The strength of gravity is different on every body in the solar system. Whether it’s the crushing weight of Jupiter or the miniscule pull of a small asteroid, this fundamental force of physics still has a major impact on the material those bodies are made up of. A new paper from researchers at the University […]
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Keeping satellites cool in space is a major challenge. Unlike on Earth, where heat can escape into the surrounding air, space is a vacuum. This means there is no air to carry heat away. As a result, electronic systems on satellites and spacecraft can quickly overheat if they do not have an effective way to […]
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The moon may look peaceful from Earth, but its surface is one of the harshest environments imaginable. It is covered by a layer of fine dust and crushed rock called lunar regolith. This material is not like the soil we have on Earth. It contains sharp particles of rock and glass that can damage equipment, […]
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The planet’s name is HD 189733b. It is one of the closest extrasolar planets to Earth that can be studied in detail, and one of the most thoroughly characterised exoplanets in the astronomical literature. Its star, HD 189733, sits in the constellation Vulpecula, the Little Fox, north of the celestial equator. The star itself is faintly visible to a small telescope from any dark backyard in the northern hemisphere on a clear summer night. The planet is not. The planet has never been directly photographed and probably never will be by any current generation of telescope.
Everything that is known about it has been deduced from the way its star’s light changes as the planet passes in front of, alongside, and behind it. The deductions, after twenty years of accumulated work, describe one of the most hostile environments in the catalogued universe.
It was discovered on 5 October 2005 by François Bouchy and colleagues at the Haute-Provence Observatory in southern France, using the Doppler-spectroscopy method to detect the small gravitational tug of the planet on its host star. The planet is approximately the mass and size of Jupiter. It sits roughly 4.6 million kilometres from its star, which is about one-thirtieth of the distance between the Sun and Mercury. It completes one orbit every 2.2 Earth days. At that distance, it is tidally locked. The same hemisphere faces the star at all times.
On the side facing the star, it is approximately 1,000 degrees Celsius.
The astronomers who have studied HD 189733b in detail describe an atmosphere that has no analogue in the solar system. The temperature differential between the planet’s permanently lit day side and its permanently dark night side, measured by NASA’s Spitzer Space Telescope in 2007, is approximately 260 degrees Celsius. That differential drives atmospheric winds at speeds that, in the upper atmosphere on the day side, reach approximately 7,000 kilometres per hour. As the European Space Agency set out in its 2013 announcement of the planet’s confirmed colour, the wind speeds are roughly seven times the speed of sound. On Earth, by comparison, the strongest sustained surface winds ever recorded reached approximately 410 kilometres per hour during a tropical cyclone. HD 189733b’s winds are approximately seventeen times faster.
The atmosphere is composed primarily of hydrogen and helium, like Jupiter’s, but it also contains a high concentration of silicate particles. Silicates are the family of minerals that make up most of the Earth’s crust, including sand, quartz, and the basaltic rocks that form ocean floors. On HD 189733b, at atmospheric temperatures exceeding 1,000 degrees Celsius, silicate particles condense in the atmosphere from gaseous form into small molten droplets of glass.
The droplets do not fall straight down. They are driven sideways by the 7,000 km/h winds, at velocities at which a single droplet impacting a surface would carry the energy of a small artillery shell. The planet, on the side facing its star, is therefore experiencing continuous horizontal precipitation of molten glass at hurricane speeds, at temperatures that would melt aluminium.
The rain is the weather. There is no break in it.
The visual colour of an exoplanet 63 light-years away cannot be observed in the conventional sense. The planet is far too faint and far too close to its star to be photographed directly. The team that established HD 189733b’s true colour, led by Tom Evans at the University of Oxford, used a technique called secondary eclipse spectroscopy. Their paper, published in Astrophysical Journal Letters on 1 August 2013, describes the method in detail.
The Hubble Space Telescope’s Space Telescope Imaging Spectrograph observed the HD 189733 system continuously through several full orbital cycles of the planet. During each orbit, the planet passes behind the star from the telescope’s perspective. In the moments before and after the planet disappears behind the star, the telescope is receiving light from both the star and the planet. In the moments when the planet is hidden, the telescope is receiving light from the star alone. By subtracting the second measurement from the first, the team could isolate the light reflected by the planet alone.
The drop in brightness as the planet vanished behind its star was measurable specifically in the blue part of the spectrum, between 290 and 450 nanometres. The drop in the red and near-infrared was much smaller. The published interpretation is that the planet reflects blue light at approximately three to four times the rate it reflects red light, which makes it, in the visual range, a deep cobalt blue.
If a human observer could be positioned in space within the HD 189733 system, at a safe distance, the planet would appear to them as a small, deep blue point of light, almost indistinguishable in colour from the way the Earth appears to astronauts looking back from the International Space Station.
The mechanism that produces the blue colour, however, is completely different.
Earth appears blue from space for two reasons. The most obvious is the reflection of light from the oceans, which cover approximately 71 per cent of the planet’s surface. The second, less commonly understood, is Rayleigh scattering in the atmosphere. Short-wavelength light, including blue, scatters more efficiently off the molecules of nitrogen and oxygen in the air than long-wavelength light does. This is why the sky is blue. The same effect contributes to the planet’s blue appearance from orbit.
HD 189733b has no oceans and probably no liquid water of any kind. The temperatures are too high for water to exist as a liquid anywhere in the atmosphere. The blue colour comes entirely from the silicate particles. The droplets of molten glass suspended and condensing in the atmosphere scatter blue light preferentially, in much the same way that nitrogen and oxygen molecules in Earth’s atmosphere scatter blue light. The mechanism is a different kind of Rayleigh-like scattering, off particles that are themselves molten and being driven sideways by hurricane-speed winds, but the optical outcome is similar.
HD 189733b is, by the resemblance of one colour to another, a kind of cosmic mimicry. A planet that looks, from sixty-three light-years away, like Earth. A planet that, on inspection, has nothing in common with Earth except the wavelength of the light it reflects.
HD 189733b belongs to a class of exoplanets called hot Jupiters: gas giants similar in mass and composition to Jupiter, orbiting their stars at distances much closer than Mercury orbits the Sun. The first hot Jupiter was discovered in 1995. Since then, several hundred have been confirmed in the published exoplanet catalogue maintained by NASA’s Exoplanet Exploration Program. They are, on the data so far, surprisingly common in the galaxy. They are also, on the same data, completely absent from our own solar system.
The reasons hot Jupiters form, and the processes that drive them inward to such close orbits around their stars, are still subjects of active investigation. HD 189733b, because of its relative closeness to Earth and its bright host star, has become one of the most-studied hot Jupiters in the astronomical literature. The 2013 confirmation of its blue colour was the first time the visible-light colour of any exoplanet had been measured directly. The same observational programme, and follow-ups using the James Webb Space Telescope, have detected water vapour, carbon dioxide, methane, and atmospheric haze in the planet’s upper layers, building up a picture of an atmosphere chemically rich and physically violent at scales no observation of a solar-system planet has matched.
The exoplanet has, in the years since its discovery, been informally referred to in the astronomical literature as the planet where it rains glass. The wind speeds, the temperatures, and the silicate atmospheric chemistry are now well established. The geological details — whether the molten glass droplets reach the planet’s deeper layers as glass or evaporate back into vapour, whether the planet has a coherent solid core or whether its interior is a continuous fluid down to whatever pressure ultimately produces metallic hydrogen — remain open questions.
The light that the Hubble Space Telescope captured in 2013 had been travelling toward Earth since approximately 1950. The light Hubble might capture from HD 189733b today began its journey toward us during the early 1960s. The planet itself, in real time, is doing whatever it has continued to do for the four billion years it has existed. The astronomers who study it are studying its past.
If a human civilisation around HD 189733 were, at this moment, observing Earth through the same kind of telescope Hubble represents, they would be looking at images of Earth as it was in 1962. They would be receiving the light Earth was reflecting during the Cuban Missile Crisis, the early Mercury space programme, and the year before the death of John F. Kennedy.
Earth, from sixty-three light-years away, also looks like a deep blue dot.
The difference is that ours, on closer inspection, has oceans.
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Astronomers have solved one of the biggest mysteries in radio astronomy by identifying the source of a strange type of cosmic signal that has puzzled scientists for years. An international team led by researchers from the University of Sydney discovered that these unusual signals come from a rare pair of stars locked in a tight […]
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This matters for the story of KM3-230213A, the neutrino detected at 01:16 UTC on 13 February 2023 by the KM3NeT/ARCA detector at the bottom of the Mediterranean, roughly 3,450 metres below the surface off the coast of Sicily. With an estimated energy of approximately 220 petaelectronvolts, it remains the most energetic neutrino ever recorded. The previous record, from IceCube’s dataset, sat near 10 PeV. The gap between them is roughly a factor of twenty.
Where that neutrino came from has not been settled. Several candidate explanations have been put forward over the past year. A paper published in the Physical Review Letters in February 2026 argues that a population of blazars is a plausible origin. It is, as the authors are explicit in stating, one hypothesis among several. It has not been confirmed.
Inside a blazar jet, protons can be accelerated to extreme energies. When those protons interact with photons or other matter inside the jet, they produce pions, and those pions decay into neutrinos and gamma rays. This mechanism, broadly called hadronic production, is why high-energy neutrino detections and high-energy gamma-ray observations are connected: both are products of the same underlying process.
The connection also gives the hypothesis a testable constraint. Any model that invokes blazars as neutrino sources must not produce more gamma rays than have actually been observed. The extragalactic gamma-ray background has been measured carefully by the Fermi Gamma-ray Space Telescope, and a proposed blazar population cannot exceed it. This is one of the key tests the Bendahman paper applies.
The team used an open-source modelling tool called AM3 to simulate a realistic population of blazars, fixing parameters like magnetic field strength and emission region size to values established by independent observations. They then varied two quantities: the baryonic loading, which governs how much energy is carried by protons relative to electrons and therefore how many neutrinos can be produced; and the proton spectral index, which determines how the energy is distributed across the proton population. For each combination of these parameters, they calculated the expected diffuse neutrino flux and the corresponding gamma-ray output, then compared both against actual measurements from KM3NeT, IceCube, and Fermi.
They found a region of parameter space in which the blazar population could account for an event like KM3-230213A while remaining consistent with the gamma-ray constraints. The result positions blazars as physically viable. It does not identify a specific blazar source for this specific neutrino. The paper’s conclusion is that the scenario is plausible and merits further investigation, not that the question has been answered.
When a high-energy neutrino is detected, the standard follow-up procedure involves searching for an electromagnetic counterpart, a signal in radio, optical, X-ray, or gamma-ray light arriving from the same direction at approximately the same time. For KM3-230213A, no such counterpart was found. The KM3NeT collaboration conducted searches for correlations with known Galactic and extragalactic sources in the direction of the event (right ascension 94.3 degrees, declination minus 7.8 degrees) and found nothing significant.
The absence of a counterpart rules out some source scenarios more cleanly than others. A single dramatic event, a flare or an outburst from one identified object, would generally be expected to produce an accompanying electromagnetic signal. Its absence is one reason Bendahman and colleagues lean toward a diffuse origin: if the neutrino comes not from one spectacular burst but from the accumulated flux of many blazars integrated across large distances, there may be no single object to point to and no associated flare to find.
As Bendahman noted in the EurekAlert press release accompanying the paper, this reasoning does not completely exclude a point-like source. It does shift the prior toward a diffuse population explanation, which the blazar model provides.
The IceCube Neutrino Observatory at the South Pole has been collecting data since 2010 with a larger effective detection volume than KM3NeT had at the time of the event. It has not observed any neutrino comparable to KM3-230213A. This non-detection is not a minor footnote: it imposes a real constraint on the expected rate of ultra-high-energy neutrino events, and any proposed source population must be consistent with it.
The tension between KM3NeT’s detection and IceCube’s non-detection has been estimated at between two and three-and-a-half sigma across several analyses, depending on assumptions about the source spectrum and angular region. That sits below the conventional threshold for claiming a significant discrepancy, but it is not easily waved away either.
The Bendahman paper addresses this directly. Their blazar population model is tested not just against the KM3NeT observation but against IceCube’s upper limits as well. They find a scenario in which blazars can produce a neutrino flux consistent with the KM3NeT detection while the IceCube non-detection remains statistically unremarkable. The model threads the needle, but only within a specific parameter range, and only as a statistical argument about expected rates, not as a demonstrated resolution of the IceCube tension.
The blazar population hypothesis is the most recent well-developed proposal from the KM3NeT collaboration itself, but it is not the only one circulating in the literature.
The cosmogenic neutrino scenario holds that KM3-230213A was produced not at an astrophysical source but in transit, when an ultra-high-energy cosmic ray collided with a photon from the cosmic microwave background. This process, expected to produce neutrinos in a broadly similar energy range, was analysed in a companion paper to the original Nature publication. The cosmogenic explanation has the advantage of not requiring a specific identified source, but the IceCube non-detection makes any steady, isotropic source harder to sustain without careful tuning.
Separate analyses have examined whether specific known objects could be the source. One paper by researchers at Peking University and Chongqing University investigated associations with gamma-ray bursts, searching a broader region around the event’s coordinates while allowing for possible Lorentz invariance violations that might have delayed the neutrino relative to any accompanying photons. No definitive association was found. Another paper pointed to a specific blazar, PKS 0605-085, as a candidate point source, based on its proximity to the reconstructed direction. The angular uncertainty of KM3-230213A is currently around 1.5 degrees, which leaves a sizeable search cone, and PKS 0605-085 has not been confirmed as the source.
A paper published in Physical Review Letters in March 2026, by Vedran Brdar and Dibya S. Chattopadhyay, takes a different approach entirely, focusing not on where the neutrino came from but on what may have happened during its journey. Their proposal involves sterile neutrinos, hypothetical particles that do not interact via the standard weak force, oscillating into active neutrinos over the 147-kilometre path through rock and seawater to the KM3NeT detector. The same transformation would be far less likely over the 14-kilometre path to IceCube from the same sky position, potentially explaining the discrepancy between detectors. This scenario requires physics beyond the Standard Model and remains speculative. The authors describe it as a possible resolution, not a demonstrated one.
A more exotic proposal, published separately in Physical Review Letters, suggested the event could have originated in the final evaporation of a primordial black hole. This hypothesis is not supported by independent evidence and has not been taken up broadly in the follow-up literature.
KM3NeT/ARCA was operating with 21 detection strings at the time of the event, roughly ten per cent of its planned final configuration. Construction has continued since. The completed detector will cover approximately one cubic kilometre of deep water with around 200,000 photomultiplier tubes, and an online alert system is being developed to notify other observatories within seconds of a candidate high-energy event, enabling the kind of rapid multi-wavelength follow-up that might finally attach a counterpart to the next event of this kind.
The collaboration also expects a positioning system upgrade to tighten the directional reconstruction from the current 1.5 degrees to a target of around 0.1 degrees. That improvement, applied retroactively to KM3-230213A as well as to future detections, would substantially shrink the search cone and either implicate or exclude several of the current candidate sources.
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The physics of neutron stars are almost too fantastic to believe. Something the weight of two Suns compacted to a sphere the size of a city. Each teaspoon of its material would weigh billions of tons. If you’ve done any reading on the topic, you’ve heard these facts before. But despite the intense interest these […]
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After two years of analysis, the KM3NeT collaboration published its findings in Nature on 12 February 2025. The muon’s energy was estimated at 120 petaelectronvolts. The neutrino that produced it, they concluded, carried approximately 220 petaelectronvolts. No neutrino of comparable energy had ever been recorded anywhere. The previous record, held by events in IceCube’s dataset, sat around 10 petaelectronvolts. The event, designated KM3-230213A, nearly an order of magnitude higher.
This is one detection, not a population. The questions it raises are real, but a single event cannot settle them.
Neutrinos interact so rarely with matter that trillions pass through a human body every second without leaving any trace. They carry no electric charge and have almost no mass. Those properties make them useful as astronomical messengers: unlike photons or charged particles, they travel from their source to the detector in a straight line, largely unaffected by the magnetic fields and radiation that fill intergalactic space. A neutrino arriving from a distant source carries information about that source in a relatively uncorrupted form.
Energy is a key piece of that information. At 220 PeV, KM3-230213A falls into a regime that the KM3NeT collaboration describes as the first confirmed detection of a neutrino in the hundreds-of-PeV range. The detector was not full at the time: only 21 of the planned 230 detection strings were in place, with 378 optical modules operating. That the instrument caught an event of this energy while still a fraction of its eventual size is, at minimum, an indication of what the completed detector may see.
The almost horizontal trajectory of the muon, combined with its energy, rules out the possibility that the particle arrived from the atmosphere above, which is where the vast majority of muons at KM3NeT’s depth originate. The track’s geometry and energy together point to a neutrino that entered the Earth from above, interacted with rock or seawater near the detector, and produced the muon that ARCA recorded. In the collaboration’s analysis, the probability of this being an atmospheric background event is negligibly small.
The neutrino’s reconstructed direction points to celestial coordinates of right ascension 94.3 degrees and declination minus 7.8 degrees. The KM3NeT collaboration searched that region for potential sources. They found nothing significant. No known Galactic object has been proposed as a plausible source for a neutrino at this energy. The direction is also inconsistent with any nearby extragalactic source that would straightforwardly explain the observation.
Several candidate categories have been proposed in the literature since the paper’s publication. Active galactic nuclei, particularly blazars, are among them: blazars are a class of active galaxy in which a relativistic jet points toward Earth, and they have already been associated with lower-energy neutrino detections by IceCube. One analysis, published in early 2025, examined blazar candidates within the angular uncertainty of KM3-230213A and found no convincing match. A separate paper considered gamma-ray bursts as the source, including a search for associations with documented GRB events accounting for possible Lorentz invariance violations that might have delayed the neutrino’s arrival relative to the gamma-ray signal. Those searches also found no definitive association.
A third category is the cosmogenic neutrino: a particle produced not at an astrophysical source but in transit, when an ultra-high-energy cosmic ray collides with a photon from the cosmic microwave background that fills the Universe. This process, known as the GZK mechanism after the physicists who described it, is expected to produce neutrinos at very high energies, and 220 PeV sits within a plausible range. The KM3NeT collaboration’s own companion paper on the cosmogenic scenario found it consistent with the observation but not conclusive. A more exotic proposal, published in Physical Review Letters in 2026, suggested the event could have been produced by the final evaporation of a primordial black hole, an event that would generate a short burst of high-energy particles including neutrinos. That hypothesis remains speculative and is not supported by independent evidence.
None of these scenarios has been confirmed. The source of KM3-230213A is, at present, unknown.
The more awkward question about KM3-230213A is not where the neutrino came from, but why IceCube has not seen anything like it.
IceCube is the larger instrument. It operates in the ice beneath the South Pole with an instrumented volume of one cubic kilometre and has been collecting data since 2010. Its exposure, the product of effective detection area and operating time, substantially exceeds KM3NeT’s at the energy of KM3-230213A. On purely statistical grounds, IceCube should have been the first detector to observe a neutrino at this energy, and it should have observed more than one by now. It has not. IceCube’s published upper limits on the neutrino flux at these energies are, in fact, below the rate implied by a single KM3NeT event observed over the period KM3NeT was running.
Several papers have attempted to quantify this tension. Estimates vary between roughly two and three-and-a-half sigma, depending on assumptions about the neutrino’s source spectrum and the angular region of sky considered. That range sits below the conventional threshold for claiming a significant discrepancy, but it is not negligible, and it has generated a substantial volume of follow-up work.
One natural explanation is statistical: a single-event detection at an energy where event rates are expected to be very low will inevitably produce some tension with non-detection elsewhere, and the significance of that tension is sensitive to the model used to estimate expected rates. It is possible that KM3NeT was simply fortunate to catch an unusually rare event, and that IceCube’s non-detection is consistent with that rate at the two-sigma level.
A more theoretically ambitious explanation was put forward by Vedran Brdar and Dibya S. Chattopadhyay in a paper published in Physical Review Letters in February 2026. Their argument centres on a geometrical fact. The neutrino detected by KM3NeT travelled through approximately 147 kilometres of rock and seawater before reaching the detector. A neutrino arriving at IceCube from the same direction in the sky would have passed through only about 14 kilometres of ice. That difference in path length through matter, they suggest, could explain the discrepancy if the particle was originally a sterile neutrino, a hypothetical particle that does not interact via the standard weak force, which oscillated into a detectable active neutrino over the longer path to KM3NeT. The shorter path to IceCube would have been insufficient for that conversion to occur at the required rate.
The paper presents two mechanisms by which such oscillations could arise, both involving physics beyond the Standard Model. The authors are explicit that this is a proposal, not a confirmed result: the paper identifies a possible resolution, not a demonstrated one.
KM3NeT/ARCA at the time of the detection was operating with 21 detection strings out of a planned 230. The full detector will instrument approximately one cubic kilometre of deep Mediterranean water, with roughly 200,000 photomultiplier tubes distributed across the string array. At the time of writing, the collaboration has continued deploying detection units through annual marine campaigns, and an online alert system is being developed to distribute direction and timing information for interesting events shortly after detection, enabling rapid follow-up from other instruments.
The collaboration also notes that an upgrade to the detector’s positioning system will improve directional reconstruction for future events from the current angular uncertainty of approximately 1.5 degrees down to the target of around 0.1 degrees. That improvement will apply retroactively to KM3-230213A as well, potentially tightening the source region enough to either implicate or exclude specific candidate objects that currently fall within the uncertainty cone.
A parallel development worth watching is the collaboration between KM3NeT and IceCube. Both teams have indicated interest in joint analysis of the ultra-high-energy neutrino sky, combining IceCube’s larger exposure with KM3NeT’s different viewing geometry and detection medium. That comparison may eventually resolve whether the apparent tension between the two instruments reflects astrophysics, instrument response, or something more fundamental. At the moment, it remains unresolved.
KM3-230213A is a real detection. The analysis behind it is careful and the publication in Nature was peer-reviewed. The energy estimate of 220 PeV carries a 90 per cent confidence interval running from 72 PeV to 2.6 exaelectronvolts, which is wide, but even the lower bound of that range comfortably exceeds any previous neutrino detection. The event is what the KM3NeT collaboration says it is.
What it is not is a resolved scientific story. The source is unknown. The IceCube tension is real but not yet statistically decisive. The proposals for exotic physics, sterile neutrino oscillations, primordial black hole evaporation, remain speculative. The collaboration itself is measured about what can be concluded from a single event, and the papers published since the Nature article reflect the genuine uncertainty that still surrounds it.
The more useful frame, for now, is that KM3-230213A establishes that neutrinos at these energies exist and can be detected with instruments of this kind. The completed KM3NeT will see more of them. Whether the next one resolves the questions the first has raised, or adds new ones, is the part that remains to be seen.
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The paper draws on Two-Line Element data from the Space-Track database, the standard orbital-mechanics format used to track every object in Earth orbit. Ashruf’s team began with a set of 95 candidate debris objects from the 1960s, then filtered for objects in low Earth orbit below 800 km, with stable near-circular orbits and continuous data across the full period. Seventeen survived that filtering.
The list includes TIROS weather satellites, Thor rocket debris, Delta stage fragments, and two Soviet-era Cosmos objects. They orbit at inclinations ranging from roughly 48 to 99 degrees, and altitudes between about 600 and 800 km, completing a full loop of the planet every 90 to 120 minutes. Their masses range from under 20 kg to over 1,400 kg.
What they share is longevity and passivity. None have performed any orbital adjustment in over sixty years. That makes their altitude history a clean signal: whatever happened to their orbit happened because of the atmosphere, not because of anything onboard.
Solar activity is most commonly tracked through sunspot numbers, a count that correlates with the Sun’s emission of extreme ultraviolet radiation. The Sun’s eleven-year cycle moves between quiet periods, when sunspot numbers are low and the thermosphere cools and contracts, and active periods when numbers climb and the upper atmosphere heats and expands. More atmosphere at orbital altitude means more drag on any object passing through it.
Earlier research had established this general connection. What it had not established was where within a solar cycle the drag effect becomes meaningfully stronger. Ashruf’s team fitted a Gaussian curve to the sunspot record for each of the three cycles studied, then identified the point in each cycle where the 17 debris objects showed a transition from slow, gradual decay to markedly steeper decay. That point, consistent across three cycles and across the full range of objects, fell between approximately 67 and 75 per cent of the cycle’s peak sunspot number.
The authors describe this as a threshold beyond which thermospheric density increases sufficiently to drive a clear acceleration in orbital decay. Below the threshold, descent is slow and fairly uniform. Above it, the curve steepens. The threshold appears on the way up through each cycle and again on the way back down.
Cross-checking with direct measurements of extreme ultraviolet flux from the Solar and Heliospheric Observatory confirms the pattern. Within the rapid-decay windows identified by the debris data, EUV flux in the 0.1 to 50 nanometre band ran roughly 50 to 130 per cent above levels seen outside those windows. The debris records and the solar measurements point to the same mechanism: heightened EUV output heats the thermosphere, which expands upward, and the increased air density at orbital altitude increases drag.
The three cycles in the dataset were not equal in strength. Solar cycle 22, which peaked around 1989 to 1991, was the most active of the three. Cycle 23, peaking around 2000 to 2002, was moderately active. Cycle 24, peaking around 2014, was historically weak by modern standards.
The debris records reflect that hierarchy directly. Peak decay rates during cycle 22 averaged 0.59 metres per hour across the 17 objects. Cycle 23 produced a mean of 0.54 metres per hour. Cycle 24 came in at 0.25 metres per hour, roughly half of cycle 22’s pace. The staircase is clean: each successive cycle, as solar activity weakened, drove proportionally less orbital decay.
The paper also compared the correlation between decay rates and several different solar and geomagnetic indices. Solar proxies performed strongly: the F10.7 radio flux index explained about 75 per cent of the variance in decay rate across the 17 objects; sunspot numbers accounted for about 67 per cent. Geomagnetic indices fared poorly. The AE index, which tracks auroral electrojet activity driven by particle precipitation and magnetic disturbances, explained less than 2 per cent of the long-term variance. The Dst index, which measures the ring current, explained around 22 per cent. The paper’s interpretation is that geomagnetic storms matter for short-term orbital perturbations, but for sustained, long-term decay the dominant driver is solar EUV forcing of the thermosphere, not geomagnetic disturbance.
The team used ballistic coefficients derived from the cycle 22 and cycle 23 data to model what cycle 24 orbital decay should have looked like, then compared those predictions to what the TLE data actually showed. For 15 of the 17 objects, the model reproduced the observed decay profiles reasonably well, though it required a scaling factor ranging from 0.55 to 0.79 to match observed behaviour. The need for that scaling reflects known limitations in empirical atmospheric density models, particularly around the transition between solar minimum and solar maximum conditions.
Two objects did not fit at all. SAT 733, a Thor Agena D rocket body, and SAT 734, a satellite called OPS 3367A, showed persistent large discrepancies between modelled and observed decay that no scaling factor could close. Both travel near-polar orbits, at inclinations close to 99 degrees. The other 15 objects orbit between roughly 48 and 67 degrees of inclination.
The paper’s interpretation is cautious but direct: the NRLMSIS 2.0 atmospheric model, which is the standard empirical model used for this kind of orbital prediction, likely underestimates atmospheric density variability at high latitudes. The thermosphere at polar regions is influenced by geomagnetic activity in ways that are not fully captured by models built primarily around lower-latitude data. The gap matters because polar and sun-synchronous orbits are common choices for Earth-observation missions, and their reentry predictions may carry larger errors than the model currently reflects.
The practical value of a threshold is that it gives satellite operators a more specific warning indicator than a general solar forecast. Rather than tracking the entire solar cycle, operators can watch sunspot numbers relative to the expected cycle peak. When that ratio climbs past roughly two-thirds of peak, conditions enter the regime where drag-driven decay accelerates. Fuel reserves for orbit maintenance need to be adequate for that period, not just for quiet-Sun operations.
The February 2022 Starlink event sits in the background here. A moderate geomagnetic storm shortly after launch pushed 38 satellites into orbits lower than planned, and atmospheric drag was sufficient to prevent them from reaching their target altitude. Most reentered within weeks. The Starlink case involved a geomagnetic disturbance rather than sustained solar maximum conditions, so the mechanisms are not identical, but the broader point stands: drag at low Earth orbit is not a fixed baseline to plan against, and the Sun’s eleven-year cycle is the primary long-term variable.
The paper notes that missions launched near a solar maximum may consume propellant faster than mission planners expect, particularly if planning tools use average solar conditions rather than the cycle-phase-specific drag rates the new threshold identifies.
The 17 objects all orbit within the 600 to 800 km altitude range. The paper does not claim the same threshold applies at lower altitudes, where atmospheric density is higher and the relationship between solar activity and drag may behave differently. Most of the large satellite constellations being deployed now operate below 600 km, and the paper’s findings do not directly translate to that regime without further work.
The three solar cycles in the dataset were also all relatively moderate by historical standards. The most active cycles on record, including cycle 19 in the late 1950s, produced solar maxima substantially stronger than cycle 22. Whether the 67 to 75 per cent threshold holds under more extreme solar conditions is not something this data can answer.
The polar orbit modelling gap remains open. Ashruf’s team notes it explicitly as a direction for future work, and it is a real limitation for anyone predicting the reentry timing of debris in high-inclination orbits.
Better empirical models for high-latitude atmospheric density are needed, and the debris records in this study now provide one benchmark for testing them.
The paper is published as “Characterizing solar cycle influence on long-term orbital deterioration of low-earth orbiting space debris”, authored by Ayisha M. Ashruf, Ankush Bhaskar, C. Vineeth, and Tarun Kumar Pant, in Frontiers in Astronomy and Space Sciences, volume 13, published 6 May 2026. It is open access.
The post Rocket debris that has been drifting in low Earth orbit since the 1960s just helped scientists find something they had missed for decades — a specific threshold in solar activity past which space junk starts falling toward Earth measurably faster appeared first on Space Daily.
Astronomers have found thousands of worlds in faraway star systems, but one of the questions that’s been hardest to answer is the one that immediately jumps to any human mind: What does the planet look like? By analyzing subtle changes in light, researchers found the planet Kua’kua—which orbits a small star in the constellation Indus—has […]
The post Scientists get their best-ever look at distant planet’s surface appeared first on Knowridge Science Report.
A new race to the moon is emerging between the United States and China. Unlike fifty years ago, the goal is no longer just about landing and leaving, but establishing a base that allows for a sustainable presence and extended stays on the surface of our natural satellite. The objective is now to use the […]
The post Is extracting oxygen from lunar soil the future of space exploration appeared first on Knowridge Science Report.
For nearly 30 years, scientists have believed that a mysterious force called dark energy is causing the universe to expand faster and faster. But a new mathematical study suggests that dark energy may not be needed at all. Researchers from the University of California, Davis have published a paper in Proceedings of the Royal Society […]
The post Everything we thought we knew about dark energy could be wrong appeared first on Knowridge Science Report.
South Korean optical payload developer TelePIX and Indian propulsion specialist Bellatrix Aerospace have teamed up on a very low Earth orbit geospatial demonstration mission slated for 2028.
The post Bellatrix and TelePIX plan 2028 air-breathing VLEO imaging demonstration appeared first on SpaceNews.
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