PROBA-3 is Europe’s first mission dedicated to demonstrating highly precise autonomous formation flying with two independent spacecraft. Launched in December 2024, the mission operates in a highly elliptical orbit that reaches approximately 60,530 kilometres at apogee and descends to about 600 kilometres at perigee. The spacecraft spend around six hours near apogee conducting formation-flying operations, when their relative velocity and the influence of the Earth’s gravity gradient are comparatively low. During this period, they perform acquisition, rendezvous, station-keeping, retargeting and collision-avoidance manoeuvres without continuous instructions from Earth. The two vehicles are normally separated by about 144 metres, a distance that is too large to maintain during the high-speed perigee passage. They therefore break formation before entering the lower portion of the orbit and move into safe trajectories, later reassembling near apogee. This orbital choreography allows PROBA-3 to test the technologies needed for future observatories built from multiple spacecraft, including distributed telescopes, interferometers and large space structures that cannot be launched as a single rigid vehicle.

The mission’s scientific instrument, the Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun, or ASPIICS, exploits the unusual geometry. One spacecraft carries a telescope, while the other carries a precisely positioned disk known as the external occulter. When the two vehicles line up with the Sun, the occulter blocks the bright solar surface and creates an artificial eclipse for the telescope. The resulting coronagraph has an effective separation of 144 metres, greatly reducing the diffraction effects that limit conventional coronagraphs, in which the occulting disk and telescope are mounted on the same platform. ASPIICS is intended to image the faint inner corona, a region normally overwhelmed by sunlight scattered inside an instrument. The corona contains the plasma structures that drive solar eruptions and space weather, yet its innermost regions remain difficult to observe. To maintain the artificial eclipse, however, the spacecraft must remain aligned with extraordinary precision. The SPS subsystem is the final and most accurate link in a chain of metrology instruments that measures the relative three-dimensional position of the two vehicles.

The SPS hardware consists of eight silicon photomultipliers arranged around the 50-millimetre entrance pupil of the coronagraph on a circle with a radius of 55 millimetres. Silicon photomultipliers are highly sensitive photon detectors capable of converting faint optical signals into electrical measurements. Each sensor samples a different part of the penumbra—the gradual transition between the dark umbra and fully illuminated sunlight—projected by the external occulter onto the telescope’s pupil plane. If the two spacecraft are perfectly aligned, sensors placed opposite one another receive irradiance values that follow a predictable balance. A lateral displacement shifts the penumbra, changing those opposing signals in a characteristic way. Differences among the sensor readings therefore encode the position of the eclipse, while the overall illumination level provides information about the longitudinal separation between the spacecraft. In nominal operation, one group of four sensors, called Channel A, is used, while the other four in Channel B provide redundancy and opportunities for calibration. The sensor currents are converted into relative lateral coordinates and an along-the-line-of-sight coordinate, which are then supplied to the formation-flying guidance, navigation and control system.

The original onboard algorithm represents the penumbra with a pseudo-paraboloidal mathematical model. In practical terms, it approximates the irradiance as a third-order function of the horizontal and vertical distances from the centre of the umbra. The model includes linear, quadratic and cubic terms, with adjustable coefficients determined through numerical optimization and ground calibration. These coefficients can be updated during the mission to account for changes in the actual illumination profile, diffraction at the occulter edge, spacecraft tilt and other evolving conditions. The algorithm is fast because it uses the four sensor measurements together in a compact calculation. It can also be supplemented by a simple linear algorithm based on the differences between opposite sensors. The two approaches have complementary error patterns, so combining them can improve performance in selected regions. But the pseudo-paraboloid has a fundamental geometric limitation: its mathematical surface is not truly rotationally symmetric. The physical penumbra is expected to be nearly axis-symmetric when spacecraft tilt is negligible, whereas the model’s separate horizontal and vertical terms can reconstruct it unevenly, especially toward the corners of the positioning region.

That mismatch becomes important because PROBA-3’s SPS system was originally designed around extremely ambitious accuracy goals. The mission defines a requirement box extending 20 by 20 millimetres laterally and 200 millimetres longitudinally around the nominal alignment point. Under the currently relaxed formal requirements, the sensors must determine lateral position to within 0.5 millimetres and longitudinal position to within 50 millimetres in that region. A broader goal box covers a 100-by-100-millimetre lateral area and a 1,000-millimetre longitudinal range, with reduced performance expectations. Earlier in the project, the target was far more demanding: 50 micrometres laterally and 1 millimetre longitudinally inside the requirement box. An engineering review found that these specifications placed severe demands on both the electronics and the positioning algorithms, leading ESA to relax the formal limits to provide additional manufacturing margin. Nevertheless, the research team continued testing against the stricter values because ground-based processing could still benefit from extracting the highest possible precision from the mission data.

The alternative algorithm changes the geometry of the problem. Instead of describing the penumbra separately along two perpendicular axes, it calculates the radial distance of each sensor from the unknown centre of the umbra. The measured irradiance is fitted with a third-order radial polynomial containing adjustable coefficients corresponding to the fine and coarse calibration regimes used by the original algorithm. For every sensor reading, the algorithm must invert this cubic relationship to recover the sensor’s radial distance from the eclipse centre. It does so by transforming the equation into a “depressed” cubic, one without a squared term, and then applying Cardano’s method. Depending on the discriminant, a cubic can have one real solution or three real solutions. The algorithm selects the physically meaningful positive root that satisfies the boundaries of the operational region. Once the four radial distances are known, simple geometric relations recover the lateral coordinates of the umbra centre. For example, the difference between the squared distances measured by opposite sensors isolates one coordinate, while the corresponding pair isolates the other. This procedure naturally enforces circular symmetry, unlike the pseudo-paraboloidal model.

The researchers initially found that a straightforward third-order radial interpolation produced an unrealistic increase in the reconstructed irradiance near the central dark region, where the umbra should have little or no illumination. They corrected that behaviour and modified the alternative code to use both fine and coarse fitting parameters, matching the structure of the onboard algorithm and enabling a direct comparison. The fine parameters are applied within a lateral decentring of 13.8 millimetres, where the darkest portions of the penumbra demand the greatest accuracy. Coarse parameters cover the outer portions of the goal box. The longitudinal coordinate is calculated using the average irradiance inferred from the four sensors. A linear calibration can translate that average into the spacecraft separation, while a more elaborate expression includes additional fitting constants and a correction for the lateral displacement. These reconfigurable coefficients allow the algorithm to adapt to the real in-orbit penumbra rather than relying permanently on a profile established before launch.

To test the methods, the team generated a numerical grid containing one million simulated umbra positions within the requirement box. The synthetic sensor irradiances were calculated from the calibrated penumbra models and then fed back into the competing algorithms. The pseudo-paraboloidal method met the older 50-micrometre target only in a cross-shaped region near the centre, aligned with the sensor directions. Toward the corners, its error reached approximately 100 micrometres—still comfortably inside the current relaxed requirement of 0.5 millimetres, but twice the original target. The combined pseudo-paraboloidal-plus-linear method did not solve the problem across the full box; its accuracy improved near the centre but deteriorated rapidly elsewhere. By contrast, the Cardano-based alternative produced an almost uniform error pattern throughout the requirement region. Its maximum simulated lateral error remained below 0.006 millimetres, or 6 micrometres, meeting the stricter historical requirement by roughly a factor of eight. The improvement was especially visible at the edges and corners, precisely where the non-axisymmetric model was weakest.

The findings do not mean that PROBA-3 will replace its onboard software with the slower algorithm. The flight code must operate within strict limits on processor time, power consumption and response latency, making the compact pseudo-paraboloidal and linear methods attractive for real-time formation control. The alternative algorithm’s computational burden arises because it solves a separate cubic inversion for every sensor and may perform the calculation first with coarse parameters and then repeat it with fine parameters after checking the result against a threshold. In the researchers’ tests, this produced a calculation time about 50 times longer than the faster methods. That cost is acceptable in the Science Operation Center, where ground computers can process recorded measurements after transmission to Earth. There, the alternative can serve as an independent validation tool, identify systematic biases in the onboard position estimate and refine the final reconstruction of the spacecraft geometry. Such corrections could be particularly valuable when interpreting ASPIICS observations, because even tiny alignment errors can alter the apparent structure and brightness of the solar corona. By combining fast onboard control with more exact ground analysis, PROBA-3 may demonstrate not only that spacecraft can fly in formation autonomously, but also that their measurements can be sharpened after the most critical manoeuvres are complete.

Subject of Research: Alternative metrology algorithm for ESA’s PROBA-3 Shadow Position Sensors and precision formation flying.

Article Title: Design of the alternative SPS metrology algorithm for the ESA PROBA–3 formation flying mission

Image Credits: AI Generated

Keywords: PROBA-3, ESA, formation flying, Shadow Position Sensors, silicon photomultipliers, ASPIICS, solar corona, coronagraphy, Cardano’s method, spacecraft metrology, autonomous navigation

At the core of PUNCH’s design lies its unique capability to act as a single, virtual instrument spanning some 8,000 miles. This enormous synthetic aperture enables unprecedented imaging of the solar corona and its gradual transition into the solar wind—a supersonic stream of charged particles that flows out from the Sun and shapes space weather near Earth and throughout the solar system. By capturing these phenomena with exceptional clarity, PUNCH aims to unravel long-standing mysteries about the corona’s structure and the forces propelling solar wind particles outward at speeds exceeding one million miles per hour.

On April 14, 2025, two of PUNCH’s primary instruments—the Near Field Imager (NFI) and one of the Wide Field Imagers (WFI)—successfully opened their instrument doors and collected initial images. These early captures demonstrate that the onboard cameras are sharply in focus and functioning correctly, revealing the deep-field night sky set against the bright backdrop of space near the Sun’s noontime glare. Among the visible features in these images are familiar celestial landmarks such as the Taurus constellation and the iconic Pleiades cluster. Additionally, a subtle diffuse glow known as zodiacal light, caused by sunlight reflecting off microscopic dust particles orbiting within the inner solar system, is clearly discernible.

The subsequent days brought further validation of the technology as the remaining two WFI instruments commenced their first-light imaging sequences on April 16. Dr. Craig DeForest, PUNCH’s Principal Investigator and a respected voice in solar system science at SwRI, highlighted the technical achievement of having all four instruments operating flawlessly. As commissioning proceeds over the 90-day testing window managed from SwRI’s Mission Operations Center, the team is busy calibrating these instruments to unprecedented levels of precision.

Central to the mission’s scientific ambition are the four spacecraft that compose the PUNCH constellation. One of these satellites is equipped with the NFI coronagraph, developed by the U.S. Naval Research Laboratory, designed to image the Sun’s corona continuously and with high contrast. The remaining three carry SwRI’s expertly engineered WFIs, specialized “heliospheric imagers” that delve even farther, capturing the faint edge of the corona and the elusive solar wind. The challenge is immense since the brightness of solar wind features is less than one-thousandth of a percent of the star field and galactic background light captured in raw images. Extracting meaningful solar wind data thus demands meticulous removal of stars, zodiacal light, and other signals while retaining the faint electrical glow streaming from the Sun.

A remarkable innovation onboard these satellites lies in their propulsion system, featuring compact, water-powered “shot-glass-sized” rocket engines. These novel thrusters utilize electrolysis to split onboard water into hydrogen and oxygen, which are subsequently combusted to produce thrust. Each burst delivers a subtle but critical velocity adjustment, on the order of just a fraction of an inch per second, allowing the spacecraft to maintain precise constellation geometry essential for coherent imaging across vast spatial scales. This system joins the ranks of the first space missions to incorporate such safe, inert, and non-toxic propulsion technology, which contrasts with the hazardous hydrazine fuel systems traditionally used in spacecraft maneuvering.

Data acquisition within each instrument is designed for efficiency and scientific rigor. Every four minutes, the onboard cameras collect a trio of images using three distinct polarizing filters. This polarization data is crucial, enabling researchers to discern the directional movement and physical properties of coronal mass ejections (CMEs) and other solar wind structures in three dimensions. Unlike conventional coronagraphs, which have largely been confined to two-dimensional imaging of the corona alone, PUNCH’s multi-angle polarization technique promises to revolutionize our understanding of solar wind dynamics and space weather forecasting.

With the commissioning phase wrapping up by June 2025, the Science Operations Center will commence regular data reception, processing, and distribution to NASA and the global scientific community. This data pipeline is expected to provide unprecedented insights into the interplay between the Sun’s outer atmosphere and the heliosphere—the vast bubble carved out by solar wind within the interstellar medium. Researchers anticipate that the mission will enhance our ability to predict the onset and trajectory of solar storms that can disrupt satellite communications, power grids, and aviation systems.

The first light images from PUNCH also hold particular aesthetic and scientific fascination. The visible zodiacal light, often elusive in terrestrial observations, is a continuous reminder of the dust-filled environment within which our solar system resides. This delicate haze reflects sunlight and provides critical clues about the distribution and evolution of fine particulate matter, factors influencing planetary formation and solar system evolution models.

In addition to these observational breakthroughs, PUNCH serves as a pathfinder for innovative spacecraft engineering and mission architecture. The collaborative development between SwRI and the U.S. Naval Research Laboratory marries expertise in solar imaging with cutting-edge spacecraft technology, showcasing how small, coordinated satellites can deliver results previously reserved for much larger missions. This inspires new thinking on the cost-effective, adaptive deployment of constellation missions aimed at dynamic Earth and space environments.

As the mission moves from commissioning into full science operations, the PUNCH team is poised to extend humanity’s gaze deeper into the Sun’s influence, offering fresh perspectives on fundamental astrophysical questions. This mission not only addresses the complexities of solar wind origin and acceleration but also exemplifies the fusion of innovative propulsion and precise optical instrumentation, setting a precedent for future missions exploring the vast frontiers of heliophysics and beyond.

The coming months promise a wealth of data and discoveries, as PUNCH’s virtual giant eye remains fixed on the ever-changing corona, eagerly awaiting the next solar events that will illuminate the inner workings of our star’s relationship with the space that surrounds it. Scientists and space enthusiasts alike are encouraged to watch this pioneering mission’s progress, which will undoubtedly shape our understanding of the solar system for decades to come.

Subject of Research: Solar corona, Solar wind, Heliosphere, and space weather phenomena observed via advanced heliospheric imaging.

Article Title: PUNCH Mission’s First-Light Images Open New Frontiers in Solar Wind Observation

News Publication Date: April 17, 2025

Web References: https://www.swri.org/markets/earth-space/space-research-technology/space-science/heliophysics?utm_campaign=punch-first-light-pr&utm_source=eurekalert!&utm_medium=referral

Image Credits: NASA/Southwest Research Institute

Keywords: Sun, Solar wind, Heliosphere, Cameras, Artificial satellites, Scientific data, Earth systems science, Solar physics

PUNCH represents an innovative approach to solar observation, one that promises to unify previously disparate elements of understanding regarding solar dynamics. Dr. Craig DeForest, the Principal Investigator for the PUNCH mission, emphasizes that this is the first time a holistic view of the solar corona and solar wind will be achieved simultaneously. Utilizing four small, coordinated spacecraft flying in a precise configuration, PUNCH aims to synthetically generate data equivalent to what would be collected by a colossal instrument spanning 8,000 miles—a feat not feasible with any current technology.

Each of the four satellites is outfitted with sophisticated imaging equipment capable of capturing distinct aspects of the solar corona, which is the outer layer of the Sun’s atmosphere. This region, typically obscured by the Sun’s bright disc, holds vital information about solar emissions that can dramatically affect space weather. PUNCH is not intended as a singular observation platform; rather, it is orchestrated to function as a comprehensive sensor network that will operate continuously while maintaining a clear view of the atmosphere surrounding the Sun.

One key scientific tool aboard the PUNCH mission is the Narrow Field Imager, developed by the U.S. Naval Research Laboratory, designed to continuously monitor the solar corona. Meanwhile, the other three satellites will employ Wide Field Imagers engineered by SwRI, specifically configured to detect the faint emissions from the outermost layers of the solar atmosphere and the solar wind. The ambitious undertaking is expected to enhance our understanding of how coronal mass ejections (CMEs) unfold and affect Earth as they travel through the solar system.

The intricate measurements collected by PUNCH will pave the way for the first true observation of how solar wind accelerates and interacts with the solar corona. Particularly, the images captured will shed light on the complex mechanisms at play in solar heating, an area that has baffled scientists for years. By synchronizing observations from multiple vantage points, PUNCH aims to give a three-dimensional perspective on solar dynamics, offering insight that traditional methods—relying on one-dimensional measurements—could never achieve.

PUNCH will operate in a unique orbit along the terminator line, also known as the day-night line. This position will allow the satellites to stay in continuous sunlight, guaranteeing that they maintain operational efficiency while providing a steady stream of observational data. The strategic placement of the satellites is instrumental for continuously capturing the subtle variations in the corona’s behavior, without the interference from the Earth’s atmosphere or varying light conditions.

The technological innovations aboard the PUNCH spacecraft are noteworthy. The deep baffles integrated into the wide-field imagers significantly reduce the light from the Sun itself—effectively more than a trillion times—allowing the faint glimmers of solar wind emissions to be captured. Excitingly, the data processing performed on Earth will further enhance these images, drastically reducing the overwhelming background light to reveal insights into solar activity that have remained hidden for too long.

Central to PUNCH’s mission objectives is the ability to track CMEs in three dimensions as they traverse towards Earth, a significant advancement in the field of space weather forecasting. Dr. DeForest has drawn parallels between the anticipated capabilities of PUNCH and the transformative impact of geosynchronous satellites on terrestrial weather forecasting. As the data accumulates, scientists will potentially have a far more reliable means of predicting space weather events, which can impact satellite operations and even terrestrial power grids.

NASA’s Small Explorers (SMEX) program, known for its ability to facilitate robust and efficient missions, has backed PUNCH, demonstrating the agency’s commitment to advancing heliophysics science. Alongside SwRI, which oversees operations of the four spacecraft, the PUNCH mission encompasses collaborative efforts with the U.S. Naval Research Laboratory, which constructed key imaging instruments, and RAL Space from the United Kingdom, recognized for developing advanced detector systems.

As the launch date approaches, the excitement surrounding PUNCH continues to build. This mission’s novel approach to solar physics holds the promise of a more profound comprehension of fundamental processes that govern not only our solar system but potentially other star systems as well. The insights gathered through this mission could lead to significant breakthroughs, not only informing our understanding of solar dynamics but also aiding in the advancement of technology designed to mitigate space weather risks.

In conclusion, NASA’s PUNCH mission is set to redefine our understanding of solar phenomena and the complex interactions that characterize our relationship with the Sun. As this exciting venture unfolds, it will undoubtedly inspire a new generation of researchers and enthusiasts who share a passion for uncovering the mysteries of the cosmos and its myriad influences on our daily lives. The data produced by PUNCH will not only contribute to the field of astrophysics but could also foster advancements in technological applications focused on space weather, underscoring the mission’s broader significance.

Subject of Research: Solar corona and solar wind dynamics
Article Title: NASA’s PUNCH Mission Set to Unveil Secrets of the Solar Corona
News Publication Date: February 25, 2025
Web References: PUNCH Mission Video
References: Southwest Research Institute
Image Credits: Southwest Research Institute

Keywords

• Solar wind
• Spacecraft
• Planet Earth
• Measuring instruments
• Heliosphere
• Cameras