44 March/April 2022
Space
9Here’s the Thrust of It
Ion thrusters rely on a process called
ionization, pulling electrons from an
atom to form ions, to generate thrust.
Instead of ionizing expensive noble
gases, ThrustMe’s propulsion system
(see main image) turns solid iodine
into a plasma, which can then be
ionized and expelled as thrust.HOW IT WORKS: Heaters within a
specialized tank warm the solid iodine
until it sublimates, turning it into agas. That gas enters a specialized
discharge chamber surrounded by
a radio-frequency (RF) inductive
antenna. Similar to an induction
stovetop, the RF induction antenna
heats the gas and turns it into a
plasma. “It’s essentially a soup of
electrons and positive ions,” Lafleur
says. Then, a pair of high-voltage
grids extracts the positive ions from
this soup and accelerates them to
generate thrust.effort to develop a cheaper, more sustainable
propellant—one that can maneuver satellite con-
stellations through orbit safely and efficiently.
Xenon, the heaviest nonreactive noble gas, is
the propellant of choice in part because it has a
low ionization threshold, meaning it requires rela-
tively low amounts of energ y to lose an electron and
ionize. But at approximately $3,000 per kilogram,
it’s an expensive option. It’s also a finite resource,
making up less than one part per 10 million in
Earth’s atmosphere. That’s particularly troubling
given how many industries, like lighting and imag-
ing, use the gas.
Krypton, the propellant that powers SpaceX’s
Starlink satellites, is also in high demand. While
it’s cheaper than xenon, krypton isn’t as efficient.
Plus, more than half of the world’s krypton is used
in the thermal insulation of windows—a need that’s
expected to grow as the world warms and contrac-
tors work to reduce energy loss in large buildings.
“The [small-satellite] industry is almost forced to
look for alternatives,” Laf leur says.
Enter iodine. The mineral, found naturally in
many sources from soil to seaweed, is abundant—
its global production is 500 times that of xenon, and
it’s about 100 times cheaper. “It could easily accom-
modate being used on all these satellites, while still
being able to service all of the other industries that
require it,” Laf leur says. Though iodine has roughly
the same mass as xenon, its storage density is three
times higher than xenon’s and nine times higher
than krypton’s, both of which are stored as gases
in pressurized tanks. Iodine can be packed into a
smaller, unpressurized tank, making miniaturiza-
tion easier. In tests, iodine proved t wice as effective
as xenon. It’s safer to transport, and thrusters can
be shipped prefueled so buyers don’t have to spend
time sourcing propellants from local vendors. “You
can pretty much plug it in,” Laf leur says.
In February 2021, ThrustMe concluded the first
ever in-space test of an iodine ion thruster. While
in orbit, the NPT30-I2 thruster fired a total of 11
times, both to test whether it was operational and to
reposition the Spacety satellite it was maneuvering.
ThrustMe shared the results in November 2021 in
the journal Nature.
“People have talked about iodine as an option
for electric propulsion ion thrusters for almost 20
years now,” says Charlie Ryan, Ph.D., an aerospace
engineer at the U.K.’s University of Southampton.
“[This test] is a considerable step.” ThrustMe was
the first to launch and test an iodine thruster, but
it’s not the only company developing the technol-
ogy. The Italian space firm T4i launched a similar
iodine propulsion system in 2021.
Iodine isn’t necessarily a silver bullet, though.
It’s highly corrosive, so the team at ThrustMe
encased the most critical parts of the thruster in
iodine-compatible coatings made from ceramic
and polymer. And it takes 10 minutes for the sys-
tem to heat up enough to convert the solid iodine
into plasma, hampering the satellite’s ability to
make quick orbit adjustments. Laf leur says this
issue will likely improve with more power.
ThrustMe plans to scale its designs to meet the
needs of a wide variety of missions. Laf leur says
the company is working on a larger version of the
current propulsion system, as well as a design that
clusters multiple thrusters together. Both, Lafleur
says, could take a spacecraft to the moon, Mars,
or beyond.
The propulsion system can shrink, too. Many
satellites currently in orbit are too tiny to house a
propulsion system. “If you’re a small satellite with
no propulsion system and there’s a collision risk,
you can’t do anything about it,” Laf leur explains.
Future satellites could be outfitted with a minia-
ture, simplified version of the company’s iodine
thruster, powerful enough to perform avoidance
maneuvers and reenter Earth’s atmosphere, thus
limiting potential collisions involving space junk.
“We can finally offer something to small satellites
which didn’t really exist before,” he says.