An illustration showing the final stage of a possible future giant telescope being created in space using fluids. Credits: Studio Ella Maru
When it comes to telescopes, bigger is
better. Larger telescopes collect more light and allow astronomers to peer
farther into space and see distant objects in greater detail.
What if there was a way to make a
telescope 10 times – or even 100 times – bigger than before? What started
as a theoretical question is now a series of experiments to see if fluids can
be used to create lenses in microgravity. The next experiment is stowed on
the International Space Station National Laboratory waiting for the arrival of
Axiom-1 astronauts to try it out, and is part of Ax-1 Mission Specialist
Eytan Stibbe’s research portfolio.
NASA
astronaut Karen Nyberg watches a water bubble float freely between her and the
camera, showing her image refracted in the droplet. Credits:
NASA
It Starts with Fluids
All liquids have an
elastic-like force that holds them together at their surface. This force is
called surface tension. It's what allows some insects to glide across water
without sinking and gives water droplets their shape. On Earth, when droplets
of water are small enough (2 mm or smaller), surface tension overcomes gravity
and they remain perfectly spherical. If a droplet grows much larger, it gets
squished under its own weight.
But in space, blobs of
water and other liquids (after wobbling about) eventually assume a perfect
spherical shape.
Edward Balaban, principal investigator of
the Fluidic Telescope Experiment, or FLUTE, at NASA's Ames Research Center in
California's Silicon Valley teamed up with researchers at Ames, the agency's
Goddard Space Flight Center in Greenbelt, Maryland, and Technion - Israel
Institute of Technology, to explore whether it was possible to make
high-precision lenses and mirrors in space using liquids.
"We thought, why not take advantage
of the way liquids naturally behave in microgravity and apply it to the
construction of large-scale telescopes or space-manufactured optical components
that can have all kinds of uses," said Balaban. "In microgravity,
liquids take on shapes that are useful for making lenses and mirrors, so if we
make them in space, they could be used to build telescopes that are
dramatically bigger than was previously thought possible."
Ground
Tests: A Drop in the Bucket
Before taking their ideas to the skies,
the team wanted to test their ideas on the ground.
"Liquids are useful not only for
creating the lenses themselves, but also as a mechanism to eliminate the effect
of gravity in an experiment on Earth," said Moran Bercovici, an associate
professor of Mechanical Engineering at Technion.
"By injecting a liquid that can be
solidified, into circular frames submerged in water, we were able to create
lenses literally in a janitor’s bucket," said Dr. Valeri Frumkin, who
developed the method in Bercovici's group. "Polymers, which are
also used in nail salons to make acrylic nails or in adhesives like
superglue, are a natural choice for lens material. The trick is to
make sure that the water has the exact same density as the polymer we're
injecting so that the forces of buoyancy precisely oppose gravitational forces to
simulate the conditions of weightlessness."
The resulting lenses have an outstanding
surface quality comparable or even better than achievable with the best
polishing methods, and they took only a tiny fraction of the time to
construct.
“This method allows us to completely skip
any mechanical processes such as grinding or polishing," said Bercovici.
"The natural physics of fluids simply does all the work for
us."
FLUTE researchers capture data while they push synthetic oil into a circular frame (about the size of a dollar coin), momentarily forming a liquid lens during a 15 to 20-second period of microgravity on a ZeroG parabolic flight. Credits: Technion - Israel Institute of Technology
Next Up:
Testing in Microgravity
In December 2021, the team tried out their
ideas on two ZeroG parabolic flights. The flights provided 50 opportunities to
achieve 15 to 20-second periods of microgravity – long enough for the team to
form liquid lenses and capture data to analyze whether they achieved their
goals.
During the flight
researchers used pumps to push synthetic oil into a circular frame (about the
size of a dollar coin), letting the liquid fill the gap and momentarily achieve
the desired shape. The oils are similar to automotive oil, with different
levels of viscosities – or goopiness – to try out which works better.
"Sure enough, in a few seconds we
were able to create a free-standing liquid lens – until the plane lifted
upwards again and gravity kicked in and the oils oozed out," said
Bercovici. "Our experiment on the
space station will add a step to cure the fluids so they keep their
shape."
Using a laser to take ultra-fast and
ultra-exact measurements, they were able to see the precise moment the liquid
took shape.
With the first round of parabolic flights
successfully behind them, the team eagerly awaits their experiment aboard the
orbiting laboratory. Ax-1 crew member Eytan Stibbe will perform the
experiment and former NASA astronaut and Ax-1 commander Michael Lopez-Alegria
will serve as his backup. The experiment will take place entirely in
microgravity, again using liquid polymers (like the hardened lenses made in the
lab at Technion) and will use either UV light or temperature to harden them
in-orbit. The lenses will then return to Earth where researchers at Ames will
study them.
"We expect this approach will create
perfectly shaped and smooth surfaces: the best surfaces to turn into
mirrors," said Vivek Dwivedi, a FLUTE scientist at Goddard and expert in atomic layer deposition technology that could be used to make
ultra-precise telescope mirrors in space.
"If our station experiment is
successful, it will be the first time an optical component is made in
space," said Balaban. "It feels a bit like making
history."
The space station experiment is part of
the Rakia Mission, led by The Ramon Foundation, and is supported by
The Israeli Space Agency and the Israeli Ministry of Innovation, Science and
Technology. NASA's Space Technology Mission Directorate supports FLUTE through
the Center Innovation Fund and the Flight Opportunities Program. FLUTE's test aboard the space station
is sponsored by the International Space Station National Laboratory.
Water
in space behaves…differently. Credits: NASA
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