibo Observatory in Puerto Rico, operated by Cornell University.
Then the U.S. Pioneer 12, the Soviet Venera 15 and ?6 and the U.S. Magellan missions inserted
radar telescopes into orbit around Venus and mapped the place pole to pole. Each spacecraft would
transmit a radar signal to the surface and then catch it as it bounced back. From how reflective each
patch of surface was and how long it took the signal to return (shorter from mountains, longer from
valleys), a detailed map of the entire surface was slowly and painstakingly constructed.
The world so revealed turns out to be uniquely sculpted by lava flows (and, to a much lesser
degree, by wind), as described in the next chapter. The clouds and atmosphere of Venus have now
become transparent to us, and another world has been visited by the doughty robot explorers from
Earth. Our experience with Venus is now being applied elsewhere—especially to Titan, where once
again impenetrable clouds hide an enigmatic surface, and radar is beginning to give us hints of what
might lie below.
VENUS HAD LONG BEEN THOUGHT of as our sister world. It is the nearest planet to the Earth. It has
almost the same mass, size, density, and gravitational pull as the Earth does. It's a little closer to the
Sun than the Earth, but its bright clouds reflect more sunlight back to space than our clouds do. As a
first guess you might very well imagine that, under those unbroken clouds, Venus was rather like
Earth. Early scientific speculation included fetid swamps crawling with monster amphibians, like the
Earth in the Carboniferous Period; a world desert; a global petroleum sea; and a seltzer ocean dotted
here and there with limestone-encrusted islands. While based on some scientific data, these “models"
of Venus—the first dating from the beginnings of the century, the second from the 1930s, and the last
two from the raid-1950s—were little more than scientific romances, hardly constrained by the sparse
data available.
Then, in 1956, a report was published in The Astrophysical Journal by Cornell H. Mayer and his
colleagues. They had pointed a newly completed radio telescope, built in part for classified research,
on the roof of the Naval Research Laboratory in Washington, D.C., at Venus and measured the flux of
radio waves arriving at Earth. This was not radar: No radio waves were bounced off Venus. This
was listening to radio waves that Venus on its own emits to space. Venus turned out to be much
brighter than the background of distant stars and galaxies. This in itself was not very surprising. Every
object warmer than absolute zero (-273°C) gives off radiation throughout the electromagnetic
spectrum, including the radio region. You, for example, emit radio waves at an effective or
"brightness" temperature of about 35°C, and if you were in surroundings colder than you are, a
sensitive radio telescope could detect the faint radio waves you are transmitting in all directions.
Each of us is a source of cold static.