NEWS NOTES
VOYAGER 2: NASA / JPL; GRAPH: DON GURNETT / UNIVERSITY OF IOWAFOUR DECADES AFTER its launch
into the outer solar system, Voyager 2
exited the heliosphere, the cavity around
the Sun blown out by the solar wind. It
crossed over on November 5, 2018, six
years after that of the speedier Voy-
ager 1 (S&T: Apr. 2019, p. 9).
Voyager 1 returned a surprisingly
messy view of the outer boundary,
known as the heliopause, that separates
the Sun’s domain from the interstellar
medium. Voyager 2 promises a point of
comparison. In the November Nature
Astronomy, fi ve teams of astronomers
analyze the spacecraft’s report from the
frontier of the solar system.
Particles at the edge of the heliosphere
are hot and sparse — a mere 0.002 elec-
tron per cubic centimeter, Donald
Gurnett and William Kurth (both at
University of Iowa) report in their study
of Voyager 2 data. The gas and dust
between the stars, on the other hand, are
colder and denser. As Voyager 2 crossed
over, its plasma instrument noted a
20-fold increase in plasma density.
A similar instrument on Voyager 1
failed in 1980, which made its crossing
more diffi cult to recognize. However, its
indirect measurements showed a similar
jump in plasma density. Both Voyag-
ers crossed the heliopause in underSOLAR SYSTEM
Voyager 2’s View of the Solar System’s Frontiera day, corresponding to a boundary
0.005 astronomical unit (a.u.) thick.
The two spacecraft saw the density
change at roughly the same distance
from the Sun: 121.6 a.u. and 119 a.u.,
respectively. This was unexpected, since
the heliopause’s exact location ought
to change with solar activity. Voyager
had crossed the boundary as the Sun
was reaching a peak in its cycle of mag-netic activity, but by the time Voyager
crossed, solar storms had quieted. “If
we take our models at face value, we’d
expect a difference,” says Leonard
Burlaga (NASA Goddard Space Flight
Center), coinvestigator on the Voyagers’
plasma and magnetic fi eld experiments.
Another marker of the heliopause
is a sudden change in the nature of
the energetic particles measured by
the spacecraft. As Voyager 2 crossed
through the heliopause, it saw the low-
energy ions of the solar wind largely
drop away, replaced by higher-energy
galactic cosmic rays produced in distant
and long-ago supernovae.
However, even an astronomical unit
beyond the heliopause, Voyager 2 con-
tinued to detect a few lower-energy par-
ticles from the solar wind. This leakage
from the inside out was the opposite of
what Voyager 1 saw, says Edward Stone
(Caltech), the Voyagers’ project scien-
tist. “Even before [Voyager 1] left the
heliosphere, we had two episodes where
we were connected to the outside.” How
and how often particles leak in either
direction across the heliopause remains
an open question.
In addition, the Voyagers upended
expectations when neither one saw a
sudden shift in magnetic fi eld at the
heliopause. “We can dismiss this as a
coincidence in one case, but we cannot
do that twice,” Burlaga says. “There must
be some physical process that’s coor-
dinating the magnetic fi eld across this
region. That process is not understood.”
Scientists will continue to study and
compare Voyager data as the spacecraft
zip along at 3 a.u. per year. They’re
heading toward the bow wave (or per-
haps the bow shock) that forms in front
of the heliosphere as it plows through
the interstellar medium. But with only
about fi ve years or so of power left, nei-
ther one will reach this structure with
working instruments. Instead, their
main goal now is simply to measure
the undisturbed interstellar medium —
traveling as far from the heliosphere as
they can in the time they have left.
■MONICA YOUNG
Read more results from Voyager 2 at
https://is.gd/voyager2.pVoyager 1 crossed the heliopause in August
2012; heading in a different direction, Voyager
crossed in November 2018. The lines mark
the direction of plasma ow both inside and
outside the heliopause.pAs they traveled away from the Sun, the Voy-
agers encountered a surge in plasma density,
thought to mark the boundary between the
heliosphere and interstellar medium.10 FEBRUARY 2020 • SKY & TELESCOPE
0.0.01 Voyager 2119.0 a.u.121.6 a.u.Voyager 10.80 100 120 140
Distance from the Sun (a.u.)Plasma density(ions per cubic centimeter)HeliosphereVoyager 1Voyager 2HeliosphereVHeliopa
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