SPACE/COSMOS
The spin behind fading black hole flares
Stellar spin may solve a long-standing puzzle about repeating encounters between stars and supermassive black holes.
Syracuse University
image:
A hydrodynamical simulation of a star being ripped apart by the tidal forces of a supermassive black hole.
view moreCredit: Credit: NASA/ S. Gezari (JHU)/ J. Guillochon (UCSC)
At the center of most galaxies lies a supermassive black hole, with a mass millions to billions of times that of our sun and some of the most extreme gravity in the universe.
Some stars that venture too close to such black holes live to tell the tale. Rather than being completely destroyed, they survive to make repeated close passes, producing a new flare of light each time.
These repeating partial tidal disruption events (rpTDEs) give astronomers the opportunity to watch the same star and black hole interaction unfold again and again, thanks to wide-field time-domain surveys that repeatedly scan large areas of the sky for objects that change in brightness.
But in several cases, researchers have noticed a puzzling pattern: The successive flares grow progressively dimmer. For years, theoretical models couldn’t explain why.
Now, a team of astrophysicists at Syracuse University has shown that the answer may lie in a previously overlooked factor—the star’s spin.
The study, published in The Astrophysical Journal, was led by doctoral student Ananya Bandopadhyay, working with postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin—all in the Department of Physics—as well as colleagues at other institutions.
When stars meet black holes
In a typical tidal disruption event (TDE), a black hole’s tidal force—the difference in gravitational pull across a nearby star—tears the star completely apart.
As the disrupted stellar debris falls toward, or “accretes” onto, the black hole, it loses energy that is emitted in the form of light over the course of days to months.
While black holes themselves emit no light, TDEs provide a short-lived supply of fuel that lights up the surrounding region, allowing astronomers to study these otherwise invisible objects indirectly.
If a star orbiting a black hole does not come close enough to be completely ripped apart, it can, however, lose a fraction of its mass, resulting in a partial TDE. In a repeating partial TDE, the surviving core continues orbiting the black hole, losing more material with each new close pass, a few months to several years apart.
The dimming mystery
How much material a star loses during repeated encounters depends partly on its internal structure. Bandopadhyay compares a low-mass star to a fluffy meringue, which can become increasingly vulnerable to the black hole’s tidal forces. By contrast, a higher-mass star has a more centrally concentrated, onion-like star and can shed its outer layers while its dense core remains relatively unaffected, losing decreasing amounts of mass over time.
Those differences can help explain why rpTDEs don’t all behave the same way. But one pattern in particular has mystified researchers. Of the roughly 10 repeating systems identified to date, four have produced flares that grow progressively dimmer.
Decreasing mass loss might seem like an obvious explanation. But previous hydrodynamical simulations showed that, surprisingly, even as the material lost decreased with each encounter, the predicted flares retained roughly the same brightness.
“We were puzzled by this for two years,” Bandopadhyay says.
Their previous work had revealed another effect of the black hole's tidal forces.In addition to stripping material from the star, they exert a torque that causes the star to spin faster with each close encounter. As a result, although less material falls back toward the black hole, it returns over a shorter period of time, helping to keep the predicted flare at roughly the same brightness.
To reproduce the dimming astronomers were actually observing, the researchers needed what Bandopadhyay called “a new ingredient”—a star that was already spinning rapidly before its first encounter with the black hole.
The new study found that this initial rotation prevents the star from being significantly spun up during each passage. Without that additional spin-up, the timescale over which the stripped material falls back remains relatively constant. As the star loses less material with each encounter, the peak fallback rate—and the predicted brightness of the flare—can finally decline.
Tracing the star’s past
But why would the star already be spinning so rapidly?
“It is also extremely difficult to ‘bind’ a star to a supermassive black hole so tightly that it orbits the black hole in a matter of months, and yet they seem to do so in rpTDEs,” Coughlin says.
The so-called Hills mechanism may explain both. Under this scenario, a pair of closely orbiting stars passes near a supermassive black hole, which tears the binary apart, ejecting one star and capturing the other.
In a close binary, the stars can become tidally locked, causing each to rotate on its axis at the same rate that the pair orbits each other. The tighter the binary, the shorter that orbital period and the faster a tidally locked star spins. The binaries capable of leaving a captured star on the short orbit observed in rpTDEs would have to be extremely tight—also leaving a tidally locked star spinning rapidly before its capture.
“Ananya’s work demonstrates that each of these peculiarities can be explained by the same underlying phenomenon: the tidal destruction of a binary system and the capture of one of the stars,” Coughlin says. “From a theoretical standpoint, this is a major step forward in our understanding of the physics at play in these systems.”
Zooming out, Coughlin notes that Hills capture may also have produced some of the stars orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way. The new findings could therefore help explain some of the properties of stars in what he calls “our own cosmological backyard.”
Journal
The Astrophysical Journal
Researchers create a Little Big Bang: Bowling-pin-shaped nuclei shed new light on the Universe’s first moments
University of Copenhagen
image:
Event display of collisions between Neon-20 and Oxygen-16 at the CERN Large Hadron Collider.
view moreCredit: ALICE@CERN
Researchers from the University of Copenhagen have succeeded in recreating the primordial matter that filled the Universe shortly after the Big Bang – by smashing much smaller atomic nuclei together than previously thought possible. These microscopic Big Bangs could provide new insights into the Universe’s earliest moments while also helping researchers understand one of the fundamental questions in nuclear physics.
What happened in the first moments of the Universe – before the building blocks of life and the world we know today came into existence?
Physicists at the CERN research facility in Switzerland are trying to answer this question by recreating some of the extreme conditions that prevailed in the Universe during its earliest history. Now, researchers from the Niels Bohr Institute, together with colleagues from the international ALICE collaboration, have come one step closer to understanding them.
At CERN, researchers can make atomic nuclei collide at almost the speed of light, creating tiny droplets of the primordial matter that filled the Universe during its first millionth of a second. This matter is known as quark-gluon plasma and is thought to have been the earliest form of matter in the Universe.
For many years, scientists have assumed that creating this plasma required collisions between very heavy atomic nuclei such as lead. But the physicists from the Niels Bohr Institute have now succeeded in creating the primordial matter by smashing the much smaller nuclei oxygen-16 and neon-20 together.
“We have pushed the boundary for how small the atomic nuclei can be while still recreating this primordial matter – what you could call a Little Big Bang. We now know more about the fundamental conditions required for matter to transition into this extreme state,” says Associate Professor You Zhou, who led the experiment and until recently was employed at the Niels Bohr Institute at the University of Copenhagen.
He adds:
“Hopefully, this will help us better understand how the plasma behaved during the first moments of the Universe – and how it later evolved into the forms of matter that everything around us is made of.”
The research findings, produced as part of the international ALICE experiment, have just been published in the prestigious journal Physical Review Letters.
A microscopic Big Bang shaped like a bowling pin
When the atomic nuclei collide, their constituents are transformed into a tiny droplet of quark-gluon plasma that exists for a tiny fraction of a second. The droplet of extremely hot matter then expands. Researchers cannot observe the plasma directly, but they can measure the particles that the matter turns into shortly afterwards.
Here, it turns out that the movement pattern of the particles reveals the shape of the atomic nucleus. While collisions between two oxygen nuclei produce a more rounded pattern, collisions involving neon produce a bowling-pin-shaped pattern.
“The particles from the primordial matter are directly governed by the geometric shape of the atomic nucleus. If the two nuclei we smash together are spherical, we get one pattern. If they are shaped like bowling pins, we get another. By studying how the particles move after the collision, we can gain insights into atomic nuclei that are otherwise difficult for physicists to obtain,” explains Postdoctoral Researcher Emil Gorm Dahlbæk Nielsen from the Niels Bohr Institute, who is a co-author of the study.
He elaborates:
“It is a bit like shining light on an object and seeing its shadow. You cannot see the object directly, but its shadow reveals its shape. In the same way, the movement of the particles reveals the geometric shape of the atomic nuclei that was present at the beginning of the collision.”
Deep roots at the Niels Bohr Institute
The question of the shape and structure of atomic nuclei has occupied physicists for more than 70 years and has deep roots at the Niels Bohr Institute. Indeed, it was Aage Bohr’s work on the structure of the atomic nucleus that earned him the Nobel Prize in Physics in 1975.
The shape of an atomic nucleus is not merely a matter of geometry. It reveals how protons and neutrons are organised and provides important information about the strong force – one of nature’s four fundamental forces, which scientists are still working to fully understand.
Until now, researchers have primarily investigated nuclear structure at low energies, for example by studying the rotation and vibrations of atomic nuclei.
“A precise understanding of nuclear structure helps us understand the strong force. But instead of carefully investigating nuclei at low energies, we smash them together at the highest energies we can create and can now read their shape from the imprint they leave behind,” says You Zhou.
The researchers describe the potential of the approach as a possible paradigm shift. If the method can be further developed, it could provide a new way of investigating other atomic nuclei whose structures are not yet well understood.
Researchers still do not know the exact boundary for when quark-gluon plasma can be created. The next step is therefore to carry out experiments with even lighter nuclei, such as helium-4.
“What is fascinating is that we can use the same experiment both to learn about the structure of atomic nuclei and to gain a better understanding of what happened during the birth of the Universe. These two things turn out to be much more closely connected than one might initially think,” You Zhou concludes.
WHAT IS QUARK-GLUON PLASMA?
Quark-gluon plasma is an extremely hot and dense state of matter that existed during the first millionth of a second after the Big Bang. At that time, the Universe was so hot that protons and neutrons had not yet formed. Instead, their building blocks – quarks and gluons – moved freely in a kind of hot “soup”.
As the Universe expanded and cooled, quarks and gluons gradually became bound together to form, among other things, protons and neutrons. These later became the building blocks of atomic nuclei and, ultimately, the ordinary matter that makes up stars, planets and ourselves.
ABOUT THE STUDY
- The study was carried out as part of the international ALICE collaboration at CERN and has been published in Physical Review Letters as an Editors’ Suggestion.
- You Zhou, Emil Gorm Dahlbæk Nielsen and Zhiyong Lu from the Niels Bohr Institute played central roles in the work.
- The study was supported by the ERC project InitialConditions.
Journal
Physical Review Letters
Article Title
Evidence of Nuclear Geometry-Driven Anisotropic Flow in O+O and Ne+Ne Collisions at √𝑠NN=5.36 TeV
Article Publication Date
17-Aug-2026
The Apophis encounter in 2029: A rare opportunity for planetary science and defense
image:
Fig. 1. The schematic diagram of the ignition test system of an ammonium dinitramide (ADN)-based thruster.
view moreCredit: Space: Science & Technology
The near-Earth asteroid Apophis will make an extremely close flyby of Earth on April 13, 2029, at a distance of approximately 38,000 km—an event that occurs on average only once every 7,500 years, offering an unprecedented opportunity for planetary science and planetary defense. However, significant uncertainties remain regarding the physical parameters of Apophis, such as its shape, spin state, internal structure, and material composition, and predictions of tidal effects—including potential orbital changes, spin acceleration, surface material displacement, and dust ejection during the flyby—are highly dependent on model assumptions. Although multiple nations have planned exploration missions, how to coordinate global observational resources within the limited time window, optimize rapid-response mission architectures, and translate scientific understanding into effective planetary defense strategies remain critical challenges to be addressed.
In a recent review published in Space: Science & Technology, the team led by Li Jianyang from the School of Atmospheric Sciences, Sun Yat-sen University, systematically synthesizes the scientific opportunities and planetary defense value of the 2029 Apophis flyby event. The study integrates radar observations, light-curve and spectral data to summarize the current state of knowledge about Apophis, and analyzes the expected effects of the Earth flyby from four dimensions: orbital and spin changes, surface material displacement, internal structural response, and dust ejection. Through mission trajectory design and launch window calculations, the study evaluates the feasibility and scientific return of various exploration modes, including rendezvous, flyby, sample return, and impactor experiments, and reviews the approved or planned exploration missions globally, such as OSIRIS-APEX and RAMSES, along with their payload configurations. The study points out that during the flyby, surface changes at the centimeter-to-decimeter scale, spin-period alterations on the order of hours, and potential dust ejection phenomena are expected to be detectable, providing direct evidence for the structural evolution of rubble-pile asteroids. Furthermore, the coordinated multi-national detection and observation activities will serve as the first global joint exercise without an actual threat under the framework of the International Asteroid Warning Network, accumulating engineering experience for rapid response to near-Earth asteroid threats. The study also emphasizes that the Apophis event is comparable in significance to the historic moments of the Voyager mission in 1977 and the multinational joint exploration of Halley's Comet in 1986, and will lay the foundation for the scientific exploration and resource utilization of asteroids that frequently fly by within the Earth–Moon system, holding substantial strategic value for the substantive advancement of planetary defense capabilities.
First, this paper focuses on the fundamental physical state of Apophis and the scientific significance of its flyby event. With a diameter of approximately 340 meters, Apophis is classified as a potentially hazardous asteroid. Since its discovery in 2004, the possible impact risk it poses has once attracted widespread attention. As illustrated in Fig. 1, Apophis will fly past Earth on April 13, 2029, at a distance of about 38,000 km—merely one-tenth of the Earth–Moon distance. Such a close flyby event is extremely rare; an asteroid of comparable size passing Earth at such a close distance occurs on average only once every 7,500 years. Fig. 2 presents a distribution diagram of near-Earth objects in terms of flyby distance and size, clearly demonstrating the uniqueness and scientific value of the Apophis flyby event. Current knowledge of Apophis is derived primarily from radar and light-curve observations: Table 1 summarizes its physical parameters, including a size of approximately 340 m, a non-principal-axis rotation state, a rotation period of about 30.6 hours, a density of about 1.95 g/cm³, and a porosity of approximately 55%, suggesting that it may possess a rubble-pile structure; Fig. 3 presents a shape model based on radar data, exhibiting an elongated, asymmetric, and slightly bifurcated morphology. Spectroscopic observations classify it as an Sq-type asteroid, similar to ordinary chondrites, implying that its physical properties are comparable to those of asteroid Itokawa.
Second, the paper provides a detailed analysis of the expected effects of the Earth flyby on Apophis, encompassing four aspects: orbital and spin changes, surface material displacement, internal structural response, and dust ejection. In terms of orbital and spin effects, Earth's gravity will transfer Apophis from an Aten-type orbit to an Apollo-type orbit; the orbital uncertainty is expected to increase dramatically from approximately 1 km before the flyby to 6,000 km one year after the flyby, and the spin period is projected to change by −7.6 to +14.4 hours. Regarding surface material displacement, numerical simulations indicate that although tidal forces will not cause structural disruption, local material movement may occur in about 1% of the surface area (with displacements not exceeding three times the maximum particle radius), potentially exposing fresh material and altering local spectral properties. In terms of internal structural response, by combining the shape model with spin-state variations, the mass distribution and center-of-mass location of Apophis can be retrieved. Fig. 4 illustrates the trajectory of Apophis as it traverses Earth's magnetosphere, passing sequentially through different space environment regions including the magnetosheath, magnetotail, and magnetosphere, providing an important reference for understanding the interaction between asteroids and the Earth's space environment. With respect to dust ejection, Apophis's extremely low gravity allows dust particles smaller than 50 μm to be ejected from its surface; after being captured by Earth's magnetosphere, these particles form high-speed dust streams that may produce observable electromagnetic disturbances on spacecraft.
Finally, the paper systematically reviews the scientific exploration opportunities, planetary defense implications, and prospects for international cooperation. In terms of exploration missions, Fig. 5 presents the distributions of C3 energy and flyby velocity for different launch windows, indicating that within specific windows, small launch vehicles can already enable Apophis exploration. Fig. 6 presents an example trajectory with a low flyby velocity of 1.88 km/s in January 2029. Currently approved or under-development missions include NASA's OSIRIS-APEX (a rendezvous mission arriving in June 2029), ESA's RAMSES (arriving before the flyby to monitor the entire process), and Japan's DESTINY+, among others; Chinese scientists have also proposed conceptual mission designs such as ARS and CROWN/Apophis. In terms of ground-based observations, Apophis will be resolved by multiple 10-meter-class telescopes during the flyby, and the under-construction China Fuyan radar is expected to achieve meter-level resolution and sub-centimeter surface deformation detection. At the planetary defense level, this event will serve as the first global joint exercise without an actual threat under the framework of the International Asteroid Warning Network, accumulating critical experience for rapid response to near-Earth asteroid threats. The establishment of an International Year of Planetary Defense will further promote public outreach and global cooperation, making the Apophis flyby a milestone event in the history of planetary science and planetary defense.
Journal
Space: Science & Technology
Article Title
The Apophis Encounter in 2029: A Rare Opportunity for Planetary Science and Defense
Fig. 2. (A) The demonstration of the ammonium dinitramide (ADN)-based thruster and (B) schematic diagram of the ADN-based thruster.
Fig. 3. (A) The hot-fire test diagram of the thruster and (B) the thermal decomposition and combustion processes of the ammonium dinitramide (ADN)-based propellant within the thruster.
Fig. 4. The images of the (A) preignition state, (B) arc loading, (C) ignition running, and (D) hot-fire ending of the thruster.
Fig. 5. The pressure and mass flow rate curves of 30-s hot-fire test of the ammonium dinitramide (ADN)-based thruster.
Fig. 6. The voltage, current, and resistance curves of the ammonium dinitramide (ADN)-based liquid propellant in the decomposition area.
Fig. 7. Fast Fourier transform (FFT) spectra of combustion chamber pressure in a 30-s hot-fire test.
Credit
Space: Science & Technology
Spacecraft proximity operation model-based sequential coalitional observation game strategy design
Beijing Institute of Technology Press Co., Ltd
image:
Fig. 1 The definition of local vertical, local horizontal (LVLH) coordinate system.
view moreCredit: Space: Science & Technology
With the intensifying space confrontation in geostationary orbit (GEO), the U.S. GSSAP (Geosynchronous Space Situational Awareness Program) satellites have conducted hundreds of close-range observation operations on dozens of on-orbit satellites in the vicinity of GEO. The acquisition of multi-aspect information from non-cooperative targets has thus become a critical component of space security assessment. Close-range spacecraft operations, which employ relative motion configurations such as fly-around, hovering, and drifting flight, provide an effective technical means for approaching and observing targets to obtain multi-aspect payload information. However, existing studies predominantly focus on the control design of a single relative motion configuration, which fails to satisfy the complex mission requirements of multi-aspect observation. The combination of multiple models for observation presents multiple challenges, including smooth switching between modes, fuel optimization, strategy design algorithms, and analysis of payload parameter effects. Therefore, how to coordinately plan a sequential combination of multiple close-range operation models under minimum fuel constraints to efficiently acquire multi-aspect information of non-cooperative targets has become a key bottleneck in enhancing space situational awareness capabilities.
In a recent study published in Space: Science & Technology, the team led by Meng Yunhe from the School of Artificial Intelligence, Sun Yat-sen University, proposed a strategy design method for multi-model combined observation of spacecraft based on sequential coalition game theory. Based on the Clohessy–Wiltshire relative dynamics equations, the study constructs four typical close-range operation models, namely the droplet model, the coplanar fly-around model, the non-coplanar fly-around model, and the drifting flight model, and provides the solution functions for the initial motion states that yield stable relative motion configurations. To achieve smooth switching between models, a waypoint trajectory planning scheme and a multi-impulse maneuver strategy are designed based on the minimum integral squared control theory. On this basis, the sequential coalition game theory is introduced for the first time into the multi-model combined observation problem. By constructing a game tree, the algorithm selects the optimal combination of initial motion points and close-range operation models at each stage under minimum fuel constraints, thereby generating an optimal observation strategy sequence. Furthermore, a statistical analysis of the effective observation time under different observation distances and field-of-view angles is conducted. Simulation results demonstrate that the proposed algorithm can successfully generate a multi-model combined observation strategy sequence under minimum fuel constraints; the effective observation time increases with larger observation distances and field-of-view angles, and reducing the model configuration size also contributes to improved observation performance. This study provides a systematic strategy design method for multi-aspect information acquisition of non-cooperative targets, offering significant engineering reference value for space security assessment and on-orbit situational awareness.
First, this study focuses on the problem of multi-aspect observation strategy design for non-cooperative spacecraft and proposes a multi-model combined observation method based on sequential coalition game theory. As a core strategic resource, the geostationary orbit (GEO) hosts a large number of missile early warning and military communication satellites. In recent years, the U.S. GSSAP (Geosynchronous Space Situational Awareness Program) satellites have conducted close-range observation operations on dozens of on-orbit satellites in the vicinity of GEO, rendering the space security situation increasingly severe. Acquiring multi-aspect payload information of non-cooperative targets to infer their functions and mission statuses is of great significance for space security assessment. Fig. 1 defines the LVLH (Local Vertical–Local Horizontal) coordinate frame, which serves as the basis for describing relative spacecraft motion. Fig. 2 illustrates the observation cameras mounted in the ±y directions of the spacecraft; given the constraints of observation distance and field-of-view angle, the effective observation time is defined as the total duration during which the target can be observed while satisfying the prescribed constraints. Since the observation cameras on the spacecraft are fixedly installed, frequent large-angle attitude maneuvers are time-consuming and consume substantial fuel. By designing a combination sequence of multiple close-range operation models, multi-aspect information acquisition of non-cooperative targets can be achieved with minimum fuel consumption. As shown in Fig. 3, through the sequential combination of the four configurations—namely the droplet model, the coplanar fly-around model, the non-coplanar fly-around model, and the drifting flight model—the observing spacecraft can realize multi-aspect imaging.
Second, the study constructs four typical close-range operation models and designs a sequential coalition game solution algorithm. Based on the Clohessy–Wiltshire relative dynamics equations, the analytical solutions of unforced relative motion are derived, and the solution functions for the initial motion states that yield stable relative motion configurations are provided by designing the relative distance and phase angle parameters. The droplet model, characterized by symmetry and re-visit capability, enables fine observation of the target; the coplanar and non-coplanar fly-around models facilitate fly-around observations in different orbital planes; and the drifting flight model achieves close-range observation through multi-impulse control. To achieve smooth switching between models, a waypoint trajectory planning scheme is designed based on the minimum integral squared control theory. On this basis, the sequential coalition game theory is introduced for the first time into the multi-model combined observation problem. Through game tree search, the optimal combination is selected at each stage under minimum fuel constraints, generating an optimal observation strategy sequence.
Finally, the study validates the effectiveness of the proposed method through simulations and analyzes the influencing factors of effective observation time. Fig. 4 presents the three-dimensional relative trajectories of the four-model combined observation, and Table 3 lists the generated sequential coalition observation strategy set: the first stage selects the initial point and the droplet model, the second stage selects the drifting flight model, and the third and fourth stages sequentially select the coplanar and non-coplanar fly-around models. The simulation results demonstrate that the algorithm can successfully generate the optimal combined observation sequence under minimum fuel constraints. Based on the combined observation strategy, the study conducts a statistical analysis of the effective observation time. As shown in Fig. 5, the combined effects of observation distance and field-of-view angle on effective observation time are comprehensively examined; the results indicate that under the same close-range operation model configuration, increasing the observation distance and field-of-view angle significantly enhances target observability and yields longer effective observation time. Given fixed observation distance and field-of-view angle, reducing the model configuration size also contributes to improved observation performance. This study applies sequential game theory to the multi-model combined observation problem of spacecraft for the first time, providing a systematic strategy design method for multi-aspect information acquisition of non-cooperative targets, which offers significant engineering reference value for space security assessment and on-orbit situational awareness.
Journal
Space: Science & Technology
Article Title
Spacecraft Proximity Operation Model-Based Sequential Coalitional Observation Game Strategy Design
Fig. 2. Installation diagram of spacecraft observation camera. (A) Payload installation. (B) Field of view.
Fig. 3. Schematic diagram of combined observations based on proximity operation model.
Fig. 4. The relative observed trajectory of multimodel coalitional. (A) Three-dimensional relative observed trajectory. (B) The relative observed trajectory of x-y plane. (C) The relative observed trajectory of x-z plane. (D) The relative observed trajectory of y-z plane.
Fig. 5. Effect of observation distance and field of view (FOV) on effective observation time.
Credit
Space: Science & Technology
Pre-rerouting strategy to ensure business continuity for low-earth-orbit satellite network
Beijing Institute of Technology Press Co., Ltd
image:
Fig. 1. The structure of the extended Route-LSA (the red font represents the extension parts). LSA, link state advertisement.
view moreCredit: Space: Science & Technology
With the rapid development of 6G space–air–ground integrated communication networks, low Earth orbit (LEO) satellite constellations, leveraging their advantages of wide-area coverage and freedom from geographic constraints, have become key infrastructure for achieving global seamless connectivity. However, due to the relative motion among satellites in different orbits, inter-satellite links (ISLs) exhibit periodic dynamic on–off characteristics, among which a portion of the interruptions are predictable topological changes. Conventional Open Shortest Path First (OSPF) protocols compute shortest paths solely based on link distance, without considering link effective duration or bandwidth constraints, rendering them inadequate for adapting to the dynamic topological characteristics of satellite networks. Although fast reroute strategies can address sudden link failures, they consume substantial bandwidth resources and cannot provide protection for all nodes across the entire network. Therefore, how to proactively switch service paths before predictable link interruptions occur, so as to minimize service disruption and resource overhead, has become a core issue in ensuring service continuity in LEO satellite networks.
In a recent study published in Space: Science & Technology, the research team led by Huang Shanguo from the School of Electronic Engineering, Beijing University of Posts and Telecommunications, proposed a pre-rerouting strategy based on an extended OSPF protocol. The study first extends the routing information advertisement by adding two new fields—namely, "termination time of validity" and "available bandwidth"—and achieves synchronization of the link state database across the entire network through the Link Management Protocol. Furthermore, the routing metric is redefined as a weighted combination of the conventional metric and the link effective duration, enabling the Constrained Shortest Path First (CSPF) algorithm to simultaneously account for both path length and effective duration. On this basis, a pre-rerouting mechanism is established: the source node continuously monitors the remaining effective time of the label switched path (LSP); when the remaining effective time falls below the remaining service transmission duration and is less than a preset threshold, the node automatically computes the optimal backup path based on the current network state, establishes a new LSP, performs service switching, and finally releases the resources of the old path. Functional verification based on the VxWorks embedded system and FPGA hardware platform demonstrates that the pre-rerouting mechanism achieves seamless service switching before the old link interruption, with zero packet loss during the switching process. Performance verification on the EXata hardware-in-the-loop simulation platform shows that the pre-rerouting strategy attains a packet loss rate of 1%, representing a reduction of approximately 6 percentage points compared to the 7% rate of conventional rerouting schemes. This research provides an efficient, low-loss service continuity assurance solution for LEO satellite networks in dealing with predictable link interruptions, offering significant engineering application value for the stable and reliable operation of space–air–ground integrated networks.
First, this study focuses on the service interruption problem caused by dynamic changes in inter-satellite links (ISLs) within LEO satellite networks, and proposes a pre-rerouting strategy based on an extended Open Shortest Path First (OSPF) protocol. Owing to the relative motion among satellites in different orbits, ISLs exhibit periodic on–off characteristics, a portion of which can be predicted in advance based on orbital motion laws. Conventional OSPF protocols compute the shortest path solely on the basis of link distance, without accounting for constraints such as link effective duration or bandwidth availability; meanwhile, fast reroute strategies, although capable of handling sudden failures, consume substantial bandwidth resources. To address this, the study extends the routing information advertisement by adding two new fields—namely, "termination time of validity" and "available bandwidth"—to the original link state information, as shown in Fig. 1, enabling routers to acquire information on the remaining available duration and bandwidth of each link. On this basis, the routing metric computation is redefined by incorporating link effective duration as a significant weighting factor in path selection, allowing users to adjust the weighting factor according to service requirements to strike a balance between the shortest path and the longest effective duration. As illustrated in the example in Fig. 2, a path with fewer hops but an interruption after 200 seconds cannot satisfy a service duration requirement of 800 seconds; in contrast, the Constrained Shortest Path First (CSPF) algorithm can select a path with slightly more hops but an effective duration of up to 1,000 seconds, thereby validating the effectiveness of the proposed scheme in ensuring service continuity under dynamic topologies.
Second, the study elaborates on the detailed design of the trigger and execution process of the pre-rerouting strategy, and verifies its functionality through hardware experiments. As shown in Fig. 3, the source node continuously monitors the remaining effective time of the established Label Switched Path (LSP). When this remaining time falls below the remaining service transmission duration and is less than a preset time threshold, the pre-rerouting mechanism is automatically triggered: the source node recomputes the optimal path satisfying both bandwidth and duration constraints based on the current network state, reserves resources and establishes a new LSP through the constraint-based routed label distribution protocol, and releases the resources of the old path after service switching is completed. To verify this functionality, the research team built an experimental environment based on the VxWorks embedded system and an FPGA hardware platform, with the topology shown in Fig. 4, where dynamic link on–off is simulated by plugging and unplugging ports between control nodes. The functional verification results demonstrate that when the old LSP is established using a port with a shorter effective time, upon triggering of the pre-rerouting, the source node automatically constructs a new path using a port with a longer effective time, during which ping packets are transmitted normally; after the old path is released, service transmission continues without packet loss. This proves that the pre-rerouting strategy can autonomously complete path switching before link interruption and ensure service continuity.
Finally, the study quantitatively validates the performance of the pre-rerouting strategy on the EXata hardware-in-the-loop simulation platform. In the simulation topology, two alternative paths are available between the source node and the destination node, with the primary path configured with an effective duration of 100 seconds and a service duration of 500 seconds. Fig. 5 shows that the source node transmits a total of 1,000 UDP packets. In the conventional OSPF rerouting scenario, as depicted in Fig. 6, the link fails at 100 seconds, and service transmission resumes only after an interruption of approximately 20 seconds; ultimately, the destination node receives 930 packets, corresponding to a packet loss rate of 7%. In contrast, in the scenario employing the pre-rerouting strategy, as shown in Fig. 13, the service is switched to the backup path before the interruption occurs, resulting in uninterrupted transmission throughout the entire duration; the destination node receives 990 packets, with a packet loss rate of merely 1%, representing a reduction of approximately 6 percentage points compared to the conventional scheme. The experimental results demonstrate that the pre-rerouting strategy can effectively avoid service loss caused by predictable link interruptions. The study also notes that the new path established by pre-rerouting may be inferior to the original path in terms of hop count and transmission delay, and future work could further optimize transmission performance by recomputing the globally optimal path after the interruption. This strategy provides an efficient, low-loss service continuity assurance solution for LEO satellite networks in coping with dynamic topological changes.
Journal
Space: Science & Technology
Article Title
Pre-rerouting Strategy to Ensure Business Continuity for Low-Earth-Orbit Satellite Network
Fig. 2. An example of the comparison between CSPF routes and Open Shortest Path First (OSPF) routes.
Fig. 3. The flowchart of pre-rerouting strategy. LSP, label switching path; CR-LDP, Constraint-based Routing Label Distribution Protocol.
Fig. 6. Traditional OSPF scenario. (A) The number of packets received by destination nodes. (B) Packet receiving curve. CBR, constant bit rate.
When gravity lets go: Why astronauts’ lower eyelids rise in space
image:
Comparison of terrestrial and spaceflight MRD2 values.
view moreCredit: HIGHER EDUCATION PRESS
Spaceflight consistently elevates the lower eyelid, according to a new study. Analyzing 115 NASA photographs (52 Earth, 63 space) from 13 astronauts, researchers found that MRD2—the distance from the corneal center to the lower eyelid margin—decreased by a mean of 1.0 mm in microgravity (from 4.9 ± 0.7 mm on Earth to 3.9 ± 0.7 mm in space, p < 0.001). All astronauts exhibited this “reverse ptosis,” with 62% showing a change of ≥1 mm—a threshold considered clinically significant. The effect was highly consistent between eyes (r = 0.870, p < 0.001) and comparable in magnitude to changes observed during short-term parabolic flights, suggesting the phenomenon occurs rapidly and persists through long-duration missions.
The authors propose two main mechanisms. First, cephalad fluid shift—about 2 liters of fluid moves toward the head in microgravity, with approximately 50 mL redistributing into head and neck tissues—causes pretarsal swelling that mechanically pushes the lower eyelid upward. Second, elastic recoil of facial tissues plays a role: on Earth, gravity pulls tissues downward, but in microgravity, elastic forces reposition them to minimize tension, contributing to the upward displacement. Notably, the changes were similar to those seen in parabolic flights, aligning with reports of persistent periorbital edema throughout spaceflight.
While reverse ptosis can alter corneal topography, reduce visual field, and affect visual acuity, this study was not designed to establish a direct link to the dry eye symptoms reported by approximately 30% of astronauts. Limitations include the retrospective use of publicity images (often with smiling or angled faces), reliance on average corneal diameters for calibration rather than individual measurements, and absence of symptom data. The authors call for future studies with standardized imaging protocols to clarify mechanisms and health implications for extended lunar and Martian missions.
The work titled “The Effect of Microgravity on Lower Eyelid Position in Astronauts”, was published on Eye & ENT Research (published on June 17, 2026).
Journal
Eye & ENT Research
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
The Effect of Microgravity on Lower Eyelid Position in Astronauts
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