67P/Churyumov–Gerasimenko

• 1. Introduction

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  67P/Churyumov, Gerasimenko, nicknamed "Chury," is a small, oddly shaped comet that once played host to a robotic visitor from Earth. It measures roughly four and three tenths by four and one tenth kilometers at its longest and widest. A single spacecraft would go on to study nearly every part of it up close. Why does a body that small end up shaped like two stones pressed together? What happened when a spacecraft finally reached it and tried to set something down on its surface? And what did years of close study reveal about the raw materials sealed inside a comet like this one?

• 2. A Case Of Mistaken Identity

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  Svetlana Ivanovna Gerasimenko exposed a photographic plate at the Alma-Ata Astrophysical Institute on the 11th of September 1969. She was aiming her camera at a different comet, Comas Solà. Neither she nor her colleague Klim Ivanovich Churyumov, of Kyiv University's Astronomical Observatory, realized a second object had drifted onto the same plate. Churyumov spotted a faint cometary shape near the plate's edge. He assumed it was simply Comas Solà, slightly out of position. Only after returning to Kyiv and studying the plates more closely did the mistake become clear. On the 22nd of October, about a month after the photograph was taken, he looked again. The object sat roughly 1.8 degrees from where Comas Solà was supposed to be. A closer look turned up a faint image of the real Comas Solà exactly where it belonged. That proved the other object was something new. That mistake gave the comet its double name, Churyumov-Gerasimenko. Decades later, a spacecraft's cameras would reveal just how strange its shape really was.

• 3. A Body Built From Two

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  The larger of Churyumov, Gerasimenko's two lobes measures about 4.1 by 3.3 by 1.8 kilometers. A narrow neck connects it to a smaller lobe, roughly 2.6 by 2.3 by 1.8 kilometers. Together the pair weighs an estimated 10 billion tonnes. With every orbit, the comet sheds roughly a meter of surface material, give or take half a meter. That loss comes from sunlight boiling away its gas and dust. Scientists call this two-lobed form a contact binary. It formed from a gentle, low-speed collision between two separate objects, not from one body cracking apart. The proof sits in the comet's "terraces," layers of interior material exposed where outer surfaces have been stripped away. Those terraces run in different directions on each lobe, a sign that two once-independent bodies fused together. Later analysis drew on thousands of images from Rosetta's OSIRIS cameras, all taken after the comet's closest approach to the Sun. The set included 7,682 narrow-angle frames and 1,504 wide-angle frames. Together they produced a shape model built from 132.1 million facets. That model revealed Subsurface Access Points, cavities between 20 and 47 meters deep that offer a direct route to material beneath the crust. Researchers found a link between the energy reaching the bottom of these cavities and the start of short-lived sublimation jets. The sites are now considered strong candidates for a future mission that might bring pristine comet material back to Earth.

• 4. A Landscape Named For Gods

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  Twenty-six distinct regions cover Churyumov, Gerasimenko's surface, and every one carries the name of an Egyptian deity. Regions on the larger lobe are named for gods; those on the smaller lobe are named for goddesses. Scientists mapped nineteen of these regions in the northern hemisphere before the comet's equinox. Seven more followed once the southern hemisphere came into sunlight, named under the same convention. The terrain varies sharply between regions. Some, like Maat and Ash, are covered in dust. Others, including Hatmehit and Aten, form large-scale depressions, while regions such as Hapi and Imhotep are smooth. Many more, from Serqet to Wosret, are rock-like in character. The largest boulder anywhere on the surface sits in the comet's larger lobe: Cheops, measuring up to 45 meters across. It takes its name from the pyramid at Giza, because its silhouette echoes a pyramid's shape. Two other features, twin prominences described as "gates," were named after members of the Rosetta team who had died: Claudia Alexander and Angioletta Coradini. Naming a landscape after gods and lost colleagues was one thing; watching that landscape change in real time, as Rosetta soon would, was another.

• 5. A Restless Surface

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  Circular patterns on Churyumov, Gerasimenko's smooth terrain grew by a few meters a day while Rosetta watched, especially as the comet neared the Sun. A fracture running through the comet's neck widened over the same period. Boulders tens of meters across shifted position, some traveling more than 100 meters from where they started. Patches of ground disappeared entirely, uncovering features no one had seen before. Cliffs collapsed more than once during the mission. In December 2015, Rosetta's NAVCAM captured a sudden bright patch of light flaring from the comet's surface. Scientists traced it to a large cliff giving way. It marked the first time a landslide on a comet had been linked to a burst of activity. A further outburst was recorded on the 14th of November 2021. Researchers noted that at the moment of discovery the comet sat 1.23 astronomical units from the Sun. It was 0.42 astronomical units from Earth at the time. That kind of sudden brightening near perihelion would define the comet's next two close passes by the Sun.

• 6. A Path Shaped By Jupiter

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  On the 4th of February 1959, Jupiter passed just 0.0515 astronomical units from Churyumov, Gerasimenko. That close encounter pulled the comet's closest point to the Sun inward, from 2.7 astronomical units to 1.28 astronomical units, where it has stayed roughly steady ever since. Churyumov, Gerasimenko belongs to the Jupiter family of comets, bodies thought to have started out in the Kuiper belt before Jupiter's gravity dragged them into the inner solar system. Jupiter is not finished reshaping its path. In November 2220, the comet will pass about 0.14 astronomical units from Jupiter. That pass will drag its perihelion inward again, to roughly 0.8 astronomical units from the Sun.

  Before its 2009 perihelion passage, Churyumov, Gerasimenko rotated once every 12.76 hours. By the time it rounded the Sun that year, the rotation period had dropped to 12.4 hours. That change most likely came from sublimation exerting a slight torque on the nucleus.

  The comet came to perihelion again on the 13th of August 2015, when its nucleus carried an apparent magnitude of about 20. It crossed the celestial equator on the 5th of May that year, making it easiest to see from the Northern Hemisphere. Even at its brightest, while passing through the constellation Gemini, it reached only magnitude 12. A telescope was still needed to see it at all.

  The 2021 apparition was different from the one six years earlier. The comet reached perihelion on the 2nd of November 2021. Its closest approach to Earth followed ten days later, on the 12th of November, at 00:50 UTC. The distance then was 38 million miles, or about 61 million kilometers. That was the nearest Churyumov, Gerasimenko had come to Earth since 1982. This time it brightened to apparent magnitude 9, bright enough for amateur telescopes to pick up. Two outbursts punctuated that approach, one on the 29th of October and another on the 17th of November. The second was roughly two and a half times stronger than the first, judging by the extra dust it threw off. While these brightening events played out for telescopes on Earth, a spacecraft had already spent more than two years studying the comet at close range.

• 7. The Rosetta Mission

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  On the 12th of March 2003, the Hubble Space Telescope captured images of Churyumov, Gerasimenko that scientists studied closely ahead of any mission. From those images they built an overall three-dimensional model and generated computer images of its likely shape. Nearly a decade later, on the 25th of April 2012, astronomers N. Howes, G. Sostero and E. Guido used the two-meter Faulkes Telescope. They recorded the most detailed observations of the comet taken up to that point, catching it while it sat at its farthest point from the Sun.

  The Rosetta spacecraft itself had launched on the 2nd of March 2004. By the 6th of June 2014, it detected water vapor escaping the comet at a rate of roughly one liter per second. At that point Rosetta was still 360,000 kilometers from the comet and 3.9 astronomical units from the Sun. On the 14th of July, new images revealed an irregular nucleus split into two distinct lobes, sized at roughly 3.5 by 4 kilometers. Mission scientists debated two explanations at the time. Either the comet was a contact binary, or asymmetric erosion from sublimating ice had carved it into that lobed shape. By September 2015, the mission had ruled the erosion theory out entirely.

  Starting in May 2014, ground controllers slowed Rosetta by 780 meters per second through a series of thruster firings. On the 6th of August 2014, they brought its velocity relative to the comet down to just one meter per second, achieving rendezvous. Rosetta settled into orbit on the 10th of September, circling roughly 30 kilometers from the nucleus.

  The small lander Philae, weighing 220 pounds, began its descent on the 12th of November 2014. It became the first spacecraft ever to land on a comet nucleus. Gravity on the comet's surface is only about a ten-thousandth as strong as Earth's. Philae carried a cold gas thruster, harpoons, ice screws built into its landing legs, and a flywheel meant to keep it steady during descent. During the landing, the thruster and harpoons failed to fire, and the ice screws found no grip. Philae bounced twice before finally coming to rest on its third touch with the surface, two hours after first contact. The landing site was named Agilkia, after the island where the temples of Philae Island were moved once the Aswan Dam flooded their original location. Battery power dropped fast, and contact with Philae was lost on the 15th of November 2014. The European Space Operations Centre briefly reestablished contact on the 14th of June 2015, reporting a healthy spacecraft, before losing the signal again soon after.

  Finding Philae's exact resting place took nearly two more years. Measurements from the CONSERT instrument narrowed the search to an ellipse measuring about 16 by 160 meters. Further analysis by CNES-SONC narrowed the likely location using illumination conditions and the timing of contact between orbiter and lander. By April 2015, researcher Guillaume Faury had identified a strong candidate image within the OSIRIS-NAC data. A pre-landing image matched the lighting and viewing angle of a photo taken in mid-December 2014 almost perfectly. Every topographic detail lined up between the two images except for one bright spot, about two pixels wide, present only after the landing. Early ESA communications called it only the "red candidate." The match was confirmed on the 2nd of September 2016, when Rosetta flew close enough to capture high-resolution images of Philae. The lander was resting in a dark crevice, with only its body and two legs visible.

  Somewhere in the data Rosetta gathered during that stay was a full chemical fingerprint of the comet. That fingerprint would end up challenging assumptions about where the solar system's water and oxygen came from.

• 8. Chemistry, Oxygen And What's Next

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  Water vapor from Churyumov, Gerasimenko carries a ratio of deuterium to hydrogen three times higher than water found on Earth, according to Rosetta's measurements. That difference makes it unlikely that comets like this one supplied Earth's water. The vapor is also mixed with formaldehyde, at about 0.5 percent by weight, and methanol, at about 0.4 percent. Those concentrations fall within the normal range for solar system comets. On the 22nd of January 2015, NASA reported that between June and August 2014 the comet's water vapor output had increased by up to tenfold. The following day, the 23rd of January 2015, the journal Science devoted a special issue to studies of the comet.

  Measurements taken before Philae's batteries failed suggest the comet's dust layer runs up to 20 centimeters thick, sitting above hard ice or a mixture of ice and dust. Instruments aboard both Philae and Rosetta, named ROMAP and RPC-MAG, found no magnetic field in the nucleus at all. That suggested magnetism played less of a role in early solar system formation than once thought. Rosetta's ALICE spectrograph later identified the real cause of a chemical breakdown happening in the comet's coma. Electrons produced when solar radiation ionizes water molecules, not sunlight itself, break down water and carbon dioxide released from the nucleus.

  The COSAC and Ptolemy instruments on Philae detected sixteen organic compounds in the comet's material. Four of them, acetamide, acetone, methyl isocyanate and propionaldehyde, had never before been found on any comet. Astrobiologists Chandra Wickramasinghe and Max Wallis suggested that some of the comet's surface features could be explained by extraterrestrial microorganisms. Rosetta's own program scientists dismissed the idea as "pure speculation," and neither spacecraft carried instruments built to search for organisms directly. The only amino acid found was glycine, alongside related compounds methylamine and ethylamine. One of the mission's biggest surprises was the discovery of large amounts of free molecular oxygen gas around the comet. That gas had never been detected in any cometary coma before. Solar system models suggest oxygen should have reacted away into water during the hot, violent process that formed the comet about 4.6 billion years ago. Instead, measurements suggest much of the oxygen was locked into the nucleus when the comet first formed, incorporated from the start rather than created later. That primordial explanation was challenged by another discovery. Oxygen molecules can also form on the comet's surface when water collides with silicates and other oxygen-bearing materials. Detection of molecular nitrogen added another clue, suggesting the comet's icy grains formed in extremely cold conditions, below 30 kelvin. On the 3rd of July 2018, researchers proposed that the comet's surface alone could not be generating enough molecular oxygen to explain what Rosetta measured. That deepened the mystery rather than resolving it.

  One proposal for returning to the comet was CAESAR, a mission designed to collect regolith from 67P's surface and carry it back to Earth. It reached the final round of NASA's fourth New Frontiers selection, competing against one other finalist. In June 2019, NASA chose Dragonfly instead. That left Churyumov, Gerasimenko's unresolved chemistry, and the question of where its oxygen came from, for whichever mission reaches the comet next.