Space-based laser science offers the possibility of measuring things that are difficult or even impossible to measure on the ground. Such measurements are currently critical for tracking climate change (through the GRACE missions, operated by a US–Germany partnership), and will be critical for the planned space-based gravitational-wave observatory (the laser interferometer space antenna, LISA; an ESA–NASA partnership). But space-based laser science is only just getting started; what could be achieved from advances and learnings over the next 50 years is untold.
Detecting a laser pointer in the void of space
When making measurements of very small signals (such as gravitational waves) on Earth, millions of other things are also measured incidentally, such as tectonic plates shifting, trucks driving by, clouds passing overhead and birds pecking on the detector. Going to space avoids these Earthly noise sources—as the saying goes, “in space, no one can hear you scream”. It turns out it’s also very quiet from a measurement perspective. But going to the relative silence of space brings with it a slew of technical challenges.
One challenge is that space-based measurements are done by shooting lasers between satellites separated by hundreds or even millions of kilometres—the former with laser powers equivalent to only about four conventional laser pointers. Over these large distances, incredibly high-precision sensors and advanced sensing techniques are required to even spot the laser, let alone make any useful measurements.
EQUS researchers from ANU—including PhD student Callum Sambridge and Chief Investigator Kirk McKenzie—are developing technologies and techniques to track such ultraweak laser signals. In 2023, the team hit an exciting milestone in weak-power tracking, by making measurements at optical powers 100 times lower than previous research. This capability means the separation between satellites of GRACE-like missions may be measured with nanometre precision—about the size of an atom— with a vanishingly small amount of light. They also performed the first expansive set of simulations to demonstrate the viability of a new class of space-based laser mission.
The future of space lasers
The team are collaborating with laboratories across the world to translate their research to future missions. In 2024, they aim build a portable version of the experiment and visit laboratories in the US and Australia to perform a series of experiments.
They will focus on improving current measurement systems by performing tests with lasers identical to the ones they have in space.
The team’s findings also have implications for future missions using lasers in space. The latest results show the minimum operatable power level is roughly 10,000 times lower than currently accepted mission baselines. This is likely to open up a wide range of new space-mission architectures for monitoring climate change with higher accuracy and lower cost.
Their results also support the feasibility of far-future (2050s) super-large space-based gravitational-wave detectors that shoot lasers hundreds of millions of kilometres through space to listen to the sounds of supermassive black holes colliding billions of years ago.
This story was first published in the 2023 EQUS annual report, and was written by Kristen Harley.