Sky & Telescope - November 2020 - 27

Researchers are only starting to untangle the data, and
there are much more to come. "One of the most exciting
things so far - and we haven't even gotten that close yet - is
that we're seeing smaller and smaller and smaller events as
we get in closer," McComas says.

Where the Dust Never Settles
There's one other thing that Parker encounters that doesn't
actually come from the Sun: dust. The spacecraft is ﬂying
through the densest region of the solar system's zodiacal cloud.
Comets and asteroids coming too close to the Sun break up
and their fragments collide, grinding down until nothing but
micron-scale electrically charged particles remain. Due to
their interactions with sunlight, these particles slowly spiral
in toward the Sun, though when the particles become small
enough sunlight may instead push them away.
The Wide-ﬁeld Imager for Solar Probe (WISPR) is Parker's
only imager, a roughly shoebox-size telescope that peeks over
the edge of the spacecraft to capture sunlight scattering off
electrons and dust. The heat shield acts like a coronagraph,
blocking the light from the Sun itself, which is 13 to 15
orders of magnitude brighter than the corona. The images
provide context for Parker's other measurements, revealing
structures - such as large-scale solar storms or even smallscale twists of the magnetic ﬁeld - before the spacecraft ﬂies
into them and samples them directly.
Already, WISPR's PI Howard and colleagues have seen that
the emission from dust-scattered sunlight drops off in a way
that suggests a dust-free region extends out to at least 10
solar radii from the Sun's surface (S&T: Apr. 2020, p. 10). The
drop-off is so smooth, Howard says, that he doesn't think the
heat is sublimating dust grains, species by species, directly
into gaseous form. Instead, he thinks the particles are erod-

SOL A R ENERG E TIC PA RTICLES: N ASA / PRINCE TON /
JOHNS HOPK INS A PL; WISPR IM AG E: N ASA / N AVA L
RESE A RCH L A BOR ATORY / PA RK ER SOL A R PROBE

Coronal
mass ejection

Magne
tic ﬁ
eld
line
s

Energetic
protons
and ions

of
bit
Or

ol
ar
Pr
ob
e

Flare

rS
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Pa

p "PAUL REVERES" This schematic shows an explosion on the visible
surface of the Sun that ejects material out into the solar system, termed
a coronal mass ejection. IS IS scientists discovered that energetic
charged particles rushed ahead of one such eruption that Parker witnessed during its first orbits. The particles could provide advance warning to satellites and astronauts of the incoming space weather threat.

p SEEING SOLAR WIND Parker's views of the streaming solar wind
are oblique - its WISPR imager cannot stare straight at the Sun, so it
looks off to the side in the direction that Parker is traveling. Nevertheless,
the images are critical to seeing ahead of time what space environment
Parker will encounter.

ing, broken up by the pervading solar wind, and gradually
being pushed back out.
Parker's other instruments can detect dust indirectly,
too. FIELDS, for example, measures the momentary voltage
generated when a dust grain slams into the spacecraft and
vaporizes into plasma. IS‫ݪ‬IS can likewise detect dust impacts.
Jamey Szalay (Princeton) and colleagues have used these
data to conclude that the Sun is ejecting dust from the solar
system at a rate of at least half a ton per second. Ultimately,
such results could help astronomers understand planetary
formation in systems around other stars.

Every Orbit Closer
The Parker Solar Probe launched during solar minimum,
giving scientists the opportunity to study the extraordinarily
quiet Sun and its relatively undisturbed (but still churning)
ﬁelds and particles.
But even as scientists continue to pore over the data from
the initial orbits, the Sun's activity should begin to ramp up,
and solar eruptions will dump more and more energy into the
corona. The maximum in solar activity will come between
2023 and 2026 - around when Parker is at its closest to the
Sun, swinging within 9 solar radii of the visible surface.
Other telescopes will soon be joining in on the fun. The
Daniel K. Inouye Solar Telescope in Hawai'i, which is still
in the process of coming online, took its ﬁrst light images
in January, and the European Space Agency's Solar Orbiter
reached its ﬁrst perihelion in June (see page 9).
Crucially, well before solar maximum, Parker will pass
within the Alfvén radius. "We almost did it with encounter
four," Bale says. "In the next couple of orbits for sure, probably." Once inside, the spacecraft will ﬁnally "touch" the Sun.
Will scientists ﬁnd answers there? Certainly. But as McComas points out, "The point is to get more questions. Answer
the questions you've had for a while, and uncover the next,
more difﬁcult round."

¢ Sky & Telescope's News Editor MONICA YOUNG is glad to
see that the Parker Solar Probe has put on sunblock before
catching some rays.
sk yandtelescope.org * NOVEMBER 2 02 0

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Sky & Telescope - November 2020

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