🔭 astrophysics

The ASPIICS solar coronagraph aboard the Proba-3 formation flying mission. Scientific objectives and instrument design

This paper outlines the scientific objectives and design of the ASPIICS coronagraph, a novel instrument aboard the ESA's Proba-3 mission that utilizes a 144-meter formation-flying separation between an external occulter and a telescope to observe the inner solar corona with unprecedented clarity and low straylight.

Original authors: A. N. Zhukov, C. Thizy, D. Galano, B. Bourgoignie, L. Dolla, C. Jean, B. Nicula, S. Shestov, C. Galy, R. Rougeot, J. Versluys, J. Zender, P. Lamy, S. Fineschi, S. Gunar, B. Inhester, M. Mierla, P. Rud

Published 2026-07-07✓ Author reviewed ⓘ

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: A. N. Zhukov, C. Thizy, D. Galano, B. Bourgoignie, L. Dolla, C. Jean, B. Nicula, S. Shestov, C. Galy, R. Rougeot, J. Versluys, J. Zender, P. Lamy, S. Fineschi, S. Gunar, B. Inhester, M. Mierla, P. Rudawy, K. Tsinganos, S. Koutchmy, R. Howard, H. Peter, S. Vives, L. Abbo, C. Aime, K. Aleksiejuk, J. Baran, U. Bak-Steslicka, A. Bemporad, D. Berghmans, D. Besliu-Ionescu, S. Buckley, O. Buiu, G. Capobianco, I. Cimoch, E. DHuys, C. E. DeForest, M. Dziezyc, K. Fleury-Frenette, S. E. Gibson, S. Giordano, L. Golub, K. Grochowski, P. Heinzel, A. Hermans, J. Jacobs, S. Jejcic, N. Kranitis, F. Landini, D. Loreggia, J. Magdalenic, D. Maia, C. Marque, R. Melich, M. Morawski, M. Mosdorf, V. Noce, P. Orleanski, A. Paschalis, R. Peresty, L. Rodriguez, D. B. Seaton, L. Short, J. -F. Simar, M. Steslicki, R. Sorensen, G. Terrasa, N. Van Vooren, F. Verstringe, L. Zangrilli

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Idea: A Space-Based "Eclipse" Machine

Imagine trying to take a picture of a candle flame sitting right next to a blindingly bright spotlight. If you look directly at the candle, the spotlight's glare washes it out completely. This is the problem astronomers face when trying to study the solar corona (the Sun's outer atmosphere). The Sun's surface is so bright that it hides the faint, wispy corona, except during a total solar eclipse on Earth, which only lasts a few minutes and happens rarely.

Enter ASPIICS, a special camera on a mission called Proba-3. Instead of waiting for a rare eclipse, Proba-3 creates its own "artificial eclipse" in space.

How It Works: The Two-Satellite Dance

Most space telescopes are single units. Proba-3 is different; it consists of two separate spacecraft flying in a precise formation, like a dance partner pair.

The Occulter Spacecraft (The "Hand"): One satellite carries a large, circular disk (the occulter). Think of this as a giant hand held up to block the blinding spotlight (the Sun).
The Coronagraph Spacecraft (The "Eye"): The second satellite carries the camera (ASPIICS). It flies about 144 meters (472 feet) behind the first one.

Because the two satellites are so far apart, the "hand" can block the Sun perfectly without casting a shadow that is too small or distorted. This creates a long, dark tunnel of shadow where the camera sits. Inside this tunnel, the camera can see the faint corona clearly, free from the blinding glare of the Sun's surface.

The Analogy: Imagine trying to see a firefly next to a stadium floodlight. If you hold a coin close to your eye to block the light, the coin's shadow is fuzzy and small. But if you have a friend hold a large sign 100 feet away from you, the sign casts a long, sharp, dark shadow. You can stand in that shadow and see the firefly perfectly. That is exactly what Proba-3 does.

Why This Matters: Filling the "Missing Link"

For a long time, astronomers have had a "gap" in their vision of the Sun:

Close-up cameras (like those on the SDO satellite) can see the Sun's surface and the very bottom of the atmosphere, but they get blinded if they look too far out.
Far-away cameras (like LASCO on the SOHO satellite) can see the outer corona, but they miss the crucial inner part where things start to happen.

ASPIICS fills this gap. It can see the corona from just above the Sun's surface (1.099 times the Sun's radius) out to 3 times the Sun's radius. It's like having a camera that can see the entire "atmosphere" of the Sun in one continuous, clear view, from the ground level to the stratosphere.

What They Hope to Learn

The paper outlines three main mysteries ASPIICS hopes to solve:

Why is the Sun's atmosphere so hot? The Sun's surface is about 5,500°C, but the corona is millions of degrees. It's like walking away from a campfire and suddenly getting burned by a blast of heat. ASPIICS will look for tiny waves and magnetic "snaps" that might be pumping energy into the corona to heat it up.
Where does the solar wind come from? The Sun constantly blows a stream of particles (solar wind) toward Earth. Sometimes it's a gentle breeze; other times it's a hurricane. ASPIICS will watch the "source regions" to see how this wind is born and accelerated.
What triggers solar storms? Sometimes the Sun erupts, sending massive clouds of magnetized gas (Coronal Mass Ejections or CMEs) toward Earth. These can knock out satellites and power grids. ASPIICS will watch the very moment these eruptions start, trying to see the "fuse" that lights the fire.

The Instrument: A High-Tech Eye

The ASPIICS instrument is a sophisticated camera designed to handle extreme conditions:

The Lens: It uses a special set of lenses to focus the faint light of the corona onto a digital sensor.
The Filters: It doesn't just take black-and-white photos. It uses special "sunglasses" (filters) to look at specific colors of light. Some filters show the hot gas (2 million degrees), while others show cooler, dense clouds of gas (prominences).
The "Smart" Camera: The Sun's brightness varies wildly. The part near the Sun is bright; the part far away is dim. To capture both, the camera takes a series of photos with different exposure times (like taking a photo of a dark room and a bright window separately) and stitches them together into one perfect image.

The Mission Profile

Orbit: The two satellites fly in a very stretched, oval-shaped orbit around Earth. They spend about 6 hours at the top of this orbit (the apogee) where they can perform their formation flying dance.
Precision: Keeping two satellites aligned within a few millimeters over a distance of 144 meters is incredibly hard. They use lasers, cameras, and GPS to stay perfectly lined up, ensuring the shadow stays on the camera.
Duration: The mission is planned to last two years, giving scientists hundreds of hours of high-quality data—far more than any single total solar eclipse on Earth could ever provide.

Summary

In short, the ASPIICS instrument on the Proba-3 mission is a revolutionary tool that uses two satellites flying in a precise formation to create a permanent, artificial solar eclipse. This allows scientists to see the Sun's inner atmosphere with unprecedented clarity, helping them solve long-standing puzzles about why the Sun's outer layer is so hot, how the solar wind is created, and what causes dangerous space weather storms.

Technical Summary: The ASPIICS Solar Coronagraph aboard the Proba-3 Formation Flying Mission

Problem Statement
The solar corona, the source of the solar wind and coronal mass ejections (CMEs), remains poorly understood in the "middle corona" region (approximately 1.1 to 3 solar radii, $R_\odot$ ). This region represents a critical observational gap: Extreme Ultraviolet (EUV) imagers (e.g., SDO/AIA) typically observe the lower corona up to $\sim1.27 R_\odot$ , while classical externally occulted coronagraphs (e.g., SOHO/LASCO C2, STEREO/COR2) generally begin observations at $\sim2.2\text{--}2.5 R_\odot$ . Ground-based observations are limited by atmospheric scattering and short eclipse durations. Furthermore, existing space-based coronagraphs struggle with high levels of straylight when observing close to the solar limb, obscuring faint coronal structures and reducing the signal-to-noise ratio necessary to study dynamic phenomena like wave propagation and CME acceleration.

Methodology and Instrument Design
The paper describes the ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) instrument, launched on December 5, 2024, aboard the European Space Agency's Proba-3 mission. The core innovation of the mission is a formation-flying architecture consisting of two spacecraft:

Occulter Spacecraft (OSC): Carries a circular external occulter disk (radius $R_{EO} = 0.71088$ m) with a toroidal edge.
Coronagraph Spacecraft (CSC): Carries the ASPIICS telescope.

The two spacecraft maintain a precise formation with an inter-satellite distance (ISD) of approximately 144 meters during a 6-hour window centered on the apogee of their highly elliptical orbit (19.63-hour period). This distance represents a two-order-of-magnitude increase compared to classical externally occulted coronagraphs (where $D \sim 1$ m).

Key Technical Features:

Straylight Suppression: The large separation ( $D \approx 144$ m) significantly reduces the intensity of diffracted light from the external occulter, allowing observations down to $1.099 R_\odot$ with low straylight.
Optical System: ASPIICS is a Lyot coronagraph with a 50 mm entrance pupil and a focal length of 734.6 mm. It utilizes an internal occulter and a Lyot stop to block diffracted light.
Detectors and Filters: The instrument uses a $2048 \times 2048$ $2048 \times 2048$ pixel CMOS Active Pixel Sensor (APS) with a pixel scale of $2.817''$ $2.81 7^{''}$ . It employs a filter wheel with six slots:
- Wideband (WBF): 5363–5658 Å (centered at 5510 Å) for total brightness and polarized brightness (with 0°, 60°, 120° polarizers).
- Narrowband Fe XIV: Centered at 5304 Å (green line) for $\sim2$ MK plasma.
- Narrowband He I D3: Centered at 5877 Å for prominence plasma ( $\sim0.01$ MK).
Metrology: Formation flying is maintained using GNSS, a Visual-Based System (VBS), and a Fine Lateral and Longitudinal Sensor (FLLS) using laser retro-reflectors. ASPIICS also employs a Shadow Position Sensor (SPS) and an Occulter Position Sensor Emitter (OPSE) to monitor the alignment of the external occulter relative to the telescope aperture.
Data Processing: To handle the high dynamic range of the corona, the instrument uses a "tile map" approach, dividing the detector into $64 \times 64$ pixel tiles. Different exposure times are applied to different tiles, and quality flags determine which tiles are retained for downlink.

Results and Performance
Ground-based calibration campaigns (Belgium and Italy, 2020–2021) confirmed the instrument's performance:

Resolution: The system achieves a spatial resolution of $2.817''$ per pixel (linear resolution $\sim4085$ km at $1 R_\odot$ ), limited by the pixel size in the unvignetted field of view ( $>1.174 R_\odot$ ).
Straylight: Theoretical models and commissioning data confirm that diffracted light is two orders of magnitude weaker than the coronal signal, validating the design's ability to observe the inner corona.
Radiometry: The radiometric sensitivity was measured across all channels, with bias and dark current characterized (bias $\sim2050$ DN, dark current $\sim3.3$ DN/s).
Operational Cadence: The nominal synoptic cadence is 1 minute, with the capability for higher cadences (down to 2 seconds) using reduced fields of view.

Key Contributions and Scientific Objectives
ASPIICS is designed to fill the observational gap between EUV imagers and classical coronagraphs, enabling seamless coverage from $1.099 R_\odot$ to $3 R_\odot$ . The paper outlines four primary scientific objectives:

Quiescent Corona Structure: Investigating the nature of the corona on different scales, including the transition from closed to open magnetic field lines, and determining the magnetic configuration of streamers and coronal holes.
Coronal Heating: Measuring wave properties (Alfvén waves, density fluctuations) with high cadence to determine their contribution to coronal heating, particularly in the magnetically closed corona.
Solar Wind Acceleration: Studying the origin of the slow solar wind, including interchange reconnection, jets, and turbulence, by observing small-scale dynamics in the wind source regions.
CME Initiation and Evolution: Providing high-resolution, high-cadence observations of CME onset, acceleration, and restructuring in the low corona ( $<3 R_\odot$ ). This includes analyzing the three-part CME structure, current sheets, and the connection between flares and CMEs.

Significance
The paper claims that ASPIICS represents a significant advance in solar space coronagraphy. By leveraging the unique 144-meter formation flying separation, it achieves a level of straylight rejection and inner-corona access ( $1.099 R_\odot$ ) previously unattainable in space. This capability allows for the first time the seamless observation of CME dynamics from their initiation near the solar surface through their acceleration phase, bridging the gap between EUV and white-light observations. The instrument is expected to provide crucial data to resolve outstanding problems regarding the origin of the slow solar wind, the physical mechanisms of coronal heating, and the initiation of CMEs and associated shocks. The mission serves both as a scientific tool for solar physics and a technology demonstrator for precise formation flying.