Sky & Telescope - June 2020 - 9

SUN

European Solar Orbiter
Launches
THE EUROPEAN SPACE AGENCY'S

Solar Orbiter took to the skies on February 9th, the ﬁrst space-science mission
to launch in 2020. The orbiter joins a
growing armada of solar telescopes,
including the Parker Solar Probe
(S&T: Apr. 2020, p. 10) and the groundbased Daniel K. Inouye Solar Telescope,
which saw ﬁrst light in January. Solar
Orbiter will begin collecting data in
May as it cruises toward the Sun, but
full science operations won't begin until
November 2021.
Solar Orbiter will be the ﬁrst mission
to image the Sun's poles. (Ulysses, a
joint ESA/NASA mission, launched in
1990 to study the Sun's polar regions
but didn't carry cameras.) With the
help of three gravitational assists, one

p An artist's impression shows the Solar Orbiter
spacecraft approaching the Sun.

maneuver past Earth and two past
Venus, the spacecraft will enter an
initial orbit inclined 17° to the ecliptic plane. From this vantage point,
Solar Orbiter will study the the Sun's
extended atmosphere and the birthplace
of the solar wind.
Several further gravity assists past
Venus will gradually steepen the inclination, putting it at 33° by 2029. The

closest pass by the Sun, at 0.28 astronomical unit, will take the spacecraft
well inside Mercury's orbit.
Ten instruments are hitching a ride,
including the NASA-contributed Heliospheric Imager, which will take the ﬁrst
high-resolution images of the Sun from
up close. Other instruments include an
X-ray spectrometer and telescope, two
ultraviolet detectors, and an instrument
to survey the Sun's magnetic ﬁelds. A
Sun-blocking coronagraph named Metis
will also make key observations of the
inner corona.
This new mission overlaps with -
and is complementary to - NASA's
Parker Solar Probe. Parker studies the
Sun from close up, but it can't image the
Sun directly. Solar Orbiter will image
the Sun from an inclination that allows
study of the poles, offering a comprehensive view of the Sun's activity.
■ DAVID DICKINSON

SOLAR SYSTEM

SOL A R ORBITER: ESA / ATG -MEDIA L A B;
PLU TO: N ASA / JHU A PL / SWRI / A LE X PA RK ER

Pluto's Icy Heart "Beats," Driving Planet-Scale Winds
SPUTNIK PLANITIA, a basin that makes
up the left lobe of Pluto's "heart,"
may drive subtle winds over the entire
dwarf planet, Tanguy Bertrand (NASA
Ames Research Center) and colleagues
suggest. They published their ﬁndings
in the February Journal of Geophysical
Research: Planets.
Among the many surprises Pluto presented to scientists was a global atmosphere. Hints on the surface, such as
probable wind streaks west of Sputnik
Planitia, reveal the thin atmosphere's
impact. Bertrand and colleagues examined what could drive these winds using
high-resolution climate simulations.
In what Bertrand's team nicknames
Pluto's "heartbeat," nitrogen ice in
Sputnik Planitia sublimates in Pluto's
daytime and condenses back into ice
at nighttime. However, because Pluto's
spin is tilted with respect to its orbit
around the Sun so that it's almost on
its side, the northern edge of Sputnik
Planitia is in constant daylight. As a
result, the nitrogen there is constantly

sublimating, inducing a southerly wind.
Pluto's rotation deﬂects that wind
slightly westward, until it nears the
basin's mountainous western boundary, where it returns southward. At the
southeastern edge of Sputnik Planitia,
which lies closer to the part of Pluto
under polar night, nitrogen condenses
back into ice. Here, the winds turn
northerly again, completing the counterclockwise spiral.
The simulations show that from
Sputnik Planitia, nitrogen gas rises to
higher altitudes, ﬂowing from northern
summer to southern winter. But just
as an ice skater's spin slows when they
spread their arms, Bertrand explains,
the nitrogen molecules slow down
as they move from north to south,
because they're moving farther from
the planet's rotational axis. As a result,
high-altitude nitrogen ends up moving more slowly than the dwarf planet
below it. The consequence is that the
winds on Pluto blow westward, opposite
the planet's rotation - a pattern unique

p Sputnik Planitia, a 3-km-deep basin, makes
up the left lobe of Pluto's "heart."

among solar system worlds (except
maybe Neptune's moon Triton).
"Sputnik Planitia may be as important for Pluto's climate as the ocean
is for Earth's climate," Bertrand said.
William McKinnon (Washington
University in St. Louis), who was not
involved with the study, agrees that
Sputnik Planitia is the "source and
sink" of Pluto's atmosphere.
■ JULIE FREYDLIN

* Watch simulations of Pluto's winds at
https://is.gd/plutowinds.
sk yandtelescope.org * JUNE 2 02 0

https://www.is.gd/plutowinds http://www.skyandtelescope.org

Sky & Telescope - June 2020

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