Centauri Dreams

Scaling up our space telescopes calls for new thinking. Consider this: The Hubble telescope has a primary mirror of 2.4 meters. The James Webb Space Telescope takes us to 6.5 meters. But as we begin to get results from missions like TESS and JWST (assuming the latter gets off safely), we’re going to need much more to see our most interesting targets. Imagine what could be done with a 30-meter space telescope, and ponder the challenge of constructing it.

This is what Cornell University’s Dmitry Savransky has been doing, developing a NIAC study that looks at modular design and self-assembly in space. Savransky’s notions take me back to a much earlier era, when people like Bob Forward talked about massive structures in space that dwarf any engineering project we’ve yet attempted. Forward saw these projects — his vast Fresnel lens between the orbits of Saturn and Uranus 1000 kilometers in diameter, for example — as ultimately achievable, but his primary concern was to be sure the physics worked.

Image: Cornell’s Dmitry Savransky. Credit: Cornell University.

Rather than imagining spacesuited work crews in the tens of thousands welding metal to metal, I have long thought that the only way to conceive of structures on this scale is through nanotechnology and self-assembly, harvesting materials from the asteroid belt and achieving a robotic cascade of structural growth that, once programmed, would run with human oversight. So the idea of self-assembly in space on the scale of a 30-meter telescope is music to my ears.

But that kind of technology is in the future. Where do we stand today? Here’s Cornell’s Mason Peck on the matter:

“As autonomous spacecraft become more common, and as we continue to improve how we build very small spacecraft, it makes a lot of sense to ask Savransky’s question: Is it possible to build a space telescope that can see farther, and better, using only inexpensive small components that self-assemble in orbit?”

Peck’s credentials here are hard to surpass, given his extensive work on the nanocraft he calls ‘sprites,’ which are essentially spacecraft on a chip. You can see why Peck’s insights have proven so valuable to the Breakthrough Starshot concept, which envisions sending swarms of tiny payloads to nearby stars (exactly which stars, as we saw yesterday, is a question that has yet to be decided). But just how do we do self-assembly at our current level of technology?

What Savransky has in mind is discussed in the précis on his ideas published by NASA along with the other Phase I projects for 2018. The concept: Every part of the telescope, and that includes not just the primary and secondary mirrors but the support structures and the sunshield, are to be constructed from mass-produced spacecraft modules. Each module is a hexagonal spacecraft about 1 meter in diameter that houses an active mirror assembly.

We’re not at the level of nanoengineering, but the self-assembly is ingenious. Rather than trying to launch a single, massive space telescope, the heavy lifting is done by launching separate modules as payloads of opportunity. Each of the modules has the ability to navigate on its own to the Sun-Earth L2 point using a deployable solar sail. The concept wastes no materials — the planar telescope sunshield is built out of the solar sails during the assembly process.

Image: Graphic depiction of Modular Active Self-Assembling Space Telescope Swarms. The schematic shows one module, lower left, and what the finished telescope might look like, with approximately 1,000 modules assembled together. Credit: D. Savransky/Cornell University.

Construction of the 30-meter space telescope is thus as much a matter of programming as it is of mechanics. But Savransky’s $125,000 Phase 1 grant could be on to something big, a solution for the scaling problem we face with space instrumentation. Says its creator:

“James Webb is going to be the largest astrophysical observatory we’ve ever put in space, and it’s incredibly difficult. So going up in scale, to 10 meters or 12 meters or potentially even 30 meters, it seems almost impossible to conceive how you would build those telescopes the same way we’ve been building them.”

Whether or not modular self-assembly in the Savransky mode makes it through the initial feasibility study — this is essentially what Phase I NIAC work is all about — will determine whether the idea progresses up the ladder to a more detailed and focused Phase II. More specifically, the NIAC précis lays out where the scientist hopes to go in Phase I:

In the NIAC Phase I, we propose to carry out detailed simulations of the spacecraft flight and rendezvous dynamics in order to set requirements on the solar sail area and loading, along with analyses of the mirror assembly to validate the ability to achieve the required surface figure in the assembled primary and secondary mirrors.

And if we eventually build space telescopes in the 30-meter class? Mapping the surface of Earth-like planets around nearby stars becomes a possibility, as does resolved imaging of stellar populations at a level never before possible. Self-assembly in and of itself, if demonstrated through missions like this one, obviously becomes a major factor in future mission design. Up next for Savransky is a NIAC orientation meeting in early June as the study begins.

Avi Loeb’s always interesting work has recently taken us into the realm of target selection for exoplanet surveys. Where should we be putting our time and money in the search for life elsewhere, and what can we do to maximize both the credibility of the investigation and the funding that it demands? These sound like pedestrian matters compared to the excitement of discovery — finding Proxima b was a lot more exciting than watching any congressional committee debate over NASA’s priorities.

But Proxima Centauri b, that fascinating world around the nearest star, fits neatly into this narrative, for reasons that contrast nicely with its own nearest neighbors, Centauri A and B. The Centauri stars are an obvious target, and one that Loeb has devoted considerable time to assessing, given his deep involvement with Breakthrough Starshot’s study of a mission there. If we find planets around Centauri A or B, are they our priority? Just where does Proxima b fit in?

And what of fascinating systems like TRAPPIST-1? We have an abundance of evidently rocky planets there, but should we prioritize this M-dwarf over G-class systems found by TESS?

Lacking infinite resources, we have to decide what to allocate where, and that means that target selection for upcoming observations, both in space and on the ground, is a priority. As SETI has demonstrated, the federal dollar can vanish, especially after years of null results. It would be disastrous if the search for non-technological life met a similar fate. All this has prompted Loeb and Manasvi Lingam, likewise at Harvard, to think about how we can maximize our chances for success, applying the kind of cost-benefit analyses often used in economics.

Their paper has just appeared in The Astrophysical Journal Letters. The search for biosignatures of necessity confronts the real world of financing and public perception. Assumptions are everything, here discussed in terms of the statistical ‘priors’ that go into making the calculation. If we had to make a choice between solar-mass stars and M-dwarfs and if (crucially) all other things were equal — i.e., if we were not constrained by our instruments and could observe each type of star with high fidelity — then Sun-like stars obviously win.

Why? Because we know of only one life-bearing world, and that is our own. Thus in a time of scarce resources, it would make sense to put our funds into searching around this kind of star. But we can’t do away with the observational problems that exist, and given these, M-dwarfs stand out as the better target because given the current state of the art, we can more readily hope to delve into their atmospheres with our rapidly evolving observatories, probing their chemistry and looking for biosignatures. Earth-class planets around M-dwarfs offer us deep transits and the earliest prospects for using transmission spectroscopy to study them.

We’re thus pulled in different directions, based on our priors. From the paper;

If we consider a flat prior, where the probability of life is independent of the choice of star, focusing on planets around M-dwarfs is more advantageous because the detection of biosignatures becomes much easier. On the other hand, there is mounting evidence, especially based on considerations of space weather, that the potential habitability of Earth-analogs around M-dwarfs might be much lower relative to their counterparts around G-type stars. Hence, if we adopt a prior where the habitability is selectively suppressed around low-mass stars, we conclude that it would be more advantageous to focus on the search for life on planets orbiting Sun-like stars relative to those around M-dwarfs.

What to do? Obviously, press on in our analysis of both kinds of stars as the possible home for life. We’ve looked many times in these pages at the problems M-dwarfs present, usually in terms of tidal locking and the stellar flare activity common especially to younger stars. As the paper notes, the growing concern about space weather, which can involve stripping of the atmosphere itself or its serious degradation, makes the question even more problematic.

Image: This diagram compares the planets of our inner solar system to Kepler-186, a five-planet system about 500 light-years from Earth in the constellation Cygnus. The planets of Kepler-186 orbit an M dwarf, a star that is half the size and mass of the Sun. What share of our resources should go into investigating M-dwarfs as opposed to solar-type stars? Credit: NASA Ames/SETI Institute/JPL-Caltech.

We have no final answers here, but as the analysis continues, Loeb and Lingam are pointing out that if habitability around M-dwarfs is effectively suppressed by factors like these, then prioritizing planets around solar-type stars makes the most sense. This is not to say that we would not, in an ideal world, press on with strong efforts in both directions. But I think the authors’ point is telling: To achieve the funding levels we need, the community must be conducting a search that is both credible and can demonstrate a measure of success.

Will our search evolve in the direction of solar-type stars? Here Alpha Centauri makes an interesting bellwether. We have the aforementioned Proxima Centauri b, an Earth-mass planet in the habitable zone of a close red dwarf. But as the angular separation between the primary stars Centauri A and B continues to increase, we should finally be able to make a call on Earth-mass planets orbiting either or both of these stars, the G-class A and the K-class B.

Finding a planet in the habitable zone of one of the primary Centauri stars would present us with an abundance of targets closer to us than any other stellar system. The ongoing work on assessing M-dwarfs and the habitability issues they raise will help us decide where to concentrate our resources. Maximizing the chances for detection may eventually lead us to choices other than the most readily studied stars, and back to stars more like our own.

The paper is Loeb & Lingam, “Optimal Target Stars in the Search for Life,” The Astrophysical Journal Letters Vol. 857, No. 2 (20 April 2018). Full text.

Water delivery to the inner Solar System is crucial for life to develop, for worlds like our own must have formed dry, well within the ‘snowline.’ We need a mechanism to bring volatiles from the ice-rich regions beyond 3 AU or so, and while much attention has been paid to comets, we’ve been learning more about asteroids as a second delivery option, for isotopic measurements have shown that Earth’s water has similarities to water bound up in carbonaceous asteroids.

Focusing on asteroid delivery, Pete Schultz (Brown University) and colleague Terik Daly, a postdoctoral researcher at Johns Hopkins University, have confronted the issues raised by early system impacts in a series of experiments. The results appear in the journal Science Advances. Says Schultz:

“Impact models tell us that impactors should completely devolatilize at many of the impact speeds common in the solar system, meaning all the water they contain just boils off in the heat of the impact. But nature has a tendency to be more interesting than our models, which is why we need to do experiments.”

Daly and Schultz found the equipment they needed to study volatile delivery at the Vertical Gun Range at NASA Ames. Their methodology was to fire marble-sized projectiles similar in composition to water-rich carbonaceous chondrite asteroids at a dry target made of pumice power. The speed at impact is some 5 kilometers per second, producing debris that can be analyzed in search of water traces.

Image: Hypervelocity impact experiments, like the one shown here, reveal key clues about how impacts deliver water to asteroids, moons, and planets. In this experiment, a water-rich impactor collides with a bone-dry pumice target at around 18,000 kilometers per hour. The target was designed to rupture partway through the experiment in order to capture materials for analysis. This high-speed video, taken at 130,000 frames per second, slows down the action, which in real time is over in less than a second. Credit: Schultz Lab / Brown University.

The results are a useful window into water delivery. The heat of the impact destroys much of the impactor, while a vapor plume then forms that contains water that was inside the impactor. Inside the plume itself, melted materials and breccias — particles of shattered rock re-formed within a fine-grained matrix — contain some of the original water in recaptured form.The original impactor may be gone, in other words, but a portion of its internal water can survive.

The implications for the early Solar System are clear, as the paper notes:

The fact that the amorphous, glassy component—not projectile survivors—constitutes the primary reservoir for impact-delivered water is critical for extrapolating these experiments. Impact melt production increases with impact speed. If impact melt derived primarily from the target successfully traps water during collisions among planetary bodies (as it does in experiments), then higher-speed impacts may still deliver significant quantities of water.

Image: Samples of impact glasses created during an impact experiment. In impact experiments, these glasses capture surprisingly large amounts of water delivered by water-rich, asteroid-like impactors. Credit: Schultz Lab / Brown University.

The authors calculate that carbonaceous chondrite impactors should be able to deliver up to 30 percent of their internal water to silicate bodies under conditions of impact speeds and angles that we would expect during the early phases of planet formation. Impacts at velocities high enough to vaporize the volatiles still allow for the recapture of those volatiles through impact melts and breccias, so water can be incorporated into the growing planetesimals.

“[T]hese new experiments raise the possibility that growing terrestrial planets trap water in their interiors as they grow, which would profoundly affect their geodynamical evolution,” the authors write. It’s a finding that also helps us explain water distribution later on in the system, such as water ice found on the Moon’s surface in the rays of the Tycho crater, or asteroid-derived water that could account for ice deposits in the polar regions of Mercury.

“The point is that this gives us a mechanism for how water can stick around after these asteroid impacts,” Schultz adds. “And it shows why experiments are so important because this is something that models have missed.”

The paper is Daly & Schultz, “The delivery of water by impacts from planetary accretion to present,” Science Advances Vol. 4, No. 4 (25 April 2018). Full text.

I usually get up while it’s still dark and take a walk. The idea is to shake the night’s dreams out of my head, listen to the birds waking up and pull in a lot of fresh air, all conducive to thinking about what I want to write that day. Last fall I kept noticing the glow before morning twilight that marked the zodiacal cloud, faint enough to be lost in moonlight and challenging to see when competing with city lights. But catch the right conditions and its diffuse glow is apparent, as in the photograph below, a striking example of zodiacal light’s effect.

Image: Sometimes mistaken for light pollution, zodiacal light is sunlight that is reflected by zodiacal dust. It is most visible several hours after sunset on dark, cloudless nights surrounding the spring and fall equinoxes, when the Earth’s equator is aligned with the plane of the solar system. Credit: Malcol.

What we’re seeing, especially at times when the ecliptic is at its largest angle to the horizon, hence autumn and spring, is the light of the Sun reflecting off dust in the Solar System. Most of the material in this interplanetary dust cloud is concentrated along the plane of the system, and as you would expect, similar dust clouds are to be found around other stars. The Spitzer Space Telescope, for instance, has found evidence for a strong dust cloud around the star HD 69830, presumably the result of collisions within the system. No planet has yet been found there.

We often talk about interstellar gas and dust as serious issues for spacecraft moving at a substantial percentage of the speed of light. But what about interplanetary dust? The complications it poses involve how we see exoplanets, particularly those in the habitable zone.

Imagine zodiacal light perhaps a thousand times brighter than our own, enough to outshine the Milky Way. The challenge such light presents to astronomers are potentially serious but not well quantified, which is one reason why researchers using the Large Binocular Telescope Interferometer (LBTI) on Arizona’s Mt. Graham has been at work in a program called HOSTS — the Hunt for Observable Signatures of Terrestrial Systems. The team’s paper in the Astrophysical Journal gives us a look at the survey’s early results.

“There is dust in our own solar system,” says Philip Hinz, the lead for the HOSTS Survey team and associate professor of astronomy at the University of Arizona. “We want to characterize stars that are similar to our own solar system, because that’s our best guess as to what other planetary systems might have life.”

Image: This artist’s concept illustrates what the night sky might look like from a hypothetical alien planet in a star system with an asteroid belt 25 times as massive as the one in our own solar system (alien system above, ours below. Credit: NASA/JPL-Caltech.

Steve Ertel (University of Arizona) is lead author of the paper, which delves into the question of how much dust within a stellar system can affect our ability to see planets within it. All of this goes into planning for future space telescopes, with the HOSTS survey examining the issue for 30 nearby stars. What we learn from the paper is that exozodiacal dust in the stars surveyed is typically less than 15 times the amount found in our own Solar System’s habitable zone.

But planets with larger dust volumes become seriously problematic. Epsilon Eridani, long of interest because of its proximity to the Sun (10.5 light years) is one of these. Says Ertel:

“It is very nearby. It’s a star very similar to our sun. It would be a very nice target to look at, but we figured out that it would not be a good idea. You would not be able to see an Earth-like planet around it.”

Even so, Epsilon Eridani offers us a useful study in planet formation, albeit one with serious challenges for observers. This is from the paper, referring to a previously studied dust clump in the system, which could indicate:

…local dust production in the known asteroid belt and potential shepherding by a planet interior to the belt which could also be creating the clump. There is a long history of planet claims for ?Eri, but radial velocity detection is complicated by stellar activity induced jitter. The existence of the planet claimed by Hatzes et al. (2000) and Benedict et al. (2006) has been debated in the literature (Anglada-Escudé & Butler 2012; Howard & Fulton 2016), it is possible that a planet of period 6.8 – 7.3 yr and mass 0.6 – 1.55 M_Jup does orbit the star. Attempts to infer the presence of outer planets based on the ring structure are problematic due to the uncertain nature of the intrinsic disk morphology.

This is interesting stuff, although as the paper is at pains to note, we are very early in the study of dust distribution at this level. The issue can tell us something about the possibility of planets within a star system. Given the standard model — that dust is formed during asteroid collisions and spirals inward so that it is distributed throughout the entire system — the survey turned up at least one surprising result. We’ve known for some time that the star Vega has a large belt of cold dust in about the same relation to Vega as the Kuiper Belt is to our system. There is also a disk of hot dust very close to the star. From the paper:

A most puzzling result is our non-detection of warm dust around Vega, for which massive asteroid belt and Kuiper belt analogs have been detected in the mIR to fIR and a large amount of hot dust has been detected in the nIR. This raises the question of what mechanism clears the region between ?0.5 AU and ?5 AU from the star of dust.

We have as yet to detect planets around Vega, but the lack of habitable zone dust may be telling us about a massive planet whose gravitational influence could be clearing this area or, as Ertel notes, several Earth-mass planets. Several other stars in the survey showed, unlike Vega, no dust belts close to the star or far from it, but large amounts of warm dust in the habitable zone. A massive asteroid belt producing numerous collisions could be the culprit in such cases.

Overall, the HOSTS survey to this point has been able to make four new detections of habitable zone dust among its 30 stars; among these, three are the first to be found around Sun-like stars, and two occur around stars without any previous detections of circumstellar dust. The paper notes that the survey’s sensitivity is five to ten times better than previous results. Future exo-imaging attempts will be well served by extending the survey to a larger sample of stars, so we’ll know how the quantity of dust in a given system affects our ability to see HZ planets.

The paper is Ertel et al., “The HOSTS survey – Exozodiacal dust measurements for 30 stars,” Astrophysical Journal Vol. 155, No. 5 (17 April 2018). Abstract / preprint.