Sky & Telescope - July 2023 - 17

URANUS IMAGES: SCIENCE: NASA, ESA, STSCI, AMY SIMON (NASA-GSFC), MICHAEL H. WONG (UC BERKELEY), IMAGE
PROCESSING: JOSEPH DEPASQUALE (STSCI); ORBIT DIAGRAM: BEATRIZ INGLESSIS / S&T, SOURCE: M. SHOWALTER AND
M. GORDON / SETI INSTITUTE
Uranus orbits the Sun while lying on its side (why? we don't
know), and it experiences extreme illumination changes as
a result. In January 1986, Uranus was close to its southern
summer solstice, with the Sun overhead at a latitude of 82°
south. Uranus's southern hemisphere baked in continuous
sunlight. The continuous radiation built a thick haze that
obscured atmospheric activity beneath it. At the same time,
the north poles of both planet and moons stayed in perpetual
darkness, hiding them from Voyager 2's view.
Nearly 22 years later, in December 2007, Uranus passed
2014
through an equinox. The shift of seasons brought dramatic
changes to Uranus' atmosphere, which lit up with storms and
belts visible from Earth. Now, in 2023, atmospheric activity
is shutting down again as the planet approaches its 2030
solstice, plunging the southern hemisphere into darkness.
The next equinox comes in February 2050. So the best
chance to see the most dynamic state of Uranus's atmosphere,
to study the rings at a full range of solar illumination
angles, and to see all of the satellites lit pole-to-pole with
sunlight comes in the 2040s, as Uranus approaches equinox.
Each year after 2050 will hide more and more of the north
poles of both the planet and its moons in winter darkness
and will produce more atmospheric haze, making Uranus
harder to interpret.
Neptune poses no such time crunch. Its less extreme axial
tilt makes arrival at a specific time of year less urgent, and
although its weather changes as seasons shift, it produces
dark storms and bright " scooters " year-round. Neptune's year
2022
CHANGING SEASONS Left: In November 2014, seven years after
the northern hemisphere's spring equinox, storm clouds of methane ice
crystals appear at mid-northern latitudes. Right: Eight years later, with
the Sun beating down on high northern latitudes, a thick, smog-like haze
has built up over the north pole.
is so long, with each season lasting 40 years, that any mission
we launch in the next half-century will see the planet at a
different season than Voyager 2 did.
What Would a Mission to Uranus Look Like?
At the moment, the proposed Uranus mission is mostly a
list of questions, both about the science and the spacecraft
design. NASA hasn't yet committed to sending it, either.
Nevertheless, planetary scientists are throwing themselves
into the discussion, advocating for which mysteries they most
want to solve and how they would learn the answers.
We can speculate on what the mission might look like,
2050
Southern spring
equinox
1986
Southern summer
solstice
N
S
2030
Northern summer
solstice
N
N
Northern
hemisphere
in darkness
S
S
Southern
hemisphere
in darkness
N
Southern polar haze
thins and fades.
Low-latitude storms
appear in both
hemispheres, continue
through equinox
S
2007
Northern spring
equinox
Not to scale
URANIAN SEASONS Because of its rotation axis's extreme tilt - 98° from the orbital plane - Uranus experiences dramatic seasonal
changes over the course of its year. When Voyager 2 flew by in 1986, the northern hemisphere was in complete winter darkness. By
sending a spacecraft to rendezvous with the ice giant in the 2040s, scientists hope to catch the storms and weather systems that pop up
around equinoxes.
sk yand tele scope .o r g * JULY 2023 17
Storms fade,
northern polar
haze grows

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Sky & Telescope - July 2023

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