46 ASTRONOMY • JULY 2018
wenty Thousand Leagues Under the
Sea had Nemo’s Nautilus. Vo y a g e t o
the Bottom of the Sea featured the Seaview.
Fantastic Voyage even sent a submarine into
the human body at the capable hands of
Captain Bill Owens. Fiction often presages
fact, and it seems that submarines may turn out to be
the new hotness in planetary exploration, although
their fearless skippers will remain back on Earth.
The solar system is awash with
oceans. Unlike the majority of Earth’s
seas, these ubiquitous brines are locked
away beneath ice crusts. But they’re
there, lurking under the surfaces of
Ganymede, Europa, Enceladus, and per-
haps other worlds like Titan, Dione, and
Pluto. These maritime sites may provide
the best venues for life beyond Earth, but
getting to them is the tricky part. In the
case of Jupiter’s ice moon Europa, its
60-mile-deep (100 kilometers) ocean
ebbs and f lows beneath miles of solid ice,
which blocks direct access.
Many engineers now believe that the
best way to search for life in these deep
waters is to deploy a modern-day
Nautilus. But sending a probe to Jupiter’s
moon, and safely down to the surface,
through the ice, and into the ocean, is a
daunting prospect to say the least.
It’s just the kind of prospect that fires
the visions of engineers, and a few brave
souls are taking the vision beyond imagi-
nation and into the practical world. One
of them is William Stone, the founder of
Stone Aerospace, a Texas-based company
that develops both the tools and systems
needed to explore the modern frontier of
space. Recently, Stone led a team atop
Alaska’s Matanuska Glacier. There, engi-
neers tested a cryobot (a robot that can
penetrate ice) called the Very deep
Autonomous Laser-powered Kilowatt-
class Yo-yoing Robotic Ice Explorer
(VALKYRIE). VALKYRIE is one of many
submersible robotic explorers that engi-
neers are studying, hoping one day to
investigate the oceans of other worlds.
Europa, ho!
As a target for any kind of probe,
including a submarine, Europa presents
some formidable roadblocks. Jupiter’s
ice moon contains the most extensive
ocean known in the solar system, but
its location presents problems for future
explorers. Temperatures are bitterly cold,
and distances for communication with
Earth are daunting, too. The ice crust
capping the ocean may be 18 to 30 miles
(30 to 50 km) thick, and once any probe
arrives at the ice-sea boundary, it must
safely deploy its submarine. Says Stone,
“It’s a very tough problem. At Europa,
you’re operating at 100 kelvins, in a hard
vacuum. Most techniques for getting intothat ice with a drill or a melt probe are
not going to work.”
Engineering firms have approached
the ice crust problem in a host of ways.
Drills are efficient only to certain depths,
and debris often fouls the mechanism or
fills the tunnel left behind, blocking the
probe’s signals to the surface. Other
designs have tried using heating ele-
ments to melt through the ice. But as
water fills the column behind the probe,
it freezes and again blocks radio signals,
and the power needed to melt through
may be prohibitive. Another approach
heats water and pumps it through jets to
melt the ice ahead, though the refreezing
water behind the probe is still a problem.
Stone Aerospace came up with a dif-
ferent solution. Its VALKYRIE probe
operates neither by drilling, nor by hot
water. Instead, a laser takes advantage of
the fact that “certain frequencies trans-
mit power through liquid water, and yet
absorb through ice,” Stone explains.
Last year, Stone Aerospace built
the first laser-powered ice penetrator,
on a probe called Archimedes. The
Archimedes system effectively takes the
light of an industrial laser operating at
1,070 nanometers and expands that into
a collimated beam the width of the
probe. “The vehicle needs to be long and
skinny. Ultimately, you realize that this
thing starts to look like a hot dog.
Physics forces you to have the smallest
possible diameter. You end up with a
long train,” says Stone. As scientists care-
fully select the focal length of the laser
optics, the probe can increase or
decrease its rate of speed.
Other research groups have also
drawn plans for planetary subs. Sweden’s
Uppsala University is exploring a sub-
mersible the size of two soda cans, while
Georgia Tech’s Icefin follows an elon-
gated design. Britney Schmidt, an assis-
tant professor in the School of Earth and
Atmospheric Sciences at Georgia Tech,
brought a team to drill a hole in
Antarctica’s Ross Ice Shelf.
The Icefin robot entered
the water and
descended to the ocean
f loor, following a f light
profile identical to aNASA’s Titan Mare
Explorer aims to
analyze organic
compounds found
on Saturn’s largest
moon. MICHAEL CARROLL