ATOM & COSMOS
Spacecraft sees hints of hydrogen wall
New Horizons data may mark where the sun’s influence wanes
BY LISA GROSSMAN
The New Horizons spacecraft has spotted an ultraviolet glow that seems to
emanate from near the edge of the solar
system. That glow may come from a
long-sought wall of hydrogen that represents where the sun’s influence wanes,
the New Horizons team reports online
August 7 in Geophysical Research Letters.
“We’re seeing the threshold between
being in the solar neighborhood and
being in the galaxy,” says team member
Leslie Young of the Southwest Research
Institute, based in Boulder, Colo.
Even before New Horizons flew past
Pluto in 2015 (SN: 8/8/15, p. 6), the
space was using its ultraviolet telescope
to look for signs of the hydrogen wall.
As the sun moves through the galaxy, it
produces a constant stream of charged
particles called the solar wind, which
inflates a bubble around the solar system called the heliosphere. Just beyond
the edge of that bubble, about 100 times
farther from the sun than the Earth is,
uncharged hydrogen atoms in interstellar space should slow when they collide
with solar wind particles. That buildup,
or wall, of hydrogen, should scatter
UV light in a distinctive way.
The two Voyager spacecraft saw signs
of such light scattering 30 years ago. One
of those craft has since exited the heliosphere and punched into interstellar
space (SN: 10/19/13, p. 19).
New Horizons is the first spacecraft in
a position to double-check the Voyagers’
observations. It scanned for UV light
seven times from 2007 to 2017, space sci-
entist Randy Gladstone of the Southwest
Research Institute in San Antonio and
colleagues report. As the spacecraft trav-
eled, it saw the UV light change in a way
that supports the decades-old observa-
tions. All three spacecraft saw more UV
light farther from the sun than expected if
there’s no wall. But the team cautions that
the light could also be from an unknown
source farther away in the galaxy.
“It’s really exciting if these data are
able to distinguish the hydrogen wall,”
says space scientist David McComas of
Princeton University. That could help
researchers figure out the shape and variability of the solar system’s boundary.
After New Horizons flies past the outer
solar system object Ultima Thule on New
Year’s Day 2019, the spacecraft will continue to look for the wall about twice each
year until the mission’s end, hopefully,
Gladstone says, 10 to 15 years from now.
If the UV light drops off at some point,
then New Horizons may have left the
wall in its rearview mirror. But if the
light never fades, then its source could
be farther ahead — somewhere deeper in
space, says team member Wayne Pryor
of Central Arizona College in Coolidge. s
MATH & TECHNOLOG Y
Desalination filter gets an upgrade
Smoothing out the rough patches of a material used to filter saltwater could
make desalination more affordable, researchers report in the Aug. 17 Science.
Desalination plants typically strain salt out of seawater by pumping the
water through polyamide films riddled with pores that allow water molecules
to pass through but not sodium ions. Yet, organic matter and some waterborne
particles can clog the films’ pockmarked surfaces. Operators must replace the
membranes frequently or install expensive equipment to remove contaminants.
Now researchers have made a supersmooth membrane without the divots
that trap troublesome particles. Manufacturers normally create salt-filtering
films by dipping porous plastic sheets into chemical baths containing polyamide ingredients. Molecules glom onto the sheet, building up a thin polymer
membrane. But that method doesn’t allow for control over texture, says Jeffrey
McCutcheon, a chemical engineer at the University of Connecticut in Storrs.
McCutcheon and colleagues sprayed polyamide building blocks, molecular
layer by layer, onto aluminum foil sheets. Films (a micrograph of one, above
right) were up to 40 times as smooth as their commercial counterparts (left).
The team has yet to test how clean the films stay over time. — Maria Temming
The sun’s journey
through the galaxy may
build a wall of hydrogen
near the edge of the solar
system (curved line to the left
in this illustration).