A Sandia National Laboratories scientist has demonstrated how heliostats, large mirrors used to collect and concentrate solar energy during the daytime, can be repurposed during nighttime hours to search for asteroids, spacecraft, and other Near-Earth Objects (NEOs).
The researcher believes his approach could also help the U.S. Space Force and NASA track spacecraft, satellites, and other objects in cislunar orbit, a task that remains a challenge for modern tracking systems.
Light Sensitivity of Heliostats Could Aid Detection of Near-Earth Objects
According to a statement emailed to The Debrief and a separate story on the organization’s website, when Dr. John Sandusky first joined Sandia almost 20 years ago, he was asked if the heliostats at the National Solar Thermal Test Facility could perform detailed imaging. While his original response was no, since they did not possess high enough “optical quality” to form a viable image, the fact that the heliostats do collect a lot of light, and his previous education in laser communications, made him start to reconsider his position.
After some basic analysis, the NASA Jet Propulsion Laboratory veteran wondered if the heliostats might collect enough light to be useful for something other than detailed imaging. Specifically, he wondered if they might collect enough light during nighttime operations to detect frequency differences in light as smaller objects, such as NEOs, passed in front of stars.
“Even frequency changes as small as one-one-millionth of one cycle per second are measurable thanks to frequency standards that can be commercially procured and locked to references like GPS signaling,” Sandusky said. “I knew it was possible to measure the very small rate differences of an asteroid passing relative to the stars.”
To apply this ability to proactively search for asteroids and other near-Earth objects, Sandusky realized he simply needed to “sweep” the heliostat at a fixed rate relative to the stars. If successful, he calculated that anything moving relative to the stars “will appear at a neighboring frequency but still be separable.” Compared to the costly current methods for NEO detection, which involve searching for streaks of light in stellar imagery, and the even larger expense of building and maintaining new observatories, he believed this technique could provide a highly accurate, low-cost alternative for planetary defense.
“The heliostat fields don’t have a night job,” Sandusky explained. “They just sit there unused. The nation has an opportunity to give them a night job at a relatively low cost for finding near-Earth objects,”
After a nearly 20-year wait, the Sandia Labs scientist was able to use “Laboratory Directed Research and Development” funding to perform an initial round of nighttime experiments that support his nearly two-decade-old idea.
All-Night Experiments and ‘a Lot of Time Waiting’
According to the statement from Sandia, Sandusky spent several summer nights at the solar testing facility evaluating the fundamentals of his concept. For the purposes of these experiments, only one of the heliostats was used for testing, although several were employed simultaneously, which would theoretically improve performance.
After climbing the 200-foot solar tower each night at dusk, Sandusky would set up standard optical instruments designed to detect the small amount of light that the targeted heliostat was concentrating on the facility’s solar tower. Over the ensuing hours, the Sandia scientist said he would periodically change the heliostat’s direction, so it would “sweep back and forth” roughly once per minute.
“Solar towers collect a million watts of sunlight,” Sandusky said. “At night, we want to collect a femtowatt, which is a millionth of a billionth of a watt of power of sunlight that’s scattered off of asteroids.”
“I’m trying to detect the asteroid by its speed relative to the stars,” he added.
Combined with existing software designed to track the heliostat’s direction relative to the stars, Sandusky suspected the methodical sweeping method should offer a high quality of data. Still, he admitted, the all-night effort was a slow and tedious process.
“You spend a lot of time waiting,” he said. “There was about 20 minutes between the collection of data points. I would collect data until dawn.”
Heliostats could offer advanced warning of Potential asteroid impacts
After examining the data, Sandusky said his proof of concept was a success. The combination of tools, software, and patience showed that the heliostat was able to detect the minimal amounts of light they would need to spot when searching for asteroids, spacecraft, or other NEOs.
“No surprise,” he said. “They can see stars.”
Although this particular series of all-night experiments was not designed to spot asteroids, Sandusky said it demonstrated that the heliostats can be accurately swept back and forth to respond to the stars and detect their light signatures.
In the Sandia statement, the researcher described his method as “early-stage technology.” However, he also said a more mature form would still be significantly more cost-effective than current NEO hunting techniques. In its fully developed form, such a NEO tracking system could provide critical data for planetary defense.
“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 explained.
When discussing potential applications beyond tracking potentially dangerous NEOs, Sandusky suggested the heliostat’s sensitivity may make it uniquely equipped to track objects orbiting the Moon.
“It may help the U.S. Space Force with its job of trying to find spacecraft, especially in the cislunar area,” he explained. “Orbits near the moon can be difficult to track from the ground.”
Waiting for Feedback and Searching for Opportunities
After presenting his findings at the International Society for Optics and Photonics (SPIE) and publishing a paper detailing his research, Sandusky said he is excited to see what input his peers “in optics and the asteroid hunting community,” might offer. Although successful, the researcher said feedback from peers “provides an opportunity to understand what the concerns are about how this technology will work,” and helps guide future experiments to further verify and improve the technique.
“We’re looking for opportunities to scale up from one heliostat to many and try to demonstrate that we can help find near-Earth objects,” Sandusky said. “We also want to demonstrate we can scale up the technology to detect even smaller asteroids.”
Another option involves using a heliostat to find a previously discovered exoplanet. Whether or not such an effort was successful, it would add to the potential applications and help Sandusky determine the technology’s limitations.
The study “Prospect for cislunar spacecraft and near-earth asteroid detection using heliostat fields at night” was published in SPIE Proceedings.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.