December 8, 2025 — A Southwest Research Institute (SwRI) research project is using data from NASA’s Polarimeter to Unify the Corona and Heliosphere (PUNCH) spacecraft to track the interstellar comet 3I/ATLAS. It tracked the comet for many weeks when it was behind the Sun as seen from Earth. The comet’s position made impossible to observe from Earth except by telescopes designed to look at the Sun. PUNCH, which launched in March 2025, is a constellation of four small spacecraft that act as a single virtual instrument 8,000 miles across to image the solar corona, the Sun’s outer atmosphere, as it transitions into the solar wind that fills and defines our solar system. The PUNCH mission is led by SwRI’s Dr. Craig DeForest.
Earlier this year, SwRI’s Dr. Kevin Walsh and Dr. Simon Porter initiated an internally funded project to use PUNCH’s data, which is publicly available, to search the solar system for previously undiscovered asteroids that are difficult to detect because they appear near the Sun from Earth’s viewpoint.
“PUNCH is a wide-field imager, not a telescope: our spatial resolution is about the same as a human’s eyesight, but far more sensitive than the human eye, of course,” DeForest said. “But even so, PUNCH is not optimized for viewing faint astronomical objects. In a single PUNCH image, the faintest thing we can see is 100 times fainter than anything you can see with your eyes on a moonless night in the desert.”
However, PUNCH does offer some advantages, including a very wide field of view to track comets for long periods as they move across the inner solar system. PUNCH also collects vast amounts of data.
“The nature of the PUNCH mission is different from almost all asteroid surveys. It stares at the Sun and the regions around it,” Walsh said. “Space telescopes are designed to never look at the Sun, because it would require changing their internal characteristics for protection. But that’s exactly what PUNCH was designed to do. It just happens to fill this interesting niche in the asteroid world that we think will be really valuable.”
In July, just as Walsh and Porter were nearing completion of their project, 3I/ATLAS became the third officially recognized interstellar object to cross into our solar system. They quickly realized they had to switch gears.
“As soon as it was discovered, we checked its predicted trajectory and realized it would spend almost two full months in the PUNCH field of view,” Walsh said. “For some of that time, telescopes would be able to track it. But for several weeks, the comet was behind the Sun as seen from Earth. Nothing else could see it. Through most of October, nothing else was routinely measuring brightnesses or collecting images. It was basically unobservable for everything except PUNCH.”
Walsh and SwRI Senior Research Analyst Dr. Simon Porter created images and movies of the comet from PUNCH data. The spacecraft collects roughly one image per minute of 3I/ATLAS, making it possible to see the evolution of the tail and its interaction with the solar wind continuously for weeks at a time.
“I’m thrilled that PUNCH was so uniquely capable of viewing 3I/ATLAS at that position. PUNCH is designed for imaging large, distributed objects such as a coronal mass ejection traveling across the inner solar system, so finding that it’s also good for very faint small objects is a nice bonus,” DeForest said.
Because PUNCH is also a polarimeter, it can measure the polarization of light by the comet’s tail, shedding light on the distribution of particles coming off the comet head as it heats up in the bright sunlight. The combination of high time cadence, polarization and continuous coverage minute-by-minute for months provides unique observations.
“Nothing else could monitor the comet for a time except for PUNCH,” Walsh said. “And the community is desperate to know every little thing this comet does.”
Walsh and Porter plan to eventually publish their results.
“This is only the third ever interstellar object we’ve seen pass through the solar system. What we’ve learned is that there have probably been a lot of them before, we just couldn’t detect them,” Walsh said. “So, this is the beginning of a whole new field of study. It gives us a chance to study cometary bodies — the building blocks of planets and potentially the building blocks of life in other places across the galaxy.”
For more information, visit Heliophysics or contact Joanna Quintanilla, +1 210 522 2073, Communications Department, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238-5166.