by Paul Gilster | Jun 21, 2019 | Outer Solar System |
Cassini’s productivity at Saturn continues to provide fodder for scientific papers and encouragement for the builders of complex missions, who have seen enough data gathered by this one to guarantee continuing insights into the ringed planet for years to come. The June 14 issue of Science offers up four papers (citations below) that show results from four of the spacecraft’s instruments, including startling views of the main rings.
The data examined in the Science papers were gathered during Cassini’s ring-grazing orbits from December, 2016 to April of 2017 as well as during the ‘Grand Finale’ between April and September of 2017, when Cassini flew closer than ever before to the giant planet’s cloud tops. Consider the image below, showing an infrared view as captured by the spacecraft’s Visible and Infrared Mapping Spectrometer (VIMS), with (at the left) the natural color view taken as a composite by Cassini’s Imaging Science Subsystem.
Imaging the rings in visible and near-infrared wavelengths, VIMS surprised scientists by finding weak water ice bands in the outer parts of the A ring, a seeming contradiction given the highly reflective nature of this area, which has been taken to indicate less contaminated ice and stronger water ice bands. While water ice is dominant in the rings, the spectral map shows no ammonia or methane ice, nor does it see organic compounds. This is an oddity given that organics have been found flowing out of the D ring into Saturn’s atmosphere.
“If organics were there in large amounts — at least in the main A, B and C rings — we’d see them,” says Phil Nicholson, Cassini VIMS scientist of Cornell University in Ithaca, New York. “I’m not convinced yet that they are a major component of the main rings.”
Image: The false-color image at right shows spectral mapping of Saturn’s A, B and C rings, captured by Cassini’s Visible and Infrared Mapping Spectrometer (VIMS). It displays an infrared view of the rings, rather than an image in visible light. The blue-green areas are the regions with the purest water ice and/or largest grain size (primarily the A and B rings), while the reddish color indicates increasing amounts of non-icy material and/or smaller grain sizes (primarily in the C ring and Cassini Division). At left, the same image is overlaid on a natural-color mosaic of Saturn taken by Cassini’s Imaging Science Subsystem. Credit: NASA/JPL-Caltech/University of Arizona/CNRS/LPG-Nantes. Saturn image credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic.
The rings, now believed to have formed much later than the planet itself, continue to deliver complexities that will feed into future models of ring evolution. The last phase of Cassini’s mission is, in the words of project scientist Linda Spilker (JPL), author of one of the papers in Science, “like turning the power up one more notch on what we could see in the rings,” a higher resolution that even as it answers some questions raises still more.
At the outer edge of the main rings, for example, impact-generated streaks appear in the F ring that are of approximately equal length and similar orientation, suggesting a string of impactors striking at the same time. The implication: The F ring, at least, is shaped by materials already orbiting Saturn rather than by materials the planet encounters in its orbit of the Sun. Small moons embedded in the rings can be considered in that sense to be sculpting them, according to Cassini scientist Matt Tiscareno (SETI Institute). Here we might see useful analogs in young exoplanet systems, where stellar systems form out of circumstellar disks that are in turn shaped by the emerging planets within them, processes out of which our own Solar System formed.
We also see how remarkably ring textures can differ even in segments close to each other, as seen in the image below. Teasing out the interactions that shaped the patterns found here will take us deeply into chemistry and temperature changes as mapped by Cassini.
Image: New images of Saturn’s rings show how textures differ even in close proximity of one another. The image on the right has been filtered so that the newly visible straw-like textures and clumps are more visible. Credit: NASA/JPL-Caltech/Space Science Institute.
Now have a look at Daphnis, one of the embedded moons, which is here shown creating three waves in the outer edge of the Keeler Gap, at the outer edge of the A ring. The spacecraft was at a shallow 15 degree angle above the rings when the image was taken. The images here were taken in visible light using Cassini’s narrow-angle camera at a distance of about 28,000 kilometers from Daphnis and at a Sun-Daphnis-spacecraft phase angle of 71 degrees.
Image: This enhanced-color image mosaic shows Daphnis, one of the moons embedded in Saturn’s rings, in the Keeler Gap on the sunlit side of the rings. Daphnis is seen kicking up three waves in the gap’s outer edge. Three wave crests of diminishing sizes trail the moon. In each successive crest, the shape of the wave changes as the ring particles within the crest interact and collide with each other. A thin strand of ring material to the lower left of Daphnis is newly visible in this image, and there are intricate features that also hadn’t been previously observed in the third wave crest downstream. Credit: NASA/JPL-Caltech/Space Science Institute / Tilmann Denk (Freie Universität, Berlin).
And finally, this image showing the third wave crest, which displays multiple strands of ring material, an indication of the effects of Daphnis on the ring. Here we are again in visible light, now at a distance of 23,000 kilometers and at a phase angle (Sun-Daphnis-spacecraft) of 94 degrees, with an image scale of 160 meters per pixel.
Image: A closeup of the third wave crest (toward the far left in the color image) focusing on detail scientists hadn’t seen before, with multiple strands of ring material visible. The image shows how the ring material behaves after losing the structure Daphnis triggered and goes back to interacting with itself. The ribbon of material that appears to be protruding into the gap in the right-hand portion of the image is probably actually soaring above the ring plane. Toward the left-hand part of the image, that same ribbon of material dives below the ring plane and becomes obscured by the main part of the rings, which is why it disappears from view. Credit: NASA/JPL-Caltech/Space Science Institute.
I had never thought of Saturn as providing a kind of astrophysical laboratory that could illuminate processes at work in the early Solar System, but this is what the four papers in Science suggest, for here we see the kinds of interactions that take place in disk structures that contain larger accreted objects. The Cassini imagery and data will always symbolize the era of early Solar System reconnaissance on which we are embarked. Imagine what we might do with a next-generation craft, a Cassini follow-up around Uranus or Neptune.
The four papers on the Cassini ‘Grand Finale’ all appear in the June 14 issue of Science, Vol. 364, Issue 6445, and can be accessed here. They are Tiscareno et al., “Close-range remote sensing of Saturn’s rings during Cassini’s ring-grazing orbits and Grand Finale”; Buratti et al., “Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus”; Militzer et al., “Measurement and implications of Saturn’s gravity field and ring mass”; and Spilker, “Cassini-Huygens’ exploration of the Saturn system: 13 years of discovery.”
by Paul Gilster | Jun 20, 2019 | Asteroid and Comet Deflection |
Ever since the passage of interstellar interloper ‘Oumuamua, we’ve become aware of the opportunities presented by objects entering our system from interstellar space, at the same time wishing we had the resources at hand to investigate them close-up. Andreas Hein and colleagues at the Initiative for Interstellar Studies have examined the possibilities for reaching ‘Oumuamua through Project Lyra (see Project Lyra: Sending a Spacecraft to 1I/’Oumuamua), a study that also takes in the kind of future infrastructure that could allow us to react to the next such object.
Now comes the interesting news that the European Space Agency is developing a mission called Comet Interceptor, one capable of visiting a long-period comet coming into the inner system from the Oort Cloud, but just as capable of reaching an interstellar visitor. The idea revolves around not a single spacecraft, but a combination of three. The composite vehicle would be capable of orbiting the L2 Lagrange point 1.5 million kilometers from Earth until it finds a suitable target. At that point, it would journey to the object and separate into three modules.
Image: Comet Interceptor has been selected as ESA’s new fast-class mission. It will be the first spacecraft to visit a truly pristine comet or other interstellar object that is only just starting its journey into the inner Solar System. The spacecraft will wait at the Sun-Earth Lagrange point L2, which is 1.5 million kilometres ‘behind’ Earth as viewed from the Sun. It will travel to an as-yet undiscovered comet, making a flyby of the chosen target when it is on the approach to Earth’s orbit. The mission comprises three spacecraft that will perform simultaneous observations from multiple points around the comet. Credit: ESA.
Each module will be equipped with a science payload that complements the instrumentation on the others, offering insights into cometary gas and dust and the plasma environment near the object through a mass spectrometer along with dust, field and plasma instruments. Thus we get ‘multi-point’ measurements offering insights into cometary interactions with the solar wind, the stream of plasma from the Sun that itself is constantly changing in velocity and intensity.
This is a fundamentally different concept from previous missions like Giotto and Rosetta. Giotto flew within 600 kilometers of Comet 1P/Halley in 1986, with another pass by Comet Grigg-Skjellerup in 1992. Rosetta targeted Comet 67P/Churyumov-Gerasimenko in a highly successful mission in 2014. Both comets are short-period objects with periods of less than 200 years, with 67P/Churyumov-Gerasimenko orbiting every 6.5 years and Halley every 76.
In both cases, the comet’s frequent passage into the inner system has meant changes to the surface. What Comet Interceptor is looking for is a first-time visitor, one whose materials should be relatively unprocessed since the earliest days of the system. But ESA is also thinking about interstellar objects like ‘Oumuamua as potential destinations, for the mission has the luxury of being able to choose its target from its stable vantage point at L2. Given the success of the Pan-STARRS effort at finding new comets and the construction of the Large Synoptic Survey Telescope in Chile, slated to reach first light in 2020, we should have no shortage of targets.
ESA director of science Günther Hasinger describes the mission in context:
“Pristine or dynamically new comets are entirely uncharted and make compelling targets for close-range spacecraft exploration to better understand the diversity and evolution of comets. The huge scientific achievements of Giotto and Rosetta – our legacy missions to comets – are unrivalled, but now it is time to build upon their successes and visit a pristine comet, or be ready for the next ‘Oumuamua-like interstellar object.”
Image: Kuiper Belt and Oort Cloud in context. Credit: ESA.
In official terminology, Comet Interceptor is an F-class mission, the ‘F’ standing for ‘fast,’ as in ‘fast implementation’ — the total development time from selection of the mission to readiness to launch is to be eight years. But we might also consider it in terms of ‘fast response,’ just what is needed to reach objects that appear with no prior warning. This category of mission will have a launch mass of less than 1,000 kilograms. Comet Interceptor is now seen going into space along with exoplanet hunter ARIEL in 2028, both missions being delivered to L2.
by Paul Gilster | Jun 19, 2019 | Astrobiology and SETI |
On Monday I was talking about the rise of open access scientific journals, using the European Space Agency’s Acta Futura as just one example. The phenomenal arXiv service, not itself a journal but a repository for preprints of upcoming papers, is already well known in these pages. Now we have the largest public release of SETI data in the history of the field, a heartening follow-through on a trend that broadens the audience for scientific research.
Breakthrough Listen is presenting two publications in the scientific literature (available as full text, citation below) describing the results of three years of radio and optical observations, along with the availability of a petabyte of data from its work at the Green Bank instrument in West Virginia and the Parkes Radio Telescope in Australia. This covers a sample of 1327 nearby stars (within 160 light years from Earth) and builds on the team’s results on 692 stars as presented in 2017.
No signs of extraterrestrial civilizations turn up in the analysis, says Parkes project scientist Danny Price, who emphasizes that the search will continue:
“This data release is a tremendous milestone for the Breakthrough Listen team. We scoured thousands of hours of observations of nearby stars, across billions of frequency channels. We found no evidence of artificial signals from beyond Earth, but this doesn’t mean there isn’t intelligent life out there: we may just not have looked in the right place yet, or peered deep enough to detect faint signals.”
Image: The Green Bank site in West Virginia, where Breakthrough Listen observations continue. Credit: NRAO/AUI.
There are reasons why making such data public benefits the SETI effort. Both within the public and the astronomical community, those interested can now download the results of these observations and examine them independently. Those with programming skills may well develop algorithms for the detection of signals and filtering out of background noise that improve on the current model. And there may be information within the datasets that will prove useful in the investigation of unrelated astrophysical phenomena.
The existing tools developed by the Breakthrough Listen science team at the Berkeley SETI Research Center (BSRC) include both radio frequency searches as well as optical scans and algorithms designed to flag unexplained astrophysical phenomena. Go to this UC-Berkeley page for the overview, including the two just released papers. Likewise available to the public are software tools used in the analysis such as blimpy (for loading raw format data files), and turboSETI (for running Doppler drift searches). The datasets are examined in the analysis paper by Dr. Price and made available at the Breakthrough Listen Open Data Archive and via BSRC (more search options available at the latter).
For those wanting to get into data crunching themselves, the second paper (lead author Matt Lebofsky at Berkeley) goes into the intricacies of the current analysis, the tools used, the data formats and the archival systems now in play. “While we have been making smaller subsets of data public before in varying forms and contexts,” says Lebofsky, “we are excited and proud to offer this first cohesive collection along with an instruction manual, so everybody can dig in and help us search. And we’re just getting started – there’s much more to come!”
Considering the complexities involved in creating a search ‘pipeline’ that can scan through billions of radio channels, the more eyes on search algorithms and filtering techniques, the better. Thus far the detected signals have come from human technologies, with the Breakthrough Listen team filtering for narrow-band signals showing a Doppler drift, meaning they change in frequency with time because of their motion with respect to the telescope.
A second filter in the pipeline removes signals that do not appear to originate from a fixed point on the sky. The application of both techniques reduces millions of signals down to a comparative few, all of which have been examined and found to be human-generated frequency interference. From the Price paper, which notes that in its search for narrowband signals showing Doppler drift, 51 million hits emerged, with 6154 that cleared the automated filtering process, leading to a final round of manual inspection and cross-referencing against known sources of interference:
…these observations constitute the most comprehensive survey for radio evidence of advanced life around nearby stars ever undertaken, improving on the results of Enriquez et al. (2017) in both sensitivity and number of stars. Together with other recent work from the resurgent SETI community, we are beginning to put rigorous and clearly defined limits on the behavior of advanced life in the universe. We note that significant additional observational and theoretical work remains to be done before we are able to make general statements about the prevalence of technologically capable species.
Be aware that the archive also includes data from Breakthrough Listen observations of the first repeating fast radio burst ever detected, FRB 121102, as well as scans of the ‘Oumuamua object, along with optical data from the Automated Planet Finder at Lick Observatory. The search of nearby stars continues while also being expanded into the galactic disk at Parkes, and a one-million star sample with the MeerKAT telescope in South Africa is forthcoming.
The papers are Price et al., “The Breakthrough Listen Search for Intelligent Life: Observations of 1327 Nearby Stars over 1.1-3.4 GHz,” submitted to The Astrophysical Journal (preprint) and Lebofsky et al., “The Breakthrough Listen Search for Intelligent Life: Public Data, Formats, Reduction and Archiving,” submitted to Publications of the Astronomical Society of the Pacific (preprint).
by Paul Gilster | Jun 18, 2019 | Exoplanetary Science |
We rarely talk about Teegarden’s Star when mentioning interesting objects near the Solar System, probably because the star was only discovered in 2003 and until now had not been known to host planets. Today we learn, however, that an international team led by the University of Göttingen has found two planets close to Earth mass in what it considers to be the habitable zone around the tiny star. Interestingly, from where the system is located, any local astronomers would be able to see the planets of our Solar System in transit across the face of the Sun, about which more in a moment.
One of the reasons that this comparatively nearby star has been so late to be discovered is its size. We are dealing with an M-class red dwarf, this one in the constellation Aries, and no more than 12.5 light years from us. It took three years of patient radial velocity monitoring to track down planets around a star that is only about 2700 degrees Celsius in temperature, and fully 10 times lighter than the Sun. What we now have are two planet candidates, each with a minimum mass 1.1 times that of Earth, with orbital periods of 4.91 and 11.4 days respectively.
These are, according to the paper, “…the first Earth-mass planets around an ultra-cool dwarf for which the masses have been determined using radial velocities.”
Because no transits have been detected, the scientists have no information on planetary radii, and therefore estimated them based on various possible compositions, from rocky to gaseous mini-Neptune, finding that the resulting radii differ by a factor of about three. The other stellar and planetary parameters were plugged into the Earth Similarity Index (ESI), which compares key parameters to those of Earth. If these worlds are not mini-Neptunes, we get this (from the paper):
Except for the case of a mini-Neptune composition, the two planets have a high ESI. For a potentially rocky composition, the ESI value is 0.94 and 0.8 for planets b and c, respectively. This makes Teegarden’s Star b the planet with currently the highest ESI value. However, the ESI is only an estimate, and different weighting of the parameters may lead to changing ESIs. This ESI definition, for example, does not take into account the stellar spectral energy distribution and the resulting planetary atmospheric composition, which very likely have an effect on habitability.
So we push at the boundaries of what we still don’t know. Lead author Matthias Zechmeister (University of Göttingen) noted the resemblance between these two worlds and the inner planets of our Solar System, saying “They are only slightly heavier than Earth and are located in the so-called habitable zone, where water can be present in liquid form.” I asked Dr. Zechmeister to amplify on the habitability zone finding, to which he replied:
You may have noted that PHL (http://phl.upr.edu/press-releases/pr_draft_tee4321) ranks Teegarden b now as the exoplanet most similar to Earth. The conditions are good for liquid water on the surface, given a similar insolation and mass as Earth’s. Still, we cannot be sure to 100%. We have measured “only” the mass (which is a minimum mass, but true masses are statistically only ~16% higher). So we do not know the true chemical composition, though a rocky composition is probable (see Fig. 12 in the paper for other compositions).
Dr. Zechmeister also made note of the fact that Teegarden’s Star is about 8 billion years old, roughly twice the age of the Sun, allowing plenty of time for interesting things to develop if life ever took hold there. And he raised a caution re the habitability issue, noting that the diminutive star is what he refers to as ‘an extreme host,’ a type of star about which we still have a great deal to learn.
So small is Teegarden’s Star that it is not far above the upper size limit for brown dwarfs, often considered to be somewhere between 60 and 90 Jupiter masses, and at magnitude 15, it demands a large telescope to see it at all. In fact, it was actually discovered in 2003 via stored data in the Near-Earth Asteroid Tracking (NEAT) program, and had been logged in our data earlier, turning up on photographic plates from the Palomar Sky Survey taken in 1951.
Image: Comparison habitable zone in Teff [effective temperaeture] – HZ diagram. Credit: C. Harman.
Now ponder this: Radial velocity detections have produced more than 800 exoplanets, but few have been found around old, cool M-dwarfs. In fact, we have only two other planet hosts with effective temperatures cooler than 3,000 K, and one of these is Proxima Centauri, while the other is TRAPPIST-1, around which fully seven transiting planets are known to exist. The authors of the paper on the Teegarden’s Star work consider the lack of planet detections around very late-type stars the result of observational bias owing to the faintness of the objects at visible wavelengths.
From the paper:
Both planets have a minimum mass close to one Earth mass, and given a rocky, partially iron, or water composition, they are expected to have Earth-like radii. Additionally, they are close to or within the conservative HZ, or in other words, they are potentially habitable. Our age estimate of 8 Gyr implies that these planets are about twice as old as the solar system. Interestingly, our solar system currently is within the transit zone as seen from Teegarden’s Star. For any potential Teegardians, the Earth will be observable as a transiting planet from 2044 until 2496.
A later note from co-author Guillem Anglada-Escudé, the discoverer of Proxima Centauri b, unpacks this further. The transit that would be visible from this system in 2044 would have occurred in our Solar System in 2032, factoring in the 12 year light travel time. There have been SETI discussions regarding the possibility of conducting communication attempts of stars whose planets would see our own world in transit around the Sun, so any such attempt would need to take place in 2032 or later, with the earliest potential response expected around 2056.
Image: Top 19 potentially habitable exoplanets, sorted by distance from Earth. Credit: A. Mendez (PHL).
The team behind this work used data from CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs), an effort using two separate spectrographs located at the 3.5m telescope at the Calar Alto Observatory in Almeria, Spain. The goal of the project, conducted by a consortium of German and Spanish institutions, is to carry out a survey of approximately 300 late-type main-sequence stars with the goal of detecting low-mass planets in their habitable zones.
We have not, in other words, heard the last from CARMENES.
The paper is Zechmeister et al. “The CARMENES search for exoplanets around M dwarfs – Two temperate Earth-mass planet candidates around Teegarden’s Star,” accepted at Astronomy & Astrophysics 2019 (abstract).
by Paul Gilster | Jun 17, 2019 | Culture and Society |
Given the difficulties that persist in retrieving many good papers from behind publisher firewalls, I’m always glad to see open access journals plying their trade. Let me call your attention in particular to Acta Futura, which comes out of the scientists working with the European Space Agency’s Advanced Concepts Team. Acta Future defines itself as multidisciplinary in scope with a focus on the long-term development of space science.
Hence the list of topics is wide, as the website notes, “…ranging from fundamental physics to biomimetics, mission analysis, computational intelligence, neuroscience, as well as artificial intelligence or energy systems,” and this does not exhaust the range of possibilities. If you’re interested in browsing through or searching the archives, click here for a page with the appropriate links as well as information on how to submit papers to Acta Futura.
I’ve had ESA’s Advanced Concepts Team on my mind this weekend because long-time Centauri Dreams reader David Wojciech passed along news of the upcoming ACT interstellar workshop, beginning this week on the 20th, and carrying over to the following day. The conference anticipates an upcoming issue of Acta Futura devoted to interstellar exploration. The venue will be Erasmus Highbay at ESTEC, Noordwijk, Netherlands.
Topics to be covered in depth are:
- Advanced propulsion technologies for interstellar probes
- Communication for interstellar exploration
- Concepts and ideas surrounding world ships: sociology, ethics, anthropology, language development
- Technologies and measure to keep humans alive, healthy and productive during long duration spaceflight
And I note that registration is free of charge until June 18, so if you’re fortunate enough to be in range of the meeting, be sure to check in at the conference webpage. I’m seeing plenty of good material here, from Pontus Brandt’s “A Pragmatic Interstellar Probe in the 2030s,” to Andreas Hein’s “World ships: feasibility and rationale” and Michael Hippke’s “Interstellar communication.” But look at the whole list (available here), which includes Philip Lubin on laser propulsion, Rob Swinney on fusion, Angelo Vermeulen on evolvable spacecraft and Ugo Lafont on self-healing materials. Most of these will be familiar names to regular Centauri Dreams readers.
Image: A Bussard ramjet in flight, as imagined for ESA’s Innovative Technologies from Science Fiction project. Credit: ESA/Manchu.
There was a time not so long ago when posting news of an upcoming interstellar conference would be relevant only to those within immediate geographic range. The entire field of interstellar studies, in fact, was once an afterthought at conferences largely devoted to other matters, usually discussed only at the end. These days, a rising interest in interstellar possibilities leads to conferences whose papers will be readily accessible in an open access journal like Acta Futura. We’ll have a look at these when the materials reach publication stage.
