Turning Solar Mirrors into Space Guardians
By adapting software and sensors, engineers have transformed daylight power collectors into instruments capable of tracking faint signals from distant astral objects.
Written by Nicole Imeson
Built to harvest solar energy by day, heliostats may soon safeguard Earth and track spacecraft by night. Researchers at Sandia National Laboratories recently explored how these motor-controlled mirrors, normally used to concentrate the sun’s thermal energy to generate steam, supply heat, or store energy, could also detect faint light from asteroids and spacecraft in the cislunar corridor.
Traditional tracking techniques relied on long-exposure photographs, demanding observatory infrastructure with high installation and maintenance costs. Existing heliostats would sweep across the night sky in a trapezoid pattern for a set period, called T. Fixed stars reflected light in a predictable rhythm tied to T, while an asteroid would shift position against the mirror and reflect earlier or later with each pass, noted as T + ΔT. Even a fraction of a second in ΔT would reveal motion. With power spectrum analysis, scientists could separate the asteroid’s faint signal from the steady light of the stars.
While heliostats can collect energy from the sun, Sandia scientist John Sandusky put the heliostats to work at night. His findings could help detect near-Earth objects, such as asteroids. Photo: Craig Fritz/Sandia National Laboratories
“Every time the heliostat sweeps, it repeats a pattern of stars, creating a consistent frequency. A dim, slowly moving asteroid will appear at a slightly shifted frequency, allowing us to find it,” explained John Sandusky, optical engineer at Sandia National Laboratory.
To help detect these faint signals, researchers tested a chopper wheel that repeatedly interrupted incoming light between the lens and sensor. This steady rhythm allowed instruments to pick out tiny signals from background noise, letting scientists explore new ways to track extremely faint objects even when small timing instabilities occurred.
Heliostats equipped to begin the night shift. Photo: Sandia National Laboratories
Outfitting heliostats with motion sensors such as accelerometers could improve the precision of the asteroid detection system. Each mirror carried a large reflecting area, and its rhythmic sweep demands extreme accuracy to isolate the faint asteroid light against starlight. Accelerometers could track motion in real time and feed data back into the system, allowing researchers to correct for vibrations and other disruptions. The success of this approach hinges on how smoothly each heliostat field operates. Fields that cannot maintain steady, accurate sweeps risked blurring or distorting the faint asteroid signal.
“It may also turn out that we can use the heliostats as they are, if their motion proves stable enough. We just haven’t gotten far enough yet to know for sure,” Sandusky explained. Heliostats require at least an hour for a successful measurement, unlike telescopes that capture short exposures over several minutes. Additional factors, such as cloud cover, could further affect their accuracy.
Asteroid detection demands instruments that measure extraordinarily small amounts of light, down to a femtowatt, or a millionth of a billionth of a watt. For perspective, an LED averaged about seven watts and a cell phone camera about one watt. Small, distant asteroids reflected only faint traces of sunlight, making them difficult to detect. At night, heliostats concentrate light with enough precision to capture femtowatt-level signals, giving scientists a tool sensitive enough to track these elusive objects.
The heliostat testing field at sunset. Photo: Sandia National Laboratories
Asteroid Magnitudes
Scientists measured asteroids by absolute magnitude, a scale that compared brightness under equal distances from the sun and Earth. This method allows them to estimate size based on reflected light. Most detected asteroids fell between magnitudes 11 and 19, with lower numbers marking brighter, larger objects. NASA’s ATLAS (Asteroid Terrestrial-impact Last Alert System) tracked many of these bodies, spotting some as dim as magnitude 19 while missing others as bright as 14.
“We’re aiming to reach at least magnitude 20 detection with the Sandia heliostat fields. More heliostats increase the limiting magnitude, in principle. If we grow our array from 212 to 60,000 heliostats, the detection magnitude will continue to improve,” Sandusky said.
“Every time the heliostat sweeps, it repeats a pattern of stars, creating a consistent frequency. A dim, slowly moving asteroid will appear at a slightly shifted frequency, allowing us to find it.”
—John Sandusky, optical engineer at Sandia National Laboratory
Photo: Craig Fritz/Sandia National Laboratories
Cislunar Corridor
The conceptional, three-dimensional region stretching between Earth and the moon is called the cislunar corridor. It’s where spacecraft maintain persistent detection and coverage of Earth while countering the gravitational pull of the Earth, moon, and sun, referred to as a “three-body problem.”
This corridor extends beyond the geosynchronous orbit (GSO), a defined ring around Earth at roughly 35,786 km (22,236 miles) above the equator. Objects in the GSO match Earth’s rotation and return to the same point in the sky each day. The cislunar corridor extends to the moon’s orbit at an average of 384,400 km (238,855 miles) from Earth, about 10 times farther than the GSO. Objects in this region appear to move across the sky over time, making them more challenging to track.
Expanding heliostat monitoring could have broader impacts beyond asteroid detection. As more satellites and lunar missions entered the cislunar corridor, low-cost, scalable heliostat arrays offered a way to track objects and improve space situational awareness. Early detection of faint asteroids could also strengthen planetary defense, giving researchers more time to assess potential threats.
Sandia plans to test larger arrays and push sensitivity limits while studying field-to-field variations and environmental factors that could affect performance.
This approach provides a cost-effective complement to traditional observatories and expanded monitoring into the cislunar corridor. With each mirror sweep, heliostats may soon not only advance clean energy applications but also help researchers better understand and protect the space around Earth.
Nicole Imeson is an engineer and writer in Calgary, Alta.
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