ASTRONEWS
Io
0
Europa
(^41) Ganymede
131
Callisto
141
10 ASTRONOMY • DECEMBER 2018
MINORITY RULES. Although protons make up only 5 percent of a neutron star, the particles are disproportionately
active when they interact with the more numerous neutrons, likely determining several of the star’s key properties.
W
hile the Sun might seem an easy
target for astronomers, it still har-
bors numerous mysteries. One is
the nature of its corona — the
thin, outermost layer of the Sun’s atmo-
sphere, visible from Earth only during a total
solar eclipse. Stretching millions of miles
and reaching temperatures above 2 million
degrees Fahrenheit (1 million degrees
Celsius), the corona is perplexingly 300
times hotter than the Sun’s so-called surface,
1,000 miles (1,600 kilometers) below.
Launched August 12 from Cape
Canaveral, Florida, NASA’s Parker Solar
Probe will become the first spacecraft to f ly
through the blistering gas, returning mea-
surements from within this poorly under-
stood region of our star’s atmosphere. The
probe will loop the Sun more than 20 times
in seven years, approaching as close as
3.9 million miles (6.2 million km) — seven
times closer than previous spacecraft, and
about one-tenth Mercury’s distance from
the Sun.
TWO MAJOR MYSTERIES
The mechanism behind the corona’s
extreme temperatures is unknown. Leading
theories propose that either electromag-
netic waves or bomblike “nanof lares,”
smaller versions of solar f lares, could be
responsible. To find a definitive answer,
astronomers need measurements that other
probes can’t record.
“All of our work over the years has
culminated to this point: We realized we
can never fully solve the coronal heating
problem until we send a probe to make
measurements in the corona itself,” said
Parker Solar Probe deputy project scientist
Nour Raouafi of the Johns Hopkins
University Applied Physics Laboratory, in a
press release.
The corona is also responsible for the
solar wind, an outf low of charged particles
accelerated by the extreme temperatures to
speeds of around 1 million mph (1.6 million
kilometers per hour). The solar wind causes,
among other effects, aurorae. Because
astronomers don’t understand what’s heat-
ing the corona, they also don’t understand
entirely how the solar wind is generated.
Solar wind disturbances can affect Earth in
many ways, from increased auroral activity
NASA PROBE WILL TOUCH THE SUN
to electrical blackouts and disruptions in
satellite function and communications.
“The Parker Solar Probe will help us do
a much better job of predicting when a dis-
turbance in the solar wind could hit Earth,”
said Justin Kasper, a professor of climate
and space sciences and engineering at the
University of Michigan.
A LONG TIME COMING
The idea for a probe that would touch the
Sun was proposed 60 years ago, making the
Parker Solar Probe NASA’s oldest success-
fully launched mission proposal. Originally
called Solar Probe Plus, the spacecraft was
renamed in 2017 for solar astrophysicist
Eugene Parker. It is the first time the agency
has named a mission after a living person.
The car-sized probe carries four instru-
ment suites protected by a nearly 8-foot
(2.4 meters) heat shield of 4.5-inch-thick
(11 centimeters) foam and carbon compos-
ite. The shield will reach temperatures of
2,500 F (1,370 C) on its Sun-facing side,
but the instruments behind it will remain
a comfortable 85 F (30 C).
“For scientists like myself, the reward of
the long, hard work will be the unique set of
measurements returned by Parker,” said
Adam Szabo, the probe’s mission scientist at
NASA’s Goddard Space Flight Center. “The
solar corona is one of the last places in the
solar system where no spacecraft has visited
before. It gives me the sense of excitement
of an explorer.” — Alison Klesman
The Parker Solar Probe will solve
some of our star’s biggest mysteries.
B E AT TH E H E AT. The Parker Solar Probe’s disk-shaped heat shield will protect the probe’s instruments from
the searing heat as it flies through the corona to collect data on this mysterious portion of the Sun’s atmosphere.
JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY
HOW MANY CRATERS ON JUPITER’S MOONS?
METEORITE MARKS. The Italian astronomer
Galileo Galilei discovered the four largest moons of
Jupiter in 1610. As telescopes improved, we began to
see features on those moons, although we couldn’t
spot them clearly. Then, in 1979, the two Voyager
spacecraft flew by the jovian system, and we began
to map their surfaces. Subsequent spacecraft have
added to the picture. — Michael E. Bakich
FAST
FAC T
Io has no sizable craters because
its volcanoes continually
restructure the moon’s surface.
ASTRONOMY
: ROEN KELLY