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Catastrophic disruption of asteroid 2023 CX1 and implications for planetary defence
No full textInternational audienceMitigation of the threat from airbursting asteroids requires an understanding of the potential risk they pose for the ground. How asteroids release their kinetic energy in the atmosphere is not well understood due to the rarity of large impacts. Here we present a comprehensive, space-to-laboratory characterization of an impact of an L chondrite, which represents a common type of Earth-impacting asteroid. Small asteroid 2023 CX1 was detected in space and predicted to impact over Normandy, France, on 13 February 2023. Observations from several independent sensors and reduction techniques revealed an unusual but potentially high-risk fragmentation behaviour. The nearly spherical 650 +/- 160 kg (72 +/- 6 cm diameter) asteroid catastrophically fragmented at a dynamic pressure of 4 MPa around 28 km altitude, releasing 98% of its total energy in a concentrated region of the atmosphere. The resulting shock wave was spherical, not cylindrical, and released more energy closer to the ground. This type of fragmentation increases the risk of substantial damage at ground level. These results warrant consideration for a planetary defence strategy for cases where a >3-4 MPa dynamic pressure is expected, including planning for evacuation of areas beneath anticipated disruption locations
A new reference surface albedo map of Mars : An improved OMEGA/Mars Express albedo map at 1.08 µm
No full textInternational audienceIntroduction : Knowing the surface albedo of a celestial body is crucial not only for understanding its composition, but also as a key input for radiative transfer models and retrieval algorithms used to derive atmospheric parameters such as aerosol optical depth. Within the MIRS (MMX InfraRed Spectrometer, 0.9–3.6 µm) team, we are specifically investigating the use of surface albedo maps to support upcoming observations of the Martian atmosphere by this near-infrared imaging spectrometer [1]. MIRS is one of the scientific instruments aboard the JAXA Martian Moons eXploration (MMX) mission [2], scheduled for launch in 2026. While the primary goal of MMX is to explore the Martian moons Phobos and Deimos and return samples to Earth by 2031, the mission also includes a significant atmospheric science component. MIRS will contribute to this by performing high-temporal resolution observations of dust storms, ice clouds, and water vapor, thereby improving our understanding of dust and water transport processes in the Martian atmosphere [1]. In preparation for the mission, it is essential to simulate MIRS observations. We are developing a Martian aerosol retrieval algorithms by building a look-up table of Mars atmosphere spectra simulated using a radiative transfer code with different parameters from which we will be able to compare MIRS observations and deducing aerosol properties such as optical depths (see details in [3]). Building this spectra database requires a global albedo map of the Martian surface in the MIRS spectral range (0.9–3.6 µm). One available dataset is the albedo map at 1.08 µm from the OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) instrument onboard the Mars Express spacecraft [4], but it contains some gaps in the data coverage (figure 1.b). The goal of this work is to develop a method to fill these gaps to generate a complete map.Method : This method processes and merges the OMEGA albedo map (figure 1.b) (in near-IR at 1.08 µm) with data from another instrument: MARCI (Mars Color Imager) onboard the Mars Climate Orbiter [5]. Since OMEGA and MARCI have different spatial resolutions, the first step was to ensure that the two datasets could be combined effectively. First, we co-register the MARCI map (figure 1.a) onto the OMEGA grid, since they originally operate on different coordinate systems using 2D linear interpolation. Then we compute the pixel-by-pixel differences between the interpolated MARCI map and the OMEGA map, considering only valid/filled OMEGA pixels which will be used to convert MARCI UV-visible albedo values to OMEGA near-IR albedo values. To address the missing/dark pixels in the OMEGA map (Figure 1b), the script locates the nearest valid pixel for each dark pixel using the Euclidean distance [6]. It then retrieves the corresponding albedo difference from the earlier step based on the closest valid pixel. This local correction is then applied to the MARCI value at the same location, effectively estimating a new albedo value for the previously dark OMEGA pixel, resulting in a corrected OMEGA albedo map without missing data (figure 1.c). After this step, some artifacts, seen as abnormally bright pixels, remain. After filling in the dark pixels, a new difference map is calculated to compare the updated OMEGA and MARCI maps. Then, the focus shifts to correcting these abnormally bright pixels within the latitude range of 60°S to 60°N, which will be of interest for MMX observations. The map is divided into 20x20 pixels blocks, and in any block with abnormally bright pixels, those pixels are replaced using the same method described above for the dark pixels. The application of a Gaussian smoothing [6] in the final step helps to smooth the remaining artifacts by reducing abrupt changes and making the map more physically plausible (figure 1.d).Discussion : This process allows us to produce a complete and reliable albedo map (figure 1.d), especially in high contrast albedo regions. The use of local corrections ensures that regional characteristics are preserved, and the step-wise correction of both dark and bright artifacts enhances the scientific quality of the dataset. However, it’s important to acknowledge that this method involves some assumptions, especially where no OMEGA data existed, and while the corrected values are likely reasonable, they do not come from direct measurements.Figure 1. a) The MARCI albedo map in UV/Visible. b) The OMEGA albedo map at 1.08 µm. c) The OMEGA albedo map after filling all the dark pixels with the MARCI map. d) The final OMEGA albedo map after correcting both dark pixels and white pixels (artifacts), and smoothed with a Gaussian filter. Conclusion : In summary, this method completes and corrects the OMEGA near-IR albedo map using MARCI UV-visible albedo map, to generate a reference OMEGA surface albedo map of Mars at 1.08 µm. The final output is a high-coverage albedo dataset, which will be useful for accurate atmospheric simulations and spectral matching during the MMX mission. This dataset may further benefit diverse planetary science studies, such as investigations of Martian surface composition. It could also support future mission planning by providing improved surface characterization in regions of interest. Acknowledgments :We thank M. Wolff who provided us the MARCI albedo map which came from a personal communication with S. W. Lee. We thank the MMX JAXA teams for their efforts and CNES for the financial support and collaboration to build the MIRS instrument. References :[1] Barucci M. A. et al. (2021) Earth, Plan. and Space, 73, 211.[2] Kuramoto K. et al. (2022) Earth, Plan. and Space, 74, 12.[3] Leseigneur Y. et al. (2025) EPSC-DPS2025, abstract[4] Ody A. et al. (2012) Journal of Geophysical Research, Vol. 117.[5] Genda H. et al. (2024) Earth, Plan. and Space, 76, 8.[6] Virtanen P. et al. (2020) Nat. Methods, 17, 261–272
Optimising reference library selection for reference-star differential imaging of discs with SPHERE/IRDIS
No full textInternational audienceContext. The direct detection of circumstellar discs through high-contrast imaging provides key insights into the history and dynamics of planetary systems. Pole-on discs, especially faint debris discs, are difficult to detect and require careful consideration during post-processing to remove stellar residuals from the data while preserving the disc signal. Reference-star differential imaging (RDI) serves as one of the primary post-processing methods for disc observations; however, the impact of the reference library on the detection sensitivity of discs has yet to be fully explored.Aims. We aim to explore different reference library selection metrics in order to develop a method of reference frame selection that is optimised for pole-on discs to be used for the upcoming large-scale RDI reduction of archival SPHERE/IRDIS observations in the search of new discs.Methods. We performed RDI post-processing based on principal component analysis on 20 targets without discs and with varying observational conditions and seven targets with discs, using reference libraries built from frames that were preselected to best match different observational, atmospheric, and stellar parameters of the science frames. The contrast of the disc-free reductions was measured, and forward modelling was used to estimate the signal loss from over-subtraction using synthetic pole-on discs with two different widths and four different radii. The signal-to-noise ratio (S/N) of the real disc targets was measured.Results. Diverse reference libraries built using subsets of frames that closely matched different parameters achieved the best disc S/N and smallest deviation from the best contrast of each target, outperforming libraries built using a single criteria as a selection metric. Libraries built using frame-to-frame Pearson correlation coefficient alone as a selection criterion achieved the best mean contrast overall. Both selection metrics performed consistently well for all disc radii and observational conditions. We also found that reference libraries built using frames observed close in time to the science frame performed well for discs at small separations, giving the best contrast for ~30% of the targets at a radius of 20 px
Differentiating the Acceleration Mechanisms in the Slow and Alfvénic Slow Solar Wind
No full textInternational audienceIn the corona, plasma is accelerated to hundreds of kilometers per second and heated to temperatures hundreds of times hotter than the Sun's surface before it escapes to form the solar wind. Decades of space-based experiments have shown that the energization process does not stop after it escapes. Instead, the solar wind continues to accelerate, and it cools far more slowly than a freely expanding adiabatic gas. Recent work suggests that fast solar wind requires additional momentum beyond what can be provided by the observed thermal pressure gradients alone, whereas it is sufficient for the slowest wind. The additional acceleration for fast wind can be provided through an Alfvén wave pressure gradient. Beyond this fast/slow categorization, however, a subset of slow solar wind exhibits high Alfvénicity that suggests that Alfvén waves could play a larger role in its acceleration compared to conventional slow wind outflows. Through a well-timed conjunction between Solar Orbiter and Parker Solar Probe (PSP), we trace the energetics of slow wind to compare with a neighboring Alfvénic slow solar wind stream. An analysis that integrates remote and heliospheric properties and modeling of the two distinct solar wind streams finds that Alfvénic slow solar wind behaves like fast wind, where a wave pressure gradient is required to reconcile its full acceleration, while non-Alfvénic slow wind can be driven by its nonadiabatic electron and proton thermal pressure gradients. Derived coronal conditions of the source region indicate good model compatibility, but extended coronal observations are required to effectively trace solar wind energetics below PSP's orbit
PHANGS-ML: The Universal Relation between PAH Band and Optical Line Ratios across Nearby Star-forming Galaxies
No full textInternational audienceThe structure and chemistry of the dusty interstellar medium (ISM) are shaped by complex processes that depend on the local radiation field, gas composition, and dust grain properties. Of particular importance are polycyclic aromatic hydrocarbons (PAHs), which emit strong vibrational bands in the mid-infrared, and play a key role in the ISM energy balance. We recently identified global correlations between PAH band and optical line ratios across three nearby galaxies, suggesting a connection between PAH heating and gas ionization throughout the ISM. In this work, we perform a census of the PAH heating–gas ionization connection using ∼700,000 independent pixels that probe scales of 40–150 pc in 19 nearby star-forming galaxies from the PHANGS survey. We find a universal relation between PAH(11.3 μm/7.7 μm) and ([S II]/Hα) with a slope of ∼0.2 and a scatter of ∼0.025 dex. The only exception is a group of anomalous pixels that show unusually high (11.3 μm/7.7 μm) PAH ratios in regions with old stellar populations and high starlight-to-dust emission ratios. Their mid-infrared spectra resemble those of elliptical galaxies. Active galactic nucleus hosts show modestly steeper slopes, with a ∼10% increase in PAH(11.3 μm/7.7 μm) in the diffuse gas on kiloparsec scales. This universal relation implies an emerging simplicity in the complex ISM, with a sequence that is driven by a single varying property: the spectral shape of the interstellar radiation field. This suggests that other properties, such as gas-phase abundances, gas ionization parameter, and grain charge distribution, are relatively uniform in all but specific cases
The kangaroo's first hop: the early fast cooling phase of EP250108a/SN 2025kg
No full textInternational audienceFast X-ray transients (FXTs) are a rare and poorly understood population of events. Previously difficult to detect in real time, the launch of the Einstein Probe with its wide field X-ray telescope has led to a rapid expansion in the sample and allowed the exploration of these transients across the electromagnetic spectrum. EP250108a is a recently detected example linked to an optical counterpart, SN 2025kg, or 'the kangaroo'. Together with a companion paper (Rastinejad et al. 2025), we present our observing campaign and analysis of this event. In this letter, we focus on the early evolution of the optical counterpart over the first six days, including our measurement of the redshift of . We find that the source is well-modelled by a rapidly expanding cooling blackbody. We show the observed X-ray and radio properties are consistent with a collapsar-powered jet that is low energy ( erg) and/or fails to break out of the dense material surrounding it. The optical emission therefore likely arises from a shocked cocoon resulting from the trapped jet; however, we also examine the possibility that it emerges from the shock produced as the supernova ejecta expand into a dense shell of circumstellar material. We compare to other supernovae and fast transients showing similar features, finding significant similarities with SN 2006aj and SN 2020bvc. This suggests trapped jets could be more common than previously thought and SN 2025kg may herald a larger sample of similar transients
First Identification and Precise Spectral Measurement of the Proton Component in the Cosmic-Ray `Knee'
No full textInternational audienceWe report the first high-purity identification of cosmic-ray (CR) protons and a precise measurement of their energy spectrum from 0.15 to 12 PeV using the Large High Altitude Air Shower Observatory (LHAASO). Abundant event statistics, combined with the simultaneous detection of electrons/photons, muons, and Cherenkov light in air showers, enable spectroscopic measurements with statistical and systematic accuracy comparable to satellite data at lower energies. The proton spectrum shows significant hardening relative to low-energy extrapolations, culminating at 3 PeV, followed by sharp softening. This distinct spectral structure - closely aligned with the knee in the all-particle spectrum - points to the emergence of a new CR component at PeV energies, likely linked to the dozens of PeVatrons recently discovered by LHAASO, and offers crucial clues to the origin of Galactic cosmic rays
Polarization Analysis of Type III Langmuir/Z-mode Waves with Coherent Magnetic Component Observations by Solar Orbiter
No full textInternational audienceObservations from the Solar Orbiter spacecraft provide unique insights into the interaction between electron beams and the plasma background in the source regions of type III radio emissions. We analyze this interaction by examining the high-frequency electric and magnetic components of in situ wave measurements, focusing on their polarization properties. Using electron data from onboard instruments, we model the electron velocity distribution function and numerically solve the dispersion relation. We compare the predicted polarization of the electric and magnetic components with the observations. Our findings are consistent with propagation in the Langmuir/Z-mode at an oblique wavevector. We explain the magnetic component and transverse polarization by the presence of small density fluctuations, without the need for mode conversion
Design of ultrahigh flux components for Yb transportable systems at LTE
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The Paris Meudon ground based support to the NASA Solar Maximum Mission in the eighties
No full textThe Solar Maximum Mission of NASA was one of the first satellites with on board digitization of observations. It was launched for the solar maximum of cycle 21 (1980) in order to study the solar activity. It carried many instruments, such as coronagraphs, X and γ ray detectors, an Ultra Violet spectrometer and a radiometer. Ground based support was offered by many institutes, such as Paris Meudon observatory under the form of systematic observations or coordinated campaigns with specific instruments. We present here the Meudon Solar Tower (MST) and magnetograph which offered in the eighties a major contribution with observations of velocity and magnetic fields of the photosphere and chromosphere, while SMM was observing the transition region and corona above