Centauri Dreams
Imagining and Planning Interstellar Exploration
Going Interstellar in Europe
Foundations of Interstellar Studies Workshop in UK
A workshop on interstellar flight titled Foundations of Interstellar Studies is to take place from 27 to 30 June of this year in the town of Charfield, Gloucestershire, United Kingdom, at the current headquarters of the Initiative for Interstellar Studies. This follows an initial ‘foundations’ conference in 2017 that was held at City College New York and the Harvard Club of New York; future conferences, “run jointly between several organisations depending on the host country,” are planned on a roughly two-year schedule. I immediately warmed to the theme that the Initiative for Interstellar Studies (i4IS) introduced by quoting Robert H. Goddard:
How many more years I shall be able to work on the problem I do not know; I hope, as long as I live. There can be no thought of finishing, for ‘aiming at the stars’ both literally and figuratively, is a problem to occupy generations, so that no matter how much progress one makes, there is always the thrill of just beginning.
Browsing through the conference materials I note that, with reference to famous physics conferences like Shelter Island, Pocono and Oldstone, the emphasis is on both academic rigor but also informal conversation, a format that i4IS president Kelvin Long hopes will energize the interstellar community. The aim is “to get researchers together and to maximize the social interaction time for idea swapping and information exchange and it is expected that the ideas and discussions (and maybe even calculations) should continue into the evening social sessions.”
The three days of discussions in this year’s conference will take place at the Bone Mill, which has been the i4IS headquarters since 2017. This is beautiful country, as those of you who have been to the Cotswolds will already know, in the village of Charfield, near Wootton-under-Edge, in the English county of Gloucestershire. The three themes under focus, each with a day devoted to it:
- Living in Deep Space
- Advanced Propulsion Technology & Missions
- Building Architectural Megastructures
In addition to the formal scientific proceedings, there will be an opening social event on the evening of Thursday 27th June, starting at 18:00 hours at the Bone Mill. There will also be a formal dinner on Saturday 29th June starting at 19:00 hours at a venue to be announced.
An invitation will be made to submit papers from selected authors post-conference, to the Journal of the British Interplanetary Society (JBIS) and/or publication in the official conference proceedings. For more on the Foundations of Interstellar Studies Workshop 2019, including maps and information on accommodation, go to https://www.fisw.space/fisw-2019.
Horizon 2061 Synthesis Workshop in Toulouse
2061 will commemorate an interesting year in space exploration. It is the centennial not just of the first human flight into space by Yuri Gagarin but also of the speech by which John Kennedy propelled the US aerospace community into a determined drive for a lunar landing. But we might also add another memorable factor. In 2061, Comet Halley makes its return. The last time we saw Halley was in 1986, when five spacecraft ranging in origin from the European Space Agency to the Soviet Union and France as well as Japan studied the comet in the inner system.
Thus we had the first comet observed in detail by spacecraft, giving us information about the cometary nucleus, the coma and the tail, helping us understand cometary structure. The fact that the Halley expeditions were so determinedly multinational (although the studied US solar sail never materialized) gives impetus to an effort called Planetary Exploration Horizon 2061 which, according to its founders is creating a long-term analysis of four primary areas of space exploration, all of these addressed from a determinedly international perspective.
From the Horizon 2061 website:
By 2061, all the “frontiers” (or outer boundaries) of exploration should have moved dramatically outwards: human exploration might have reached Mars and perhaps the main asteroid belt; sample return missions should have reached, beyond the asteroid belt, the Trojan asteroids on the orbit of Jupiter and the icy moons of Jupiter and Saturn; robotic exploration should have reached the very local interstellar medium, well beyond the outer shock of the heliosphere, thus opening the way towards the closest stars and their planetary systems.
Thus the “the four pillars of planetary exploration” Horizon 2061 is examining:
- Major scientific questions on planetary systems;
- Representative space missions that answering these questions will demand;
- Enabling technologies needed to make these missions happen;
- Ground- and space-based infrastructure needed for mission support.
The overall goal:
[The year 2061] symbolically represents our intention to encompass both robotic and human exploration in the same perspective. Its distant horizon, located well beyond the usual horizons of the planning exercises of space agencies and of their standing committees, which generally address shorter time scales, avoids any possible confusion with them and is intended to trigger a joint foresight thinking of the scientific and technology communities of planetary exploration that will free the imagination of the planetary scientists, who are invited to formulate what they think are the most relevant and important scientific questions independently of the a priori technical feasibility of answering them; of the engineers and technology experts, who are invited to explore innovative technical solutions that will make it possible to fly by 2061 the challenging space missions that will allow us to address these questions.
Space missions are designed, the Horizon 2061 proponents note, around a Science Traceability Matrix (STM) in which mission science questions and objectives define the instruments needed, the mission profile and the kind of platform on which the mission will be flown. Unlike single missions, though, Horizon 2061 intends to write the STMs for a set of representative missions that will investigate everything from the origin of planetary systems to the detection of life. Observations to be made and destinations within the Solar System where such measurements can be performed will determine the type of space missions that emerge from this matrix.
Two meetings have already occurred, the first in Bern in September of 2016, the second in Lausanne in April of 2018. Coming now is the next step, devoted to the synthesis of the exercise. This will take place in an international colloquium hosted by the Université Paul Sabatier in Toulouse from June 5th to 7th, 2019.
The primary organizers will be the Institut de Recherche en Astrophysique et Planétologie (IRAP) and the Observatoire Midi-Pyrénées (OMP). This colloquium, placed under the sponsorship of COSPAR [Committee on Space Research, established by the International Council for Science in 1958], will complete the design of the four pillars and initiate the drafting of the final report, which will be edited and published under the auspices of COSPAR.
Tentative conclusions from the colloquium will be presented for discussion at the joint EPSC-DPS meeting (European Planetary Science Conference – AAS Division for Planetary Sciences) in Geneva (September 15th to 20th, 2019), and later for discussion and final approval at the COSPAR General Assembly (Sydney, August 15th to 23rd, 2020).
Meeting agenda, registration and other materials are available at https://h2061-tlse.sciencesconf.org/.
Proxima Centauri c?
A possible second planet around Proxima Centauri raises all kind of questions. I wasn’t able to make it to Breakthrough Discuss this year, but I’ve gone over the presentation made by Mario Damasso of Turin Observatory and Fabio Del Sordo of the University of Crete, recounting their excellent radial velocity analysis of the star. Proxima c is a fascinating world, if it’s there, because it would be a super-Earth in a distant (and cold) 1.5 AU orbit of a dim red star. Exactly how it formed and whether it migrated to its current position could occupy us for a long time.
But is it there? The first difficulty has to do with stellar activity, which Damasso and Del Sordo were careful to screen out; it’s one of the major problem areas for radial velocity work in this kind of environment, for red dwarf stars are often quite active. During the question and answer session, another key question emerged: We know from Kepler that many stars are orbited by multiple planets, and there is no reason to assume that Proxima Centauri has but one.
The question: If there are other, smaller worlds in play here, could the effect of their combined masses produce a ‘phantom’ Proxima c in the orbit Damasso and Del Sordo have discussed?
The two astronomers are completely open to this possibility, and point to the need for follow-up observations with ESPRESSO, not to mention the useful Gaia measurements that could give us even more detail. Flare activity is always an issue in any case — it may have affected the results of Anglada et al. in 2018 (citation below), when researchers found possibly two inner dust belts and one outer belt around the star (see Proxima Centauri Dust Indicates a Complicated System). The Damasso and Del Sordo work is comprehensive as far as it can go, but both were careful to note that we are dealing solely with a candidate, not a confirmed world. And it could well be the result of other, unseen planets affecting the star as well as stellar noise.
This work draws on the earlier Proxima Centauri radial velocity dataset compiled by Guillem Anglada-Escudé (University of London) and team, but folds in an additional 61 RV observations, with considerable attention to the question of filtering out the 85 day rotation period of the parent star and the associated noise of stellar surface perturbations. The instrument in play is the European Southern Observatory’s High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph at La Silla.
I suspect we’re going to find a number of small worlds around Proxima Centauri, so we’ll see how their gravitational interactions might affect the spectroscopic data and hence the confirmation of the current candidate. But if this detection is confirmed, this is what we’ve found: The planet would mass about six Earths — remember that because this is radial velocity, we can only measure a minimum mass, because we don’t know planetary inclination — and would orbit Proxima Centauri with a period of 1900 days at 1.5 AU. Not exactly a habitable place for the likes of our species. Del Sordo estimates temperatures there would be about 40 K.
We may know, via Gaia, whether Proxima Centauri c is an actual world by the end of this year. A key follow up question is, can we snag a direct image in visible light? If so, it would mark the first such detection of a planet outside our Solar System, the imaged worlds found thus far having been discovered via infrared. There is plenty, in other words, to like about the hypothetical Proxima Centauri c, provided it’s really there. Waiting a few more months could give us a firm answer.
On another matter, as a great admirer of Thoreau, I was pleased that Damasso and Del Sordo quoted him at the beginning of their presentation, and to good effect: “If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them.” That’s a good metaphor for RV studies as exceedingly delicate as these. I’ll add a favorite bit from one of Thoreau’s poems:
For lore that’s deep must deeply studied be,
As from deep wells men read star-poetry…
There’s poetry indeed in the spectroscopic data of our nearest star, if we can just tease out its meaning. And here’s an image that might evoke a bit of poetry to close today’s entry.
Image: Rigil Kentaurus is the bright star near the top of this broad southern skyscape. Of course it’s probably better known as Alpha Centauri, nearest star system to the Sun. Below it sprawls a dark nebula complex. The obscuring interstellar dust clouds include Sandqvist catalog clouds 169 and 172 in silhouette against the rich starfields along the southern Milky Way. Rigil Kent is a mere 4.37 light-years away, but the dusty dark nebulae lie at the edge of the starforming Circinus-West molecular cloud about 2,500 light-years distant. The wide-field of view spans over 12 degrees (24 full moons) across southern skies. Credit & Copyright: Roberto Colombari.
The paper on dust belts around Proxima Centauri is from Guillem Anglada, “ALMA Discovery of Dust Belts Around Proxima Centauri,” Astrophysical Journal Letters Vol. 850 No. 1 (15 November 2017) (abstract). (Note: This is not Guillem Anglada-Escudé, despite the similarity in names!) The Damasso and Del Sordo paper is as yet unpublished, though undergoing peer review. Video of their presentation is available at https://www.youtube.com/watch?v=DLzzg9p0-AI&t=15648s (go to about 4:16:45 on the video).
Reflections on Messier 87’s Black Hole
Messier 87, a massive elliptical galaxy in the Virgo cluster, is some 55 million light years from Earth, and even though the black hole at its center has a mass 6.5 billion times that of the Sun, it’s a relatively small object, about the size of our Solar System. Resolving an image of that black hole is, says the University of Arizona’s Dimitrios Psaltis, like “taking a picture of a doughnut placed on the surface of the moon.” But the M87 black hole is one of the largest we could see from Earth, making it a natural target for observations, in this case using radio telescopes working at a frequency of 230 GHz, corresponding to a wavelength of 1.3mm.
A decade ago, working with Avery Broderick, Harvard’s Avi Loeb highlighted the advantages of M87 as an observational target, finding it in many ways preferable to the black hole at the heart of our own Milky Way:
M87 provides a promising second target for the emerging millimeter and submillimeter VLBI capability. Its presence in the Northern sky simplifies its observation and results in better baseline coverage than available for Sgr A*. In addition, its large black hole mass, and correspondingly long dynamical timescale, makes possible the use of Earth aperture synthesis, even during periods of substantial variability.
That paper, “Imaging the Black Hole Silhouette of M87: Implications for Jet Formation and Black Hole Spin,” is worth revisiting (abstract), for those intrigued with how these observations get made and the kinds of things we can learn from them.
I was reminded, when I first saw the now famous image, of the nature of M87 itself. Elliptical galaxies, unlike our barred spiral Milky Way, show slow rates of star formation, their primary population being older stars, and as you would imagine, they contain little gas and dust, while also housing a large number of globular clusters. Back in 2012, I ran across a paper by Falguni Suthar and Christopher McKay (NASA Ames) assessing habitability in such galaxies. What an environment to set a science fiction story! Consider the image below before we cut to the black hole image that is now center stage in the news, because here’s the context:
Image: A composite of visible (or optical), radio, and X-ray data of the giant elliptical galaxy, M87. M87 lies at a distance of 55 million light years and is the largest galaxy in the Virgo cluster of galaxies. Bright jets moving at close to the speed of light are seen at all wavelengths coming from the massive black hole at the center of the galaxy. It has also been identified with the strong radio source, Virgo A, and is a powerful source of X-rays as it resides near the center of a hot, X-ray emitting cloud that extends over much of the Virgo cluster. The extended radio emission consists of plumes of fast-moving gas from the jets rising into the X-ray emitting cluster medium. Credit: X-ray: NASA/CXC/CfA/W. Forman et al.; Radio: NRAO/AUI/NSF/W. Cotton; Optical: NASA/ESA/Hubble Heritage Team (STScI/AURA), and R. Gendler.
Could life survive in environments like this? I bring this up again as background, but also because yesterday we looked at the question of hardy microorganisms and their ability to withstand high levels of X-ray and UV radiation. Here’s what McKay and Suthar said in 2012:
Complex life forms are sensitive to ionizing radiation and changes in atmospheric chemistry that might result. However, microbial life forms, e.g. Deinococcus radiodurans, can withstand high doses of radiation and are more ?exible in terms of atmospheric composition. Furthermore, microbial life in subsurface environments would be effectively shielded from space radiation. Thus, while a high level of radiation from nearby supernovae may be inimical to complex life, it would not extinguish microbial life.
It’s fascinating to me that we’ve begun studying such questions on a galactic scale. Fascinating too that we’re now peering into the heart of an active galaxy to reveal its powerhouse black hole. By now the image is familiar, but let’s see it again because it’s just extraordinary.
Image: Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun. Credit: Event Horizon Telescope Collaboration.
One thing I saw little attention given to in the coverage was that the Event Horizon Telescope, which produced the image, was supplemented by work from spacecraft. Remember that the EHT is comprised of telescopes located around the surface of our planet, to produce a planet-scale interferometer capable of making such an observation. But the Chandra X-ray spacecraft was also involved, as was the Nuclear Spectroscopic Telescope Array (NuSTAR), and the Neil Gehrels Swift Observatory. All of these, working at X-ray wavelengths, observed the M87 black hole at the same time it was under study by the EHT in April of 2017.
I point to this because while the space assets could not image the black hole, data from them were used to measure the brightness of the M87 jet, particles driven by an enormous energy boost from the black hole itself and surging away from it at nearly the speed of light. The hope here is that X-rays can help us measure particle events near the event horizon to coordinate with the black hole images. Also involved in space was the Neutron star Interior Composition Explorer (NICER), a NASA experiment on the International Space Station that looked at the center of the Milky Way and the black hole known as Sgr A*. Part of the EHT’s mandate is to study the origin of jets like this one, so these extraordinary interactions now become visible.
As to the ground-based observatories of the EHT themselves, what an accomplishment! The international team involved totalled over 200 astronomers, whose work is presented in a special issue of Astrophysical Journal Letters. In the black hole work, the EHT used an array of eight radio telescopes with worldwide coverage, from the Antarctic to Spain, Chile and Hawaii, all located in high-altitude settings where conditions are ideal for observation.
Jonathan Weintroub (CfA) coordinates the EHT’s Instrument Development Group:
“The resolution of the EHT depends on the separation between the telescopes, termed the baseline, as well as the short millimeter radio wavelengths observed. The finest resolution in the EHT comes from the longest baseline, which for M87 stretches from Hawai’i to Spain. To optimize the long baseline sensitivity, making detections possible, we developed a specialized system which adds together the signals from all available SMA dishes on Maunakea. In this mode, the SMA acts as a single EHT station.”
Spectacular. The very long baseline interferometry creates a virtual dish that is planet-sized, able to resolve an object to 20 micro-arcseconds. Working with a conjunction of four nights that would produce clear seeing for all eight observatories, the telescopes took in massive amounts of data — 5,000 trillion bytes of data in all — saved on 1,000 storage disks. Transmitting all that information for subsequent processing was ruled out, for air transport from FedEx could take the hard disks onto which the data had been recorded to a single location much faster. These are signals that needed to be aligned within trillionths of a second to achieve a valid result.
The resulting imagery is the payoff. The central dark region is surrounded by a ring of light, as Einstein’s equations led scientists to expect. We can’t, of course, see the black hole itself, but plasma emitted from its accretion disk, where matter piles up as material falls into the black hole, is heated to billions of degrees and accelerated almost to lightspeed. We get an image of the black hole’s shadow’ that is about 2.5 times larger than the event horizon. M87’s event horizon is thought to be some 25 billion miles across, making it 3 times the size of Pluto’s orbit.
“Once we were sure we had imaged the shadow, we could compare our observations to extensive computer models that include the physics of warped space, superheated matter and strong magnetic fields. Many of the features of the observed image match our theoretical understanding surprisingly well,“ said Luciano Rezzolla, professor for theoretical astrophysics at Goethe University and a researcher on the EHT. “This makes us confident about the interpretation of our observations, including our estimation of the black hole’s mass.“
Image: This artist’s impression depicts the paths of photons in the vicinity of a black hole. The gravitational bending and capture of light by the event horizon is the cause of the shadow captured by the Event Horizon Telescope. Credit: Nicolle R. Fuller/NSF.
This is a black hole massive enough that a planet orbiting it could move around it within a week while traveling, says MIT’s Geoffrey Crew, close to the speed of light. Crew’s colleague Vincent Fish, also at MIT’s Haystack Observatory, amplifies on the point:
“People tend to view the sky as something static, that things don’t change in the heavens, or if they do, it’s on timescales that are longer than a human lifetime. But what we find for M87 is, at the very fine detail we have, objects change on the timescale of days. In the future, we can perhaps produce movies of these sources. Today we’re seeing the starting frames.”
Now that’s something worth waiting for, movies of the accretion disk caught in the tortured spacetime of a galaxy’s central black hole. M87 anchors a jet stretching tens of thousands of light years, so we’re talking about seeing the dynamics of the jet’s interactions with the black hole. Fine-tuning EHT methods and expanding its sites points in the direction of further breakthrough imagery.
But what an accomplishment we’ve already achieved via instruments all over the world — ALMA and APEX in Chile, the IRAM 30 meter telescope in Spain, the James Clerk Maxwell telescope and the Submillimeter Array (both in Hawaii), the Large Millimeter Telescope (LMT) in Mexico, the Submillimeter Telescope (SMT) in Arizona and the South Pole Telescope (SPT) in Antarctica.
The papers are The Event Horizon Telescope Collaboration et al., “First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole,” Astrophysical Journal Letters Vol. 875, No. 1 (10 April 2019) (abstract); and from the same issue: “First M87 Event Horizon Telescope Results. II. Array and Instrumentation” (abstract); “First M87 Event Horizon Telescope Results. III. Data Processing and Calibration” (abstract); “First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole” (abstract); “First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring” (abstract); and “First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole” (abstract). The paper on M87 and galactic habitability is Suthar & McKay, “The Galactic Habitable Zone in Elliptical Galaxies,” International Journal of Astrobiology, published online 16 February 2012 (abstract).
M-Dwarfs: Weighing UV Radiation and Habitability
With 250 times more X-ray radiation than Earth receives and high levels of ultraviolet, would Proxima b, that tantalizing, Earth-sized world around the nearest star, have any chance for habitability? The answer, according to Jack O’Malley-James and Lisa Kaltenegger (Cornell University) is yes, and in fact, the duo argue that life under these conditions could deploy a number of possible strategies for dealing with the radiation influx. Their conclusions appear in a new paper in Monthly Notices of the Royal Astronomical Society.
Kaltenegger is director of Cornell’s Carl Sagan Institute, where O’Malley-James serves as a research associate. Modeling surface environments on four exoplanets that are prone to frequent flares — Proxima-b, TRAPPIST-1e, Ross-128b and LHS-1140b — Kaltenegger and O’Malley-James examined different atmospheric solutions that could suppress UV damage in living cells.
Thin atmospheres and a lack of ozone protection fail to block UV radiation well, no surprise there, and such atmospheres do not measure up favorably when compared to atmospheres like that of the Earth today. But go back four billion years and we find that the modeled planets receive radiation in the UV significantly lower than what the Earth experienced in that era of its development. Earth was at that time uninhabitable by human standards — had any humans been available — but life had indeed emerged and continued to thrive. Thus the authors write that UV radiation “…should not be a limiting factor for the habitability of planets orbiting M stars.”
Image: The intense radiation environments around nearby M stars could favor habitable worlds resembling younger versions of Earth. Credit: Jack O’Malley-James/Cornell University.
The extremophile Deinococcus radiodurans is key to this study, for it is one of the most radiation-resistant organisms known. By varying the UV wavelengths, the scientists assessed the mortality rates of the organism, in which it becomes clear that some wavelengths of UV are more damaging to biological molecules than others. From the paper:
…we use this as a benchmark against which to compare the habitability of the different radiation models. This action spectrum compares the effectiveness of different wavelengths of UV radiation at inducing a 90 per?cent mortality rate. It highlights which wavelengths have the most damaging irradiation for biological molecules: for example, the action spectrum in Fig. 4 shows that a dosage of UV radiation at 360?nm would need to be three orders of magnitude higher than a dosage of radiation at 260?nm to produce similar mortality rates in a population of this organism.
Image: This is Figure 4 from the paper. Caption: Relative biological effectiveness of UV surface radiation on Proxima-b. (A) The biological effectiveness of UV on DNA and the radiation-resistant microorganism D. radiodurans (Voet et al. 1963; Diffey 1991) quantifies the relative effectiveness of different wavelengths of UV radiation to cause DNA destruction or, for D. radiodurans, mortality, which increases with decreasing wavelength. Biological effectiveness of UV damage for (B) oxygenic atmospheres and (C) anoxic atmosphere models shown as convolution of the surface UV flux and action spectrum over wavelength (solid line shows flaring, dashed line quiescent star), compared to present-day Earth (red solid) and early Earth (3.9 billion years ago) (red dashed). Credit: Lisa Kaltenegger/Jack O’Malley-James/Cornell University.
We can’t rule out organisms below ground or living in water or rock, not to mention such survival characteristics as biofluorescence or protective pigments. We know of microorganisms that can tolerate full solar UV in space exposure experiments, using protective cells or pigments as effective UV screens. Biofluorescence offers protection against radiation because UV can be upshifted to longer wavelengths that produce less harm. The authors think protective biofluorescence would be at its most useful during the intense UV flux of flares, although a constant level of high UV might produce continuous fluorescence.
Here we have a potential biosignature, cited by the authors in a previous paper:
Because biofluorescence is independent of the visible flux of the host star and only dependent on the UV flux of the star, emitted biofluorescence can increase the visible flux of a planet orbiting an active M-star by several orders of magnitude (O’Malley-James & Kaltenegger 2018) during a flare.
We may get our first look at such atmospheres by observing ozone, which is potentially detectable by the James Webb Space Telescope. On the other hand, a high-enough level of UV could also produce a biosphere below ground that would present, if any, only the weakest of biosignatures. Even so, the authors conclude that nearby planets around M-dwarfs like those studied here are serious candidates for biosignature examination by future observatories.
While a multitude of factors ultimately determine an individual planet’s habitability our results demonstrate that high UV radiation levels may not be a limiting factor. The compositions of the atmospheres of our nearest habitable exoplanets are currently unknown; however, if the atmospheres of these worlds resemble the composition of Earth’s atmosphere through geological time, UV surface radiation would not be a limiting factor to the ability of these planets to host life. Even for planets with eroded or anoxic atmospheres orbiting active, flaring M stars the surface UV radiation in our models remains below that of the early Earth for all cases modelled. Therefore, rather than ruling these worlds out in our search for life, they provide an intriguing environment for the search for life and even for searching for alternative biosignatures that could exist under high-UV surface conditions.
The paper is O’Malley-James & Kaltenegger, “Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems,” Monthly Notices of the Royal Astronomical Society Vol. 485 Issue 4 (June 2019), pp. 5598-5603 (full text).
A Major Hubble Survey of the Kuiper Belt
You’ll recall that well before New Horizons completed its primary mission at Pluto/Charon, the search was on for a Kuiper Belt Object that could serve as its next destination. Eventually we found Ultima Thule (2014 MU-69), from which priceless data were gathered at the beginning of January. Finding the target wasn’t easy given the distances involved and the small size of the relevant objects, which is why the Hubble Space Telescope was brought into the search.
The starfield in Sagittarius is crowded as we look toward galactic center, but despite the efforts of both the 8.2-meter Subaru telescope in Hawaii and the 6.5-meter Magellan telescopes in Chile, no KBOs among those found were within range of New Horizons. It was Hubble that made the difference, and Hubble which will presumably return a second target, if indeed the New Horizons team is granted an extended mission that can reach it. It’s worth noting, too, that it was Hubble that helped New Horizons in its discovery of Pluto’s smaller four moons, while also performing searches of the system for any dust rings that could harm the mission.
KBOs have never been heated by the Sun, so they provide the most pristine sample available of the earliest days of system formation. What we’ve learned about the Kuiper Belt so far is that there are a large number of binary objects within it, and as Southwest Research Institute scientist Alex Parker notes, many of these consist of two objects of similar mass. Parker will lead a new survey on the Kuiper Belt awarded to SwRI by the Space Telescope Science Institute (STScI), one that will put the emphasis on characterizing these binary populations.
“These binary systems are powerful tracers of the processes that built the planets,” says Parker. “We will use Hubble to test the theory that many planetesimals formed as binary systems from the get-go, and that today’s Kuiper Belt binaries did not come from mergers of initially solitary objects.”
Image: The SwRI-led Origins Legacy Survey will search for Kuiper Belt objects such as those shown in this artist’s illustration of a widely separated binary. Credit: Courtesy of Southwest Research Institute and Alex H. Parker.
Called the Solar System Origins Legacy Survey (SSOLS), the project represents the largest Hubble Solar System program ever, with 206 Hubble orbits around Earth allocated to it. SSOLS is conceived as a way to examine the primordial planetesimal disk with new and archival data. At stake are differing models of planetesimal formation, which predict different size and color distributions for solitary KBOs and their binary cousins.
The process of accretion would imply objects formed in isolation, later merging into binaries. In this case, the objects in binary systems would likely show dissimilar colors and a different size distribution than single KBOs. But if a process of rapid collapse was at work, producing some binary systems and some single KBOs quickly, then the expectation is for both objects in a binary system to have a similar surface color and a size distribution similar to what we find among solitary objects. At present, Hubble is the only instrument that can measure the binary occurrence rate in the Kuiper Belt, as well as the binary separation and color distribution.
SSOLS will characterize the binary and color properties of 221 KBOs, drawing on objects observed by the two largest Kuiper Belt surveys yet conducted, the Outer Solar System Origins Survey (OSSOS) and Canada-France Ecliptic Plane Survey (CFEPS). This earlier work becomes the framework within which the binary characterization of KBOs can proceed. For more, see the SSOLS website at https://www.ssols.space/, and ponder the need for the next outer system spacecraft that can take us into the realm New Horizons continues to explore.
Hayabusa2 Impactor Deployment
Putting a crater on an asteroid is no small matter, for it allows us to gather samples to further nail down the object’s composition. The Japan Aerospace Exploration Agency (JAXA) has achieved the feat on asteroid Ryugu using the Small Carry-on Impactor (SCI) carried by the Hayabusa2 spacecraft. Confirmation of the crater and details about its size will be forthcoming, but fortunately the spacecraft’s DCAM3 camera was able to record the event.
Following Hayabusa2 on Twitter (@haya2e_jaxa) is often the best way to keep up with operations at Ryugu (even as @OSIRISREx puts you inside that mission). The fact that we have two spacecraft in current operations around asteroids should be cause for continuing celebration. From the Hayabusa2 Twitter feed:
[SCI] The deployable camera, DCAM3, successfully photographed the ejector from when the SCI collided with Ryugu’s surface. This is the world’s first collision experiment with an asteroid! In the future, we will examine the crater formed and how the ejector dispersed. pic.twitter.com/eLm6ztM4VX
— HAYABUSA2@JAXA (@haya2e_jaxa) April 5, 2019
And with a closer look plus JAXA caption:
Image: This image captured by the camera separated from Hayabusa2 (DCAM3) shows ejection from Ryugu’s surface, which was caused by the collision of the SCI against Ryugu. Image taken at 11:36 a.m., April 5, 2019 (Indicated by the camera, Japan time). Credit: JAXA, Kobe University, Chiba Institute of Technology, The University of Occupational and Environmental Health, Kochi University, Aichi Toho University, The University of Aizu, and Tokyo University of Science.
The spacecraft protected itself before impact by moving to the other side of Ryugu to avoid any debris stirred by the collision. And while Hayabusa2 has already gathered one sample from the asteroid’s surface, the material gathered as a result of the impact should give scientists the opportunity to study what is below the surface, pristine material that dates back to the early days of the Solar System. Sample return is currently scheduled for late 2020.
As to the asteroid’s composition, the early data from Hayabusa2 have already proven useful. Says Seiji Sugita (University of Tokyo), author of a recent paper on the asteroid:
“Just a few months after we received the first data we have already made some tantalising discoveries. The primary one being the amount of water, or lack of it, Ryugu seems to possess. It’s far dryer than we expected, and given Ryugu is quite young (by asteroid standards) at around 100 million years old, this suggests its parent body was much largely devoid of water too.”
Image: Ryugu is a C-type asteroid — rich in carbon — about 900m wide. Credit: © 2019 Seiji Sugita et al., Science.
In a March 19 news conference, Sugita told an audience at the Lunar and Planetary Science Conference that Ryugu is now thought to be a fragment of one of two more distant asteroids, Eulalia or Polana. The breakup is thought to have occurred 700 million years ago. The best match in color — Ryugu is an extremely dark object — is with these two main belt asteroids, with the scientist pegging the likelihood of the relationship as high as 90 percent.
Both the visible-light camera and a near-infrared spectrometer aboard the spacecraft confirm the dearth of water, a significant result given that asteroids are thought to have supplied water to the early Earth, along with comets as well as the circumstellar disk of the system itself. Ryugu’s meager water stands in contrast to what OSIRIS-REx has found at asteroid Bennu. Although both asteroids appear similar, covered in boulders and presenting challenges to lander missions, Bennu contains considerably more water.
The paper examines a range of possibilities to explain this, but concludes that the general uniformity in color across Ryugu’s surface points to a parent asteroid that experienced internal heating caused by radioactive decay of Aluminium-26. As the authors note: “Internal heating can warm a large fraction of the volume of the parent body relatively uniformly, leaving a small volume of outer layer relatively cool.” The paper continues:
Although multiple scenarios for the evolution of Ryugu’s parent body remain viable, our comparison between Hayabusa2 remote-sensing data, meteoritic samples and asteroids leads us to prefer the scenario of parent-body partial dehydration due to internal heating. This scenario suggests that asteroids that accreted materials which condensed at ?150 K (the H2O condensation temperature under typical solar nebula conditions) must have either formed early enough to contain high concentrations of radiogenic species, such as 26Al, or formed close to the Sun where they experienced other heating mechanisms). The degree of internal heating would constrain the location and/or timing of the snow line (i.e., the dividing line between H2O condensation and evaporation) in the early Solar System.
Thus the different traits of seemingly similar asteroids like Ryugu and Bennu offer plentiful ground for studying the astrophysical processes that shaped each. The paper is Sugita et al., “The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes,” Science 19 March 2019 (abstract).
Follow with RSS or E-Mail
