From a position 1.5 million km away from Earth, located between our planet and the Sun, SOHO has an uninterrupted view of the solar surface. Since launch, it has provided an almost unbroken record of solar activity spanning nearly three full 11-year solar cycles.

“It is testament to the ingenuity of our engineers, operators and scientists, and to international collaboration, that this mission has exceeded all expectations,” says Prof. Carole Mundell, ESA Director of Science. “SOHO has overcome nail-biting challenges to become one of the longest-operating space missions of all time.”

“The SOHO mission is a great example of the incredible partnerships between NASA and ESA,” adds Nicky Fox, associate administrator, Science Mission Directorate at NASA Headquarters in Washington. “Congratulations to the NASA and ESA teams on an amazing thirty years working together.”

Despite its achievements, SOHO’s journey has not been smooth. About two-and-a-half years after launch, the spacecraft experienced a major malfunction that sent it into an uncontrolled spin and severed communications with Earth. For three months, an international team worked continuously to find the spacecraft and bring it back to life.

Another serious setback occurred in November and December 1998, when its stabilizing gyroscopes failed. Engineers quickly developed new software, and by February 1999, SOHO could operate without gyroscopes. This breakthrough allowed it to continue gathering data that would go on to reshape the field of solar science.

“SOHO pioneered new fields in solar science. It is a game-changer in the study of space weather, providing real-time monitoring of the Sun to forecast potentially dangerous solar storms heading towards Earth, and its legacy continues to guide future missions,” says Daniel Müller, ESA Project Scientist for SOHO and Solar Orbiter.

“SOHO is still producing high-quality data on a daily basis, and with hundreds of papers being published every year, its scientific productivity remains very high.”

Daniel’s new paper ‘SOHO’s 30-year legacy of observing the Sun’ was published in Nature Astronomy on December 2, 2025.

Here are five highlights from the last five years:

1. A single plasma conveyor belt

SOHO became a leading instrument in the development of helioseismology, which studies how sound waves move through the Sun in a way similar to how seismic waves reveal the interior of Earth. Early in the mission, SOHO captured the first images of plasma flows (electrically charged material) beneath the solar surface, giving scientists an unprecedented look inside the Sun.

Because SOHO has remained active for so long, researchers have been able to answer a long-standing question: plasma circulates in a single loop, or cell, within each hemisphere of the Sun, instead of in multiple cells as once believed.

The observations show that plasma takes roughly 22 years to complete this full circuit. It rises from regions near the equator toward the poles before sinking deep inside the Sun and moving back toward the equator. This timing matches the Sun’s magnetic cycle and helps explain why sunspots, which are produced when strong magnetic fields break through the surface, tend to appear progressively closer to the equator as each solar cycle unfolds.

2. Does the Sun shine steadily?

Measuring the energy emitted by the Sun is essential for understanding how solar heating influences Earth’s atmosphere and climate. SOHO’s long-running record, paired with earlier datasets, now provides nearly fifty years of high-quality observations.

Scientists have found that the Sun’s total energy output changes very little, varying by only about 0.06% over the solar cycle. However, its extreme ultraviolet radiation changes far more significantly, doubling between solar minimum and solar maximum. This form of radiation affects the chemistry and temperature in Earth’s upper atmosphere, but it is not a direct cause of the long-term warming trend near the planet’s surface.

3. Solar storm monitoring made law

SOHO has played such a pivotal role in the development of real-time space weather monitoring systems that it was signed into United States law in October 2020.

The ‘Promoting Research and Observations of Space Weather to Improve the Forecasting of Tomorrow’ (PROSWIFT) act specifically mentions SOHO’s Large Angle and Spectrometric Coronagraph (LASCO) instrument.

LASCO is a coronagraph, a telescope with a disc masking the centre of view. By blocking out the direct light coming from the Sun, the instrument can see light from the surrounding atmosphere, called the corona. This allows us to see coronal mass ejections — large eruptions of solar material and magnetic fields — as they set off from the Sun, providing up to three days warning of potentially disruptive incoming space weather reaching Earth.

4. 5,000 comets — and counting!

The telescope’s prowess as a comet hunter was unplanned, but turned out to be an unexpected success. Thanks to the screening effect of SOHO’s coronagraph, ‘sungrazer’ comets — those that approach the Sun at very close distances — also become visible.

Not all comets seen by SOHO are sungrazers. For example, it also beautifully captured Comet Tsuchinshan-ATLAS, also called the Great Comet of 2024, a non-periodic comet from the outer reaches of the Solar System.

SOHO discovered its 5,000th comet in March 2024, making it the most prolific comet-discoverer in history. Most of these have been found by citizen scientists worldwide through the Sungrazer Project. The observations have provided valuable data on the movement, composition and dust production of comets.

5. Enabling future discoveries

SOHO’s longevity and accomplishments have influenced the design, goals, and collaborative approach of new solar observatories. It has also helped set standards for open data and international cooperation in solar research.

For example, the ESA-led Solar Orbiter mission is imaging the solar poles from higher latitude and flying much closer to the Sun, with many of its instruments being successors of SOHO’s. Similarly, NASA’s Solar Dynamics Observatory carries improved versions of SOHO’s instruments to continue the legacy that SOHO began in areas of full-disc imaging and helioseismology. Moreover, SOHO frequently contributes to ‘multipoint’ measurements, providing essential context for Solar Orbiter and NASA’s Parker Solar Probe as they fly along their own unique orbits around the Sun.

Even more recently, ESA’s Proba-3 took to the skies to open up new views of the Sun’s faint corona, while the Agency’s upcoming Vigil mission will be the first to monitor the Sun from the ‘side’, detecting solar storms before they roll into SOHO’s line-of-sight.

“SOHO is an all-round shining success, thanks to the dedication of the teams keeping this incredible machine flying,” says Daniel. “Its science remains valuable and relevant, serving generations of scientists, and I’m certain that its legacy will continue to guide solar science for decades to come.”

“As space activity accelerates, from mega-constellations of satellites to future lunar and Mars missions, we must make sure exploration doesn’t repeat the mistakes made on Earth,” says senior author and chemical engineer Jin Xuan of the University of Surrey. “A truly sustainable space future starts with technologies, materials and systems working together.”

Growing debris and the problem of abandoned satellites

The environmental toll continues long after launch. Most spacecraft and satellites are never recycled, which means that large amounts of material are permanently lost when missions end. Many older satellites are shifted into “graveyard orbits,” while others become drifting orbital debris that can disrupt the operation of active systems.

The authors argue that this approach cannot continue, especially with the increasing pace of private space missions. They highlight the need for a circular space economy, a model in which materials and equipment are created with reuse, repair, and recycling in mind. They also note that industries such as personal electronics and automotive manufacturing have already adopted similar ideas with considerable success.

“Our motivation was to bring the conversation about circularity into the space domain, where it’s long overdue,” says Xuan. “Circular economy thinking is transforming materials and manufacturing on Earth, but it’s rarely applied to satellites, rockets, or space habitats.”

Applying the 3 Rs to spacecraft, satellites, and space stations

According to the team, the foundation of a circular space economy lies in the 3 Rs: reduce, reuse, and recycle. Reducing waste would begin with building satellites and spacecraft that last longer and can be fixed more easily in space. They also suggest turning space stations into multifunctional centers where spacecraft can refuel, undergo repairs, or even have new components manufactured, which could cut down on the number of launches required.

The authors add that bringing spacecraft and space stations safely back to Earth for reuse would require better recovery systems, including technologies such as parachutes and airbags. They point out that equipment in space experiences significant wear because of extreme temperatures and radiation, so any part intended for reuse would need to pass strict safety checks.

Recovering orbital debris and using advanced technology for safer space operations

The researchers also recommend new efforts to gather orbital debris, such as using robotic arms or nets to collect fragments so the materials can be recycled. This would also help prevent collisions that create even more debris.

Data-driven tools will play an important role in this transition, the authors say. Information gathered from spacecraft could guide improvements in design and help limit waste, while simulation tools may reduce the need for expensive physical testing. They add that AI systems could help spacecraft and satellites avoid dangerous debris in real time.

Transforming the entire space system through innovation and global cooperation

The authors emphasize that a circular space economy represents a major shift in how the space sector works. Instead of focusing on single pieces of hardware, the entire system needs to be considered at once, from the materials used to how spacecraft are operated and retired.

“We need innovation at every level, from materials that can be reused or recycled in orbit and modular spacecraft that can be upgraded instead of discarded, to data systems that track how hardware ages in space,” says Xuan.

“But just as importantly, we need international collaboration and policy frameworks to encourage reuse and recovery beyond Earth. The next phase is about connecting chemistry, design, and governance to turn sustainability into the default model for space.”

This research received support from the UK Engineering and Physical Sciences Research Council, the Leverhulme Trust, and the Surrey-Adelaide Partnership Fund.

According to a study published Dec. 1 in the Proceedings of the National Academy of Sciences, researchers from CU Boulder and their collaborators report that billions of years ago, the young planet’s atmosphere may have been generating sulfur-based molecules that are known today as important components for life.

This discovery challenges the long-standing idea that these sulfur molecules formed only after life had already taken hold on Earth.

“Our study could help us understand the evolution of life at its earliest stages,” said first author Nate Reed, a postdoctoral fellow at NASA who conducted the research while working in the Department of Chemistry and the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU Boulder.

Sulfur’s Importance and Why the Findings Matter

Sulfur, much like carbon, is a vital element found in every form of life, from bacteria to humans. It appears in certain amino acids, which serve as the basic building blocks of proteins.

Although sulfur was present in the early atmosphere, most scientists believed that organic sulfur molecules, such as amino acids, arose only after living organisms were already present and producing them.

Earlier attempts to simulate early Earth conditions often failed to generate meaningful amounts of sulfur biomolecules before life existed. When these molecules did appear, they formed only under unusual or highly specific conditions that were unlikely to have been common across the planet.

Because of this background, the scientific community reacted strongly when the James Webb Space Telescope detected dimethyl sulfide, a sulfur compound produced by marine algae on present-day Earth, in the atmosphere of an exoplanet called K2-18b. Many considered it a possible sign of life.

New Experiments Reveal Atmospheric Chemistry at Work

However, previous work by Reed and senior author Ellie Browne, a chemistry professor and CIRES fellow, showed that dimethyl sulfide could form naturally in the lab using only light and simple atmospheric gases. This indicated that the molecule might appear even on worlds without life.

In their latest experiment, Browne, Reed, and their team tested what Earth’s early sky might have been capable of producing. They illuminated a mixture of methane, carbon dioxide, hydrogen sulfide, and nitrogen to recreate atmospheric conditions from before life emerged.

Working with sulfur is challenging, Browne noted. The element sticks to laboratory equipment, and in the atmosphere, sulfur-based molecules are present at extremely low levels compared to CO2 and nitrogen. “You have to have equipment that can measure incredibly tiny quantities of the products,” she said.

Using a very sensitive mass spectrometer to identify and measure chemical compounds, the researchers discovered that their early Earth simulation produced a wide range of sulfur biomolecules. These included the amino acids cysteine and taurine, along with coenzyme M, which plays a key role in metabolism.

A Sky Capable of Supporting a Growing Ecosystem

The team then estimated how much cysteine an entire ancient atmosphere might generate. Their calculations suggested that early Earth’s sky could have produced enough cysteine to support about one octillion (one followed by 27 zeros) cells. By comparison, modern Earth contains roughly one nonillion (one followed by 30 zeros) cells.

“While it’s not as many as what’s present now, that was still a lot of cysteine in an environment without life. It might be enough for a budding global ecosystem, where life is just getting started,” Reed said.

The researchers propose that these atmospheric biomolecules may have fallen to the surface through rainfall, potentially delivering the chemistry needed to help life begin.

“Life probably required some very specialized conditions to get started, like near volcanoes or hydrothermal vents with complex chemistry,” Browne said. “We used to think life had to start completely from scratch, but our results suggest some of these more complex molecules were already widespread under non-specialized conditions, which might have made it a little easier for life to get going.”

Dating to around 250 B.C., and possibly earlier in some sections, the structure is believed to be one of the earliest known examples of large-scale Roman architecture outside of temples and defensive walls.

Mogetta, who chairs Mizzou’s Department of Classics, Archaeology and Religion, explained that monumental construction served purposes beyond practical use. He noted that it also functioned as a strong form of political messaging.

“This discovery gives us a rare look at how the early Romans experimented with city planning,” he said. “Its location — at the center of the city near the main crossroads — suggests it may have been a monumental pool that was part of the city’s forum, or the heart of public life in Roman towns. Since archaeologists still don’t fully know what the early Roman Forum truly looked like, Gabii provides an invaluable window into its development.”

Connections to Earlier Excavations and Greek Influence

This new basin builds on the team’s previous findings at Gabii. One earlier feature, known as the “Area F Building,” is a terraced complex carved into the slope of the ancient volcanic crater around which the settlement formed.

Taken together, these discoveries indicate that early Roman builders drew significant inspiration from Greek architectural traditions. Greek cities featured paved civic spaces, dramatic terraces and grand gathering areas that communicated both cultural prestige and political power. Early Romans appear to have adapted many of these ideas for their own urban centers.

Why Gabii Matters for Understanding Early Rome

Gabii occupies a unique place in the study of Roman history. Mogetta explained that “while Rome’s earliest layers were buried beneath centuries of later construction, Gabii — a once-powerful neighbor and rival of Rome, first settled in the Early Iron Age — was largely abandoned by 50 B.C. and later reoccupied on a much smaller scale.” Because of this, he said, the ancient city’s original layout and building foundations remain unusually intact, offering a clear view of early Roman urban life.

Italy’s Ministry of Culture has designated the area as an archaeological park, now managed as part of the Musei e Parchi Archeologici di Praeneste e Gabii. This status has allowed teams of researchers, including the international Gabii Project, to systematically excavate and study the site. Mogetta became the director of this research group last year.

Next Steps in Excavation and Investigation

Supported by the General Directorate of Museums in Italy, the Gabii Project plans to continue excavating the basin and the surrounding stone-paved zone next summer. Researchers also intend to study a nearby “anomaly” detected through thermal imaging. Early indications suggest it could be a temple or another large civic structure.

“If it’s a temple, it could help us explain some of the artifacts we’ve already found in the abandonment levels of the basin, such as intact vessels, lamps, perfume containers and cups inscribed with unusual markings,” Mogetta said. “Some of these objects may have been deliberately placed there as religious offerings or discarded in connection with the ritual closing of the pool around 50 C.E. — thus underscoring the crucial role played by water management in ancient cities.”

The continuation of the Gabii Project aims to protect the site’s heritage while allowing scholars and visitors to better understand its history.

Unanswered Questions About Early Roman Society

One major question researchers hope to address is whether civic areas were developed before religious buildings, or the other way around. The answer could shed light on whether political activities or spiritual practices held greater influence in shaping the earliest Roman monumental landscapes.

By gathering evidence from the basin and surrounding structures, Mogetta and his team aim to reconstruct the full story of Gabii, from its growth and height of power to its later decline. Their work also contributes to a broader understanding of early Roman architecture and how it helped shape the urban world that followed.

White dwarfs frequently exist in binary systems, where two stars orbit each other. Most of these pairs are extremely old on galactic timescales and have cooled to temperatures near 4,000 degrees Kelvin. Recent observations, however, have uncovered a group of short period binaries in which the stars complete an orbit in less than an hour. These fast moving pairs do not match established predictions, since many appear to be roughly twice the expected size and have temperatures between 10,000 and 30,000 degrees Kelvin.

Investigating the Role of Tidal Heating

This unexpected behavior led a research team headed by Lucy Olivia McNeill of Kyoto University to examine the influence of tidal forces in these systems. Tides frequently distort objects that share close orbits, affecting how those orbits evolve over time.

“Tidal heating has had some success in explaining temperatures of Hot Jupiters and their orbital properties with their host stars. So we wondered: to what extent can tidal heating explain the temperatures of white dwarfs in short period binaries?” asks McNeill.

To explore this question, the researchers developed a theoretical model designed to estimate how much white dwarfs heat up in short period binaries. The model was built to be widely applicable, making it possible to estimate both the temperature history and the future orbital changes of white dwarfs in these systems.

Tidal Forces Reshape White Dwarf Evolution

The team’s analysis showed that tidal interactions can play a major role in how these stars evolve. In particular, the gravitational pull from a smaller white dwarf can raise internal heat within a larger but less massive companion. This added heat causes the star to expand and pushes its surface temperature to at least 10,000 degrees Kelvin.

Because of this expansion, the researchers propose that white dwarfs are likely to be twice the size predicted by standard theory at the point where they begin exchanging material, a stage known as mass transfer. As a result, these short period pairs may start interacting at orbital periods that are three times longer than scientists previously believed.

“We expected tidal heating would increase the temperatures of these white dwarfs, but we were surprised to see how much the orbital period reduces for the oldest white dwarfs when their Roche lobes come into contact,” says McNeill.

Implications for Stellar Explosions and Future Research

White dwarfs in extremely tight orbits will eventually interact and emit gravitational radiation. Systems of this kind are considered possible origins of type Ia supernovae and cataclysmic variables, two dramatic and scientifically important cosmic events.

Looking ahead, the team aims to apply their model to binary systems made of carbon-oxygen white dwarfs. Their goal is to better understand the potential pathways leading to type Ia explosions, especially whether realistic temperature predictions support the double degenerate (merger) scenario.

The research team, led by Dr. Huang Huang of the Laoshan Laboratory in Qingdao and including geochemist Dr. Marcus Gutjahr from GEOMAR, set out to reconstruct how far Antarctic Bottom Water (AABW) extended through the Southern Ocean over the past 32,000 years.

“We wanted to understand how the influence of Antarctic Bottom Water, the coldest and densest water mass in the global ocean, changed during the last deglaciation, and what role it played in the global carbon cycle,” says Huang, who completed his PhD at GEOMAR in 2019 and now works as a scientist in Qingdao, China.

Sediment cores and chemical fingerprints in the deep sea

To tackle this question, the scientists examined nine sediment cores collected from the Atlantic and Indian sectors of the Southern Ocean. The cores came from water depths between about 2,200 and 5,000 meters and from locations spread widely across the region. By analyzing the isotopic composition of the trace metal neodymium preserved in the sediments, which reflects the chemistry of the surrounding seawater, they could reconstruct how Antarctic Bottom Water changed through time on the scale of tens of thousands of years.

“Dissolved neodymium and its isotopic fingerprint in seawater are excellent indicators of the origin of deep-water masses,” explains Dr. Marcus Gutjahr. “In earlier studies, we noticed that the neodymium signature in the deep South Atlantic only reached its modern composition around 12,000 years ago. However, sediments from the last Ice Age showed values that are not found anywhere in the Southern Ocean today. Initially, we thought the method was flawed or that there was something wrong with the sediment core. But the real question was: What could generate such a signal? Such an exotic isotopic signature can only develop when deep water remains almost motionless for extended periods. In such circumstances, benthic fluxes — chemical inputs from the seafloor — dominate the isotopic imprint in marine sediments.”

Stagnant deep waters, carbon storage and the last Ice Age

During the last Ice Age, the cold and very dense deep water that currently forms around Antarctica did not spread as widely as it does today. Instead, much of the deep Southern Ocean was filled with carbon-rich waters that originated in the Pacific, a glacial precursor to today’s Circumpolar Deep Water (CDW). In the study, CDW is described as carbon-rich because it circulates in the deep ocean for long periods with limited contact with the surface. This isolation allowed large amounts of dissolved carbon to remain locked in the deep ocean, helping to keep atmospheric CO2 levels relatively low.

As Earth warmed and ice sheets retreated between roughly 18,000 and 10,000 years ago, the volume of Antarctic Bottom Water increased in two clear phases. These expansion phases occurred at the same time as known warming events in Antarctica. With more vertical mixing in the Southern Ocean, deep waters that had stored carbon for long periods were brought closer to the surface, allowing that carbon to escape into the atmosphere.

“The expansion of the AABW is linked to several processes,” explains Gutjahr. “Warming around Antarctica reduced sea-ice cover, resulting in more meltwater entering the Southern Ocean. The Antarctic Bottom Water formed during this transitional climate period had a lower density due to reduced salinity. This late-glacial AABW was able to spread further through the Southern Ocean, destabilizing the existing water-mass structure and enhancing exchanges between deep and surface waters.”

Previously, many scientists assumed that changes in the North Atlantic, particularly the formation of North Atlantic Deep Water (NADW), were the main drivers of shifts in deep-water circulation in the South Atlantic. The new results suggest that this northern influence was more restricted than earlier thought. Instead, the replacement of a glacial, carbon-rich deep-water mass by newly formed Antarctic Bottom Water appears to have been crucial for the rise in atmospheric CO2 toward the end of the last Ice Age.

Southern Ocean heat, Antarctic ice loss and today’s climate

“Comparisons with the past are always imperfect,” says Gutjahr, “but ultimately it comes down to how much energy is in the system. If we understand how the ocean responded to warming in the past, we can better grasp what is happening today as Antarctic ice shelves continue to melt.”

Because of its vast size and unique circulation, the Southern Ocean plays a major role in controlling the global climate. Over the past 50 years, waters deeper than about 1,000 meters around Antarctica have warmed significantly faster than much of the rest of the world’s oceans. To work out how this rapid deep-ocean warming affects the ability of the ocean to absorb and release carbon dioxide, scientists must track physical and biogeochemical changes over long timescales and incorporate them into climate models.

“I want to properly understand the modern ocean in order to interpret signals from the past,” Gutjahr says. “If we can trace how Antarctic Bottom Water has changed over the last few thousand years, we can assess more accurately how rapidly the Antarctic Ice Sheet may continue to lose mass in the future.”

Paleoclimate data obtained from sediment cores are indispensable for this, offering insights into past climates that were warmer than today and helping to improve projections of future climate change.