It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Scientists using the European Space Agency’s Mars Express, in combination with a state-of-the-art meteorological model of the Red Planet, have found that there may be some very exotic physics behind Mars’s most curious cloud.
Every spring and summer in Mars’s southern hemisphere, during the martian dusty season, we see a spectacular cloud emerge: the Arsia Mons Elongated Cloud (AMEC), the most visually striking cloud on the Red Planet.
This white wisp of water ice emerges downwind of the 20-km-tall Arsia Mons volcano. It forms, grows, and fades on a daily basis, stretching out for up to 1800 km – nearly twice the length of the UK – before quickly evaporating. This cycle repeats every morning for several months.
Mars Express first revealed the AMEC in 2018, and has viewed the recurrent cloud repeatedly since. Researchers have explored its evolution, vivid dynamics and intriguing behaviour, determining it to be an orographic cloud: a type of cloud also seen on Earth that forms as wind flows past the craggy topography of a volcano or mountain.
However, when trying to model exactly how the cloud springs to life, our simulations simply didn't reproduce what we see in the images – until now.
“To create the AMEC in our modelling, we found that we needed to include some exotic physics… physics that, while included in textbooks, is treated as theoretical and usually thought not to happen in nature. It certainly hasn’t been seen in action before,” says Jorge Hernández-Bernal of LMD/CNRS/Sorbonne Université in Paris, France, lead author of the new study. “Once we included this physics in our simulations, the AMEC emerged just as we hoped.”
“Wholly unexpected”: a rare process in action
On Earth and elsewhere, clouds typically form when moist air cools and water vapour condenses into liquid droplets or icy crystals. This usually happens via a process known as heterogeneous nucleation, which requires specks of other ‘stuff’ to be present in the atmosphere for vapour to cling and condense onto – salt, pollen, soot, or dust, for instance. On Mars, it’s thought to be dust.
“For the AMEC, it seems that cloud formation takes place without needing any of this ‘stuff’,” adds Jorge. “Water vapour turns directly into icy cloud particles without any middle step. It’s akin to droplets of condensation appearing in the middle of a room, rather than on a window. We call this homogeneous nucleation, and we’ve never seen it before in a planetary atmosphere. It’s wholly unexpected.”
Scientists had suggested that this process might take place in the upper atmospheres of Earth and Venus, but it hasn’t been spotted. Its rarity is due to it requiring exceptional circumstances, with extreme relative humidity levels of over 100 000 times those usually experienced in our daily life on Earth. “We’ve not seen these conditions on Mars before, but our finding now strongly suggests that the planet’s humidity can indeed reach these extreme levels,” says Jorge.
Jorge and colleagues found that the AMEC sits in a unique position where Mars’s thin atmosphere and the towering height of the nearby Arsia Mons volcano come together to create the conditions needed to see this rare process in action.
As winds flow past Arsia Mons, the volcano’s bulk triggers a powerful wave that lifts moist parcels of air several kilometres up into the air in just a few minutes. This cools the atmosphere rapidly, causing temperatures to drop by 30 degrees in just 10 minutes and relative humidity levels to spike. Water vapour then spontaneously freezes directly into cloud particles, forming the AMEC.
A win for Mars Express
While some aspects of the modelled cloud don’t exactly match the observations, “the result is remarkable,” says Jorge. “We don’t have nearly as much information about Mars’s atmosphere as we do about Earth's, so reproducing the AMEC to this degree is a big success for the model.”
The researchers based their modelling on data from the three cameras aboard Mars Express: the Visual Monitoring Camera, High Resolution Stereo Camera, and OMEGA. Mars Express can image large swathes of the martian surface at high resolution, and it is one of the few Mars-orbiting spacecraft – alongside ESA’s ExoMars Trace Gas Orbiter – able to observe during morning hours, when the AMEC is present.
“Mars Express can also track how the cloud is changing on timescales of mere hours, which gives us an unrivalled view of short-lived phenomena on the planet,” says ESA Mars Express Project Scientist Colin Wilson. “Overall, this finding is a true accomplishment for the mission and its scientists: Mars Express discovered the AMEC, has followed up and monitored it for years, and is now helping reveal the secrets of its formation.”
Beyond furthering our understanding of atmospheric processes on Mars and elsewhere, the result highlights that we should not discount unlikely processes when exploring the planets of the Universe (including exoplanets). Colin adds: “While clouds on Earth and Mars seem to be governed by the same ‘rules’, understanding this exotic martian cloud required exotic physics – and this may be true elsewhere in the cosmos.”
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Notes for editors
Mars Express has imaged the AMEC using both the lower-resolution VMC camera and – as in this release – with the high-resolution HRSC. This marks the first ESA release featuring HRSC imagery of the AMEC, and shows the cloud in striking detail.
Earth will be long gone by then, but new research shows the outer planets will be thrown out too, and a billion times earlier than expected, according to astrophysics research.
The outer planets of the solar system will be thrown into chaos roughly a billion years after the Sun dies, a new study has found, cutting their expected life span a billion-fold.
Previous estimates put the stability of the giant planets' orbits at around a quintillion years, far longer than the current age of the universe.
Researchers at California’s Institute of Technology (Caltech) were investigating how the sun’s slow death would impact the outer planets including Jupiter, Saturn, Uranus and Neptune.
It was also previously believed by many scientists, including Isaac Newton, that the sun would shed its mass smoothly as it slowly died, transitioning first into a Red Giant and later a White Dwarf.
Earth, Venus and Mercury would be the first to go, consumed by the Sun as it expanded into a Red Giant, with Mars the lone survivor, sitting far enough away to escape the engulfment.
Earlier studies found the outer planets would survive this process, with their orbits widening to about twice their current size as the Sun lost mass. Their eventual break-up was expected to come only from passing stars, over 30 to 100 billion years.
The new findings suggest this picture is wrong. Instead, the outer planets will be thrown into disorder roughly one billion years after the Sun becomes a white dwarf, which is still billions of years away.
When the Sun becomes a white dwarf, it rapidly cools and shrinks to half its current size, but this will not be a smooth process and previously thought.
Relate
The research argues the star will lose its mass in violent, random ejections known as “stochastic kicks,” rather than a calm, consistent release.
The new findings suggest this picture is wrong. Instead, the outer planets will be thrown into disorder roughly one billion years after the Sun becomes a white dwarf, which is still billions of years away.
The solar system is 4.57 billion years old, and the Sun has around five billion years left before it swells into a red giant. Scientists had long assumed the giant planets would outlast it by an almost unimaginable stretch of time.
The research was carried out using high-performance supercomputers and hundreds of advanced N-body numeral simulations, with no AI tools involved whatsoever.
The researchers programmed a virtual model of the solar system containing the Sun, Jupiter, Saturn, Uranus and Neptune, which the supercomputers then calculated and simulated the velocity and position over billions of years.
What made this year analysis different from previous research was the scripting of the sun losing its mass as it died via random, violent, episodic bursts, with hundreds of different model iterations tested in order to come to a consensus conclusion.
The findings were reported in a study called ‘Terminal Instability of the Solar System Triggered by Stochastic Solar Mass Loss’, published in The Astrophysical Journal Letters.
Monday, September 28, 2026
SPACE/COSMOS
Great news from Saturn’s moon Enceladus in the search for life in space
What are the chances of finding extraterrestrial life in our solar system? Professor Frank Postberg, planetary scientist at Freie Universität Berlin, has just published a study in Science Advances with a team of international researchers, in which they present new evidence that it is easier to determine the constituents of the ocean hidden under Enceladus’s icy surface than previously assumed. Postberg was also part of a second study published on the same day, in which he and another planetary scientist from Freie Universität, Dr. Nozair Khawaja, contributed to research that reveals that certain microorganisms could actually tolerate the conditions in Enceladus’s ocean better than was previously thought. These results increase the probability of finding evidence of life on Saturn’s moon.
What Makes Enceladus so Attractive in the Search for Life in Space?
Enceladus is considered to be one of the most promising places to search for extraterrestrial life in our solar system. Researchers suspect that under the moon’s icy crust there is a global ocean of liquid water and a rocky core further below. Due to cryovolcanic activity, gigantic plumes break through cracks in the crust at the moon’s south pole, ejecting ice particles hundreds of kilometers into space.
NASA’s Cassini spacecraft passed through these plumes multiple times to analyze its composition. Enceladus’s ocean is the only extraterrestrial “body of water” from which scientists have been able to directly analyze samples. The samples revealed traces of various salts and organic compounds. Furthermore, previous analyses conducted by Cassini gave indications of hydrothermal processes on the seafloor and other conditions that are significant in supporting life.
Enceladus Ice Plumes Separate and Sort the Ocean’s Components
Postberg’s study, “Cassini CDA Observes Compositional Segregation of Enceladus’ Ice Grains from Slow Freezing and Fragmentation of Oceanic Spray,” includes surprising findings about what happens to the ocean water on its way into space. The international team of scientists working on the study used Cassini data, long-term laboratory experiments, and theoretical models to reconstruct the process.
Droplets form at the ocean’s surface as bubbles filled with gas float up and pop. Water vapor then carries the droplets through cracks in the ice shell out into space. Until now, scientists believed that they froze instantaneously, however these new findings reveal that they freeze slowly. Due to this slow process, most components (including dissolved ones) separate from each other. Salts and organic materials are thus distributed at different locations inside each freezing droplet. Various types of previously dissolved salts also get segregated in the process; for example, sodium chloride (table salt) separates from sodium carbonate.
On their way up, the frozen droplets are accelerated to speeds of up to 1,000 km/h. If they smash into the walls of the icy cracks, they break into fragments only a few micrometers in size before they shoot into space. This results in the ice particles often consisting of just one highly concentrated, previously segregated, substance.
“Enceladus actually does a lot of the work for us in preparing samples for analysis that usually take a lot of effort in chemical labs on Earth,” says Postberg, who led the study. “The oceanic constituents are separated from each other and simultaneously concentrated into individual ice particles.”
Making It Easier to Find Signs of Life
When probing Enceladus’s plumes, this mechanism is not just helpful in characterizing the ocean as a potential habitat for life, it is also particularly interesting in the search for biosignatures, i.e., measurable indications of life. If one of these ocean droplets contained components from alien microbes, they could be segregated from other components in the freezing process. After fragmentation, the microbial material would potentially only be contained in a small fraction of ice particles; however, in those ice particles it would be found in high concentration and in relatively pure form.
“That is great news in the search for life,” says Postberg. “Future spacecrafts will have to analyze many individual ice particles in the plume. But if they come across one with microbial material in it, they could identify biosignatures in the particle relatively easy with already available technology.”
This discovery could have important implications for future space missions to Enceladus, such as the ESA’s L4 mission, which is currently in planning. The mission will be specifically looking for signs of life on Saturn’s moon. Postberg’s lab at Freie Universität Berlin has previously conducted laboratory studies that demonstrated the capacity for specialized instruments to detect microbial cellular material in individual particles from the ice plumes.
Could Life Really Exist on Enceladus?
A recent study is also providing new insights into this question. On the same day Postberg’s article appeared in Science Advances, another article in the same journal was published by scientists at Ludwig-Maximilians-Universität München (LMU): “Enceladus-Like Geochemistry Fuels Methanogenesis under Extreme CO₂-Limitation.” Postberg and another colleague from Freie Universität Berlin, Dr. Nozair Khawaja, contributed to this study. In it, scientists were able to reproduce the conditions of Enceladus’s ocean in a laboratory. The ocean has a very low concentration of oxygen, a very high concentration of carbonate, and is very alkaline (with pH values of 10 or 11). After recreating these conditions, including its hydrothermal interaction with the rocky ocean floor, they introduced Methanothermococcus okinawensis into the contained environment. This microorganism is a methane-producing archaean that normally lives near deep-sea hydrothermal vents on Earth. They do not need oxygen, which is rare on Enceladus, to survive. Instead their metabolism requires only hydrogen and carbon dioxide.
The results were surprising: The organism failed to grow in an optimum laboratory medium at such a high pH, lacking dissolved carbon dioxide. By contrast, in the Enceladus simulant it continued to grow, producing methane using hydrogen generated by water-rock reactions. Under the simulated Enceladus conditions, the microorganisms were even able to adapt their metabolism to the low amounts of carbon dioxide. “This was really a surprise to us,” Khawaja said. “This was an experiment for which we did not expect such a successful outcome.”
Taken together, the two studies in Science Advances shed new light on the search for extraterrestrial life. “On Enceladus the specific geochemical conditions might allow one of the oldest known metabolic systems on Earth to work, even in very alkaline environments,” Postberg says. “While that doesn’t mean that there is life on Saturn’s moon, our first study shows that – in the event that there is – future space missions might have a good chance of finding traces if they analyze individual ice grains from Enceladus’s plume.”
Freie Universität Researchers Investigating the Emergence of Life in Space
What are the prerequisites for planets and moons to provide suitable conditions for the formation and preservation of life? This is a central research topic for planetary scientists at Freie Universität Berlin. A new Collaborative Research Center funded by the German Research Foundation (DFG), CRC1759 “Habitability as a Fundamental Planetary Process,” was launched in July 2026. The research center is led by Professor Lena Noack with Professor Frank Postberg as her deputy, both of whom are planetary scientists at Freie Universität Berlin. The research center will study the processes that could have enabled life to emerge in space in the first place, on Enceladus and other celestial bodies, both in our solar system and beyond.
Cassini CDA Observes Compositional Segregation of Enceladus’ Ice Grains from Slow Freezing and Fragmentation of Oceanic Spray
Article Publication Date
25-Sep-2026
New study detects ammonia and unexpected chill on a distant giant planet
A UMD-led team found water, methane and a rare ammonia signal in the atmosphere of HATS-6 b, a large planet orbiting a tiny star—raising new questions about how giant planets form
Using the James Webb Space Telescope (JWST), a team of astronomers led by the University of Maryland identified water, methane and ammonia in the atmosphere of HATS-6 b, a giant planet 500 light-years away from Earth. Observations captured by JWST also revealed that HATS-6 b may be significantly cooler than expected compared to standard calculations of its temperature—suggesting that the planet’s relationship with its star may be far more complicated than previously thought.
The team’s findings published in the Astronomical Journal on September 8, 2026, raise new questions about how giant planets form and evolve.
HATS-6 b is roughly the size of Jupiter, completing an orbit every three days around an M dwarf—a small, cool, reddish star. Planets are built from the leftover disk of gas and dust that surrounds such stars after their formation, and a smaller star leaves behind only small disks. But HATS-6 b’s gigantic size doesn’t quite fit into that rule, considering how small the star it orbits is.
“These smaller stars don’t have enough material or enough time to create planets as big as Jupiter and as big as Saturn,” explained the study’s lead author Giannina Guzmán Caloca, an astronomy Ph.D. candidate at UMD. “So, the fact that HATS-6 b can exist is really interesting because it shouldn’t be possible with what we know.”
Astronomers know only about 40 such planets. HATS-6 b is one of seven being studied in a JWST program called Giant Exoplanets around M-dwarf Stars (GEMS). The program is designed to compare these outliers against the better-understood giants circling stars like our sun.
“Every one of these planets is a challenge to formation theory,” Guzmán Caloca said. “By measuring what their atmospheres are made of, we can start to ask whether they were built the same way as the hot Jupiters around sun-like stars or whether something different is going on.”
Using a technique called transmission spectroscopy, which involves watching starlight filter through a planet’s atmosphere, the team identified four molecules in HATS-6 b’s air: water, methane, ammonia and carbon dioxide. The discovery marks only the second time the technique detected ammonia on a distant world.
“Carbon, hydrogen and oxygen are all things that have been previously found in atmospheres of giant planets outside our solar system, but ammonia is something almost never detected before,” Guzmán Caloca explained. “It’s an entirely new molecule to think about.”
Because nitrogen-bearing molecules like ammonia should be more abundant in cooler giant planets than in scorching hot Jupiter-like ones, the discovery supports the theory that planets that orbit M-dwarf stars may be a chemically distinct population.
HATS-6 b's unexpected temperature raises another major question. The commonly cited temperature for HATS-6 b—near 800 degrees Fahrenheit—is not an exact measurement but a calculation that assumes the planet absorbs all the light its star delivers and spreads that heat evenly. But scientists’ early analyses returned temperatures closer to 250 degrees Fahrenheit, a figure that’s physically improbable for a planet orbiting its star every three days.
“If the planet is genuinely that cool, it means that something is probably reflecting a great deal of starlight back into space before it can warm anything,” Guzmán Caloca explained. “The likeliest explanation is cloud and haze wrapping the planet the way they wrap Venus.”
The team’s results have implications beyond just HATS-6 b. Because a planet’s temperature is folded into every calculation of what its atmosphere contains, a discrepancy this large raises questions about how reliably astronomers can read the atmospheres of planets orbiting small, active stars.
For Guzmán Caloca and her team, many mysteries remain. Longer-wavelength observations could identify other unexplained signals and test whether clouds explain a planet's unexpectedly low temperature. Astronomers have found more than 6,000 planets beyond our solar system, and many of them look nothing like the ones closest to Earth. Reading their atmospheres—what they're made of, how they formed, which ones resemble Jupiter and which resemble nothing at all—can help scientists determine whether other solar systems were created through formation mechanisms similar to our own.
“What is our context? And how rare or how common are we?” Guzmán Caloca asked. “This is one planet out of thousands, but the way I like to think about it is that Earth is also one planet out of thousands and yet it holds everything that ever lived here.”
This research was supported by NASA through a grant from the Space Telescope Science Institute for JWST program GO 3171, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract (NAS 5-03127). The observations were obtained from the Mikulski Archive for Space Telescopes (MAST) at STScI, with support for MAST for non-HST data provided by the NASA Office of Space Science (NNX09AF08G) and other grants and contracts. Additional support was provided by NASA Headquarters through an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by ORAU under contract with NASA, and under NASA award (80GSFC24M0006); by the National Science and Technology Council of Taiwan (114-2112-M-001-065-MY3); and by an Academia Sinica Career Development Award (AS-CDA-115-M03). The text does not necessarily represent the views of these organizations.
Scientists have long speculated why Venus, with a similar size, mass, and structure to Earth, doesn’t have a moon. New UC Riverside research shows it’s because our hungry twin likely swallowed it.
In the past, scientists have theorized that Venus possibly had a moon at one time that got hit with something massive, obliterating it. Other theories centered around the idea that Venus never underwent a collision that formed a moon in the first place.
The new research, published in The Astrophysical Journal, shows that neither theory fits. “My study shows Venus didn’t require a catastrophe to arrive at what we can see today,” said UCR astrophysicist and lead author Stephen Kane. “It turns out the gravity of the planet itself combined with the rate at which it spins naturally caused the moon to collapse on top of it.”
Scientists can precisely measure the distance between Earth and the moon because NASA’s Apollo 11 landing left mirrors there. “We know that the moon is slowly moving away from the planet, at a rate of around four centimeters per year,” Kane said. This distancing, Kane said, is happening because Earth spins at a relatively fast 24 hours per rotation. The energy from this spinning gets transferred to the moon, causing it to distance itself.
Venus has the exact opposite situation. It takes that planet 243 Earth days to complete a single rotation. So, rather than gradually moving away, the slow spin and the planet’s gravity would cause a moon to spiral inward toward a collision.
To test this idea, Kane wrote computer models based on the physics of how planetary bodies interact through gravity. He first reproduced the evolution of Earth and its moon to make sure the model accurately represented a known system.
Then he varied Venus’ rotation rate and the size of its hypothetical moons, testing moons with masses ranging from half to ten times the mass of Earth’s moon. In most simulations, the result was the same: the moon crashed into Venus. And the more massive the moon, the faster it crashed.
“When I made this discovery, I was shocked,” Kane said. “I thought surely the broad range of scenarios I was exploring would lead to a variety of results. But it all went pretty much in the same direction.”
The finding does not prove that Venus once had a moon. Kane believes it may have, but whether one ever formed remains an open question. The study does show that if Venus had a moon, it could not have continued existing indefinitely.
Finding physical evidence of such a collision would be difficult. Roughly 80% of Venus’ surface is similar in age, evidence of a major resurfacing event about a billion years ago that erased much of the planet’s earlier geological history.
Evidence could instead lie beneath the surface. Scientists believe Earth’s moon formed after a massive collision early in the planet’s history, and seismic studies have revealed unusual structures deep inside Earth that may be remnants of that event. Similar measurements on Venus could offer clues about whether it once absorbed a moon.
Such a collision could also help scientists understand another longstanding mystery about Venus: whether Earth’s nearest planetary neighbor was ever capable of hosting life.
A moon crashing into Venus would have transferred enormous energy and angular momentum to the planet, potentially changing its rotation, geology, and climate. If Venus once had oceans or other conditions favorable to life, such an impact could have altered the course of the planet’s evolution.
The implications extend beyond Venus. Scientists searching for potentially habitable worlds around other stars often look for planets resembling Earth and consider the presence of a moon one factor that could influence habitability.
Earth’s moon drives tides, may have helped keep the planet geologically active, and has profoundly influenced Earth’s evolution. Scientists do not know, however, whether a large moon is necessary for life.
“My feeling is there are benefits to having a moon, but it isn’t required for habitability,” Kane said. “The moon has definitely changed the way Earth has evolved through time, but we don’t fully know how important that role is.”
Kane’s findings suggest that even planets capable of forming moons may not be able to keep them. Slowly rotating worlds could send their moons spiraling toward their surfaces, dramatically changing the planets in the process.
“When people think about Earth twins around other stars, one question they ask is, ‘Does it have a moon?’” Kane said. “My study shows a disturbing scenario for many of those cases. If these planets don’t rotate fast enough, the moon will crash to the surface, and that would change the course of history for those planets.”
Cambridge, MA (September 14, 2026) —A new study of nine nearby galaxies has shown that actively growing supermassive black holes may contribute to star formation, instead preventing star formation, in their host galaxies.
The research, based on observations from the VLT/MUSE instrument, shows that active galactic nuclei (AGN), or bright regions powered by material falling into a supermassive black hole, are associated with star-forming rings or arcs, cone-shaped regions of energized gas, and fast “shocks,” which occur when energy outflows interact with surrounding gas.
The findings give a new perspective on how AGN feedback could influence the growth and evolution of galaxies.
“Once we resolved them, we could see that they not only accrete things, but they also eject things,” said Peixin Zhu, graduate student and astronomer at the Center for Astrophysics. “The injection and accretion are linked with each other.”
The study focused on galaxies whose central black holes are actively accreting, or pulling in, nearby material.
“We’re seeing that black holes are not just consuming material at the centers of galaxies, but they’re actively reshaping their surroundings,” said Lisa Kewley, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian, director of the Center, and Zhu’s advisor. “This work helps us understand a complex feedback cycle that plays an important role in galaxy evolution.”
The team used a new three-dimensional diagnostic technique to distinguish among three sources: star formation, radiation from the active black hole and excitation by shocks. Shock excitation occurs when high-speed outflows from the central black hole collide with the interstellar medium.
The galaxy NGC1386, shown in grayscale on the left and zoomed in to the central region on the right. The colors represent star formation in red, black-hole radiation in blue, and shocks in yellow. Image courtesy Peixin Zhu.
Across the sample, the researchers found that star-forming rings or arcs appeared at distances of about 0.8 to 6 kiloparsecs from the galactic center. Ionized cones of black-hole radiation extended outward from the galaxies’ discs, while central regions dominated by fast shocks often extended perpendicular to those bicones.
The researchers also found evidence for shocks in directions perpendicular to the AGN bicones. Zhu notes that these shocks are broadly consistent with interactions between AGN jets and the interstellar medium, although winds from the active black hole may also contribute, particularly in galaxies with lower-power jets.
“The most interesting phenomena about shocks is that they always go perpendicular to where the black hole’s injected outflows go,” Peixin said. “It is very common, and we see it consistently appearing across the whole nine galaxies.”
The study’s results combined high-resolution observations and detailed theoretical models. The MUSE instrument provided spatially-resolved optical data, while the state-of-the-art theoretical models built by Zhu and her colleagues, astrophysicists Lisa Kewley of the Center for Astrophysics and Ralph Sutherland of the Australian National University, allowed them to compare observations with predictions for black hole activity, star formation and shocks. Chandra X-ray observations also independently supported the researchers’ interpretation.
The results demonstrate that actively growing black holes have a complex cycle of accretion, outflow and interaction with their surrounding galaxies. By separating the effects of black hole radiation, star formation and shocks, the study provides a clearer view of that cycle and its relationship to star formation.
About the Center for Astrophysics | Harvard & Smithsonian
The Center for Astrophysics | Harvard & Smithsonian is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory designed to ask, and ultimately answer, humanity’s greatest unresolved questions about the universe.
The galaxy NGC1386, shown in grayscale on the left and zoomed in to the central region on the right. The colors represent star formation in red, black-hole radiation in blue, and shocks in yellow. Image courtesy Peixin Zhu.
With growing concerns over the impact threat of near-Earth small bodies and the utilization of space resources, the orbital evolution of binary asteroid systems has become an important subject in space science. When describing the full two-body dynamics of such systems, the gravitational potential of irregular bodies is determined by both their positions and attitudes, leading to coupling between translational and rotational motions, and the resulting model complexity significantly exceeds that of the classical two-body problem. Although existing numerical methods, such as explicit Runge–Kutta integrators, are widely used, they fail to preserve the symplectic structure and Lie group geometric properties of the system. In long-term simulations, key conserved quantities such as total energy and angular momentum are prone to numerical drift, severely undermining the long-term credibility of orbital predictions. While Lie group variational integrators can preserve geometric structure to a certain extent, their implicit equations are formulated on Lie group elements, which incurs substantial computational overhead. Therefore, how to construct a high-performance integration method that simultaneously achieves numerical accuracy, structure preservation, and computational efficiency has become a core challenge in the long-term evolutionary simulation of binary asteroid systems.
In a recent study published in Space: Science & Technology, the team led by Guo Yongxin from the College of Physics, Liaoning University, proposed a numerical method for the full two-body dynamics of a double-dumbbell system based on the Hamel variational integrator. The study employs the special Euclidean group SE(3) to uniformly describe the translational and rotational motions of the dumbbell-shaped rigid bodies, introduces a body-fixed coordinate frame attached to the second dumbbell, reduces the system motion to the evolution of relative position and attitude, and constructs discrete equations of motion on the Lie algebra based on the discrete Hamilton's principle, thereby achieving an efficient iterative scheme for the Hamel variational integrator. Simulation results demonstrate that in regular-shaped examples, the proposed method outperforms Lie group variational integrators in energy conservation and orthogonality preservation, and, since the implicit equations are formulated on the Lie algebra rather than on Lie group elements, it incurs lower computational cost. In irregular-shaped examples, the total energy error and rotation matrix orthogonality error of the proposed method are significantly superior to those of the Runge–Kutta method, which fails to maintain orthogonality structure and consequently leads to substantial accumulation of force and torque computation errors over time. Irregular shape also induces noticeable deviations in the barycentric trajectory of the double-dumbbell system along specific directions, indicating that the irregular geometry of celestial bodies has a non-negligible effect on full two-body motion. This study provides a numerical scheme that balances structure preservation and computational efficiency for high-precision long-term orbital evolution of binary asteroid systems, offering significant methodological support for planetary defense mission planning and long-term dynamical prediction of binary systems.
First, this paper focuses on the full two-body dynamics problem in the orbital evolution of binary asteroid systems. Binary asteroids constitute a significant proportion of near-Earth asteroids, with approximately 16% of near-Earth asteroids possibly being binary systems; the assessment of their impact threat and the planning of exploration missions both rely on high-precision predictions of the long-term dynamical behavior of such systems. In the full two-body problem, the gravitational potential of irregular bodies is determined by both their positions and attitudes, resulting in coupling between translational and rotational motions, and the model complexity is substantially greater than that of the classical two-body problem. Owing to the relatively small separation between the two asteroids in a binary system and the modest difference in their masses, each binary system can be modeled as a rigid dumbbell, thereby establishing a double-dumbbell full two-body system as shown in Fig. 1. The system consists of two dumbbell-shaped rigid bodies, each composed of two asteroids connected by a massless rod. To describe the configuration of the dumbbells, the study employs the special Euclidean group SE(3) to uniformly represent position and attitude, and introduces a body-fixed coordinate frame attached to the second dumbbell, reducing the system motion to the evolution of the relative position and attitude of the first dumbbell with respect to the second. This modeling approach avoids the singularity issues inherent in Euler angle representations and naturally captures the coupling effects between translation and rotation.
Second, based on the discrete Hamilton's principle, the paper constructs a Hamel variational integrator tailored for the double-dumbbell system. The core idea of the Hamel variational integrator is to utilize the body-frame motion framework provided by the left-invariant vector fields on the Lie group, expressing both the continuous and discrete equations of motion entirely on the Lie algebra. Fig. 2 illustrates the computational procedure for the double-dumbbell system: first, the continuous Euler–Lagrange equations and Hamilton equations are derived from Hamilton's principle; subsequently, the discrete Hamilton's principle is employed to construct the discrete Lagrangian, yielding the discrete equations of motion on the Lie algebra. Since all equations are formulated at the Lie algebra level, the computational complexity of the implicit equations is lower than that of Lie group variational integrators based on Lie group elements. Through the discrete Legendre transform, an explicit iterative scheme in the Hamiltonian form is established; within each time step, the state update is accomplished by solving the implicit equations on the Lie algebra, sequentially obtaining the evolution of the relative position, relative attitude, and each momentum variable. This method preserves both the symplectic and Lie group structures of the system, while maintaining long-term exact conservation of total energy and angular momentum.
Finally, this paper systematically validates the performance of the Hamel variational integrator through numerical simulations of both regular-shaped and irregular-shaped dumbbell models. Fig. 3 presents the energy evolution over time, in which the kinetic and potential energies exhibit periodic interconversion; at the ninth unit of time, the two dumbbells reach their closest distance, corresponding to minimum potential energy and maximum kinetic energy, while the total energy remains constant. Figs. 4 and 5 compare the three methods in terms of energy error and orthogonality error, respectively: both the Hamel variational integrator and the Lie group variational integrator outperform the Runge–Kutta method, with the Hamel variational integrator demonstrating superior structure-preserving performance. The CPU time comparison in Fig. 6 reveals that the Hamel variational integrator achieves slightly higher computational efficiency than the Lie group variational integrator, as its implicit equations are formulated on the Lie algebra rather than on Lie group elements. In the irregular-shaped examples, the Runge–Kutta method fails to preserve the orthogonality of the rotation matrix, leading to significant accumulation of force and torque computation errors over time, whereas the Hamel variational integrator consistently maintains low errors. By comparing the barycentric trajectories of the two dumbbell models, it is found that the irregular shape induces a noticeable deviation in the y-direction of the barycentric trajectory, indicating that the irregular geometry of celestial bodies exerts a non-negligible influence on full two-body motion. This study provides a numerical method that balances structure preservation and computational efficiency for long-term high-precision orbital evolution of binary asteroid systems.
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.
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.
