Messengers of ice and time

Long before telescopes or space missions, our ancestors gazed up in wonder at comets that appeared in the night sky. These ghostly visitors were seen as omens, messengers, or divine portents. Today, we know comets not as harbingers but as ancient assemblages of ice and dust left over from the dawn of the Solar System — time capsules carrying water and organic compounds that predate Earth itself. 

Our last mission to visit a comet was the European Space Agency’s Rosetta mission, which orbited comet 67P/Churyumov-Gerasimenko from 2014 to 2016. Rosetta became the first spacecraft to accompany a comet as it journeyed around the Sun, studying how sunlight transformed its surface and atmosphere in real time. Rosetta’s suite of instruments — including the ROSINA mass spectrometer, whose science team was led by Kathrin Altwegg at the University of Bern — revealed an unexpected richness of organic molecules, reshaping our view of comets as potential carriers of the ingredients for life on Earth. 

“The organics in comets like 67P were mostly produced before the Solar System was born,” said Altwegg. “These organics are universal. We can even observe some of them in dark molecular clouds and star-forming regions. This means whatever led to life on Earth can happen elsewhere in the Universe.” 

The results on water were more nuanced. Rosetta found that the deuterium-to-hydrogen ratio in comet 67P’s water was not a match for Earth’s oceans. Yet other comets carry water with Earth-like chemistry, suggesting that certain types of comets may have contributed to Earth’s water, alongside asteroids.

The Space Age has transformed our understanding of comets and their potential role in delivering water and organics to Earth. Yet the few we’ve visited with spacecraft have been reshaped by repeated journeys through the inner Solar System. What secrets would a pristine comet hold? What could a sample-return mission reveal? Do comets have more to teach about our origins?

An abundance of organics

When the Sun and planets formed from a giant cloud of gas and dust 4.6 billion years ago, the leftover material became the asteroids and comets we see today. Close to the young Sun, it was too hot for volatile ices to survive. But farther out, temperatures dropped low enough for icy planetesimals to form. 

Some of these icy bodies settled into the Kuiper belt, home to short-period comets that periodically enter the inner Solar System on orbits lasting up to about 200 years. Others were thrown outward by the giant planets into elongated, tilted paths that form the scattered disk, a transitional region extending far beyond the Kuiper belt. The most dramatically flung objects populated the distant, spherical Oort cloud, which may stretch halfway to our nearest star, Proxima Centauri. If Proxima has its own Oort cloud, its outer edges might overlap with ours.

So far, all the comets we’ve visited — Giacobini-Zinner, Halley, Grigg-Skjellerup, Borrelly, Wild 2, Tempel 1, Hartley 2, and 67P — have been short-period comets, baked and altered by repeated cycles of sunlight. But even these dynamic worlds have revealed remarkable chemistry. 

At 67P, Rosetta’s ROSINA instrument detected glycine, an amino acid found in proteins, and phosphorus, a key ingredient in DNA and cell membranes. In a recent paper, Altwegg and her co-authors found heterocycles, ring-shaped molecules that play crucial roles in the biochemistry of carbon-based life. Another analysis led by Altwegg used 67P’s composition to extrapolate how much organic material comets could have carried to Earth billions of years ago. The conclusion: Comet-delivered organics could have matched or equaled the total biomass on Earth today, increasing the likelihood that comets played a role in life’s origins. 

While Rosetta revolutionized comet science, Altwegg said key mysteries remain. “We mostly investigated the coma — the dust and gas from the surface. We still don’t know how the ice and dust are distributed inside or how homogeneous comets really are,” she said. “To answer that, we need to visit more comets.”

Comet Interceptor: A waiting game

Ideally, scientists would study a pristine comet — one entering the inner Solar System from the Oort cloud for the first time before sunlight cooks its surface and drives away its volatile ices. That’s the goal of Comet Interceptor, an ESA mission set to launch in 2029. The spacecraft will park itself at the Sun–Earth L2 point, about 1.5 million kilometers (930,000 miles) away, ready to intercept a long-period comet or even an interstellar object.

“We can realistically wait for a few years,” said Colin Snodgrass, an interdisciplinary scientist on the mission and a professor of planetary astronomy at the University of Edinburgh. “The limit comes from the budget cap rather than any physical one — the mission is meant to be done inside of six years, including waiting, cruise, encounter, and data downlink.”

If an interstellar object such as the headline-making comet 3I/ATLAS appears, Snodgrass said the team would jump at the chance to rendezvous with it. “If there’s an interstellar object we can actually reach, there’s unanimous support for going for it. It would be too good an opportunity to miss,” he said. “But it’s unlikely we can reach one with our limited fuel budget — it would need to come quite close to Earth.” 

Even among potential long-period comets, selecting a target is complex and fraught with trade-offs. “There are lots of competing factors,” Snodgrass said. “We’d want one whose orbit suggests a higher probability of being dynamically new, and a larger nucleus — those tend to be more active.” 

The mission’s ultimate goal is to compare pristine comets to ones already altered by sunlight. “We want to learn about the processes involved in comet evolution and their formation,” Snodgrass said. “Whether comets show signs of forming early or later in the Solar System’s history tells us a lot about how solid bodies formed in the planetary disk.” 

If no suitable new comet or interstellar object appears in time, Comet Interceptor can redirect to a known short-period comet as a guaranteed target, ensuring valuable science no matter what the Cosmos delivers.

Warm comets

Not every comet fits neatly into “short” or “long” categories. Some orbit within the main asteroid belt, while others skim the Sun or fade into dormant, rocky relics. These transitional worlds blur the line between asteroids and comets, revealing how one population evolves into the other. 

In 2023, the James Webb Space Telescope observed main-belt comet 238P/Read and detected water vapor — the first unambiguous detection of water in a comet that orbits entirely within the asteroid belt. But JWST also found something unexpected: no carbon dioxide, a staple ingredient of most comets. 

“Main-belt comets like 238P/Read help us understand where and how water is distributed in the inner Solar System and how long the water can survive,” said Heidi Hammel, vice president of The Planetary Society’s board of directors, who used JWST to study comet Read. Meteorites and telescope data had long suggested that some asteroids formed in the presence of water, she said, “but we haven’t known how long that water can last in the asteroid belt region.” 

JWST’s sensitivity finally provided the answer. “The JWST observations demonstrated that water is responsible for its activity, but they also showed that Read lacks carbon dioxide,” Hammel said. “Either it formed in a warm, close-in part of the Solar System or it formed with carbon dioxide, but the gas has been baked out while the comet orbited the Sun in the asteroid belt.” 

Both scenarios imply something important: 238P/Read is not an interloper from distant comet reservoirs — it has been part of the inner Solar System for a very long time. “The detection of water indicates that water ice from the primordial Solar System can be preserved in that region for a very long time,” Hammel said. 

Opportunities to study these comets are rare. “At last count, there were just 11 main-belt comets,” Hammel noted. JWST is the only observatory sensitive enough to study their chemistry in detail — and the competition for observing time is intense. “The most recent call for proposals garnered a record-breaking number of requests,” she said. “Fingers crossed that main-belt comets and other types of comets make the cut!”

What lies beneath 

Comets are geologically active worlds: porous mixtures of dust, rock, and ice with cliffs, pits, and plains sculpted by sunlight. But what lies beneath their surface? How ices, dust, and organics are mixed and whether they form layers or a patchwork remains one of the biggest mysteries in comet science. 

A proposed NASA mission called CAESAR (Comet Astrobiology Exploration Sample Return) would have attempted to answer some of those questions. CAESAR aimed to revisit 67P, collecting a sample and returning it to Earth. While it wouldn’t have probed the comet’s interior directly, it would have offered the clearest window yet into its composition — the mix of ices, dust, and complex organics that Rosetta could only sample in situ. CAESAR was ultimately passed over in favor of NASA’s Dragonfly mission to Titan, but it remains a blueprint for possible comet sample-return efforts. 

Another mission concept, Comet Hopper, would have taken a more mobile approach. Designed to land and hop to multiple sites on a single comet, it could have drilled beneath the surface and analyzed material from different regions. “Such a lander could finally tell us how the ice and dust are distributed — whether they’re layered around grains, mixed in pores, or separated into larger voids,” said Altwegg. “It was a Discovery-class concept that made it to NASA’s top three but wasn’t selected at the time.” 

Altwegg sees efforts like Comet Hopper and CAESAR as stepping stones to more advanced future missions. “Ideally, we’d have a cryogenic sample-return mission that keeps the material frozen at around 30 kelvins all the way back to Earth,” she said. “That’s not feasible today, but a mobile lander could move, drill, and analyze samples deep down. That’s how we could finally answer how comets formed and maybe even find more amino acids, perhaps with the same chirality as those on Earth.” 

Each comet we study is a messenger from the Solar System’s earliest days. They carry the raw ingredients that built planets, oceans, and perhaps life itself, preserved in ice and dust for billions of years. By examining these icy travelers, we glimpse the processes that shaped Earth and the worlds around us. We can’t return to those first moments, but through comets, we can hear the whispers of our cosmic origins.