When the sun dips below the horizon, vast fields of heliostats—the giant, computer-controlled mirrors that normally focus sunlight onto a central receiver for clean power—sit dormant. But what if those idle mirrors could moonlight as a giant, distributed telescope array? That’s exactly the idea Dr. John Sandusky of Sandia National Laboratories unveiled at the 2024 Optical Engineering + Applications conference: repurpose heliostat fields after dark to detect near-Earth asteroids and even lost spacecraft between Earth and the Moon.

At the heart of Sandusky’s proposal is a deceptively simple insight: heliostats are already engineered to point with exquisite precision—often better than a few arc-seconds—to track the sun’s path. By reprogramming that same tracking system at night to follow stars instead, the mirrors can reflect faint starlight down to a central tower, where sensitive instruments monitor subtle shifts in frequency. Any object moving relative to the star field—say, a small asteroid drifting through the beam—will impart a tiny Doppler shift in the reflected light. “Even frequency changes as small as one-one-millionth of a cycle per second are measurable,” Sandusky noted, tapping into Sandia’s high-resolution photonic sensors.

Sandia’s National Solar Thermal Test Facility in Albuquerque boasts 212 heliostats spread around a 200-foot tower. This summer, Sandusky commandeered one mirror for a proof-of-concept. Over several nights, the team had the heliostat sweep a small patch of sky, reflecting starlight down the tower’s optics. The goal wasn’t to capture an asteroid on camera, but rather to demonstrate that the control system could maintain the required pointing stability and that the sensors could resolve femtowatts of optical power—the faint glow you’d expect from objects millions of miles away.

Although no asteroids were flagged in this initial trial, success was measured by validating the methodology: the heliostat moved smoothly, the beam stayed locked on target, and the frequency-shift detection pipeline registered stable baselines. Sandusky quipped, “Solar towers collect a million watts of sunlight by day; at night, we’re chasing a millionth of a billionth of a watt off an asteroid.”

Traditional asteroid surveys rely on long-exposure imaging: telescopes track the sky for minutes at a time, capturing streaks as moving objects pass among the stars. Sandusky’s approach sidesteps imaging altogether. Instead, it analyzes the photocurrent’s power spectrum at sub-milliHertz resolution. When a moving object enters the reflected beam’s field of view, its relative motion alters the frequency of the incoming light ever so slightly. That shift, transposed into the radio domain, stands out against the static starlight background.

This “frequency-domain” technique draws on over two decades of Sandusky’s conceptual work, now finally funded through Sandia’s Laboratory Directed Research and Development program. According to a SPIE conference paper, he modeled how sweeping one heliostat could detect both asteroids and cislunar spacecraft by noting Doppler shifts as small as 10⁻⁶ Hz.

Why bother? Planetary defense programs such as NASA’s ATLAS (Asteroid Terrestrial-impact Last Alert System) maintain a fleet of telescopes dedicated to scanning for potential threats. But there simply aren’t enough eyes on the sky; even large survey telescopes cover only fractions of the heavens each night. Heliostats offer a low-cost, readily available supplement—fields of mirrors already built at scale, with minimal incremental infrastructure needed to tap them after hours.

“If we knew ahead of time that an asteroid was coming and where it might hit, we’d have a better chance to prepare and reduce the potential damage,” Sandusky said. And because many solar thermal plants sit idle overnight, adding this capability could turn “wasted” assets into a national network of planetary-defense sentinels.

Beyond asteroids, the method shows promise for monitoring spacecraft in the newly strategic cislunar space between Earth and the Moon. That region is becoming increasingly crowded with satellites, probes, and commercial missions—a potential blind spot for ground-based radars and telescopes. By coordinating multiple heliostat fields, Sandusky envisions detecting and tracking objects as they drift through lunar orbits. “The heliostat fields don’t have a night job. They just sit there unused,” he explained. “We might as well use them.”

Scaling from a single mirror to dozens or hundreds introduces logistical hurdles. Each heliostat must feed into a coherent network of detectors, requiring precise calibration of optical paths, timing, and data fusion. Atmospheric turbulence, temperature gradients, and local light pollution could all degrade signal quality. Moreover, turning a commercial solar power facility into a dual-use surveillance asset raises regulatory and security considerations, especially if defense applications are pursued.

Still, Sandusky’s team is already plotting the next steps: extending nighttime trials to multiple heliostats, refining data-processing algorithms, and exploring partnerships with solar-thermal plant operators. If all goes well, these mirrors might soon patrol the night sky, not for sunshine, but for the small rocks and spacecraft that slip through conventional observatories.

In an era when the Earth’s neighborhood is crowded with both natural wanderers and human-made satellites, innovative sensing solutions are in high demand. By giving heliostats a second life after dark, Sandusky’s work could turn idle solar-energy infrastructure into a force multiplier for space situational awareness and planetary defense. As dusk falls on solar power plants around the world, those reflective arrays may soon be busier than ever—tracking not the sun, but the silent voyagers of our cosmic shores.