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For a brief window about 2.1 billion years after the Big Bang, a hyperluminous quasar at the centre of a still-forming galaxy cluster puffed out a halo of X-ray-bright plasma at roughly 20 million kelvin, a temperature the local universe reserves for the intracluster medium that fills mature clusters today. A 634-kilosecond (about 176 hours) Chandra observation of the protocluster MQN01, reported by Andrea Travascio, Sebastiano Cantalupo and collaborators in the second paper of their X-ray series, has now caught that halo in the act of forming. After subtracting the bright point-spread function of the central quasar, the team is left with about 66 net X-ray counts in the soft 0.5 to 2 kiloelectronvolt band, an 8-sigma detection of extended plasma reaching out to at least 30 kiloparsecs, roughly 100,000 light-years, around quasar CTS G18.01.

Mature galaxy clusters hold most of their baryons not in galaxies but in a diffuse, X-ray-glowing plasma called the intracluster medium (ICM). That plasma is what makes clusters visible to X-ray observatories and what gives cluster cores their characteristic pressures, entropies and cooling times. The unanswered question has been when, exactly, that gas first becomes hot. At redshifts above about 3, almost every extended X-ray feature that telescopes have managed to pick out around a forming cluster has come from a radio-loud galaxy, where relativistic jets launder cosmic microwave background photons into X-rays through inverse Compton scattering. The thermal hot-gas component, the one that will become the future ICM, has been the part hiding.

MQN01 is the first clean case where the hot component shows up on its own. The central quasar is radio-quiet, so there is no jet to blame. Photoionization by the quasar cannot account for the morphology or spectrum. After a long checklist of alternatives, the only model that fits is thermal emission from a hot, optically thin plasma in collisional ionisation equilibrium. The detection therefore pins down a stage of cluster evolution that theorists have been modelling but have never before measured directly: cold gas pouring into the deep potential well of a forming halo and being shock-heated to X-ray temperatures as it falls.

The MQN01 field has been on Cantalupo’s target list for almost a decade. It hosts one of the brightest Lyman-alpha nebulae known at high redshift, mapped first by Borisova et al. in 2016 and now extended across more than two arcminutes by deeper VLT/MUSE adaptive-optics pointings. The same field also contains the largest galaxy overdensity seen at z > 3 and the highest concentration of rapidly accreting supermassive black holes of any surveyed protocluster, with six X-ray-detected active galactic nuclei inside a comoving volume of about 16 square megaparsecs (paper I, arXiv 2410.03933).

To follow up on the AGN census, Cantalupo’s Cosmic Web group at the University of Milan-Bicocca obtained 634 kiloseconds of Cycle 23 Chandra ACIS-I time on the field. Paper II, published in Astronomy and Astrophysics on 27 July 2026 and distributed as arXiv 2508.20074, focuses not on the point sources but on what is left when you subtract them. The team built a careful point-spread-function model for each observation, using simulated Chandra PSFs that account for the instrument response and spectral shape of quasar CTS G18.01, and subtracted it from the merged soft-band image.

What emerged is a soft, isotropic glow that follows a classical beta-model surface density profile with a steep inner slope (beta approximately 2.1) and a core radius near 36 kiloparsecs. Joint spatial and spectral fitting pins the gas temperature at kT about 1.8 plus or minus 0.4 kiloelectronvolts, which is about 20 million kelvin. The implied virial halo mass is about 3 plus or minus 1 times 10 to the 13 solar masses, and the hot gas inside the virial radius carries about 2.6 times 10 to the 12 solar masses, roughly 56 percent of the cosmological baryon budget expected for a halo of that mass.

The INAF press release of 27 July 2026 and a German-language scinexx write-up on 20 August 2026 popularised the work as the first clear image of a forming cluster atmosphere. A Space.com piece by Robert Lea on 29 July 2026 highlighted the temperature comparison: at 36 million degrees Fahrenheit the gas is hotter in absolute terms than anything in today’s nearby clusters, while being at an earlier point in its cooling life.

The detection method is the conceptual heart of the paper. Quasar CTS G18.01 dumps most of its 0.5 to 2 keV photons into a single pixel region, but a few percent leak into the surrounding 2- to 4-arcsecond annulus, which at z = 3.25 corresponds to about 15 to 30 kiloparsecs in projected distance. The team’s approach is borrowed from studies of local Seyfert galaxies: build a high-fidelity PSF from CIAO’s simulate_psf tool, weighted by the per-observation exposure and spectral shape, subtract it from the data, and look at what is left. The annulus holds 172 total counts in the 0.5 to 10 keV band and 96 in the soft band. After PSF subtraction, about 66 net counts remain, a significance near 8 sigma.

The spectral and spatial fits are performed jointly with an MCMC walk using emcee. The thermal component is modelled with xsmekal, a plasma code that includes bremsstrahlung, recombination continuum and line emission for highly ionised species in collisional ionisation equilibrium. Metallicity and temperature are tied across four concentric radial bins, while the normalisations scale with the surface density of a beta-model profile. The best fit yields kT approximately 1.8 plus or minus 0.4 keV and a hot gas mass fraction inside the virial radius of about 8 percent of the dynamical mass, or 56 percent of the available cosmological baryon budget.

The pressures and densities implied by that fit are one to two orders of magnitude above today’s clusters. Cooling times in the inner 15 to 30 kiloparsecs sit between 1 and 10 times the local free-fall time, which means the gas would be locally unstable if there were no heating source. The team argues that gravitational shocking, cool gas dropping into the halo’s deep potential well and being heated by the accretion shock, is enough to do the work. Quasar feedback cannot be ruled out, but the soft X-ray spectrum and the lack of any excess in the hard band make a feedback-dominated scenario hard to sustain.

The same field carries a 200-kiloparsec Lyman-alpha nebula mapped by Borisova et al. 2016 and recently extended with MUSE AO mosaics. The soft X-ray emission sits inside the inner 6 times 10 to the minus 18 erg per second per square centimetre per square arcsecond contour of that nebula, suggesting that the hot phase coexists with the cold phase rather than displacing it. The thermal pressure of the X-ray-emitting plasma is high enough to confine the cold clumps that produce the Lyman-alpha emission, which is one of the better pieces of evidence yet that the multiphase circumgalactic medium at cosmic noon can be pressure-confined rather than radiation-confined.

The combined view pairs the wide-field F150W2 mosaic of the protocluster with a zoom panel in which the Lyman-alpha nebula mapped by Borisova et al. 2016 appears in blue and the new soft-X-ray halo appears in red. The black disc in the zoom masks the quasar itself.

A radio-quiet quasar 11 billion light-years away has given astronomers the first direct look at hot, X-ray-bright plasma in the act of becoming the intracluster medium. The Chandra data show about 100,000 light-years of 20-million-kelvin gas around the quasar, with the pressure and density expected for a halo that is still accreting cool gas through gravitational shocks. Whether MQN01 is a typical waypoint for protoclusters or an unusually bright outlier is the open question; the team is working through archival X-ray data on hundreds of additional high-redshift quasars and has Atacama Large Millimeter/submillimeter Array time queued to test the gas conditions independently. If the picture holds up across a larger sample, the textbook story of how cluster atmospheres form will finally have a measurement at the epoch where it begins.

There is a version of our galaxy’s biography in which the Milky Way built itself quietly out of its own gas, and only later began collecting debris from smaller neighbours. That version is now much harder to defend.

On 17 August 2026, a team led by Davide Massari of the Astrophysics and Space Science Observatory of Bologna published evidence in Nature Astronomy that the young Milky Way absorbed a dwarf galaxy roughly 11.8 billion years ago, about two billion years after the Big Bang. The dwarf itself is gone, torn apart and mixed into the inner galaxy long before Earth existed. What survives is a set of globular clusters, dense balls of tens of thousands to a few million stars, whose ages and chemical compositions do not match anything the Milky Way could have made on its own.

The team named the vanished progenitor Low-energy-Kraken-Heracles, or LKH, a name that stitches together three separate strands of earlier work that had each argued for an early merger without being able to date it. As the ESA/Hubble release puts it, the result extends the firm merger history of our galaxy 1.8 billion years farther back than it reached before.

Galaxies in the standard picture grow hierarchically. Small systems fall together under gravity, the larger one strips the smaller apart, and the victim’s stars are redistributed through the survivor. The stars keep their chemical fingerprints and, to a degree, their orbits, so a galaxy carries a fossil record of everything it has eaten. That idea is not new, but confirming it in our own galaxy took decades of survey work: the 2026 Kavli Prize in Astrophysics went to Vasily Belokurov, Amina Helmi and Rodrigo Ibata for exactly this, uncovering the fossil evidence that the Milky Way was built through hierarchical accretion.

Until now the confident part of that record stopped around 10 billion years ago, at the merger with the dwarf galaxy Gaia-Sausage-Enceladus (GSE), which rearranged the structure of the galactic disk. The most recent large merger, with the Sagittarius dwarf, began more than 6 billion years ago and is still in progress. Earlier than GSE, the picture went soft. When the Milky Way was young it was small, so an incoming galaxy was not a minor satellite but a comparable building block, and the collisions were correspondingly messy. Signatures from that era have had billions of years to be smeared out.

That is the epoch where galaxy formation is currently most interesting and least constrained. The James Webb Space Telescope can image galaxies as they looked more than 12 billion years ago, but rarely resolves their individual stars. Galactic archaeology works the other way around, examining surviving stars in exquisite detail without ever seeing the galaxy they came from. Sven Buder of the Australian National University framed the pairing neatly in The Conversation: distant astronomy gives snapshots of young galaxies, galactic archaeology gives their fossils.

The team analysed Hubble observations of 39 globular clusters within the inner 20,000 light-years of the galaxy, the region where traces of the most ancient mergers should still be sitting. Going in, they expected two families: clusters that formed inside the young Milky Way, and clusters delivered by GSE about 10 billion years ago.

They found three. Plotting each cluster’s precise age against its metallicity, the abundance of elements heavier than helium, produced three separate age-metallicity sequences rather than two. One tracks the Milky Way’s own progenitor. One tracks GSE. Between them sits a third sequence whose clusters are consistently older than the GSE group and younger than the clusters born in place, regardless of their metal content.

An intermediate sequence is not an ambiguous result. Two overlapping populations can blur together, but a distinct third track implies a distinct chemical history, which means a distinct galaxy. From the sequence’s position the team estimated that the merger happened about 1.8 billion years before GSE, and that the incoming system carried roughly 500 million times the Sun’s mass in stars, comparable in stellar mass to GSE itself. Most of that material was deposited within the inner 6 kiloparsecs, which is why the evidence shows up in the bulge region rather than the outer halo.

“Our home is the Milky Way galaxy, but we do not know how our house was built,” Massari said in the NASA release. “In this paper we discover where the first significant batch of bricks came from: a dwarf galaxy that we call LKH.”

The naming is a deliberate act of bookkeeping. Previous work had identified a “Kraken” merger, a bulge population called “Heracles”, and a low-energy group of globular clusters, all pointing at an early accretion event from different directions and all difficult to reconcile with each other. By folding the three labels into one, the authors are arguing that the earlier claims were describing the same object with different instruments, and that the debate over whether such an event happened can now close.

Co-author Chiara Zerbinati of the University of Bologna credited the instrument pairing. High resolution and deep Hubble imaging gave the ages and metal content at unprecedented precision, she said, and combining that with Gaia measurements made it possible to separate one population of clusters from the rest.

The measurement rests on a specific and unglamorous piece of stellar astrophysics. A globular cluster’s stars formed at roughly the same time from the same gas, so on a colour-magnitude diagram they trace a single main sequence with a turnoff point where the most massive surviving stars are just exhausting their core hydrogen. The luminosity of that turnoff drops as the cluster ages, which makes it a clock. Reading it in absolute terms is hard, because the answer depends on distance, reddening by intervening dust, and the physics inside stellar models. Reading it in relative terms across many clusters observed by the same instrument in the same filters is considerably more robust, because the systematic errors largely cancel.

That is the leverage here. The absolute age of any one cluster still carries substantial model uncertainty, but the ordering of clusters relative to each other can be pinned down far more tightly, and ordering is what separates two age-metallicity sequences that overlap in metallicity. Hubble’s stable, well-characterised photometry over decades of archival imaging is what makes the relative comparison possible at all.

Metallicity supplies the second axis. A galaxy enriches its own gas as generations of stars produce heavier elements and die, so within a single galaxy age and metallicity climb together along a characteristic curve. A small galaxy enriches slowly and at low metallicity; a massive one runs the sequence faster and further. The shape of a sequence therefore encodes the progenitor’s mass, which is how a 500-million-solar-mass estimate comes out of a set of star cluster ages.

Gaia contributes the dynamics. Orbital energies and angular momenta computed from its astrometry let clusters be sorted by where they sit in the galactic potential, and the low-energy, tightly bound population is precisely the one expected from a merger that dumped its mass into the inner galaxy.

The limits are honest ones. Globular clusters are an incomplete census, since some early galaxies made few and others lost the ones they had, and turning a sequence into a progenitor mass requires models.

The concrete claim is narrow and testable: three sequences, 39 clusters, one merger dated 1.8 billion years before GSE, one progenitor mass near 5 x 10^8 solar masses. Fernando Aguado-Agelet of the University of Vigo and the University of La Laguna noted that Hubble is now observing globular clusters that have never been studied before, which is the obvious way to test whether a fourth sequence is hiding in the sample. The team’s stated goal is to characterise every massive merger in the galaxy’s history by this method.

The broader consequence lands on an argument about the first two billion years. Some earlier work held that the Milky Way’s earliest phase was defined by stars born in place. If a galaxy of GSE’s stellar mass arrived at 11.8 billion years ago, then accreted stars were part of the inner galaxy from nearly the beginning, and any model of the bulge that assumes purely local formation is missing a component that was there before the disk settled.

Astronomers who study the oldest stars in the Milky Way spend a lot of time reading chemical barcodes. When those stars formed, the universe had not yet built up much beyond hydrogen, helium, and a thin seasoning of lithium, so almost every heavier element in their atmospheres had to be made inside an earlier generation of stars and then scattered. The relative amounts of those elements tell you which stellar process actually did the forging, and one element in particular, strontium, has refused to behave. New measurements of a nuclear reaction on a related element, krypton-88, now narrow the gap between what the models predict and what the oldest stars show, and the result, published in Communications Physics on 8 June 2026 by Caley Harris, Artemis Spyrou, and a team of collaborators from twelve institutions in the United States, Canada, and Europe, was carried out at the Facility for Rare Isotope Beams (FRIB) at Michigan State University and the ATLAS accelerator at Argonne National Laboratory (FRIB newsroom; Nature paper). What made the experiment possible was a workaround for one of the hardest problems in nuclear astrophysics: most of the reactions that matter for stellar chemistry happen too rarely, and on too short-lived an isotope, to fire a beam straight at a target and count the products. The team measured a different isotope instead and worked backwards.

Why strontium is interesting starts with the fact that the periodic table heavier than iron cannot be built by normal stellar burning. Iron sits at the bottom of the curve of binding energy per nucleon, so fusing more nuclei releases less energy than it costs, and stellar cores cannot climb back up that hill. The solution, laid out in a famous 1957 review by Burbidge, Burbidge, Fowler, and Hoyle, was neutron capture. If a stellar environment can flood a seed nucleus with neutrons fast enough, the nucleus swallows them one at a time and walks up the chart before it has time to beta-decay. That framework identified two regimes: the slow neutron-capture process, or s-process, which works at relatively low neutron densities on long timescales, and the rapid neutron-capture process, or r-process, which runs at extreme neutron densities during, for example, neutron star mergers. Together, those two accounted for nearly everything heavier than iron. By the 1990s, however, surveys of metal-poor halo stars started turning up abundance patterns that neither the s-process nor the r-process could reproduce. Some of those stars had roughly solar amounts of strontium, yet barely any of the heavier elements that should have come along with it under either classical process. In between the two regimes sits a third option, the intermediate neutron-capture process, or i-process, with neutron densities around 10^13 to 10^15 neutrons per cubic centimeter and timescales measured in minutes rather than years. i-process models explain many of these strange patterns, but they consistently come up short on strontium, by factors of several, compared to what the old stars actually show, and modelers had flagged one specific reaction, the neutron capture on krypton-88, as the largest single uncertainty.

The team installed FRIB’s Summing NaI detector, called SuN, at the ATLAS facility to make that measurement. They fired a beam at a target to produce krypton-89, which is krypton-88 with one extra neutron glued on, then watched the gamma rays that krypton-89 emits as it relaxes. Those gamma-ray energies and intensities carry information about the structure of the nucleus and, crucially, about how readily krypton-88 would absorb a neutron in the first place. It is the same logic doctors use when they infer blood chemistry from a downstream signal rather than a direct needle stick: the measurement is indirect, but the inference is solid if the underlying physics is well understood. The result was not what theory had predicted. The measured krypton-88 neutron-capture rate came in consistently lower than the values most nuclear-theory calculations had been using, in some temperature regimes by nearly an order of magnitude. In parallel, the team’s reaction-rate uncertainty dropped from at least a factor of eight down to about a factor of three, which is still wide but no longer the dominant term in the strontium calculation. When the team plugged the new rate into three different i-process models, all of them produced more strontium than before, and the agreement with the stellar observations improved in every case.

The deeper question is what is happening inside the krypton-88 nucleus to slow down the neutron capture. Neutron-capture cross sections depend on the density and spacing of the nuclear energy levels just above the neutron separation energy. If those levels happen to be sparse or have the wrong parity, the incoming neutron has a hard time finding a resonance to grab onto, and the reaction rate drops. Direct measurements are impossible because krypton-88 itself is unstable, so the team turned to its more stable neighbor krypton-89 and used the Oslo method, a well-established technique that extracts level densities and gamma-ray strength functions from a single set of gamma-ray spectra. The SuN detector’s whole purpose is to capture those spectra efficiently; ATLAS provides a clean krypton-89 beam; together the two facilities make the indirect measurement practical. The takeaway is that nuclear structure details in one isotope can shift stellar predictions by a factor of two or more, and the only way to know which way the shift goes is to measure.

It is worth pausing on why strontium specifically shows up so much in this story. Strontium is the lightest element whose production has historically been a clean observational test of which neutron-capture process was active. Heavier elements like barium or lead have so many overlapping production paths that the signatures blur, but strontium sits near the edge of the s-process distribution and far from the r-process peak, so its abundance is sensitive to whatever process is producing the seeds in the middle neutron-density regime. Old stars with strong carbon and slow-neutron features but unusual strontium-to-barium ratios are exactly the kind of objects the i-process was invented to explain, and the FRIB result is the first time an experiment has directly tested the nuclear physics input rather than relying on theoretical estimates. What happens next is mostly back in the hands of the modelers, as Falk Herwig, a co-author at the University of Victoria, framed it in the press release: with the main nuclear uncertainty addressed, the i-process community can turn to the astrophysics, the neutron densities, the timing of the burning episodes inside candidate stars, and the conditions needed to match what the oldest stars actually show. Candidate i-process sites include very low-metallicity asymptotic giant branch stars, post-AGB stars, rapidly accreting white dwarfs, and the helium core flash in low-mass stars, and the new measurement gives all of them a more honest set of inputs to work with. The paper does not claim to have closed the strontium question, only to have removed the single largest nuclear physics unknown behind it. The remaining gap is astrophysical, and that is what the field will spend the next several years tightening.

For decades the thermosphere, the broad band of electrically neutral gas that sits between roughly 100 and 1,000 km above the ground, has been one of the most expensive places on Earth to study. A handful of in-situ instruments on satellites like ESA’s SWARM constellation have nibbled at it one orbit at a time. Atmospheric models fill the gaps, but with rising space traffic the gaps have grown political: collision avoidance, reentry forecasting, and space-weather alerts all need accurate thermospheric density on short timescales, and the measurements feeding those models are sparse. A team at Kyoto University has now shown that the same orbital data SpaceX publishes for free to coordinate its Starlink constellation can be turned into a continent-scale thermospheric density snapshot, using a method borrowed from medical imaging. The result, published 30 July 2026 in Earth, Planets and Space, is the first latitude-by-longitude map of thermospheric density reconstructed entirely from the orbital decay of commercial broadband satellites.

The thermosphere matters because every spacecraft in low Earth orbit lives inside it. Even at 500 km altitude the gas is thin enough that an astronaut on an EVA suit would consider it vacuum, but it is dense enough that a 250-kilogram satellite experiences a steady, measurable aerodynamic drag. That drag pulls satellites down, which is why the International Space Station and thousands of debris fragments eventually reenter. It is also why precise orbit prediction is so hard: a small change in thermospheric density can shave minutes off a predicted pass, and during a geomagnetic storm a single high-speed plasma plume from the Sun can change density at satellite altitude by a factor of two in less than an hour.

The economic stakes have grown with Starlink, OneWeb, Kuiper, and the Chinese Guowang constellation. As of mid-2026 there are roughly 9,000 active satellites in low Earth orbit, with another 50,000 pieces of tracked debris larger than 10 cm. Every collision risk assessment, every conjunction warning, every debris-reentry prediction depends on knowing how dense the air is along each object’s path. Today, most operational systems rely on either accelerometer data from a few sentinel satellites or on empirical density models calibrated against historical storms, which means they tend to miss local structure in the thermosphere: the day-night asymmetry, the geomagnetic-latitude bite, the traveling atmospheric disturbances that follow a sunrise terminator around the globe.

A method that turns routine commercial satellite tracking into a near-real-time thermospheric density map would change that. It would also change what counts as “ground truth” for the physics of the upper atmosphere: the same data set that lets SpaceX coordinate its own orbital slot allocation could now be re-used by every atmospheric scientist with a Python notebook and access to a public ephemeris feed.

The Kyoto University group, led by space engineer Mamoru Yamamoto with first author Takuya Sori, had already pushed in this direction. In an earlier paper they showed that they could estimate how thermospheric density varies over time and altitude using public Two-Line Element (TLE) data, the short text records that amateur satellite trackers use to find the ISS. The technique was time-honored: a satellite in a decaying orbit loses energy at a rate set by the local air density, so inverting the decay curve gives density as a function of height and time.

The 2026 paper takes the next step. Instead of treating each satellite as a single measurement, the team treated the constellation as a moving grid of sensors. For roughly 1,200 Starlink satellites at an altitude of 482 km, they calculated orbital energy loss from publicly broadcast ephemeris messages over a chosen interval and fed the resulting drag estimates into a tomographic inversion. Tomography, the same family of math that turns a CT scanner’s one-dimensional projections into a 3-D image, is good at reconstructing a 2-D field from many intersecting 1-D lines. Here the lines are satellite ground tracks across the globe.

The reconstruction, anchored on a single day of data from 1 September 2025, produced a full two-dimensional density map of the thermosphere at roughly 500 km altitude, with peaks over the sunlit mid-latitudes and lower density on the night side. When the Kyoto team compared their tomographic result against density profiles recorded along their tracks by the European Space Agency’s SWARM satellites, the agreement was within the noise floor of the comparison: both showed the same high-density lobes over the daytime side of the Earth, and the same minima near the dawn terminator.

The work, formally titled “Tomography of thermospheric density from Starlink Ephemeris: initial report”, appeared in Earth, Planets and Space on 30 July 2026. Kyoto University’s research news office published a press release on 4 August, and the result was picked up across space and Earth-science outlets including ScienceDaily and Phys.org. A KURENAI repository copy is available without paywall.

The “ephemeris” in the new paper is not the same thing as a TLE. A TLE is a compact orbital state vector published by the U.S. Space Force’s Space-Track catalog; it is good to roughly a kilometer of position accuracy and is updated a few times a day. SpaceX publishes a higher-precision ephemeris for each Starlink satellite, broadcast continuously over the same radio link the satellites use for internet traffic, which includes predicted position and velocity to a fraction of a meter. It is the kind of data that, until now, was treated as operational infrastructure rather than science data.

Drag at 482 km is dominated by collisions with atomic oxygen, the dominant species in the upper thermosphere. The momentum transfer rate per unit density for a satellite at typical Starlink ballistic coefficients has been measured in orbit many times; the Kyoto team leans on those calibrations rather than computing from first principles. The bigger problem is geometry. A single satellite samples density along a single line on the globe. To reconstruct a 2-D field you need many satellites crossing many different latitudes and local solar times in the same interval. With about 1,200 Starlinks in a near-polar orbit that circles the Earth every 90 minutes or so, the constellation completed more than a hundred full ground tracks during the chosen 24-hour window, which is enough to populate every 5-degree latitude-longitude cell at least a few times.

The “tomographic inversion” itself is a weighted least-squares fit with a smoothness prior. Weights come from each satellite’s expected drag uncertainty (a function of mass, frontal area, and atmospheric composition model), and the smoothness prior prevents the optimizer from inventing small-scale structure the data cannot resolve. The output is a single snapshot, not a movie. A time series would need the inversion repeated on a rolling window.

Independent validation mattered. The European Space Agency’s SWARM mission, launched in 2013, has been measuring thermospheric density along its three orbital tracks for over a decade. SWARM reads a non-gravitational acceleration directly with an onboard accelerometer, which is the gold standard for in-situ density measurements. On 1 September 2025 SWARM’s tracks happened to fall inside the latitude-longitude cells the Kyoto inversion had populated. The two density fields agreed at the level expected from SWARM’s own noise floor, plus some extra uncertainty the Kyoto team acknowledged from their assumption that every Starlink satellite has the same effective ballistic coefficient.

The remaining bottleneck is uniform ballistic coefficient. Real Starlinks vary slightly in mass and orientation, and not all of them have their ephemeris precision documented publicly. The Kyoto group flags this as the next refinement to chase.

Three numbers summarize the work: 1,200 satellites, 482 km altitude, one day of data. Together they describe the smallest possible demonstration that a global commercial broadband network can be repurposed into a free, passive, science-grade sensor for the upper atmosphere.

A near-real-time implementation would need only an automatic ephemeris ingestion pipeline and a routine inversion, something the team has indicated they are working on. If such a pipeline goes operational, atmospheric modelers will have far more ground truth on the quiet-day thermosphere than they have today, and the same data will feed directly into space-weather products that already serve satellite operators.

For now the result is a proof of concept: enough to show it works on one day, not yet a system that rewires upper-atmosphere science.

Lana Williams, a PhD researcher at Lancaster University, took the stage at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham on July 21 with a result that nobody had set out to find. Her team had been doing a routine comparison: does a solar energetic particle (SEP) event (a pulse of high-energy protons and ions flung out by a solar flare or coronal mass ejection) change the temperature of Mars’ lower atmosphere? Of the five long-duration SEP events the team examined, four showed nothing. The fifth, which arrived while the 2018 planet-encircling dust storm was still expanding, showed a measured warming of roughly 50 °C (about 90 °F) at altitudes of 75 to 125 km. That is a large signal in atmospheric physics, and it appeared in the one event of five where Mars’ surface was already being scoured by a global dust storm (Royal Astronomical Society press release; Phys.org coverage).

The team used two complementary spacecraft that are normally thought of as studying different altitudes. NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter, which has been watching the Sun-Mars interaction since 2014, contributed observations of how the incoming particles deposited their energy in the upper atmosphere. ESA and Roscosmos’ ExoMars Trace Gas Orbiter (TGO), in operation since October 2016, contributed the simultaneous thermal profile of the middle and lower atmosphere. By stacking the two datasets against the Mars Climate Database, the team could see whether observed temperatures diverged from the climatological mean (Sci.News).

Mars is not a protected planet. With no global magnetic field, its upper atmosphere takes the solar wind and SEP events head-on, and the planet’s atmospheric escape rate has been one of the long-running questions in comparative planetology. A radiation-driven temperature pulse in the lower atmosphere is a different kind of signal than the better-studied escape of hydrogen and oxygen from the top of the atmosphere. It means energy deposited above is being communicated downward into the region where dust, clouds, and weather live, and the existing models of that region do not necessarily assume such a connection.

If the pattern holds, the result has practical consequences. Dust storms are the worst time to be operating solar-powered equipment on the surface, and they already coincide with the kind of optical depth that ends rover missions, the 2018 storm being the one that finally silenced Opportunity. Adding a SEP-driven heating pulse on top of an already-stressed atmosphere changes the radiation environment and the upper-tropospheric temperature profile, with consequences for any future crew or uncrewed habitat. The work also offers a partial answer to a question that has been quietly bothering Mars atmospheric scientists: the models did not always match the 2018 TGO temperature record as well as one would like, and a SEP component may explain some of that residual.

Williams and collaborators at the University of Leicester and the Instituto de Astrofísica de Andalucía–CSIC in Granada selected five SEP events spread across the period when MAVEN and TGO were both returning science data. The event selection was the careful part: the team required each SEP to be long-duration, so that any thermal response would have time to propagate, and required both spacecraft to be making compatible observations at the same time. The first four events, occurring in quieter atmospheric conditions, did not produce a statistically meaningful divergence from the Mars Climate Database’s predicted temperature profile. In the team’s own words, “we expected that these highly energetic particles might have some effect on temperatures in Mars’ lower atmosphere, but in four of the five events we studied we found no clear evidence of heating.” The fifth, in June 2018, broke the pattern by a wide margin (Royal Astronomical Society press release; Tech Times).

The June 2018 event is the storm that ended Opportunity’s mission. NASA formally declared the rover’s end in February 2019 after more than a thousand recovery commands went unanswered, and the global dust storm that began on May 30, 2018 grew into a planet-encircling event by mid-June, with optical depths in some regions reaching tau values of 5 to 10, enough to blot out direct sunlight at the surface (2018 Mars global dust storm, Wikipedia; Space.com retrospective). It is also the same storm during which MAVEN’s Neutral Gas and Ion Mass Spectrometer measured water vapor 20 times higher than usual, lifting moisture to altitudes where it could be broken apart by ions and lost to space. That measurement was published in 2020 and is now the standard reference for the role of dust storms in Mars’ long-term water loss (NASA SVS infographic, MAVEN team).

A second-order detail has to be acknowledged. NASA’s MAVEN lost contact with Earth on December 6, 2025, and a February 2026 review board concluded the spacecraft was not recoverable. NASA formally ended the mission in May 2026 (MAVEN, Wikipedia; Reuters, May 2026). The Lancaster study relies on archived MAVEN data, so the analysis itself is not affected by the spacecraft’s loss, but the result arrives at a moment when the only orbiter dedicated to studying the Sun-Mars interaction is no longer in operation. Future SEP-Mars coupling studies will lean even more heavily on TGO and on Mars Reconnaissance Orbiter’s continuing aeronomy observations.

The 50 °C / 90 °F warming at 75 to 125 km altitude is not, on the face of it, what one would expect. SEPs deposit their energy high in the atmosphere, mostly through ionization of CO₂, which is the dominant species on Mars. In a quiet atmosphere, that energy is radiated away or conducted to space faster than it can build a temperature signal in the middle atmosphere. The Lancaster team’s working hypothesis is that the suspended dust changes the radiative budget of the lower and middle atmosphere. Dust absorbs upwelling infrared and re-emits it; it also provides a denser population of ice-coated grain surfaces on which water can condense. The net effect is to keep the middle atmosphere warmer than the climatology would predict, so when an SEP event dumps a pulse of energy from above, the column is already closer to a thermal threshold and the additional heating becomes visible. This is a “two stressors, one signal” pattern that has analogues on Earth in stratospheric ozone chemistry and aerosol-radiation interactions, but the Mars case is cleaner because the two drivers are so well separated in time and altitude (Gizmodo coverage of the talk; Phys.org).

For the engineering side, the result matters because MAVEN and TGO were not designed to look for this signal. MAVEN’s strength is in-situ ion and neutral measurements in the upper atmosphere, with instruments like NGIMS and IUVS. TGO carries NOMAD and ACS for atmospheric spectroscopy, plus a radio-science experiment that uses the spacecraft’s occultation link to derive vertical temperature profiles. Putting the two together for an SEP-dust synergy study is the kind of cross-mission analysis that only became possible once both archives had matured. The lesson for future Mars exploration architecture is the same one that keeps coming back: the value of long-lived orbital assets is realized years after their prime mission ends.

A methodological caveat is worth flagging. The Lancaster team is presenting one matching event out of five. The result is suggestive but it is not yet a population-level claim. The Royal Astronomical Society’s framing, “first study hints,” is doing real work in the press release (Royal Astronomical Society press release). Confirming or refuting the synergy will require a second matching event, and global dust storms that produce them happen roughly once every three to four Mars years, so the next opportunity may not come until late in Solar Cycle 26 or into Cycle 27.

The take-home is that Mars’ lower atmosphere is not a passive target for whatever the Sun and the surface throw at it; it is a coupled system in which a dust-driven radiative perturbation can change the visibility of a solar-driven energetic perturbation. That coupling was not in the models in 2018, and it should be in the next round. The Lancaster team has put a concrete, well-instrumented case on the table and is asking the field to plan for the next match. The answer will come on a Mars year measured in years, not months, and the MAVEN and TGO archives are now the legacy infrastructure on which that answer will be built.

On July 9, 2026, the Federal Communications Commission granted a license to a small California company called Reflect Orbital to launch a satellite the size of a dining table, fold out an 18-meter sheet of mirrored Mylar 625 km above the ground, and aim a moving five-kilometer-wide spot of sunlight at whatever patch of Earth it wants to illuminate. The license covers a single satellite, Eärendil-1, and a single demonstration pass. The wider plan is as many as 50,000 such spacecraft by 2035.

The FCC’s order authorized radio spectrum for the spacecraft and the data link that will run the test. It did not authorize the act of bouncing sunlight at people, because that act is not a communications function and the agency does not regulate light. That distinction is the hinge on which the entire controversy swings, and it is why the American Astronomical Society called itself “dismayed” within hours of the announcement (The Debrief, July 16, 2026).

Astronomy is one of the few sciences whose primary signal is the absence of unwanted light. Ground-based optical telescopes depend on a sky that is, in their exposure windows, mostly dark. The Vera C. Rubin Observatory in Chile is currently running the largest astronomical survey ever attempted, stacking thousands of wide-field exposures from a 3.2-gigapixel camera over ten years. Every photon that does not come from a star, galaxy, or asteroid is a contaminant. Each Starlink train and imaging constellation has already cost the field time and money, and astronomers have spent the past five years learning how to subtract that signal from their data.

Eärendil-1 is a different kind of contaminant. It is not a point source streaking through a single exposure. It is a deliberate, modulated light source that the operator can point at any ground patch within range. Tony Tyson, the chief scientist of the Vera C. Rubin Observatory, told a National Academies meeting on June 4 that he considered the plan “even crazier” than the broadband constellations, because the thin-film reflectors would scatter sunlight rather than aim it precisely. “Imagine the sky full of moons,” he said (SpaceNews, July 10, 2026).

The European Southern Observatory, which operates some of the largest optical telescopes on Earth in Chile’s Atacama Desert, weighed in on July 1 with a calculation: a full 50,000-satellite Reflect Orbital constellation would multiply the background sky brightness at its facilities by a factor of three to four. Betty Kioko, an institutional affairs officer at the observatory, called the technology “an existential threat” to optical astronomy. Reflect Orbital’s FCC application drew nearly 1,900 public comments, almost all of them critical, before the agency ruled.

Reflect Orbital is based in Hawthorne, California, a few miles from SpaceX headquarters. The company calls itself “the sunlight company.” Its premise is that the Earth’s day-night terminator is, from a commercial standpoint, a wasted hour at the start of every morning and another at the end of every evening, and that an orbital mirror could redirect a small slice of sunlight onto a solar farm or work site during those hours. The chief executive, Ben Nowack, has said the satellites could extend the operating hours of terrestrial solar plants before sunrise and after sunset.

The FCC order, issued in the standard format used for experimental satellite licenses, gives the company permission to use the radio frequencies needed to control the spacecraft and telemeter its health data back to the ground. It does not, as the agency itself noted, authorize the optical payload. The 142-kilogram Eärendil-1 will carry a thin-film square reflector 18 meters on a side, made of aluminized Mylar, folded into the satellite’s body for launch and unfurled once the spacecraft reaches an altitude of 600 to 650 kilometers in a near-polar orbit inclined at 88 degrees. Reflect Orbital has said the satellite will ride to orbit on a SpaceX rideshare mission later in 2026.

The operational concept is to keep the reflector pointed at the Sun while continuously tilting its face so that the reflected beam sweeps across the dark side of the Earth below. A single satellite at 625 km can hold a five-kilometer-wide spot on the ground for several minutes at a time. With dozens or hundreds of satellites spaced around the orbit, the spot could be passed from one to the next and held for hours. The company has also said the light produced is “not bright enough to start fires or harm eyes, even when viewed through a telescope, and cannot be concentrated past maximum natural sunlight irradiance,” and that it will maintain exclusion zones around astronomical observatories and turn the beam off instantly on command.

The American Astronomical Society’s statement, released the same day as the FCC decision, was direct. It warned of damage to sensitive research instruments and flash-blinding risks to pilots and drivers, and noted that the company’s own FCC filings had included language about the possibility of permanent eye damage to anyone looking through a mid-sized telescope at the wrong moment. The society cited research suggesting that a full constellation could double or triple nighttime sky brightness even at remote observatories through atmospheric scattering of the reflected light.

The FCC’s response to those arguments was procedural. The agency’s order concluded that “concerns about Eärendil-1’s impacts on optical astronomy fall outside our review and authorization of the space station and are not a basis for denial of or additional conditions on Reflect Orbital’s operations.” It added that the company had committed to working with NASA and the National Science Foundation to address astronomical concerns. The agency framed its role narrowly: it regulates spectrum, not photons, and concluded that making spectrum available for new space activities serves the public interest.

A flat reflector in orbit cannot turn night into day, but it can shift the natural terminator. The Earth rotates once every 23 hours and 56 minutes, so any point on the equator spends roughly twelve hours in sunlight and twelve in darkness. A satellite in a sun-synchronous orbit at 625 km circles the planet about fifteen times a day, and on roughly half of those passes the spacecraft is on the lit side. The geometry that matters is the specular reflection angle: the mirror must be oriented so its normal vector bisects the angle between the incoming Sun and the outgoing ground spot, and that angle changes continuously as the satellite moves along its orbit.

The reflector itself is the simplest part. Aluminized Mylar at this scale has a mass per unit area of roughly 50 grams per square meter [unverified], so an 18-by-18-meter sheet weighs on the order of 16 kilograms. The hard engineering problems are attitude control and pointing stability. To hold a five-kilometer spot from 625 km up, the angle of the mirror must be controlled to within roughly 0.1 degrees [unverified]. Any vibration, thermal distortion, or gravitational sag in the unfurled sheet will smear the spot.

The scale of the proposed constellation is what worries astronomers most. Fifty thousand satellites at 625 km would put roughly one reflector every 100 kilometers along every orbit. Even with each satellite dark for half its orbit, the cumulative effect on the night sky would be a steady, moving pattern of bright objects that no terrestrial observatory could subtract out with software. The light is not radio interference, which can be filtered; it is broadband visible light in the same band the telescopes are trying to measure.

The FCC has approved a single test satellite, not a constellation. Reflect Orbital has until the end of 2026 to fly Eärendil-1, deploy its reflector, and demonstrate that the beam can be aimed precisely enough to stay inside the intended target zone. If the demonstration succeeds, the company will return to the FCC and likely to other agencies for the spectrum, orbital debris, and launch approvals needed for a full constellation. If it fails, the controversy loses its anchor. In the meantime, the American Astronomical Society, the European Southern Observatory, and the Vera C. Rubin Observatory argue that the test alone has already shifted the burden of proof, because the same engineering that lets one satellite hold a beam on a five-kilometer spot is the engineering that, multiplied by fifty thousand, lights up the night sky over the world’s best observatories.