Page 31 - Astronomy - October 2017 USA

9
WHAT IF?

When Voyager 2 flew by the Saturn sys-
tem in August 1981, one of its goals was
to further study Enceladus. Unfortunately,
the spacecraft’s swiveling platform mal-
functioned for several hours, and its five
instruments were out of commission dur-
ing closest approach to Saturn and its
inner moons.
That meant the science team didn’t
get all the images and measurements
they were hoping for, including close-up
mosaics of Enceladus. During the flyby,
that moon and the Sun aligned in such
a way to illuminate Enceladus’ south
polar region, where scientists now
know geysers of water and organic
materials spew.
Would sunlight have glinted off
the icy particles in those jets, revealing
their presence in 1981? “I always wonder:
Would we have found Enceladus’ plumes
with Voyager had the Voyager 2 scan
platform not gotten stuck?” asks Linda
Spilker. Instead, the Cassini spacecraft
discovered the geysers more than two
decades later, during a 2005 flyby. — L. K.

What a difference a generation of tech- 10 7. Enceladus is one of Saturn’s most
nology design makes: Today, scientists have compelling moons. Voyager 2 passed
much closer to the moon than Voyager
hundreds of occultation observations from 1, and it took this image August 25,
the Cassini spacecraft, which studied the 1981, from a distance of 74,000 miles
Saturn system from 2004 until September (119,000km). Looking at data from the
of this year. Cassini’s 13 years of observa- probes, planetary scientists became
intrigued by Enceladus’ widely varying
tions provided answers about how the terrain. Swaths of the moon appeared
moons and Saturn itself shape the rings. devoid of craters, indicating ages in
those areas of less than a few hundred
Saturn’s satellites million years. NASA/JPL-CALTECH
Before either Voyager arrived at Saturn, 8. Voyager 1 sent back images of Dione
most of the planet’s moons were no more on November 12, 1980, from 149,000
than pinpricks of light. Ground-based tele- miles (240,000km), that showed
numerous impact craters marred by
scopes couldn’t resolve their surfaces, so fractures. These fractures appeared as
scientists had little information about the bright, wispy streaks, and range from
immense variation these worlds hold. Voyager scientists stationed at JPL, recalls tens to hundreds of thousands of miles
First up was Titan, the planet’s largest planetary scientist Carolyn Porco, that liq- long. NASA/JPL
moon. Voyager 1 made its closest approach uid nitrogen might exist on the surface of 9. More than 30 years after Voyager’s
to the orange sphere in the late hours of Titan. “It turned out the initial analysis was last glimpse of Titan, Cassini finally
November 11, 1980, when it flew less than incorrect,” she says. “But I’ll never forget revealed the moon’s surface via infrared
imaging November 13, 2015. The surface
310,000 miles (500,000km) from the moon. the indescribable thrill of hearing that is surprisingly Earth-like, with features
Scientists hoped to see through the thick rumor. It felt, for a moment, like all of us . . . that include sand dunes, mountain
atmosphere to learn about the surface, but were crewmates on the starship Enterprise, ranges, and liquid hydrocarbon seas.
Titan’s mysteries weren’t so easy to solve. and we had just come upon the most alien NASA/JPL/UNIVERSITY OF ARIZONA/UNIVERSITY OF IDAHO
The visible and infrared cameras could not of worlds yet seen. We were indeed plan- 10. Haze layers sit just above Titan’s
penetrate the clouds. Fortunately, research- etary explorers.” Porco later explored the thick atmosphere, at heights of
ers could get a radio signal to the surface Saturnian system as a member of the about 300 miles (500km), above the
and back, and used it to calculate the atmo- Cassini team. moon’s surface in this Voyager image.
NASA/JPL-CALTECH
sphere’s density: 1.6 times that of Earth. What Voyager did reveal of Titan,
During the analysis of the radio data, a though — knowledge of its atmosphere’s
hushed rumor spread among the dozens of density and composition, the possibility of

WWW.ASTRONOMY.COM 31