Unlike the gas giants
Jupiter and Saturn, the
solar system’s more
distant ice giants have largely received
the cold shoulder from robotic space-
craft. But thanks to a fortunate planetary
alignment that occurs only once every
175 years, NASA’s ambitious Voyager 2
mission f lew by the solar system’s sev-
enth planet, Uranus, in 1986.
Like both its bloated inner siblings,
Uranus hosts a ring system, though it is
much fainter than that of Saturn. The
rings around Uranus were initially dis-
covered in 1977 by astronomers aboard
the Kuiper Airborne Observatory, an
airplane equipped with an infrared
telescope. But Voyager 2 studied themSTATS
Mass: 14.5 Earth masses
Equatorial diameter: 31,760 miles
(51,120 km)
Average temperature: –323 F (–197 C)
Rotation period (day):
17 hours 15 minutes (retrograde)
Orbital period (year): 84 Earth years
Moons: At least 27 moonsURANUS
BELOW: Thanks to adaptive optics, the Keck
Telescope obtained these infrared views of the two
hemispheres of Uranus and its faint ring system in- The ice giant’s south pole is facing left in both
images. W.W. KECK OBSERVATORY/LAWRENCE SROMOVSKY (UNIVERSITY OF
WISCONSIN-MADISON)
in unprecedented detail. The mission
also uncovered 10 new moons and
clocked the planet’s atmosphere zipping
around the world at speeds approaching
450 mph (725 km/h). Before continuing
on to Neptune, Voyager 2 also captured
informative images of some of the ice
giant’s largest moons: Titania,
Miranda, Umbriel, Oberon,
and Ariel.
But no other craft has
visited Uranus since.
That’s disappointing,
considering all the
mysteries the
planet still
holds. Not
least of
which: Why
is Uranus’
rotationaxis tilted nearly 100° to the plane of the
solar system, making it orbit the Sun not
like a spinning top, but more like a roll-
ing ball? No matter the cause (the leading
theory is an ancient planetary collision),
we do know that this unique orientation
gives Uranus the most extreme seasons in
the solar system. One pole is bathed in
constant sunlight while the other is veiled
in darkness for some 21 years at a time.
Uranus’ magnetic field is also lopsided,
tilted some 60° relative to its spin axis, so
the planet’s rotation twists its magnetic
field lines into a bizarre corkscrew shape.
Another unresolved mystery about
Uranus is its structure. The blue-green
hue of its swirling atmosphere (primar-
ily made of hydrogen and helium) is the
result of trace methane gas, which more
readily absorbs red light. But as you ven-
ture deeper beneath the planet’s cloud
tops, things get murkier. Scientists think
that about 80 percent of the planet exists
in the form of hot and dense mantle lay-
ers composed of super-pressurized
water, ammonia, and methane f luids,
which surround a small core of icy rock.
The jury’s still out on that, however.
Maybe another mission to Uranus is
in order?22 ASTRONOMY • DECEMBER 2021
Voyager 2 arrived
at Uranus in 1986,
returning views of a
celeste orb with very
subtle features. Still,
the spacecraft’s
instruments shed light
on myriad mysteries.
NASA/JPL-CALTECH