LTE - Observatoire de Paris - PSL - Centre de recherche en astronomie et astrophysique

Numerical simulations of rings around irregular bodies: the unexpected role of resonance 1/3

Sylvie Lemaître — 2026-04-10T13:41:01Z

An international team, comprising a researcher from the Paris Observatory – PSL, Sorbonne University and the CNRS, and a researcher from the University of Oulu in Finland, has studied ring dynamics using N-body collision simulations. These simulations show that the 1/3 spin-orbit resonance can effectively confine rings around irregular bodies such as Chariklo by transferring the energy provided by this resonance to free Lindblad modes, a process never previously observed. This work is the subject of a paper published in the journal Astronomy & Astrophysics on 26 March 2026.

In recent years, rings have been discovered around small bodies in the solar system, such as Centaur-type objects like Chariklo and Chiron, or trans-Neptunian objects like Haumea and Quaoar. Unlike the giant planets, which are practically axisymmetric, these small bodies are irregular, creating strong spin-orbit resonances between the central object and its rings. In fact, several of the rings discovered are close to a 1/3 spin-orbit resonance with the body, in which the body completes three rotations whilst a particle in the rings completes one orbital revolution.

This is not the first time that resonances have been linked to the presence of confined rings. In fact, several of the rings of Saturn, Uranus and Neptune are confined by first-order m/m-1 resonances with a satellite, whereby the satellite completes m-1 revolutions whilst the particle completes m. These resonances, known as Lindblad resonances, were studied in the 1960s in the context of galactic dynamics. They force the formation of spiral arms in the disc, and in the case of rings, they can lead to the confinement of narrow rings.

In principle, the effect of a second-order m/m+2 resonance (i.e. 1/3 for m=1) is destructive. Indeed, unlike Lindblad resonances, the 1/3 resonance causes the current lines forced by this resonance to cross, a highly dangerous situation for a collisional ring. It is as if motorways were being built that cross without traffic lights! This is illustrated in the figure opposite, where we see the central body with a mass anomaly (small circle attached to the central body), surrounded on the left by the periodic orbit of a particle in a 1/2 spin-orbit resonance with the central body. This orbit does not intersect itself, unlike the periodic orbit corresponding to the 1/3 resonance on the right, which has a self-intersection point (blue dot).

The authors of the paper used N-body simulations, in which tens of thousands of particles with radii of a few tens of metres undergo inelastic collisions, whilst being perturbed by an irregular central body. This irregularity is modelled by a mass anomaly representing, for example, a mountain or a crater on the surface of Chariklo; see the figure above. This mass anomaly is quantified by a dimensionless parameter µ, which measures its mass relative to that of the central body.

Following an excitation phase that leads to the self-crossing of the current line excited by the 1/3 resonance, a surprising phenomenon occurs: the ring transfers the energy it receives to free Lindblad modes, thereby eliminating the self-crossing of the orbits. Even more surprisingly, these free modes ultimately lead to the radial confinement of the ring.

This process is illustrated in the figure above. It is the result of an N-body simulation involving 40,000 particles with a radius of 25 metres undergoing inelastic collisions and perturbed by the 1/3 resonance. The positions of the particles are shown in a longitude-radius diagram, with the dotted line indicating the radius of the 1/3 resonance. In the case of Chariklo, the radius range covered here (from 2.03 to 2.12) corresponds to a radial distance of approximately 18 km centred on the orbital radius of the resonance towards a radius of 2.08, corresponding to 400 km for Chariklo. The time shown is in units of Chariklo's rotation period (approximately 7 hours), i.e. approximately 130, 136 and 205 years for the left, centre and right panels, respectively. On the left, we see the current line forced by the 1/3 resonance, with self-crossing, followed by a transition period in the centre panel, which leads on the right to a strongly confined ring, which now avoids any self-crossing. Fourier analysis of this ring shows that it results from the superposition of Lindblad free modes.

By varying the mass anomaly µ and the particle radius, and using dimensional analysis, the authors show that Chariklo's main ring can be confined with mass anomalies of the order of µ≈10⁻³, assuming particles of the order of one metre in size. This value corresponds to a mountain (or crater) approximately 10 km high (or deep) on the surface of Chariklo, which is physically plausible for a small body with a diameter of approximately 250 km.

Votre navigateur ne supporte pas la vidéo HTML5.

This animation shows the confinement of a ring around the 1/3 resonance with Chariklo. It is the result of an N-body simulation involving 40,000 particles with a radius of 25 metres undergoing inelastic collisions and perturbed by the 1/3 resonance with Chariklo, with Chariklo completing three rotations whilst the particles complete one orbital revolution. The ring's arc is shown in perspective, with the distance on the x-axis (2 to 2.12) corresponding to approximately 24 km. The elapsed time in years since the start of the simulation is shown at the bottom right of the image.

This mechanism could also apply to the rings of Haumea and Quaoar, given that the latter also has a ring that is close to a 5/7 resonance with the central body. Like the 1/3 resonance, it is of order two, and therefore has the same topological structure, with self-crossings of the periodic orbits, and possibly the same behaviour as Chariklo's ring.

Reference

The article is published under the title : "Rings around irregular bodies II. Numerical simulations of the 1/3 spin-orbit resonance confinement and applications to Chariklo" dated 26 March 2026, https://doi.org/10.1051/0004-6361/202556946.
Another article setting out the theoretical aspects of resonances, by Sicardy, Salo, El Moutamid, Renner and Souami, was published on 25 November 2025, https://doi.org/10.1051/0004-6361/202556950.
This research was partly funded by the "Roche" project of the French National Research Agency ANR-23-CE49-0012. It is the result of scientific research carried out in France, at the Paris Observatory – PSL in Laboratoire Temps et Espace (Paris Observatory – PSL / CNRS / Sorbonne University / University of Lille) and at the University of Oulu in Finland.

Contacts

Bruno Sicardy, Professor Emeritus SU, LTE
+33 (0) 1 40 51 23 34
bruno.sicardy@observatoiredeparis.psl.eu

Heikki Salo, Space Physics & Astronomy Research unit, University of Oulu
90014 Oulu, Finland
Heikki.salo@oulu.fi

Poisson, Arago, Young, Fresnel, Sommerfeld, Pluto and Triton: a strange story of the central flash

Sylvie Lemaître — 2026-03-25T13:23:39Z

A French team, comprising a researcher from the Paris Observatory – PSL, Sorbonne University and the CNRS, and a researcher from Jean Monnet University in Saint-Étienne, has studied the structure of the central flashes produced during stellar occultations by the atmospheres of Pluto and Triton, which act as immense lenses focusing starlight towards Earth. This work is the subject of a paper published in the journal Astronomy & Astrophysics on 17 March 2026.

In the early 19th century, a number of experiments, including those conducted by Thomas Young and Augustin-Jean Fresnel, led to the triumph of the wave theory of light. However, not all scientists were convinced. Siméon Denis Poisson, for instance, a proponent of Isaac Newton's corpuscular theory of light, was staunchly opposed to it. The controversy intensified with a question put forward by the Academy of Sciences in 1818. On that occasion, Poisson had noted that one of the consequences of Fresnel's wave theory would be the presence of a bright spot at the centre of the shadow cast by a circular opaque disc, due to constructive interference of the wave. This struck him as absurd: how could light illuminate the centre of a completely black shadow? Especially since no one had ever observed such a spot… However, François Arago was able to produce one using a small metal disc. The jury members were convinced, and the Academy's prize was awarded to Fresnel in 1819.

In memory of these debates, the observed spot is now known as the Poisson spot, or the Poisson-Arago spot, or the Fresnel spot. It made an unexpected reappearance during calculations carried out to describe the ‘central flashes' observed during stellar occultations by objects such as Pluto or Triton (see figure opposite). The question was: what is the structure of the flash produced by a perfectly spherical and transparent atmosphere? Can its amplitude be infinite? What role does diffraction play?

The calculations describing the flash involve path integrals and make use of Sommerfeld's lemma, which is fundamental to quantum mechanics. This is only logical: photons are, by their very nature, quantum objects! Building on this, the authors consider several scenarios, starting with a spherical occluder without an atmosphere, of radius R₀, and a star assumed to be point-like. This yields the classical Poisson spot, whose maximum irradiance is equal to that received from the star outside the shadow. The Poisson spot is described by the square of the Bessel function of the first kind and zero order, J₀. It is an oscillating function that reaches unity at the centre of the shadow and has a width close to λ∆/(2R₀), where λ is the observation wavelength and ∆ is the geocentric distance of the occulter. This peak is also surrounded by fringes, equally spaced at approximately λ∆/(2R₀).

The figure opposite shows the structure of the shadow cast by an opaque spherical body with a radius of 10 km situated at Pluto's level and observed in the visible spectrum. Note the classic Fresnel fringes on the left and right edges, caused by the object's abrupt and opaque limb.

If we introduce a thin atmosphere, but one too weak to focus the light rays towards the centre of the shadow, not only does the Poisson spot remain, but it is amplified by a factor of (R₀/r₀)², where r₀ < R₀ is the radius of the shadow cast by the body, taking into account the refraction of light rays due to the atmosphere of the occulter. The figure opposite shows the theoretical structure of the shadow of Pluto, whose atmospheric pressure has been arbitrarily reduced to one-tenth of its current value, and observed in millimetre waves. The Fresnel fringes mentioned above can be seen at the edges of the shadow, as well as the central Poisson spot amplified by a factor of (R₀/r₀)².

If the atmosphere is dense enough to focus the light rays towards the centre of the shadow, r₀ no longer exists. Calculations then show that diffraction imposes a finite flash height of approximately (2π)²RH/(λ∆), where R is the radius of the layer causing the central flash (close to R₀), and H is the height scale of the atmosphere. For Pluto and Triton (with 0.01 mbar at the surface), this flash height can reach very large values in the visible spectrum, ranging from 10⁴ à 10⁵ times the star's luminance outside the occultation. At the same time, the width of the flash projected onto Earth λ∆/(2R) is extremely small, on the order of a metre, making it impossible to resolve with current technology. However, as this width is proportional to λ, observations made at millimetre wavelengths could resolve the flash and the fringes surrounding it, which would then have sizes on the order of a kilometre.

The figure opposite shows a theoretical curve of a Pluto occultation observed in the visible spectrum. The Fresnel fringes are now too faint to be observed. Around the flash, which reaches very high off-scale values ( 10⁴), rapid fluctuations caused by Poisson fringes can be seen. These fringes result from interference between the primary and secondary stellar images produced by Pluto's atmosphere, and are therefore identical to the interference produced in Young's double-slit experiment, but with a subtle additional phase quadrature correlated with the fact that the light rays corresponding to the secondary image pass through the axial caustic just before reaching the observer's front plane.

That said, other effects must be taken into account when describing the flash. For example, the size of the occulted star is finite. When projected onto the occulting body, this size is usually of the order of one kilometre. This implies that diffraction effects are smoothed out by the stellar diameter: in the case of a thin atmosphere, the height of the central flash is then less than (R₀/r₀)²; in the case of a dense atmosphere such as Pluto's at present, calculations show that despite this smoothing, the flash can still reach heights more than 50 times the star's initial signal. Another effect to take into account is the possible distortion of the atmosphere relative to the spherical model. In this case, the Poisson spot is replaced by a caustic surrounded by fringes, a subject for future study…

Reference

The article is published under the title : “Central flashes during stellar occultations.
Effects of diffraction, interferences, and stellar diameter” on 17 March 2026, https://doi.org/10.1051/0004-6361/202555548

This research is the result of scientific work carried out in France at the Paris Observatory – PSL, in the Laboratoire Temps et Espace (Paris Observatory – PSL / CNRS / Sorbonne University / University of Lille), and at Jean Monnet University in Saint-Étienne, CNRS, in the Institut d'Optique Graduate School, and the Laboratoire Hubert Curien , UMR 5516.

Contacts

Bruno Sicardy
Professor Emeritus SU, LTE
+33 (0) 1 40 51 23 34
bruno.sicardy@observatoiredeparis.psl.eu

Luc Dettwiller
Contributor to Laboratoire Hubert Curien,
Institut d'Optique, Saint-Etienne
dettwiller.luc@gmail.com

Two summers and one winter, three rare events: the ‘Iberian trilogy' of eclipses (2026–2028)

Sylvie Lemaître — 2026-03-03T14:32:49Z

Imagine: within three years, the Moon will repeatedly ‘target' Southern Europe. As a result, Spain becomes the best place in Europe to experience exceptional solar eclipses – a total eclipse at sunset (2026), a total eclipse in the morning (2027), and then a spectacular ‘ring of fire' at dusk (2028).

12 August 2026: Totale, the ‘summer night' at dusk

Visibility map of the total solar eclipse on 12 August 2026. Credits: LTE

Movement of the Moon's shadow over the Earth during the total solar eclipse on 12 August 2026. The black dot represents the locations where the eclipse will be total. Credits: LTE

In the middle of summer, the Sun is about to perform a real disappearing act. The band of totality crosses northern Spain (and also passes through Iceland): there, for about 1 to 2 minutes depending on the location (max 1 min 50 s in northern Spain, up to 2 min 18 s on the central line before reaching Europe), the day changes - the sky turns blue, the air cools, and the solar corona appears as a living halo around a black disc. And it all happens quickly: the shadow moves at about 3,429 km/h, like a dark wave breaking silently. In France, it will be a partial eclipse in the evening, perfect for observing on the way back from the beach or a picnic: the light becomes strange, as if someone had slowly turned down the world's dimmer switch. And for ‘easily accessible' Europe, it's a highly anticipated event: the first eclipse visible from mainland Europe since 1999, the return of a great spectacle close to home.

2 August 2027: Total, the great ‘long'

Visibility map of the total solar eclipse on 2 August 2027. Credits: LTE

Movement of the Moon's shadow over the Earth during the total solar eclipse on 2 August 2027. The black dot represents the locations where the eclipse will be total. Credits: LTE

A year later, another dramatic turn of events... but this time in the morning, when the sky is clear and the shadow advances across the continent at a speed of 2,398 km/h. The totality grazes Europe via southern Spain, towards Andalusia and the Strait of Gibraltar: there, the darkness can last several minutes (up to nearly 5 minutes in the best areas: 4 min 49 s in Ceuta, more than 4 min 30 s in Tarifa), long enough to feel the silence descend and see the horizon take on the colours of twilight across 360°. Further along its path (particularly towards Egypt), the eclipse reaches a maximum of around 6 min 23 s – the second longest total eclipse of the 21st century and the longest over easily accessible land, said to leave ‘time to look', as if time itself were looking: time to be surprised, time to understand, time to get goosebumps. In France, it will be seen as partial, like a crescent nibbling at the Sun, but the event itself will take place a few hours' drive away. And to exceed this duration, we will have to wait a very long time, nearly a century: the next longest total eclipse is announced for 3 June 2114, with a maximum of 6 min 32 s, before the very long one on 13 June 2132, approaching the maximum duration of a total eclipse when all the optimal celestial conditions are met, i.e. 7 min.

26 January 2028: Annular eclipse, the “ring of fire” at dusk

Visibility map of the annular solar eclipse on 26 January 2028. Credits: LTE

Movement of the Moon's shadow over the Earth during the annular solar eclipse of 26 January 2028. Credits LTE

The final act, in the middle of winter: not a night in broad daylight, but a celestial gem. On 26 January, the Moon moves directly in front of the Sun without covering it completely: it leaves a bright, thin, sharp, almost unreal ring – the famous ‘ring of fire'. The ring-shaped band once again passes over Portugal and Spain (with a passage towards Gibraltar and northern Morocco) and, as an added bonus, the phenomenon occurs in the late afternoon: a twilight spectacle, where the ring can last for several minutes, with a global maximum of around 10 minutes, which is very rare for an annular eclipse and therefore exceptional (the maximum ‘theoretical' duration of an annular eclipse is around 12 min 35 s). Here, the relative slowness of the shadow is part of the magic: nearly 1,709 km/h - and it is also one of the main reasons why the ring can linger for so long. In France, it will be a partial eclipse: a foretaste. But in Iberia, with a clear horizon, we will be able to see the Sun wearing its ring – as if the sky were signalling, in light, the end of this trilogy.

Learn more

Total solar eclipse on 12 August 2026
Total solar eclipse on 8 August 2027
Annular solar eclipse on 26 January 2028

The EQUIP-G project takes off

Sylvie Lemaître — 2026-03-03T13:48:42Z

Towards a permanent European entity managing a shared fleet of quantum gravimeters accessible to all and a network of ground reference stations.

Gravity measurements are a unique tool for subsurface imaging, as they provide direct information on underground mass variations. They also play an essential role in establishing terrestrial spatial references. Gravimetry can be used to map and monitor subsurface dynamics, and has numerous applications: geothermal energy, monitoring climate change, groundwater and volcanoes, and underground gas storage.

After decades of research, development and industrial transfer, quantum technology has reached a high level of maturity and it is now possible to benefit from operational quantum gravimeters in the field, offering numerous advantages over the devices used until now.

Quantum gravimeter deployed in Greenland in August 2025 as part of the EQUIP-G project.
Credits Tim Jensen, DTU

The EQUIP-G consortium is deploying a fleet of quantum gravimeters across Europe and offering free loans to the geophysical and geodetic community to use this technology. By 2029, the project will provide the European Commission with details and recommendations concerning a future fleet of shared instruments (terrestrial or onboard quantum gravimeters). This will involve managing requests from various public institutions, monitoring the instruments and providing training. Finally, it will also involve finding the best solution for storing and making available all the data, which will be public.

The future entity would be similar to a research infrastructure, with challenges similar to those faced by astronomers who reserve observation time and collect data when sharing telescope time.

How does a gravimeter work?

A quantum gravimeter is a sensor that replicates Newton's apple experiment. Several times per second, atoms are trapped and cooled by laser to form a small sample of matter. This test mass is then released in free fall into a vacuum. The acceleration experienced by the atoms is measured by a vertical reference laser using the principle of atomic interferometry, which allows precision measurements to be made by interfering waves of matter manipulated by laser pulses. The result corresponds to the acceleration of Earth's gravity, g.

The LTE laboratory's LNE-OP service was one of the pioneers of interferometry worldwide in the early 2000s, and part of this research was successfully transferred to industry in the 2010s. Active research on the subject continues to be conducted by the Atomic Interferometry and Inertial Sensors team.

Who is behind this project ?

Following a call for proposals from the European Commission (HORIZON-CL4-2024-DIGITAL-EMERGING-02), the EQUIP-G project, comprising a consortium of 20 partners from 11 European countries, has been selected to develop and deploy a network of quantum gravimeters in Europe by 2029. The project is led by the CNRS and headed by Sébastien Merlet and Jean Lautier-Gaud, members of the LTE.

Learn more

The project website.
Atomic Interferometry Team Web Page du LTE.
Video, broadcast by ARTE, describing how the quantum instrument works on Mount Etna.

Publication: Les dentelles noires de l'Etna

Sylvie Lemaître — 2026-03-03T12:55:10Z

On 20 February, a book entitled Les dentelles noires de l'Etna was published, written by Jean Lautier-Gaud, a member of the LTE staff. In it, he recounts an experiment carried out by an LTE research team.

In this novel, Jean Lautier-Gaud recounts the scientific and human adventure in which he participated to install a quantum gravimeter at ‘Dentelles noires', the observatory station located closest to the craters of Mount Etna. This sensor, based on the work of the LTE's Atomic Interferometry and Inertial Sensors team, is used to study and monitor volcanoes. The author discusses the project and the novel.

« My thesis with Arnaud Landragin at LTE focused on developing a compact quantum gravimeter for field measurements within the Atomic Interferometry and Inertial Sensors team. In 2014, we published a major result paving the way for the use of these instruments outside laboratories.

The technologies developed by the team were transferred by the CNRS to the start-up Muquans, which turned them into a commercial product, enabling one of these sensors to be installed on Mount Etna, an expedition in which I personally took part.

I ended up writing a book on the subject, in which I revisit the human and scientific adventure of climbing. Popular science on the subjects of metrology, quantum sensors and geophysics features prominently in the book. The novel was released on 20 February in bookshops, published by Transboréal.

Today, the deployment of quantum gravimeters in the field continues, notably as part of the EQUIP-G project, supported by the CNRS and led by the LTE. »

Format : 12 × 18 cm – 224 pages
Editor : Transboréal
ISBN : 978-2-36157-366-9

Learn more
Le projet EQUIP-G
Link to the publisher Transboréal
Link to the article about the scientific work behind this experiment

What time is it on the Moon?

Sylvie Lemaître — 2026-02-17T13:38:33Z

Exploration of the Moon and its surroundings has seen renewed interest over the past decade, as evidenced by the Artemis (NASA), LunaNET (NASA/ESA) and Moonlight (ESA) programmes. Their objective is to establish a permanent presence around and on the Moon, which requires the implementation of infrastructure dedicated to navigation, positioning and telecommunications in the vicinity of our satellite. To do this, it is necessary to agree in advance on the use of a coordinated time scale, playing the same role as Coordinated Universal Time (UTC) on Earth, and enabling the various lunar players to exchange information and compare their measurements.

The concept of coordinated time is provided by the theory of relativity and should not be confused with the more familiar concept of proper time. The latter has a local dimension and represents the time physically indicated by an ideal clock. In contrast, coordinated time is purely conventional, but has a global scope, meaning that it is defined and can be used anywhere and by any observer. Two distant observers can thus compare the primacy of their respective local dating by converting (via a mathematical procedure) their measurement of proper time into the same coordinated time scale.

In the lunar environment, three coordinated time scales may be of practical interest. The first, E1, is the most fundamental time scale; it is naturally given by the theory of general relativity: it is the lunocentric coordinated time. The duration of one second of E1 coincides with one second of a clock located at the centre of mass of the Moon; E1 is thus the analogue for the Moon of geocentric coordinated time. The second scale (E2) is obtained by artificially applying a multiplicative factor to the duration of one second of E1, so that the second of E2 coincides with the second beaten by a clock at rest on the lunar geoid; E2 is therefore the analogue for the Moon of Earth time. Finally, the third scale (E3) is also artificially constructed by applying a multiplier to the duration of a second in E1, ensuring, this time, that the duration of a second in E3 is as close as possible to a second in the coordinated universal time scale.

The E2 scale may be advantageous if several clocks placed on the surface of the Moon wish to exchange their measurements. This is because the proper time of each lunar clock will remain close to the E2 coordinated time used for comparison, which masks the mathematical procedure of transforming proper time into coordinated time. However, as the surface of the Moon is very flat, an atomic clock will not generally be located on the lunar geoid; it will therefore not tick at the same rate as E2 and the mathematical transformation procedure cannot generally be avoided. This procedure must be applied if the coordinated time scale is E1 or E3. In all three cases, it can still be circumvented by artificially changing the frequency of the lunar clocks. The frequency corrections to be applied are of the order of 10-11, 10-13 and 10-10 (relative) for the E1, E2 and E3 scales respectively.

At first glance, E2 therefore appears to be the most advantageous. However, E2 and E3 are scales of E1, which, in the context of general relativity theory, implies that masses must also be scaled. This scaling of physical parameters is problematic in that the same mass can then be assigned several numerical values! For example, adopting E2 or E3 would require using a mass for the Earth that would not have the same numerical value as that defined naturally by the barycentric coordinated time scale.

In conclusion, since the mathematical procedure that transforms proper times into coordinate times E1, E2, and E3 cannot be avoided for clocks located on the surface of the Moon, and because E1 does not involve scaling physical parameters unlike E2 and E3, we recommend adopting the lunocentric coordinated time (i.e., E1) as the coordinated time scale associated with the lunar reference system. In the near future, this approach will be easily transposable to other bodies in the solar system, notably Mars.

Contact
Adrien Bourgoin, Astronome adjoint, LTE, Observatoire de Paris
adrien.bourgoin@obspm.fr

View the article

LTE researcher appointed as innovation ambassador

Sylvie Lemaître — 2026-02-03T15:05:36Z

The CNRS has established a network of innovation ambassadors, created in 2024. This network is composed of approximately 80 researchers from all disciplines, recognised for their innovative capabilities in their research.

One LTE researcher has been appointed to this network: Carlos Garrido Alzar (Atomic Interferometry and Inertial Sensors team – IACI/LTE). His research focuses on the development of guided atom interferometry on atom microcircuits for positioning, navigation and timing applications. This work is being carried out in collaboration with French and European manufacturers operating in the aerospace and inertial navigation sectors. It has resulted in four patent applications in quantum technologies.

The role of ambassadors is to boost innovation: to bear witness to, support and encourage all forms of research promotion through their experience, their willingness to communicate and their desire to pass on knowledge. The aim is to give meaning to fundamental research and apply its discoveries to society.

Learn more
CNRS news: Ambassadeurs de l'innovation : susciter le futur au cœur du CNRS

Contact
Carlos Garrido Alzar
carlos.garrido@obspm.fr

At the edge of Saturn: artificial intelligence in search of new moons

Sylvie Lemaître — 2025-12-29T11:40:22Z

The collaboration between the École pour l'informatique et les techniques avancées (EPITA) and the Paris Observatory reveals a fruitful use of space imagery.

The Cassini-Huygens mission, led by NASA, ESA and the Italian Space Agency (ASI), explored Saturn, its rings and its moons between 2004 and 2017. By applying advanced computer vision techniques to data from this mission, Guillaume Tochon, lecturer-researcher and head of the Image Processing and Pattern Recognition team at the EPITA Research Laboratory, Valéry Lainey, astronomer at the LTE at the Paris Observatory, and Giulio Quaglia, PhD student at the LTE, have identified a new way of exploiting images. This improves the study of cosmic rays, facilitates the search for new moons, and opens up new perspectives for the exploration of planetary systems.

When image processing meets astronomy: a fruitful collaboration

This scientific partnership began when an EPITA student started an internship at the Paris Observatory in autumn 2020.

The astronomers then learned about the school's team of experts in image analysis algorithms. For Guillaume Tochon, the collaboration is a unique opportunity: "The issues involved in space images lend themselves particularly well to processing by pattern recognition algorithms. "

Guillaume Tochon and his team are specialists in mathematical morphology and their work covers a broad spectrum. The application to space opens up a field that has yet to be explored by the team. The Cassini mission, which photographed the Saturnian system for 13 years, constitutes an immense source of data. Too vast for astronomers alone to handle, it includes hundreds of thousands of images, many of which have never been examined in detail.

Based on this raw material, Giulio Quaglia's thesis aims first and foremost to answer one question: are there still unknown moons hidden in these images?

Artificial intelligence to distinguish the invisible

Detecting tiny celestial objects is like identifying ‘spots measuring just a few pixels in these images,' says Guillaume. The difficulty lies as much in their small size as in their diversity, including stars, known or potential moons, and cosmic rays striking Cassini's sensor. These high-energy particles leave short, irregular traces that are impossible to annotate manually on thousands of images.

The team therefore created an initial semi-automatic database, a naive algorithm that identifies all bright sources and then classifies them according to their correspondence with star catalogues or moon ephemerides. Anything that does not correspond to anything already known is categorised as cosmic radiation. Despite its imperfections, this database is used to train a neural network. ‘Our bet is that it is statistically good enough,' explains Guillaume Tochon.

And this bet paid off: the AI learned to correct the initial errors. For example, when the naive algorithm confused thin portions of rings with particles, ‘the network was not fooled'.

On several dozen images annotated manually for validation, the model shows a remarkable ability to distinguish between the three categories of objects.

Above all, the results obtained on cosmic rays are striking. The spatial and temporal distribution deduced from the images is consistent with the measurements obtained by the specialised instruments on board the Cassini probe. ‘We were able to obtain similar results (...) just with a camera,' the researcher points out.

From Saturn to Jupiter: a methodology destined to travel

This breakthrough opens up several concrete possibilities. First, the search for new moons: all predictions generated by AI have been stored and can be cross-referenced with orbital models.

Second, this work directly paves the way for the JUICE (Jupiter Icy Moons Explorer) mission, a European Space Agency space mission dedicated to studying Jupiter and its major icy moons. ‘What we have developed here for Saturn will be directly transposed' to future images from the probe. The ability to analyse high-energy particles using simple cameras could significantly reduce the cost of space missions, where each additional instrument represents a technical and financial challenge.

This collaboration with the Paris Observatory illustrates EPITA's mission: to bring decisive scientific and technological expertise to other disciplines. As Guillaume Tochon points out, ‘the goal is not necessarily to innovate in methods, but to correctly transfer the methods we have mastered.'

The image thus becomes a common language between engineers and astronomers, capable of revealing what the human eye cannot detect.

Learn more

Juice, Europe's next major scientific mission.

Crédits image : NASA/JPL-Caltech/Space Science Institute

Q-PLANET project

Sylvie Lemaître — 2025-12-16T14:04:09Z

The Q-PLANET project has been selected for funding under the Chips Joint Undertaking (Chips JU) programme.

This project, in which Carlos L. Garrido Alzar of LTE is leading the Atom chips work package (€2.3 million), aims to establish a quantum pilot line for scientific and industrial innovation based on cold atoms on microcircuits.

The objective of this pilot line is the sustainable development of chips (photonic and atomic) and their enabling technologies (electronics, digital, packaging, ultra-high vacuum, and others), enabling the EU's independence in critical quantum technologies.

A step in the NAROO project

Sylvie Lemaître — 2025-11-21T12:46:12Z

On October 14, a meeting was held at the Paris Observatory in Meudon concerning the NAROO project. The aim was to organize a discussion between the various parties involved in the project following the digitization of more than 2,500 photographic plates from the Greenwich Observatory, marking the completion of a significant milestone.

The diversity of participants at this event demonstrates the importance of this international project:

Valéry Lainey (Observatoire de Paris – PSL, LTE)
Louise Devoy (Royal Observatory Greenwich)
Elizabeth Sanders (Royal Observatory Greenwich)
Laura McCann (Bodleian Libraries, University of Oxford)
Vincent Robert (Observatoire de Paris – PSL, LTE, IPSA)
Paolo Tanga (Observatoire de la Côte d'Azur – CNRS)
Carl Murray (Queen Mary University of London)
Caroline Terquem (Université Paris Cité – Observatoire de Paris, Institut d'Astrophysique de Paris).

Observatories around the world hold tens (even hundreds) of thousands of plates in their archives! Only a tiny fraction of these have been analyzed, mainly for the scientific needs of the time and those of the first space probes, using measurement methods that are now obsolete and inaccurate. There is therefore a huge reservoir of observational data, the initial or renewed analysis of which can provide first-rate scientific data covering a period of nearly a century.

What is NAROO? (New Astrometric Reduction of Old Observations) ?

NAROO is a center for digitizing astronomical photographic plates. It is built around a new-generation submicrometric scanner dedicated to measuring astrophotographic plates and analyzing old observations. It is located at the Meudon site of the Paris Observatory and is managed by a team of researchers from the LTE and IPSA, an aeronautical and space engineering school.

Photographic plates are to telescopes what film is to cameras: the analog means of capturing and preserving an image. Used since the 1890s by the first modern astronomers, they were gradually replaced in 1998 by CCD cameras and other CMOS image sensors.

NAROO is a high-tech machine and one of the few in the world capable of scanning photographic plates with sub-micron precision (one thousandth of a millimeter). The ultimate goal is to produce digital images of maximum precision in order to extract all useful astronomical information from this type of medium.

The data collected improves our understanding of the long-term astrometric positions of planets, natural satellites, and small bodies. It thus enriches and complements the data from the Gaia space mission for the development of its star catalog. Indeed, the evolution of the positions of bodies over time, which can be observed using photographic plates, makes it possible to refine the dynamics of their movement.

The scientific team works in partnership with numerous national and international observatories with the aim of digitizing their collections for scientific purposes, as was the case with the Greenwich Observatory. The digitized material will also be made available online to the professional community and the general public, with the launch of a participatory science campaign planned for the near future!

Learn more

The CNRS video presentation of NAROO
The photo report by CNRS Images

Contact

Vincent Robert, LTE/IPSA
Vincent.Robert@obspm.fr