A century ago, when American magazine science fiction was developing, the Solar System seemed a relatively tidy place. At least, it did in comparison to today. The first issue of Hugo Gernsback’s Amazing Stories serialized a reprint of Jules Verne’s 1877 novel Off on a Comet and, indeed, in those days comets were the objects most likely to move around the system. The asteroids seemed distant in their belt and in stable orbits and there was little else between the planets. There was no Pluto.
Today, of course, we seem to have debris everywhere. The main belt asteroids are joined by trojan objects like the large population around Jupiter, and there is another belt of ancient material out beyond Neptune, the Kuiper Belt. In Earth’s neighborhood, interesting objects like 2021 PJ1, whose approach to our planet occurred on August 14 at 1.7 million kilometers, remind us that there is a large population of asteroids that move in orbits well inside the main belt, and could conceivably present a danger to us, at least enough of one to demand that we keep a close eye on their trajectories.
2021 PJ1 has a certain claim to fame, being the 1,000th near-Earth asteroid to be observed by planetary radar in the past 50 years. In this technique, we bounce a radar signal off an object and examine the photonic echo. The first asteroid to be viewed in this way was 1566 Icarus, all the way back in 1968, ‘painted’ by Goldstone radar, the same facility near Barstow, California that produced the PJ1 data this summer.
Image: The 70-meter Deep Space Station 14 (DSS-14) antenna at the Deep Space Network’s Goldstone Deep Space Complex near Barstow, California, was able to measure the Doppler frequency of the radio waves that reflected off asteroid 2021 PJ1’s surface. The figure shows radar echo signal strength on the vertical axis versus Doppler frequency (in units of hertz, or Hz) on the horizontal axis. The strong spike at a value of minus 70 Hz is the reflected signal (or “echo”) from 2021 PJ1; the other, smaller spikes are receiver noise. Credit: NASA/JPL-Caltech.
I always think primarily of Arecibo when planetary radar comes to mind, and in fact its radar capabilities were a prime reason for fighting to sustain its funding before its collapse in 2020. Well over half of existing NEO radar observations were made by its 305-meter dish. But the tally of the Goldstone Deep Space Complex is impressive via its DSS-14 70-meter and 34-meter DSS-13 antennae, with 374 near-Earth asteroids to date. Moreover, the Deep Space Network’s Canberra site, working with Australian observatories including Parkes, has notched up another fourteen.
NEA 1,001 came only a week after 2021 PJ1, an object labeled 2016 AJ193 that moved by at about 3.4 million kilometers. While 2021 PJ1 was small — between 20 and 30 meters wide — 2016 AJ193, although more distant, was a much easier catch because it’s some 40 times larger, with a diameter in the range of 1.3 kilometers. Originally observed by the NEOWISE mission, this asteroid gave us a lot more than plots on a graph: Ridges, hills, concavities and possible boulders appeared in the Goldstone observations, which also determined that it rotates with a period of 3.5 hours.
Image: This animation shows asteroid 2016 AJ193 rotating as it was observed by Goldstone’s 70-meter antenna on Aug. 22, 2021. 1.3-kilometers wide, the object was the 1,001st near-Earth asteroid to be measured by planetary radar since 1968. Credit: NASA/JPL-Caltech.
The 2016 AJ193 observations were led by Shantanu Naidu (JPL), who says:
“The 2016 AJ193 approach provided an important opportunity to study the object’s properties and improve our understanding of its future motion around the Sun. It has a cometary orbit, which suggests that it may be an inactive comet. But we knew little about it before this pass, other than its size and how much sunlight its surface reflects, so we planned this observing campaign years ago.”
The significance of planetary radar for simple security is obvious. Including telescopes on the ground and in space, we’re tracking close to 27,000 near-Earth objects, characterizing them through observations like the recent Goldstone work. The more we learn about the size, shape and composition of NEOs, the better we’ll be able to resolve questions about their trajectories and any future danger they might pose. This, in company with data from asteroid missions like Hayabusa2 and OSIRIS-REx, will help us tune our threat mitigation strategies if we ever do have to nudge an NEO.
Image: This series of images captured on Aug. 22, 2021, shows asteroid 2016 AJ193 rotate as it was observed by Goldstone’s 70-meter antenna. Credit: NASA/JPL-Caltech.
In addition to its sample return mission at asteroid Bennu, OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) has tightened our projections about the object’s future trajectory. Although the impact possibility on Earth through the year 2300 is on the order of 1 in 1750 (0.057%), it’s an object we want to keep an eye on, because in 2135 Bennu will make a close approach to Earth that could nudge its trajectory in ways that are difficult to anticipate.
OSIRIS-REx spent more than two years working near the 500-meter wide asteroid, studying its mass and composition while tracking its spin and orbital trajectory. In terms of the latter, even factors as tiny as the force the spacecraft exerted during its sample collection event in October of 2020, a mere touch-and-go, had to be considered (the study confirms that the effect was negligible). Far more significant is the Yarkovsky effect, which occurs as solar heating eases on the nightside during the asteroid’s rotation and is radiated away as infrared energy, generating thrust.
It’s the tiniest of effects at Bennu, says Steve Chesley (JPL), a co-investigator on the study that has just appeared in Icarus, but of course it builds over time and has consequences for the asteroid’s path:
“The Yarkovsky effect will act on all asteroids of all sizes, and while it has been measured for a small fraction of the asteroid population from afar, OSIRIS-REx gave us the first opportunity to measure it in detail as Bennu traveled around the Sun. The effect on Bennu is equivalent to the weight of three grapes constantly acting on the asteroid – tiny, yes, but significant when determining Bennu’s future impact chances over the decades and centuries to come.”
Imagine how tricky it is to measure the cumulative effects of the Yarkovsky effect on a rotating object of uneven shape. Three close encounters in 1999, 2005 and 2011 were extensively tracked from the ground, but OSIRIS-REx data from the object itself have now allowed researchers to model an asteroid’s trajectory to the highest level of precision ever. Bennu’s future path is well known up to 2135. The question during the close encounter in that year will be whether it will pass through a dangerous ‘gravitational keyhole,’ an area where, as it responds to the effects of Earth’s gravity, the asteroid could become more likely to present an impact threat in the future. There is one ‘keyhole’ solution that would result in an Earth impact in 2182, with an impact probability of 1 in 2,700 (or about 0.037%).
In addition to the Yarkovsky factor, scientists have to consider the gravitational influence of the Sun and other objects in the Solar System including more than 300 asteroids that could have some effect. The solar wind streaming at variable rates outward from the Sun has to be considered as a source of pressure, and remember that OSIRIS-REx also discovered that Bennu was shedding rock particles, likely the result of thermal fracturing due to heating and cooling during rotation. These events along with the drag caused by interplanetary dust all affect the future trajectory.
Image: This mosaic of Bennu was created using observations made by NASA’s OSIRIS-REx spacecraft that was in close proximity to the asteroid for over two years. Credit: NASA/Goddard/University of Arizona.
The authors note that a close approach to Earth that will occur in 2037 will be the next opportunity to collect radar data and therefore gauge the accuracy of their work while improving the trajectory projections even more. In their conclusion they add this (and note that we are dealing with not one but numerous gravitational keyholes)::
…improved orbital knowledge allowed us to refine the impact hazard assessment, which we extended through 2300. The dense structure of keyholes on the B-plane of the 2135 encounter with Earth (Chesley et al., 2014) made it unlikely to avoid all possible pathways to impact. Still, the uncertainties for the 2135 encounter decreased by a factor of about 20, and so many of the most significant impacts found by Chesley et al. (2014) are now ruled out.
The fact that we have a sample return on the way adds another plus for OSIRIS-REx. Dante Lauretta (University of Arizona) is principal investigator for the mission:
“The spacecraft is now returning home, carrying a precious sample from this fascinating ancient object that will help us better understand not only the history of the solar system but also the role of sunlight in altering Bennu’s orbit since we will measure the asteroid’s thermal properties at unprecedented scales in laboratories on Earth.”
The opportunity given us by OSIRIS-REx to test models and calculate future trajectory probabilities shows what can be done with objects with even a remote chance of striking the planet. The trajectory changes coming in 2135’s close approach will be watched carefully by way of further understanding how to tighten such calculations. By then, in the highly unlikely event of a dangerous deflection toward impact, we can hope to have methods in place to force a further trajectory change of our own.
The paper is Farnocchia et al., “Ephemeris and hazard assessment for near-Earth asteroid (101955) Bennu based on OSIRIS-REx data,” published online by Icarus 10 August 2011 (full text).
An interstellar freebie like ‘Oumuamua or 2I/Borisov is priceless. We don’t need to travel light years to see it because it comes to us. Although we’re expecting to find a lot more such objects as instruments like the Vera Rubin Observatory come online, right now only two are known to have passed through our system. But only slightly less inaccessible places like the Oort Cloud also bring gifts in the form of long-period comets, and I don’t want the advent of C/2014 UN271 Bernardinelli-Bernstein to go unnoticed in these pages, given its startling size and already detected activity.
Pedro Bernardinelli (University of Pennsylvania), who along with colleague Gary Bernstein discovered the comet, estimates its nucleus as being between 100 and 200 kilometers (62 and 125 miles) long. This dwarfs Hale-Bopp, and Colin Snodgrass (University of Edinburgh) is quoted in the New York Times as saying: “With a reasonable degree of certainty, it’s the biggest comet that we’ve ever seen.”
The discovery involved reprocessing data from the Dark Energy Survey, data that had been acquired via the 4-meter Blanco instrument at Cerro Tololo in Chile between 2013 and 2019. Bernardinelli and Bernstein announced the find in June of this year.
C/2014 UN271 was at roughly the distance of Neptune when first acquired in the data, although it is now about 20 AU out, or twice the distance of Saturn. While there was no statement about activity on the comet when the discovery was announced, astronomers at Las Cumbres Observatory quickly turned their network of telescopes on the object.
Las Cumbres offers a globally distributed network of telescopes with 24-hour robotic operation that is finely tuned to detect transients. Its instruments have now revealed that C/2014 UN271 is active, with the 1-meter LCO telescope at the South African Astronomical Observatory returning images that, in keeping with the international flavor of the observing effort, drew the attention of astronomers in New Zealand.
Thus Michele Bannister (New Zealand’s University of Canterbury):
“Since we’re a team based all around the world, it just happened that it was my afternoon, while the other folks were asleep. The first image had the comet obscured by a satellite streak and my heart sank. But then the others were clear enough and gosh: there it was, definitely a beautiful little fuzzy dot, not at all crisp like its neighbouring stars!”
Comet C/2014 UN271 was indeed active, that fuzzy dot indicating a coma. Volatile ices, likely carbon dioxide and carbon monoxide, were already reacting to the scant sunlight available. The coma was active even with the comet almost three billion kilometers from the Sun. No comet has ever been observed to be active this far from Sol.
Image: Comet C/2014 UN271 (Bernardinelli-Bernstein), as seen in a synthetic color composite image made with the Las Cumbres Observatory 1-meter telescope at Sutherland, South Africa, on 22 June 2021. The diffuse cloud is the comet’s coma. Credit: LOOK/LCO.
This is one useful object, particularly since it was discovered early in its entry into the realm of the planets. It should offer astronomers over a decade of observing time. Perihelion is not expected until 2031, although when it comes, the comet will still be well beyond Saturn. Its orbit is now projected to take three million years to complete.
Image: An orbital diagram showing the path of Comet C/2014 UN271 (Bernardinelli-Bernstein) through the Solar System. The comets’ path is shown in gray when it is below the plane of the planets and in bold white when it is above the plane. Credit: NASA.
The discovery of activity on C/2014 UN271 draws attention to the LOOK Project now active at Las Cumbres. LOOK stands for LCO Outbursting Objects Key, an effort that investigates unexpected brightening of comets through burst activity and other aspects of comet evolution. Out of all this we should get a better understanding of comet outbursts, of which C/2014 UN271 is offering such a tantalizing sample.
These findings should also feed into future space missions like the European Space Agency’s Comet Interceptor, using data from this and other wide field surveys. Las Cumbres staff scientist Tim Lister explains:
“There are now a large number of surveys, such as the Zwicky Transient Facility and the upcoming Vera C. Rubin Observatory, that are monitoring parts of the sky every night. These surveys can provide alerts if one of the comets changes brightness suddenly and then we can trigger the robotic telescopes of LCO to get us more detailed data and a longer look at the changing comet while the survey moves onto other areas of the sky. The robotic telescopes and sophisticated software of LCO allow us to get images of a new event within 15 minutes of an alert. This lets us really study these outbursts as they evolve.”
That’s good news for the next interstellar object that wanders by as well. One of these days our growing network of instruments will be able to spot something like ‘Oumuamua in time for a spacecraft to pay it a visit.
Near-Earth Object Surveyor is a proposed space telescope working at infrared wavelengths, an instrument that just completed a successful mission review and now moves on to the next phase of mission development. In NASA parlance, the upcoming Key Decision Point-B moves into Preliminary Design territory. Getting a spacecraft from concept to flight is a long process, but let’s back out to the broader picture.
Planetary defense is all about finding objects that could impact the Earth with serious consequences. That means setting size targets, and on that score, we’re making progress. In 2010, NASA announced that it had identified 90 percent of all Near Earth Objects larger than 1,000 meters. That moved us to the next target, NEOs larger than 140 meters in size, a goal set by the National Aeronautics and Space Administration Act of 2005. JPL now says about 40% of NEOs within this size range have been identified.
So with this work in progress, what does NEO Surveyor bring to the table? For one thing, it makes it possible to discover asteroids on dangerous trajectories much faster than current methods allow, by including objects that could approach the Earth from directions close to the Sun, a blind spot for ground-based observatories. Amy Mainzer is survey director for NEO Surveyor at the University of Arizona:
“By searching for NEOs closer to the direction of the Sun, NEO Surveyor would help astronomers discover impact hazards that could approach Earth from the daytime sky. NEO Surveyor would also significantly enhance NASA’s ability to determine the specific sizes and characteristics of newly discovered NEOs by using infrared light, complementing ongoing observations being conducted by ground-based observatories and radar.”
Image: NEO Surveyor is a new mission proposal designed to discover and characterize most of the potentially hazardous asteroids that are near the Earth. Credit: NASA/JPL-Caltech.
It’s worth remembering that while there are currently no impact threats in the catalog for this century, unknown objects still pose problems. Nobody tracked the Chelyabinsk impactor of 2013, reminding us of the dangers of complacency and the need for better sensors, like those NEO Surveyor would deploy in the infrared. The Chelyabinsk object was about 17 meters in size, well below what we are currently cataloging.
But we continue to make progress. Mike Kelley, a NEO Surveyor program scientist at NASA headquarters, believes the spacecraft could bring the catalog of 140-meter objects to 90 percent completion within ten years of launch (in 2026, if NEO Surveyor continues to move on track).
Meanwhile, we should keep in mind missions further along in the pipeline. The Double Asteroid Redirection Test (DART) mission is up for launch later this year. This one is about active planetary defense, with the plan of using a kinetic impactor to change an asteroid’s trajectory. The target is a binary near-Earth asteroid called (65803) Didymos; more specifically, DART will hit Didymos’ moon Dimorphos head on in the fall of 2022.
Image: Illustration of how DART’s impact will alter the orbit of Dimorphos (formerly called “Didymos B”) about Didymos. Telescopes on Earth will be able to measure the change in the orbit of Dimorphos to evaluate the effectiveness of the DART impact. Credit: NASA/JPL.
Interestingly, about one sixth of the known near-Earth asteroid (NEA) population are binary or multiple-body systems. Didymos and Dimorphos are separated by about one kilometer, with the 160-meter moon tidally locked to the 780 meter primary. Let’s also note the international aspects of DART, for the mission will work hand in glove with an Italian cubesat called LICIA (Light Italian CubeSat for Imaging of Asteroid) that will observe the impact ejecta, while the European Space Agency’s Hera mission will make a post-impact survey several years after the event.
Asteroid threat mitigation is indeed a global concern, but we’re beginning to experiment with deflection strategies using actual missions. The mission page for DART explains the plan this way:
The DART demonstration has been carefully designed. The impulse of energy that DART delivers to the Didymos binary asteroid system is low and cannot disrupt the asteroid, and Didymos’s orbit does not intersect Earth’s at any point in current predictions. Furthermore, the change in Dimorphos’s orbit is designed to bring its orbit closer to Didymos. The DART mission is a demonstration of capability to respond to a potential asteroid impact threat, should one ever be discovered.
We can hope we’ll never have to use the DART strategy — or others that are under active consideration — to adjust the trajectory of a major impactor, but we obviously need to have the tools available just in case. The need to conduct such tests and to maintain active surveillance as a means of planetary defense is a driver for space technologies we shouldn’t overlook. The capability of adjusting orbits much further from home is a spur toward exploration and surveillance throughout the system.
The beauty of comet 2I/Borisov, the second interstellar object discovered in our Solar System, is that it looks and acts more or less like, well, an interstellar comet, without the puzzling characteristics of its predecessor, the still controversial ‘Oumuamua. 2I/Borisov’s cometary nature is clear in the latest observations from the European Southern Observatory’s Very Large Telescope, data from which also tell us that this is one of the most undisturbed relics of a circumstellar disk ever found. Scientists believe it never passed close to any star before its 2019 passage by the Sun.
We don’t know around which star it formed, but Stefano Bagnulo (Armagh Observatory and Planetarium, Northern Ireland), lead author of one of two new papers on the object, says that 2I/Borisov “could represent the first truly pristine comet ever observed.” Bagnulo’s team used the FORS2 instrument on the VLT (FOcal Reducer and low dispersion Spectrograph), an instrument that can take spectra as well as measuring polarization.
That later capability, called polarimetry, helps astronomers understand the chemistry of comets by studying how sunlight is polarized by a comet’s dust. The technique has been used on small bodies in the Solar System including comets, making for interesting comparisons. For 2I/Borisov’s polarimetric properties differ from Solar System comets with the exception of one, comet Hale-Bopp, which was likewise one of the most pristine comets observed to that time.
Image: Taken with the FORS2 instrument on ESO’s Very Large Telescope in late 2019, when comet 2I/Borisov passed near the Sun. Here the background stars appear as streaks of light as the telescope followed the comet’s trajectory. The colours in this composite image are the result of combining observations in different wavelength bands. Credit: ESO/O. Hainaut.
You may recall Hale-Bopp from the late 1990s, when it was a naked eye object (I remember a total stranger offering up a view in his small telescope on a nearby parking lot one night). A comet like this would be little affected by the solar wind and other radiation, thus having a composition similar to the original cloud of gas and dust that produced it. But Hale-Bopp was thought to have made one pass by the Sun before its recent visitation, while 2I/Borisov shows every sign of being a complete newcomer to the inner regions of a star.
Two interesting points from the paper:
…at the time of our observations, comet 2I/Borisov was polarimetrically homogeneous, showing no sign of active areas contributing to the coma formation. Prior to its recent perihelion passage, comet Hale-Bopp probably was near the Sun at least once, and possibly only once, ~2250?BC; at the time of that first approach, the original material was removed from the surface and active areas were open, hence Hale-Bopp could manifest activity during its recent perihelion passage. Comet 2I/Borisov instead, most likely never passed close to the Sun or any other star, and may represent the first truly pristine comet that has ever been observed.
That’s useful information in the context of what follows, as we learn in this paper that 2I/Borisov’s composition is consistent with objects that emerged close to home:
The similarity between the polarimetric properties of the two comets must depend upon the microscopic structure and composition of the aggregates, and not on their macroscopic characteristics, as the two comets are quite different in size: the analysis of the photometric profile of the inner coma suggests that comet Hale-Bopp belongs to the class of giant comets, with the diameter of the nucleus being estimated between 20 and 35?km, while 2I/Borisov’s nucleus size is ? 0.4?km. The close similarity between the polarimetric behaviour of the comet 2I/Borisov and Hale-Bopp suggests that, whatever astrophysical environment in which comet 2I/Borisov originated in, such environment had properties which led to the formation of a body bearing significant analogies with those accreted in the outer regions of our Solar System, a remarkable result on its own.
Image: Image of comet C/1995 O1 (Hale-Bopp), taken on 1997 April 04, with a 225mm f/2.0 Schmidt Camera (focal length 450mm) on Kodak Panther 400 color slide film with an exposure time of 10 minutes. The field shown is about 6.5°x6.5°. At full resolution, the stars in the image appear slightly elongated, as the camera tracked the comet during the exposure. Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria (http://www.sternwarte.at) – Own work, CC BY-SA 3.0.
In a second paper on the interstellar comet, ESO astronomer Bin Yang used data from the Atacama Large Millimeter/submillimeter Array (ALMA) to study 2I/Borisov’s dust grains. Here the key finding is that the coma of the comet surrounding the nucleus contains dust grains of one millimeter or larger. The relative amounts of carbon dioxide and water also changed as the comet neared perihelion, an indication that mixing of materials occurred where it formed. We can begin to deduce interesting things about the home system of this interstellar comet:
Our ALMA and VLT observations indicate that 2I/Borisov’s home planetary system, much like our own Solar system, had experienced efficient radial mixing from the innermost parts of its protoplanetary disk to beyond the frost line of CO. Among a number of probable mechanisms that have been proposed for the origin of ISOs [interstellar objects], gravitational interactions between planetesimals in the protoplanetary disk and growing giant planets is favored, as it can explain both the ejection of ISOs from their home systems as well as account for the strong radial transport of materials in the disk. While the most common planets in other exoplanetary systems seem to be super-Earths and mini-Neptunes, our study suggests the presence of giant planets in the home system of 2I/Borisov.
The first paper is Bagnulo, et al., “Unusual polarimetric properties for interstellar comet 2I/Borisov,” Nature Communications 12, No. 1797 (30 March 2021). Abstact/Full Text. The second paper is Bin Yang et al., “Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Borisov,” Nature Astronomy 30 March 2021. Abstract.
Were the rocky worlds of the inner Solar System depleted in carbon as they formed, the so-called ‘carbon deficit problem’? There is evidence for a system-wide carbon gradient in that era, which makes for interesting interactions between our Sun’s habitable zone and the far reaches of the system, for as the planets gradually cooled, the carbon so necessary for life as we know it would have been available only far from the Sun.
How much of a factor were early comets in bringing carbon into the inner system? This question underlies new work by Charles Woodward and colleagues. Woodward (University of Minnesota Twin Cities / Minnesota Institute of Astrophysics) focuses on Comet Catalina, which was discovered in early 2016. He sees carbon in the context of life:
“Carbon is key to learning about the origins of life. We’re still not sure if Earth could have trapped enough carbon on its own during its formation, so carbon-rich comets could have been an important source delivering this essential element that led to life as we know it.”
Image: Illustration of a comet from the Oort Cloud as it passes through the inner Solar System with dust and gas evaporating into its tail. SOFIA’s observations of Comet Catalina reveal that it is carbon-rich, suggesting that comets delivered carbon to the terrestrial planets like Earth and Mars as they formed in the early system. Credit: NASA/SOFIA/ Lynette Cook.
Let’s zoom in on this a little more closely. Volatile ices of water, carbon monoxide and carbon dioxide are found mixing with dust grains in the outer system, an indication that the young Solar System beyond the snowline was, in the authors’ words, “not entirely ‘primordial’ but was ‘polluted’ with the processed materials from the inner disk, the ‘hot nebular product.'” Or to slip the metaphor slightly, we can say that comets were salted with materials that were originally produced at higher temperatures. Comets can offer a window into this process.
The work is anything but straightforward, for although we’ve learned a lot through missions like Giotto, Rosetta/Philae and Deep Impact (including, of course, abundant telescope observations from Earth and a sample return mission called Stardust), the interplanetary dust particles we’ve been able to analyze from comets 81P/Wild 2 and 26P/Grigg-Skjellerup differ considerably. The paper explains:
The former contains material processed at high temperature (Zolensky et al. 2006), while the latter is very “primitive” (Busemann et al. 2009). For these reasons, it is necessary to determine as best as we can the properties of dust grains from a large sample of comets using remote techniques (Cochran et al. 2015). These include observations of both the thermal (spectrophotometric) and scattered light (spectrophotometric and polarimetric). The former technique provides our most direct link to the composition (mineral content) of the grains.
The research team drew on data from the Stratospheric Observatory for Infrared Astronomy (SOFIA), a Boeing 747 aircraft carrying a 2.7-meter reflecting telescope with an effective diameter of 2.5 meters. At altitude (SOFIA generally operates between 38,000 and 45,000 feet), the observatory is above 99 percent of Earth’s atmosphere, which can block infrared wavelengths. SOFIA data show Catalina as a carbon-rich object.
The paper points out that carbon dominates as well in other comets we’ve seen, both those in closer orbits (103P/Hartley 2) and Oort Cloud comets like C/2007 N3 and C/2001 HT50. It also turns out that dusty material from comet 67P/Churyumov–Gerasimenko was rich in carbon, although the authors note that comets can show changes in their silicate-to-carbon ratio, sometimes even during the course of a single night’s observations. The paper adds:
A dark refractory carbonaceous material darkens and reddens the surface of the nucleus of 67P/Churyumov–Gerasimenko. Comet C/2013 US10 (Catalina) is carbon rich. Analysis of comet C/2013 US10 (Catalina)’s grain composition and observed infrared spectral features compared to interplanetary dust particles, chondritic materials, and Stardust samples suggest that the dark carbonaceous material is well represented by the optical properties of amorphous carbon. We argue that this dark material is endemic to comets.
All this suggests that carbon delivered by comets is a part of the evolution of the early Solar System. Each carbon-rich comet we study has implications for how life may have been spurred by impacts, making the investigation of carbon-rich Oort Cloud comets a continuing priority for SOFIA, which can be deployed quickly when comets are found entering the inner system.
The paper is Woodward et al, “The Coma Dust of Comet C/2013 US10 (Catalina): A Window into Carbon in the Solar System,” The Planetary Science Journal (2021). Abstract / Full Text.