20. December 2016
We spent a century hunting for gravitational waves, until the Laser Interferometer Gravitational-Wave Observatory finally detected them in 2015.
Now astrophysicists from the European Space Agency (ESA) are in the midst of a large-scale mission to detect gravitational waves from astronomical sources. Right now, two metal cubes stuck in perfect free fall are orbiting Earth from the LISA Pathfinder (LISA is short for Laser Interferometer Space Antenna), and they’re proving the technology that’ll be part of a larger space mission. At the center of LISA Pathfinder is a piece of glass-ceramic that’s providing the support necessary to capture and relay exact measurements back to ESA scientists.
This is the story of that hockey-puck-shaped spacecraft and how that slab of glass-ceramic is aiding LISA’s mission to investigate an invisible force in the universe that Albert Einstein predicted a century ago.
Image: ESA–ATG Medialab
LISA Pathfinder’s mission
LISA Pathfinder is a proof-of-concept experiment for a much larger mission that’s still a few years away. Pathfinder sits in stable orbit about 1.5 million kilometers from its home planet, between it and the sun.
At the center of the spacecraft is the delicate equipment at the crux of the mission – two 46 cm3 solid-metal cubes in free fall. These cubes are enclosed in vacuum-sealed reactors and are shielded from all external forces except gravity. Laser interferometer mirrors constantly measure their relation to each other – a distance of 38 centimeters that only changes if they’re hit with a gravitational wave, which can alter the distance between them ever so slightly.
When a slight deviation registers, scientists from ESA and around the globe study the data to deduce if gravitational waves altered the cubes’ positions. Gravitational waves would move the cubes by mere picometers – smaller than the width of an atom.
To be clear, LISA Pathfinder’s mission isn’t to observe gravitational waves; rather, it’s testing fine instruments that one day will be tuned to more accurately detect and measure these waves. The spacecraft is the appetizer for a larger mission, dubbed Evolved Laser Interferometer Space Antenna (eLISA), whose launch date is preliminary set for 2034.
The glass that withstands 18 Gs
The interferometer mirrors key to detecting the cubes’ movements sit on an “optical bench” made of ZERODUR® glass-ceramic. This slab of glass-ceramic holds critical importance to the success of the mission: Remember, these mirrors and sensors are so finely calibrated they can accurately measure a distance as tiny as 10 picometers – one hundred millionths of a millimeter. The material ZERODUR has high structural strength, which protected the mirrors during the violent vibrations of takeoff and as they spin through space, and a near-zero coefficient of thermal expansion that stops the slab from expanding or contracting as temperatures change.
How sturdy are they? In a testing environment, this optical bench withstood 15 g sine and 18 g random vibrations – three to five times higher than the G-forces on a Space Shuttle on liftoff.
An experiment rooted in a century-old belief
Data and findings from LISA Pathfinder will shape our search for the different frequencies of gravitational waves. What do we gain from this search?
Detecting and charting these distortions in space and time give us another avenue for examining the universe. That’s because gravitational waves aren’t uniform – different waves will flutter with unique frequencies based on the size of the object that emitted them. We can use gravitational waves to study “tiny” black holes, supermassive black holes at the edge of the universe, and gain insight into the rapid expansion of the universe shortly after the Big Bang.
It took us a century to detect gravitational waves, the last unseen piece of Einstein’s general theory of relativity. ESA’s LISA mission will give us the tools to learn more about these waves — and therefore the origins of the universe – with a little help from glass.