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A Disk at an Angle (and a Remarkable View)

One of the joys of science fiction is imagining landscapes. What would it be like to stand on Titan, for example, a question that was inescapably influenced in my youth by Chesley Bonestell’s wonderful depictions, as well as novels like Larry Niven’s World of Ptavvs (1966) or Michael Swanwick’s novelette “Slow Life” (Analog, December 2002). And then, of course, there were those multi-star skies, as in Asimov’s “Nightfall” (Astounding Science Fiction September, 1941.

The Science Fiction Writers of America, incidentally, voted “Nightfall” the best science fiction story written prior to 1965, when the Nebula Awards began. I would bet almost all Centauri Dreams readers are familiar with it, but if not, it’s widely anthologized.

And now we have another visual phenomenon to contend with, a landscape and its sky that had never occurred to me. A team led by Grant Kennedy (University of Warwick, UK) has discovered the first confirmed case of a multiple star system whose surrounding disk of gas and dust circles the central stars at right angles. The work grows out of data gathered with the Atacama Large Millimeter/sub-millimeter Array (ALMA). Says Kennedy:

“Discs rich in gas and dust are seen around nearly all young stars, and we know that at least a third of the ones orbiting single stars form planets. Some of these planets end up being misaligned with the spin of the star, so we’ve been wondering whether a similar thing might be possible for circumbinary planets. A quirk of the dynamics means that a so-called polar misalignment should be possible, but until now we had no evidence of misaligned discs in which these planets might form.”

So let’s imagine a planet forming in the dust ring, just as we know planets form in the disks we’ve found around single stars. From the surface of such a world, our new science fictional setting shows us the disk as a band rising out of the horizon, with the twin stars moving in and out of the disk plane, so that we get two shadows much of the time. Our circumbinary planet in its all but perpendicular orbit of the primaries might see a scene like the one below.

Image: View from an orbiting planet. Copyright: University of Warwick/Mark Garlick. Used with permission.

The young system in question is found at HD 98800, also known as TV Crateris, in the constellation Crater, somewhere around 150 light years away from the Sun. This is actually a quadruple star system found in the TW Hydrae association. HD 98800 A is a K-class dwarf probably orbited by a red dwarf, while HD 98800 B is likewise a K-class, red dwarf pairing. A planet in this system — and bear in mind that this is a very young system, so the planet-forming process would be early — would have four nearby stars to color its landscape.

The authors do not believe such systems are rare. From the paper:

If planet formation can proceed equally efficiently in both coplanar and polar configurations, circumbinary planets on polar orbits are predicted to be nearly as common as their coplanar brethren (although these fractions may be modified by later dynamical evolution). The most eccentric binaries are the most likely to have polar disk configurations, so it is not surprising that the known transiting circumbinary planets, which are near to coplanar, are all in systems with e≤0.52, with 8 out of 9 having e<0.22… Polar disks, and perhaps planets, may be a common outcome of circumbinary disk formation, and provide motivation for systematic searches for both.

Image: View of the double star system and surrounding disc. Copyright: University of Warwick/Mark Garlick. Used with permission.

Is there, then, a large population of such unusually aligned circumbinary planets awaiting discovery? If so, we’ll have a variety of further interesting landscapes to consider, even as we ponder the kind of seasonal variations that can occur on circumbinary worlds circling a wide range of stellar classes and their own possible companions. Plenty of material here for writers, or has some far-sighted SF wordsmith already depicted such a planet? If so, please let me know in the comments.

The paper is Kennedy et al., “A Circumbinary Protoplanetary Disc in a Polar Configuration,” Nature Astronomy 14 January 2019 (abstract).

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A Closer Look at Barnard’s Star b

Barnard’s Star b, the planet announced last November around the second nearest star system to the Earth, has been the subject of intensive study by an international team led by Ignasi Ribas at the Institute of Space Studies of Catalonia (IEEC), and Institute of Space Sciences (ICE, CSIC). As announced at the recent meeting of the American Astronomical Society in Seattle, the work helps to refine the age of Barnard’s Star and examines its potential for supporting life on its known planet.

We don’t know whether there are other planets around Barnard’s Star, but the fact of Barnard Star b’s existence is significant, according to Scott Engle (Villanova University), who along with colleague Edward Guinan presented the results in Seattle. Says Engle:

“The most significant aspect of the discovery of Barnard’s Star b is that the two nearest star systems to the Sun are now known to host planets. This supports previous studies based on Kepler Mission data, inferring that planets can be very common throughout the galaxy, even numbering in the tens of billions.”

Indeed, the idea of at least one planet around every star gains currency, and in terms of our own position in the cosmos, it bears noting that many stars in our stellar neighborhood are far older than our own. On that point, the new work benefits from the running analysis performed by a Villanova program called Living with a Red Dwarf, which homes in on the radiative environments that planets around such stars would be subject to as their host evolves. The goal is to make a determination of the likelihood that complex molecules can form, and whether life can evolve.

Image: Model of the Barnard’s Star planet system (from Ribas et al. 2018) compared to the inner Solar System. Barnard b orbits at 0..404 AU from its M3.5V host star and has an equilibrium temperature of T=-168C° in its 233-day orbit. Credit: Edward Guinan, Scott Engle / Villanova University.

The gathering of photometric data on Barnard’s Star under this project goes back to 2003, determining a rotation period of 142±8 days, a value that agrees well with other recent studies. The team then used the rotation period to extract a likely age of 8.6 billion years. Estimating stellar age for low-mass stars through rotation is a field known as gyrochronology, one that has accumulated a significant history of published analysis in the past decade. The age determined here also fits other age indicators to establish a result with 1.2 billion years play on either side.

As to that interesting planet, Barnard’s Star b is a super-Earth orbiting far enough from the primary to be cold (-168 C°), with only about 2 percent of light relative to the Earth. What the researchers go on to point out in their presentation is that as a super-Earth with a minimum mass of 3.25 Earth masses, Barnard’s Star b could have a hot iron/nickel core with resulting geothermal activity. The potential, if water is present, is for liquid water under an icy surface.

Geothermal heating could support “life zones” under its surface, akin to subsurface lakes found in Antarctica,” Guinan said. “We note that the surface temperature on Jupiter’s icy moon Europa is similar to Barnard b but, because of tidal heating, Europa probably has liquid oceans under its icy surface.”

We can only speculate about such matters, and the range of outcomes depending on the mass of the planet is wide. Note the range of possibilities in the authors’ presentation, called “X-Ray, UV, Optical Irradiances and Age of Barnard’s Star’s New Super Earth Planet – ‘Can Life Find a Way’ on such a Cold Planet?”:

Although little is definitely known about geomagnetism of superearths like Barnard b, a large liquid iron core, that could strong generate geomagnetic fields, could offer protection from strong winds and coronal mass ejections when the star was young & magnetically active. However, if the mass of the Barnard b is much higher than about 7–10 M, its higher gravity could result in it retaining a thick H2 -He atmosphere and thus be a dwarf gas giant (mini-Neptune). In this case all hope for life is probably lost unless by chance Barnard b hosts an icy moon (with a subsurface ocean) that could be tidally heated like Europa.

Image: (L) Possible model of Barnard b based on geothermal heating. If water is present, geothermal heating could create a subsurface ocean where primitive life could exist. The model would be a scaled-up Europa. (R) In another scenario if the mass of the exoplanet is > 7 M, then the stronger gravity could cause the retention of its primordial H2/He atmosphere. These planets are known as Mini-Neptunes / Dwarf Gas Giants. Credit: Edward Guinan, Scott Engle / Villanova University.

To learn more, we need to image the planet, an observation that would tell us about its atmosphere, surface and potential for life. On this score, the news is promising. Barnard’s Star b has an angular separation from its host that is much larger than Proxima b from Proxima Centauri, and may well be imaged by the next generation of extremely large telescopes (ELTs). It may also prove a target for the James Webb Space Telescope or the WFIRST mission.

For more, see Toledo-Padrón, “Stellar activity analysis of Barnard’s Star: Very slow rotation and evidence for long-term activity cycle” (preprint), which includes the high-precision photometry data of Barnard’s Star used in this analysis.

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Red Dwarf Planets May Lack Needed Volatiles

We can identify a number of circumstellar disks, but most are too far away to provide internal detail, much less the kind of activity that seems to be showing up around the red dwarf AU Microscopii. For at 32 light years out in the southern constellation Microscopium, AU Microscopii is presenting us with an unusual kind of activity that may have repercussions for the question of life around red dwarf stars in general. As presented at the recent meeting of the American Astronomical Society, fast-moving blobs of material are eroding the disk.

The consequence: Icy materials and organics that might have developed in asteroids and comets may instead be pushed out of the disk, long before they could provide the infall of materials thought to have benefited planets like ours. “The Earth, we know, formed ‘dry,’ with a hot, molten surface, and accreted atmospheric water and other volatiles for hundreds of millions of years, being enriched by icy material from comets and asteroids transported from the outer solar system,” said co-investigator Glenn Schneider (Steward Observatory, Tucson, Arizona).

Image: These two NASA Hubble Space Telescope images, taken six years apart, show fast-moving blobs of material sweeping outwardly through a debris disk around the young, nearby red dwarf star AU Microscopii (AU Mic). The top image was taken in 2011; the bottom in 2017. Hubble’s Space Telescope Imaging Spectrograph (STIS) took the images in visible light. This comparison of the two images shows the six-year movement of one of the known blobs (marked by an arrow). Credit: NASA, ESA, J. Wisniewski (University of Oklahoma), C. Grady (Eureka Scientific), and G. Schneider (Steward Observatory).

Researchers estimate that the blob of material in the image above is moving at about 24,000 kilometers per hour. It would have moved more than 1.3 billion kilometers between 2011 and 2017, roughly the distance between the Earth and Saturn when the two are at their closest approach to one another. Continually pushing small particles containing water and other volatiles out of the system, such circumstellar materials could cause the AU Microscopii disk to dissipate in 1.5 million years. Each blob — and thus far the team has found six of them — is thought to mass four ten-millionths the mass of Earth.

The ejection speeds among the six identified blobs range between 14,500 kilometers per hour and 43,500 kilometers per hour, well beyond escape velocity for the star. Their current distance ranges from 1.5 billion kilometers from the star to more than 8.8 billion kilometers. AU Microscopii’s relative proximity makes it possible for Hubble to resolve substructure in at least one of the blobs, which may eventually make it possible to discover their origins.

Image: The box in the image at left highlights one blob of material extending above and below the disk. Hubble’s Space Telescope Imaging Spectrograph (STIS) took the picture in 2018, in visible light. The glare of the star, located at the center of the disk, has been blocked out by the STIS coronagraph so that astronomers can see more structure in the disk. The STIS close-up image at right reveals, for the first time, details in the blobby material, including a loop-like structure and a mushroom-shaped cap. Astronomers expect the train of blobs to clear out the disk within only 1.5 million years. The consequences are that any rocky planets could be left bone-dry and lifeless, because comets and asteroids will no longer be available to glaze the planets with water or organic compounds. Credit: NASA, ESA, J. Wisniewski (University of Oklahoma), C. Grady (Eureka Scientific), and G. Schneider (Steward Observatory).

We wind up with planets lacking the nearby volatiles to enrich them, giving us the prospect of dry, dusty worlds without life. We can add this to the other factors that challenge the emergence of life around red dwarf stars, such as possible tidal lock and the resulting climate issues, not to mention heavy ultraviolet flux from young stars that could strip away the atmosphere of planets in the habitable zone. AU Microscopii is itself 23 million years old, an infant in stellar terms. Bear in mind that red dwarfs are the most common type of stars in the galaxy.

“The fast dissipation of the disk is not something I would have expected,” says Carol Grady (Eureka Scientific, Oakland, California), a co-investigator on the Hubble observations. “Based on the observations of disks around more luminous stars, we had expected disks around fainter red dwarf stars to have a longer time span. In this system, the disk will be gone before the star is 25 million years old.”

The AU Microscopii data were gathered by the European Southern Observatory’s Very Large Telescope in Chile as well as the Hubble Space Telescope Imaging Spectrograph (STIS) by a team led by John Wisniewski (University of Oklahoma). The STIS visible light images, taken in 2010-2011, were followed up by near-infrared work at the the SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) mounted on the VLT. The work also draws on disk observations of AU Microscopii by the Hubble Advanced Camera for Surveys in 2004.

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‘Oumuamua: Future Study of Interstellar Objects

‘Oumuamua continues to inspire questions and provoke media attention, not only because of its unusual characteristics, but because of the discussion that has emerged on whether it may be a derelict (or active) technology. Harvard’s Avi Loeb examined the interstellar object in these terms in a paper with Shmuel Bialy, one we talked about at length in these pages (see ‘Oumuamua, Thin Films and Lightsails). The paper would quickly go viral.

Those who have been following his work on ‘Oumuamua will want to know about two articles in the popular press in which Loeb answers questions. From the Israeli newspaper Ha’aretz comes an interview conducted by Oded Carmeli, while at Der Spiegel Johann Grolle asks the questions. From the latter, a snippet, in which Grolle asks Loeb what the moment would be like if and when humanity discovers an extraterrestrial intelligence. Loeb’s answer raises intriguing questions:

I can’t tell you what this moment will look like. But it will be shocking. Because we are biased by our own experiences. We imagine other beings to be similar to us. But maybe they are radically different. For example, it is quite possible that we won’t encounter the life forms themselves, but rather only their artifacts. In any case, we ourselves are not designed for interstellar journeys. The only reason astronauts survive in space is that they are under the protection of the Earth’s magnetic field. Even when traveling to Mars, cosmic rays will become a major problem.

Image: Avi Loeb (center) at the Daniel K. Inoue Solar Telescope (DKIST) in June of 2017. Credit: Avi Loeb.

Intriguing, given our conversations here about artificial intelligence and the emergence of non-biological civilizations. After all, we are in the nearby galactic company of numerous stars far older than our own. Would robotic beings supplant their biological cousins, or would the scenario be more like biological beings using artilects as their way of achieving interstellar travel? Either way, Loeb’s guess is that our first evidence will be an encounter with technological debris. The interview goes on to cover the ‘Oumuamua story’s outline thus far.

Meanwhile, two new papers from Loeb have appeared, the first written with John C. Forbes. “Turning Up the Heat on ‘Oumuamua” looks at the interstellar object, whatever it is, from another angle. If we were to discover more objects like this, how could we best analyze them? In earlier work with Manasvi Lingam, Loeb examined the population of interstellar objects that could be trapped within the Solar System, slung by Jupiter into parabolic orbits around the Sun.

The number could be as high as 6,000, a figure based on the deduced abundance of interstellar objects given the fact that we observed ‘Oumuamua as early as we did with instrumentation of the sensitivity of the Pan-STARRS telescopes. The paper references work on the overall abundance of these objects performed in 2017 by Greg Laughlin (UC-Santa Cruz) and Konstantin Batygin (Caltech), as well as a 2018 paper from Aaron Do (University of Hawai’i).

Learning more could involve a flyby mission, says Loeb, but there may be a better way:

In our new paper with John Forbes we proposed instead studying the vapor produced when such objects pass close to the Sun and get evaporated by the intense solar heat. We calculated the likelihood of that happening, keeping in mind that `Oumuamua did not show any signs of a cometary tail or carbon-based gas since it did not pass close enough to the Sun.

We used the known orbit of `Oumumua and assume a population of similar interstellar objects on random orbits in the vicinity of the Sun. This provided us with a likelihood of passages close to the Sun.

These objects would be expected to show a high orbital inclination, and assuming a population of this size, they should be readily detectable by future telescopes, such as the forthcoming Daniel K. Inoue Solar Telescope (DKIST). Another marker of interstellar origin, according to the paper, would be anomalous oxygen isotope ratios. If we can find interstellar objects that pass close to the Sun, we should be able to learn something about their composition. Loeb and Forbes use Monte Carlo methods to determine that such objects collide with the Sun once every 30 years, while about two should pass within the orbit of Mercury each year.

Usefully, spectroscopic study of cometary tails is a well-practiced science. As the paper notes:

Generally these studies are able to classify comets into different groups depending on the inferred production rates of H2O, C2, CN, and NH2 as well as dynamical properties, which likely reflect formation in different parts of the protoplanetary disk (Levison 1996)… The promise of using close encounters with the sun to learn about extrasolar small bodies is that the sun has the ability to disrupt even large cometary nuclei via its intense radiation, sublimating not just surface volatiles but even silicates and iron. In principle this exposes the interiors of these objects to remote spectroscopy, which could place strong constraints on the composition of these objects.

And indeed, two comets — 96P/Machholz 1 and Yanaka (1998r) — have been found to have depleted levels of CN and C2 relative to water. Sun-grazing comets of interstellar origin, assuming we can identify them early through instrumentation like the LSST (Large Synoptic Survey Telescope) should be available for such examination, a way to probe their composition without the need for sending fast flyby missions, although the latter would obviously be useful.

In a second paper, just accepted at Research Notes of the American Astronomical Society. Loeb and Harvard colleague Amir Siraj note that ‘Oumuamua’s shape may be more extreme than we have thought. Noting that the axis ratio for the object has been pegged at between 6:1 and 10:1, the paper delves into the lightcurve, with a startling result, as Loeb explained in an email this morning:

The lightcurve of the interstellar object Oumuamua showed a net brightening by one magnitude between October and November 2017, after corrections for the changing distances to the Sun and Earth and solar phase angle, assuming isotropic uniform albedo and the canonical phase function slope value for cometary and D-class objects of -0.04 magnitude per degree. We used the change in the orientation of `Oumuamua between October and November 2017 to show that this brightening implies a more extreme shape for the object. We inferred a ratio between its brightest and dimmest phases of at least 50:1 for a cigar shape and 20:1 for a pancake-like geometry. The revised values can be avoided if the phase function slope is 3 times larger than the canonical value, implying in turn another unusual property of `Oumuamua.

Variations in albedo could be in play, although here we would be looking at sharp variations for a minor change in viewing angle of ~ 11°, which Loeb and Forbes consider a possibility, though one without precedent in previous studies of asteroids and comets.

The papers are Forbes and Loeb, “Turning Up the Heat on ‘Oumuamua,” submitted to The Astrophysical Journal Letters (preprint); and Siraj and Loeb, “‘Oumuamua’s Geometry Could be More Extreme than Previously Inferred,” accepted at Research Notes of the American Astronomical Society (full text).

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Is Most Life in the Universe Lithophilic?

Seeking life on other worlds necessarily makes us examine our assumptions about the detectability of living things in extreme environments. We’re learning that our own planet supports life in regions we once would have ruled out for survival, and as we examine such extremophiles, it makes sense to wonder how similar organisms might have emerged elsewhere. Pondering these questions in today’s essay, Centauri Dreams regular Alex Tolley asks whether we are failing to consider possibly rich biospheres that could thrive without the need for surface water.

By Alex Tolley

Image: An endolithic lifeform showing as a green layer a few millimeters inside a clear rock. The rock has been split open. Antarctica. Credit: https://en.wikipedia.org/wiki/Endolith#/media/File:Cryptoendolith.jpg, Creative Commons).

A policeman sees a drunk man searching for something under a streetlight and asks what the drunk has lost. He says he lost his keys and they both look under the streetlight together. After a few minutes the policeman asks if he is sure he lost them here, and the drunk replies, no, and that he lost them in the park. The policeman asks why he is searching here, and the drunk replies, “this is where the light is” – The Streetlight Effect

I’m going to make a bold claim that we are searching for life where the starlight can reach, and not where it is most common, in the lithosphere.

One of the outstanding big questions is whether life is common or rare in the universe. With the rapid discovery of thousands of exoplanets, the race is now on to determine if any of those planets have life. This means using spectroscopic techniques to find proxies, such as atmospheric composition, chlorophyll “red edge”, and other signatures that indicate life as we know it. There is the exciting prospect that new telescopes and instruments will give us the answer to whether life exists elsewhere within a decade or two.

The search for life on exoplanets starts with locating rocky planets in the habitable zone (HZ). The HZ is defined as potentially having liquid surface water, which requires an atmosphere dense enough to ensure that water is retained. While complex, multicellular life that visibly populates our planet is the vision most people have of life, as I have argued previously [13], it is most likely that we will detect the signatures of bacterial life, particularly archaean methanogens, as prokaryotes were the only form of life on Earth for over 85% of its existence. Most worlds in the HZ will probably look more like Venus or Mars, either too dry and/or with an insufficient atmosphere to allow surface water. Such worlds will be bypassed for more attractive Earth analogs.

This is particularly important for the most common star type, the M-dwarfs. These stars are often downgraded as having habitable planets due to the flaring of their stars which can strip atmospheres and irradiate the surface. This reduces the likelihood for life at the surface, and for many, is a showstopper.

However, if life established well below the surface, these factors affecting the surface become relatively unimportant. All stars, including M-dwarfs, may well have a retinue of living worlds, but with their life undetectable by current means.

Despite mid-20th-century hopes for multicellular life to be found on Mars or Venus, it is now clear that the surfaces of these planets are devoid of any sort of multicellular based ecosystems. Venus’ surface is too hot for any carbon-based life to survive. The various Martian orbiters and landers have found no multicellular life, and so far no unambiguous evidence of microbial life on or near the surface. The Moon is the only world where surface rock samples have been returned to Earth, and these samples suggest, unsurprisingly, that the lunar surface is sterile [10,12].

NASA’s mantra for the search for life, echoing the HZ requirement, is “Follow the water!” On its face, this makes the lunar surface unlikely as a habitat, similarly Mars, although Mars’ does have an abundance of frozen water below the surface. This leaves the subsurface icy moons as the current favorite for the discovery of life in our solar system, particularly around any hypothetical “hot vents” that mimic Earth’s.

However, when following the trail of liquid water, we now know that the Earth has a huge inventory of water in the mantle, providing a new source of water for the crustal rocks. This water is most likely primordial, sourced from the chondritic material during formation.[6,9] If the Earth has primordial water in the mantle, so might the Moon, as it was formed from the same material as the Earth. A recent analysis of lunar rocks indicates that the bulk of the water in the Moon is also primordial, with concentrations only an order of magnitude less than the water in the Earth’s mantle [1]. While we know Mars has water just below the surface, the same argument about primordial water deep within Mars also follows.

The question then becomes whether this water is in a form suitable for life. Is there a zone in these worlds where water is both liquid and at a temperature below the maximum we know terrestrial thermophiles can survive?

Table 1 below shows some estimates for Earth, Mars and the Moon where a suitable liquid water temperature range exists. The estimated thermal gradients are used to suggest the depths where life might start to be found as temperatures and pressures result in liquid water, and the maximum depth life might survive.

On Earth, the reference planet, the high thermal gradient, and warm surface suggest life can be found at any depth, up to about 5 – 6 km. The Moon, due to a low thermal gradient might only have a habitable zone starting at 15 km below the surface but reaching down to nearly 120 km. Mars is intermediate, with a habitable zone 6-29 km in extent.

Table 1. Estimates of thermal gradients and range of depths where water is liquid, but below 120C as a current approximate maximum for thermophiles

WorldSurface CThermal
gradient
Depth (km)
at 120C (with
0C at
surface)
Depth (km) at
0C with
surface temp
Depth (km) at
120C with
surface temp
Earth1420-304-603.5-5
Mars-636.4-10.6 **11-196-1018-29
Moon-18 *1.17 ***10315118

* Assumes the Moon surface temperature would be the same as the Earth without an atmosphere
** [7]
*** [8]

So we have 2 possible rocky worlds in our solar system that may have water reservoirs in their mantles due to primordial asteroids and therefore liquid water in their lithospheres deep below the surface, protected from radiation and with fairly constant temperatures within the range of terrestrial organisms. So our necessary condition of liquid water may exist in these worlds, rather than at the surface.

Given that liquid water may be found deep below the surface, is there any evidence that life exists there too?

In 1999, the iconoclast astrophysicist and astronomer Thomas Gold published a popular account of his theory that fossil fuels were not derived from biological sources, but rather from primordial methane that was contaminated by organisms living deep within the Earth’s crust.[4,5]. While his theory remains controversial, his suggestion that organisms live in the lithosphere has been proven correct. [11]. Bores have shown that microorganisms have been found living at least 4 km below the surface. It has been suggested that the biomass of these organisms may exceed that of humanity on Earth, so life in the lithosphere is not trivial compared to that on the surface of our planet.

Figure 1. Illustration of the search for life in the lithosphere. At this time, life has been found at depths of nearly 4 km, but absent at 9 km where the temperatures were too high.
1. Deep-sea, manned submersibles and remotely operated vehicles collect fluid samples that exit natural points of access to the oceanic crust, such as underwater volcanoes or hydrothermal vents. These samples contain microbes living in the crust beneath.
2. Drilling holes into the Earth’s crust allows retrieval of rock and sediment cores reaching kilometers below the surface. The holes can then be filled with monitoring equipment to make long-term measurements of the deep biosphere.
3. Deep mines provide access points for researchers to journey into the Earth’s continental crust, from where they can drill even deeper into the ground or search for microbes living in water seeping directly out of the rock.

Source: [11]

From the article:

To date, studies of crustal sites all over the world—both oceanic and continental—have documented all sorts of organisms getting by in environments that, until recently, were deemed inhospitable, with some theoretical estimates now suggesting life might survive at least 10 kilometers into the crust. And the deep biosphere doesn’t just comprise bacteria and archaea, as once thought; researchers now know that the subsurface contains various fungal species, and even the occasional animal. Following the 2011 discovery of nematode worms in a South African gold mine, an intensive two-year survey turned up members of four invertebrate phyla—flatworms, rotifers, segmented worms, and arthropods—living 1.4 kilometers below the Earth’s surface.

With our existence proof of a deep, hot biosphere in Earth, is it possible that similar life could exist in the lithospheres of other rocky worlds in our solar system, including our Moon?

Mars is particularly attractive, as there is evidence Mars was both warmer and wetter in the past. There was geologic activity as clearly evident by the Tharsis bulge and the shield volcanoes like Olympus Mons. We know there is frozen water below the surface on Mars. What we are not certain of is whether Mars’ core is still molten and hot, and what the areothermal gradient is. One of the scientific goals of the Insight lander, currently on Mars, is to determine heat flow in Mars. This will help provide the data necessary to determine the range of the habitable zone in the lithosphere.

In contrast, we do have samples of Moon rock. An analysis of the Apollo 11 samples showed that organic material was present, but there was no sign of life except for terrestrial contamination [10, 12]. Since then, very little effort has been applied to look for life in the lunar rocks. The theory that the Moon is desiccated, hostile to life, and sterile, seems to have deterred further work. The early analyses indicated that methane (CH4) is present in the Apollo 11 samples. This may be primordial or delivered subsequently by impacts from asteroids or comets. If we ever discovered pockets of natural gas, even petroleum, on the Moon, this would be a staggering confirmation of Gold’s theory.

So where should we look?

Although the Moon is in our proverbial backyard, the expected depth of liquid water starts well below the bottom of the deepest craters.. This suggests that either deep boring would be necessary, or we must hope for impact ejecta to be recoverable from the needed depths. The prospects for either seem rather remote, although scientific and commercial activities on the Moon might make this possible in this century.

Despite its remoteness, Mars may be more attractive. Sampling at the bottom of crater walls and the sides of the Valles Marineris may give us relatively easy access to samples at the needed depths. Should the transient dark marks on the sides of crater walls prove to be liquid water, we would have samples within easy reach. The recent discovery of a possible subsurface water deposit just 1.5 km beneath the surface of Mars might be another possible target to reach.

The requirement that water is a necessary, but insufficient, condition for life has focused efforts on looking for life where liquid surface water exists. Because of the available techniques, exoplanet targets will be those that satisfy the HZ requirements. While these may prove the first confirmation of extraterrestrial life, they cannot answer some of the fundamental questions that we would like to know, for example, is abiogenesis common, or rare, and is panspermia the means to spread life. For that, we will need samples of such life. For the foreseeable future, that means sampling the solar system. We have 2 nearby worlds, and Gold suggested that there might be 10 suitable Moon-sized and above worlds that might have deep biospheres [5]. That might be ample.

To date, our search for life beyond Earth has been little more than looking for fish in the waves lapping the shore. We need to search more comprehensively. I am arguing that this search needs to focus on the habitable regions of lithospheres of any suitable rocky world. We might start with signs of bacterial fossils in exposed rock strata and ejecta, and then core samples taken from boreholes to look for living organisms. Finding life, especially that from a different genesis would indicate that life is indeed ubiquitous in the universe.

References

1. Barnes, J. J., Tartèse, R., Anand, M., Mccubbin, F. M., Franchi, I. A., Starkey, N. A., & Russell, S. S. (2014). The origin of water in the primitive Moon as revealed by the lunar highlands samples. Earth and Planetary Science Letters, 390, 244-252. doi:10.1016/j.epsl.2014.01.015

2. Davies, P. C., Benner, S. A., Cleland, C. E., Lineweaver, C. H., Mckay, C. P., & Wolfe-Simon, F. (2009). Signatures of a Shadow Biosphere. Astrobiology, 9(2), 241-249. doi:10.1089/ast.2008.0251

3. Davies, P. C. (2011). ​ The eerie silence: Renewing our search for alien intelligence. ​ Boston: Mariner Books, Houghton Mifflin Harcourt.

4. Gold, T. (1992). The deep, hot biosphere. Proceedings of the National Academy of Sciences, 89(13), 6045-6049. doi:10.1073/pnas.89.13.6045

5. Gold, T. (2010). ​ The deep hot biosphere: The myth of fossil fuels. New York, NY: Copernicus Books.

6. Hallis, L. J., Huss, G. R., Nagashima, K., Taylor, G. J., Halldórsson, S. A., Hilton, D. R., . . . Meech, K. J. (2015). Evidence for primordial water in Earth’s deep mantle. Science, 350(6262), 795-797. doi:10.1126/science.aac4834

7. Hoffman N.(2001) Modern geothermal gradients on Mars and implications for subsurface liquids. Conference on the Geophysical Detection of Subsurface Water on Mars (2001)

8. Kuskov O (2018) Geochemical Constraints on the Cold and Hot Models of the Moon’s Interior: 1–Bulk Composition. Solar System Research, 2018, Vol. 52, No. 6, pp. 467–479.

9. Mccubbin, F. M., Steele, A., Hauri, E. H., Nekvasil, H., Yamashita, S., & Hemley, R. J. (2010). Nominally hydrous magmatism on the Moon. Proceedings of the National Academy of Sciences, 107(25), 11223-11228. doi:10.1073/pnas.1006677107

10. Nagy, B., Drew, C. M., Hamilton, P. B., Modzeleski, V. E., Murphy, S. M., Scott, W. M., . . . Young, M. (1970). Organic Compounds in Lunar Samples: Pyrolysis Products, Hydrocarbons, Amino Acids. Science, 167(3918), 770-773. doi:10.1126/science.167.3918.770

11. Offord, C. (2018) Life Thrives Within the Earth’s Crust. The Scientist, October 1, 2018.

12. Oyama, V. I., Merek, E. L., & Silverman, M. P. (1970). A Search for Viable Organisms in a Lunar Sample. Science,167(3918), 773-775. doi:10.1126/science.167.3918.773

13. Tolley, A Detecting Early Life on Exoplanets. Centauri Dreams, February 2018

14. Way, M. J., Genio, A. D., Kiang, N. Y., Sohl, L. E., Grinspoon, D. H., Aleinov, I., . . . Clune, T. (2016). Was Venus the first habitable world of our solar system? Geophysical Research Letters, 43(16), 8376-8383. doi:10.1002/2016gl069790

15. Woo, M. The Hunt for Earth’s Deep Hidden Oceans. Quanta Magazine, July 11, 2018

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Technosearch: An Interactive Tool for SETI

Jill Tarter, an all but iconic figure in SETI, has just launched Technosearch, an Internet tool that includes all published SETI searches from 1960 to the present. A co-founder of the SETI Institute well known for her own research as well as her advocacy on behalf of the field, Tarter presents scientists with a way to track and update all SETI searches that have been conducted, allowing users to submit their own searches and keep the database current. The tool grows out of needs she identified in her own early research, as Tarter acknowledges:

“I started keeping this search archive when I was a graduate student. Some of the original papers were presented at conferences, or appear in obscure journals that are difficult for newcomers to the SETI field to access. I’m delighted that we now have a tool that can be used by the entire community and a methodology for keeping it current.”

Image: Screenshot of the Radio List on https://technosearch.seti.org/.

Among the materials included in Technosearch are:

  • Title of the search paper
  • Name(s) of observers
  • Search date
  • Objects observed
  • Facility where the search was conducted
  • Size and sensitivity of the telescope used
  • Resolving power of the instrument
  • Time spent observing each object
  • A link to the original published research paper
  • Comments that explain the search strategy
  • Observer notes

Technosearch currently contains 102 radio searches and 38 optical searches. The tool was presented yesterday at the 2019 winter meeting of the American Astronomical Society in Seattle and will be maintained by the SETI Institute. The AAS meeting always produces interesting developments, including exoplanet investigations that I intend to discuss next week.

On Technosearch, a personal thought: No one who has not attempted a deep dive into the scholarship on SETI can know how frustrating it is to chase down lesser known investigations or details of major ones. The issue of ready availability extends to the broad field of interstellar flight research, as I learned when compiling materials for my Centauri Dreams book. The trail from conference presentation to published paper can be obscure, while materials relating to specific researchers can be scattered through library collections or spread over a range of journals, some of them with firewalls, or available only in expensive books..

I’ve long advocated for interstellar studies a return to what Robert Forward began with Eugene Mallove, a detailed bibliography whose last appearance was in the Journal of the British Interplanetary Society in 1980. Putting such a resource online opens it worldwide and strengthens a field whose online databases are in many cases incomplete and often do not include older materials. All fields of scholarship will be following this essential path even as we continue to wrestle with academic publishers over questions of access to complete texts.

Technosearch is a step forward for SETI that helps scientists work with consolidated information while building a useful archive of contemporary work going forward. Tarter developed the tool in collaboration with graduate students working with Jason Wright (Penn State), a well-known figure in Dysonian SETI, which culls astronomical data looking for the possible physical artifacts of advanced civilizations. Also in the mix is Research Experience for Undergraduates, a program supporting students in areas of research funded by the National Science Foundation.

Image: Jill Tarter and Andrew Garcia presenting the Technosearch Tool.

SETI Institute REU student Andrew Garcia worked with Tarter in the summer of 2018:

“I started helping Dr. Tarter with this project as a research opportunity during the summer. I’ve become convinced that Technosearch will become an important instrument for astronomers and amateurs interested in exploring the cosmos for indications of other technological civilizations. We can’t know where to look for evidence tomorrow if we don’t know where we have already looked. Technosearch will help us chronicle where and how we’ve looked at the sky. I would like to thank the NSF REU program and the CAMPARE program for their encouragement and support throughout this project.”

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The advantages of neutral particle beam propulsion seem clear: Whereas a laser’s photon beams can exchange momentum with the sail, neutral particle beams transfer energy and are considerably more efficient. In fact, as we saw in the first part of this essay, that efficiency can approach 100 percent. A mission concept emerges, one that reaches a nearby star in a matter of decades. But what about the particle beam generators themselves, and the hard engineering issues that demand solution? For that matter, how does the concept compare with Breakthrough Starshot? Read on as James Benford, working in collaboration with Alan Mole, describes the salient issues involved in building an interstellar infrastructure.

By James Benford and Alan Mole

We discuss the concept for a 1 kg probe that can be sent to a nearby star in about seventy years using neutral beam propulsion and a magnetic sail. We describe key elements of neutral particle beam generators, their engineering issues, cost structure and practical realities. Comparison with the Starshot laser beam-driven concept gives roughly similar costs.

Beam Generator Concept

Figure 1 Block diagram of early neutral particle beam generator [1]. Drift-Tube Linac is not shown.

Creation of the neutral particle beam begins with

1. Extraction of a negative ion beam (negative ion with attached electrons) from a plasma source; it then drifts into the first acceleration stage, the RFQ. The first element of the accelerator will appear much like the geometry shown in figure 2. Here ions are extracted from the plasma source on the left by electrostatics and brought by a converging magnetic field to the linear accelerator.

Figure 2. Ion beam on left is propagated along converging magnetic field to the linac.

2. The ion beam enters a radiofrequency quadrupole (RFQ) accelerator, a vane-like structure where the application of radiofrequency power produces a continuous gentle acceleration much like a surfer riding a wave. It also provides strong electrostatic focusing to prevent divergence growth. The structure bunches the particles in phase space.

The RFQ fulfils at the same time three different functions:

  • focusing of the particle beam by an electric quadrupole field, particularly valuable at low energy where space charge forces are strong and conventional magnetic quadrupoles are less effective;
  • adiabatic bunching of the beam: starting from the continuous beam produced by the source it creates with minimum beam loss the bunches at the basic RF frequency that are required for acceleration in the subsequent structures;
  • acceleration of the beam from the extraction energy of the source to the minimum required for injection into the following linac structure.

3. After the ions exit the RFQ at energies of a few MeV, further acceleration to increase the particle energy is done with a drift-tube linac (DTL), which consists of drift tubes separated by acceleration regions, as shown in Figure 3. Particles arriving at the gaps at the proper phase in the radiofrequency waves are given acceleration impulses. When the electric field of the wave reverses, the particles are shielded from being accelerated by passing through the drift tubes. The typical accelerating gradient is a few MeV/m.

Figure 3. Drift-Tube Linac, which consists of drift tubes separated by acceleration regions.

4. In order to maintain low emittance and produce the microradian divergence we desire, the beam is expanded considerably as it exits the accelerator. Beam handling elements must have minimal chromatic and spherical aberrations.

5. Beam pointing to be done by bending magnets with large apertures.

6. Finally, the extra electrons are stripped from the beam, making a neutral particle beam. This can be done with by stripping the electrons in a gas neutralization cell or by photodetachment with a laser beam. It may be possible to achieve 100% neutralization by a combination of methods. Thus far this high-efficiency neutralization has not been demonstrated.

Beamer Engineering

There are several possible schemes for building the beam generator. Both electrostatic and electromagnetic accelerators have been developed to produce high power beams. The most likely approach is to use linear accelerators. In the past, the cost of an electromagnetic accelerator is on the order of a person year per meter of accelerator (~1 man-year/m) but this could be larger for the more sophisticated technologies.

The power system to drive such accelerators could come from nuclear power (fission or fusion) or solar power. Furthermore, if it were to be space-based, the heavy mass of the TW-level high average power required would mean a substantially massive system in orbit. Therefore Mole’s suggestion, that the neutral beam be sited on Earth, has its attractions. There is also the question of the effects of propagating in the atmosphere, on both beam attenuation and on divergence.
If the beam generator were to be on Earth, it should all be at the highest altitude for practical operations. The Atacama Desert, for example, would offer very low humidity and half of sea level pressure. In addition, a way to reduce beam losses in the atmosphere would be to launch a hole-boring laser beam in advance just before the neutral beam. This laser would heat up a cylinder of atmosphere to lower the pressure, allowing the neutral beam to propagate with less loss. Such hole-boring exercises have been conducted in laser weapon studies and does appear to be a viable technique.

The final neutral beam can be generated by many small beam drivers or a single large beam driver. If a great number of driver devices and their associated power supplies are required, increasing the construction and maintenance expense of this portion. Of course, economies of scale will reduce the cost of individual segments of the Beamer by mass production of the system modules. Making such choices is an exercise for future engineers and designers.

Neutral particle beam generators so far have been operated in pulsed mode of at most a microsecond with pulse power equipment at high voltage. Going to continuous beams, which would be necessary for the seconds of beam operation that are required as a minimum for useful missions, would require rethinking the construction and operation of the generator. The average power requirement is quite high, and any adequate cost estimate would have to include substantial prime power and pulsed power (voltage multiplication) equipment, the major cost element in the system. Of course, it will vastly exceed the cost of the Magsails, which is an economic advantage of beamed propulsion.

However, this needs economic analysis to see what the cost optimum would actually be. Such analysis would take into account the economies of scale of a large system as well as the cost to launch into space versus the advantages of beaming from Earth.

Beamer Cost Estimates

The interstellar neutral particle beam system described here is a substantial extrapolation beyond the present state-of-the-art. Nevertheless, estimates can be made of both the capital and operating costs.

The cost of the Beamer is divided between the cost of the accelerator structure (RFQ and DTL) and the power system that drives it. For a cost estimate for the Mercury system, we assume that the present day accelerating gradient is maintained for this very high-power system. That gradient is ~ 2 MeV/m. For the mercury neutral particle beam the length of the 1.35 GeV accelerator would be 675 m.

There is an extensive technology base for drift-tube linacs; there are many in operation around the world [2]. We use as a model the well-documented 200 MeV Brooklyn National Laboratory 200 MeV ion beam system, which was completed in 1978 at a cost of $47M. It used 22 MW of radiofrequency power and was 145m long. In that era, the cost of microwave equipment was ~$1/W. The cost today is ~$3/W, so the 22 MW would cost 22 M$ then and 66 M$ today. Since the total cost of accelerator was $47 M$, the Accelerator structure would cost 47 M$ -22 M$ = $25 M$. Thus at this level the two cost elements are roughly equal. The accelerator structure then costs $25 M$/145 m = $0.17 M$ per meter in 1978. We multiply all costs by a factor of three to account for inflation to get today’s costs.

To estimate the capital cost of the mercury in NPB described here, we have the following relations:

Caccl= 0.5 M$/m x 675 m = 350 M$

Cmicrowave= 3$/W x 18 TW = 5.47 B$

Therefore the dominant cost element would be the microwave system driving the accelerator.

However, high-volume manufacturing will drive costs down. Such economies of scale are accounted for by the learning curve, the decrease in unit cost of hardware with increasing production. This is expressed as the cost reduction for each doubling of the number of units, the learning curve factor f. This factor typically varies with differing fractions of labor and automation, 0.7 < f < 1, the latter value being total automation.

It is well documented that microwave sources have an 85% learning curve, f = 0.85 based on large-scale production of antennas, magnetrons, klystrons, etc [3]. Today’s cost is about $3/W for ~1 MW systems. Note that this includes not only the microwave generating tube, but also the power system to drive that continuous power. The 18 TW power needed would require 18 million such units. Therefore the cost is ~1.1 B$. Adding together the accelerator and microwave power system, the cost will be 1.45 B$.

The electrical power to drive this large system cannot possibly come from the electrical grid of Earth. Therefore a large cost element will be the system that stores the 162 TJ of energy. (Note that the beam power starts at zero and rises with time (t2) to 18 TW at the end.) From Parkin’s estimates of the Starshot energy storage system [10], based on Li-ion batteries, we take the storage cost to be $50 per kilowatt-hour, which is $13,900 $/TJ. Consequently the cost for the energy store is ($13,900 $/TJ) 162 TJ = 2.25 B$. So the energy stores cost is comparable to that of the accelerator.

The total capital cost is

Caccl= 350 M$

Cmicrowave = 1.1 B$

Cstore= 2.25 B$

Total accelerator capital cost is 3.7 B$.

The operating cost to launch a single Magsail is of course far smaller. It is simply the cost of the spacecraft and the energy to launch it. We will assume that the cost of the spacecraft will be on the order of $10 million. The cost of the electricity at the current rate of $.10 per kilowatt-hour is $4.5 million.

Total operating cost for a single launch is ~15M$.

Comparison with Starshot

The neutral particle beam approach is conceptually similar to photon beams such as the laser-driven Starshot project. A disadvantage of reflecting photons from the sail will be that they carry away much of the energy because they exchange only momentum with the sail. Neutral particle beams transfer energy, which is much more efficient. The reflecting particles may in principle be left on moving in space after reflection and thus the efficient energy efficiency can approach 100%.

The Starshot system, a laser beam-driven 1 gram sail with the goal of reaching 0.2c, has been quantified in a detailed system model by Kevin Parkin [4]. Since both the high acceleration neutral particle beam described here and Starshot are both beam-driven high-velocity systems, we make the following comparison between their key parameters and cost elements:

Physical parameters and cost elements of beam-driven probes

 Mercury Neutral Particle Beam SystemStarshot
Sail mass1 kg1 g
Velocity0.06 c0.2 c
Beamer capital cost1.45 B$4.9 B$
Energy store cost2.25 B$3.4 B$
Total capital cost3.7 B$8.3 B$
Energy cost/launch4.5 M$7 M$
Kinetic energy1.6 1014 J1.8 1012 J
Kinetic energy/ capital cost43.2 kJ/$0.2 kJ/$

Here we have summed the accelerator and microwave power system costs for the neutral Beamer and the laser and optics cost for Starshot. A major caveat is that Parkin’s estimates have realistic efficiencies of the systems of Starshot, but our costs assume unrealistically high efficiencies.

Although they differ in detail, the two concepts give the same order of magnitude cost. However, the kinetic energy in the NPB-driven probe is 90 times that of the Starshot probe. This shows the disadvantage of reflecting photons from the sail: they carry away much of the energy because they exchange only momentum with the sail. Neutral particle beams transfer energy, which is much more efficient. The kinetic energy/capital cost ratio is 200 times greater in the NPB case.

It is instructive that the high-energy requirement of interstellar probes drives the existence of a stand–alone storage system, which is a major element in the total cost of both systems. The similarity of costs for these rather different beam- driven systems gives us some confidence that these rough estimates in this paper are credible.

Neutral Particle Beam Realities

Practical realities are always bad news. Performance of most systems degrades to below their design points because of inefficiencies of processes. Note that the beam systems described here are perfectly efficient, as determined from equation 5. That is, the beam reflects from the sailcraft with perfect efficiency, so as to stop dead, transferring all the energy to the spacecraft. The realities of neutral particle beams in the present day are substantially poorer.

To see where the problems lie, we consider a daring experiment called BEAR, conducted 30 years ago [1, 5]. A neutral particle beam generator was actually deployed and operated in space and its performance was measured.

On July 13, 1989 the Beam Experiment Aboard Rocket (BEAR) linear accelerator was successfully launched and operated in space by Los Alamos National Laborotory. The rocket trajectory was sub-orbital, reaching altitude of 220 km. The flight demonstrated that a neutral hydrogen beam could be successfully propagated in an exoatmospheric environment. The cross-section of the rocket is shown in figure 4.

Figure 4. Beam Experiment Aboard Rocket (BEAR) [1].

The accelerator, which was the result of an extensive collaboration between Los Alamos National Laboratory and industrial partners, was designed to produce a 10 rnA, 1 MeV neutral hydrogen beam in 50 microsecond pulses at 5 Hz. The major components were a 30 kev H- injector a 1 MeV radio frequency quadrupole, two 425 MHz RF amplifiers, a gas cell neutralizer, beam optics, vacuum system and controls. The beam extracted was 1 cm in diameter with a beam divergence of 1 mradian. There was no unexpected behavior such as beam instability in space.

The design was strongly constrained by the need for a light- weight rugged system that would survive the rigors of launch and operate autonomously. The payload was parachuted back to Earth. Following the flight the accelerator was recovered and successfully operated again in the laboratory.

From the paper and report describing this experiment we see substantial inefficiencies, which should guide our future expectations.

The input power to the accelerator was 620 kW for 60 µs, a 7.2 J energy input. The beam as extracted was 27 mA at 1 MeV for 50 µs, which gives 1.35 J. The efficiency therefore is 19%, so approximately 4/5 of the energy supplied was lost in the beamline shown in figure 1. The major loss was in the neutralizer which was a xenon gas injected into the beamline. The efficiency of the neutralizer was changed by varying the amount of gas injected. They obtained 50% neutral hydrogen and 25% each of negative and positive hydrogen. Therefore the neutralization process was only 50% efficient in producing a neutral beam. This accounts for most of the loss. The other losses can be accounted for by inefficiencies in the optics of the low-energy beam region and the high-energy beam region.

In the 30 years since the flight, little work on particle beams has occurred at high power levels, because of the termination of the Strategic Defense Initiative. Doubtless substantial improvements can be made in the efficiency of NPB’s, given substantial research funding. Therefore the concept in this paper, with its hundred percent efficiency of energy transfer from the electrical system to the sail, is an upper bound on the performance. Consequently the parameters in Table 1 and the capital and operating cost estimates given here are lower bounds on what would actually occur.

Conclusions

The cost model presented here is lacking in realistic efficiencies. The next level of analysis should address this lack.

We can forsee a development path: a System starts with lower speed, lower mass Magsails for faster missions in the inner solar system. As the system grows, the neutral beam System grows and technology improves. Economies of scale lead to faster missions with larger payloads. As interplanetary commerce begins to develop, making commerce operate efficiently, outcompeting the long transit times of rockets between the planets and asteroids, the System evolves [6]. Nordley and Crowl describe such a development scenario [7]. We conclude that this concept is a promising method for interstellar travel.

References

1. P. G. Oshey, T. A. Butler, M. T. Lynch, K. F. McKenna, M. B. Pongratz, T. J. Zaugg, “A Linear Accelerator In Space-The Beam Experiment Aboard Rocket”, Proceedings of the Linear Accelerator Conference 1990.

2. H. B. Knowles, “Thirty-Five Years of Drift-Tube Linac Experience” Los Alamos Scientific Laboratory Report, LA-10138-MS, 1984. See also reference 4, pg. 81.

3. J. Benford, J. A. Swegle and E. Schamiloglu, High Power Microwaves, Third Edition, pg. 77, Taylor and Francis, Boca Raton, FL, (2015).

4. K. L. G. Parkin, “The Breakthrough Starshot System Model”, Acta Astronautica 152, 370-384, 2018.

5. G. J. Nutz, “Beam Experiments Aboard a Rocket (BEAR) Project Summary’, LA-11737, 1990.

6. J, Benford, “Beam-Driven Sails and Divergence of Neutral Particle Beams” JBIS 70, pg. 449-452, 2017.

7. G. Nordley and A. J. Crowl, “Mass Beam Propulsion, An Overview”, JBIS 68, pp. 153-166, 2015.

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Beamed propulsion has clear advantages when it comes to pushing a payload up to interstellar flight speeds, which is why Breakthrough Starshot is looking at laser strategies. But what about a neutral particle beam in conjunction with a magnetic sail? We’ve discussed the possibilities before (see Interstellar Probe: The 1 KG Mission), where I wrote about Alan Mole’s paper in JBIS, followed by a critique from Jim Benford. Mole, a retired aerospace engineer, is now collaborating with plasma physicist Benford (CEO of Microwave Sciences) to examine a solution to the seemingly intractable problem of beam divergence. Getting around that issue could be a game-changer. Read on for the duo’s thoughts on sending a 1 kg probe to a nearby star system with a flight time in the range of 70 years. Part 2 of this study, outlining engineering issues and the practical realities of cost, will follow.

by James Benford and Alan Mole

We advance the concept for a 1 kg probe that can be sent to a nearby star in about seventy years using neutral beam propulsion and a magnetic sail. The concept has been challenged because the beam diameter was too large, due to inherent divergence, so that most of the beam would miss the sail. Increasing the acceleration from 1000 g’s to 100,000 g’s along with reducing the final speed from 0.1 c to 0.06 c redeems the idea. Such changes greatly reduce the acceleration distance so that the mission can be done with realistic beam spread. Magsail-beam interaction remains an aspect of this concept that needs further study, probably by simulations.

Central features of Neutral Particle Beam Propulsion

Use of a neutral particle beam to drive a Magsail was proposed by Geoffrey Landis as an alternative to photon beam-driven sails [1]. Compared to beam-driven propulsion, such as Starshot, particle beam propelled magnetic sails, Magsails, substitute a neutral particle beam for the laser and a Magsail for the ‘lightsail’, or ‘sailship’. The particle beam intercepts the spacecraft: payload and structure encircled by a magnetic loop. The loop magnetic field deflects the particle beam around it, imparting momentum to the sail. The general ‘mass beam’ approach has been reviewed by Nordley and Crowl [2].

Particle beam propelled Magsails require far less power for acceleration of a given mass. There’s also ~ 103 increase in force on the sail for a given beam power. Deceleration at the target star is possible with the Magsail but not with a laser driven sail.

The neutral particle beam approach is conceptually similar to photon beams such as the laser-driven Starshot project. A disadvantage of reflecting photons from the sail will be that they carry away much of the energy because they exchange only momentum with the sail. Neutral particle beams transfer energy, which is much more efficient. The reflecting particles may in principle be left unmoving in space after reflection and thus the efficient energy efficiency can approach 100%.

The thrust per watt beam power is maximized when the particle velocity is twice the spacecraft velocity. The Magsail, with a hoop force from the magnetic field, is an ideal structure because it is under tension. High-strength low-density fibers make this lightweight system capable of handling large forces from high accelerations. The rapidly moving magnetic field of the Magsail, seen in the frame of the beam as an electric field, ionizes the incoming neutral beam particles. Nordley and Crowl discuss on-board lasers to ionize the incoming beam, although this adds additional on-board mass and power [2]. When the dipole field of the Magsail is inclined to the beam vector the Magsail experiences a force perpendicular to the beam vector, which centers it on the particle beam, perhaps providing beam-riding stability.

Ultrahigh Acceleration

Alan Mole proposed using it to propel a lightweight probe of 1 kg [3]. The probe was accelerated to 0.1 c at 1,000 g by a neutral particle beam of power 300 GW, with 16 kA current, 18.8 MeV per particle. The particle beam intercepts a spacecraft that is a Magsail: payload and structure encircled by a magnetic loop. The loop magnetic field deflects the particle beam around it, imparting momentum to the sail, and it accelerates.

Benford showed that the beam divergence is fundamentally limited by the requirement, at the end of the acceleration process, to strip electrons from a beam of negative hydrogen ions to produce a neutral beam [4,5]. Therefore neutral beam divergence is typically a few microradians. Mole’s beam had an inherent beam divergence of 4.5 µradians.

In Mole’s work, the neutral hydrogen beam at 18.8 MeV per particle and inherent beam divergence of 4.5 µradians accelerated to two-tenths of the speed of light (0.2 c) had acceleration of 103 g’s for 50 minutes [3]. This resulted in a 411 km diameter beam spot, far larger than the Magsail diameter, which was 0.27 km. So most of the beam missed the sail.

But if we use much higher acceleration, the sail will stay within the beam until it reaches the desired final velocity, even with microradian divergence. We choose 105 g’s, 106 m/s2 to accelerate to 0.06 c, 1.8 x 107 m/s.

Numerical experiments with the model developed by Nordley [6], and later replicated by Crowl, showed that the greatest momentum delivery efficiency is when the velocity of the neutral beam is twice the sail velocity. The physics of this is straightforward: Maximum energy efficiency comes when all of the energy goes to the sail and none of it remains in the beam. For a sail that is perfectly reflective, the beam bounces off the sail at the same velocity it impinges the sail. If after reflection it is moving at zero velocity (so none of the energy is left in the beam), the initial beam velocity must be twice the sail velocity, so that it impinges on the sail at a relative velocity equal to the sail velocity.

We take the beam velocity at the end of acceleration to be the twice the final sail velocity, 0.06c The energy of a hydrogen atom is imparted by accelerating through a voltage of 6.6 MeV. The mission parameters for a hydrogen beam then become those shown in Table 1.

The lighter the particle to be accelerated, the shorter the beam driver can be at a fixed field gradient. However, lighter-particle shorter beam drivers, while they may cost less, would require a larger sail due to the higher divergence of the beam.

For a second example, a mercury beam has a minimum divergence of 0.8 µradians, but must use far higher voltage because of the larger mass [4]. Mercury beam parameters are also given in Table 1.

Table 1 Parameters of neutral particle beam-driven sail probes

Beam and Sail ParametersHydrogen BeamMercury Beam
Beam Divergence4.5 microradian0.8 microradian
Acceleration105 g’s=106 m/sec2105 g’s=106 m/sec2
Sail diameter1.46 km260 m
Sail final velocity0.06 c, 1.8 x 107 m/s0.06 c, 1.8 x 107 m/s
Acceleration distance1.6 x 105 km, 10-3 AU1.6 x 105 km, 10-3 AU
Acceleration time18 sec18 sec
Magsail mass1 kg1 kg
Kinetic energy1.6 1014 J4 1014 J
Beam peak power1.8 1013 W, 18 TW1.8 1013 W, 18 TW
Beam voltage6.76 MeV1.35 GeV
Beam current2.66 MA1.33 kA

We will see that when the beam divergence is in reality roughly 3 orders of magnitude higher than previous studies have assumed, from a nanoradian to microradian, rapidly moves the beam generator regime toward being very large and expensive.

Because in Table 1 the hydrogen beam sail diameter is so large, we will focus the rest of this discussion on the mercury beam. Even so, the mercury beam Magsail has a 260 m diameter and 1 kg mass, if the superconducting hoop has a density of steel, the thickness must be no larger than 0.44 cm, if the density of carbon, 0.8 cm.

Magsail-Beam Interaction

Note that the sail diameter given in Table 1 is taken to be simply the diameter of the divergent beam encountering the Magsail. The diameter of the reflection region produced by the magnetic field of the sail could well be somewhat larger than the superconducting hoop diameter. (Of course, early in the acceleration, the beam will hit it at the axis where the magnetic field is greatest.)

When a Magsail driven by a neutral particle beam is at the early stages of the acceleration, the beam will have a considerably smaller spot size on the Magsail than it will later and will hit it at the axis where the magnetic field is greatest. Later on, as the Magsail flies away, the beam will reach a size dictated by its divergence. A question is: does the initial beam high intensity of the beam on the magnetic field tend to push the sails magnetosphere outward radially and make the effective diameter of the Magsail larger? If it does, then the beam divergence can be a bit larger and still strike the Magsail. Or, conversely one could accelerate the Magsail for a longer time because some of the beam would still be captured.

Simulations show the field being compressed; but they are of solar wind, which is taken to be uniform across a magnetic dipole. There are no simulations of the beam smaller than the sail. One would expect the loop generated field to be compressed in the direction of motion, but it seems reasonable for it to be inflated radially, especially if charged particles are trapped in it for significant periods of time.

Andrews and Zubrin have done single particle numerical calculations that do not include modeling dynamic effects (such as field distortions from magnetic pressure) and do not include any such “inflation” of the mirror due to trapped beam ions [7].

Figure 1 is taken from the late Jordan Kare’s NIAC report [8]. (From his figure, he considered using a nuclear detonation to accelerate a Magsail, which is not relevant to our discussion.) From the left a uniform solar wind strikes the Magsail, which in our case would be a non-uniform neutral particle beam. The beam encounters the peak of magnetic field along the axis of the sail. On the right of the figure, the field is distorted, producing a plasma interface shock against the magnetic field of the Magsail. Inflation of the magnetic field due to a particle beam pressure could occur. However, the effect would be to allow the beam divergence to be only a bit larger.

Note also that in this diagram the sail is shown as dragging the payload behind it as it accelerates. If part of the particle beam reaches the payload it could create substantial damage. Consequently, it might it be better to distribute the payload around the superconducting hoop where it would have the most protection against incoming charged particles. Note also the stability of the superconducting loop on a beam of finite width has not been investigated to date. However, the Starshot program is looking at this issue extensively.

Figure 1: Interaction of streaming plasma flow with a Magsail. From Jordan Kare NIAC report [8].

The assumption that the moving magnetic field of the Magsail, seen in the frame of the beam as an electric field, ionizes the incoming neutral beam particles must be quantified.

Conclusions

Since beam divergence is fundamentally limited, high accelerations can be used to insure the sail will stay within the beam until it reaches the desired final velocity, even with microradian divergence. This leads to ultrahigh, 105 g’s, 106 m/s2 to accelerate to 0.06 c. The Starshot system, a laser beam-driven 1 gram sail with the goal of reaching 0.2c, has been quantified in a detailed system model by Kevin Parkin [9]. It too uses 105-106 g’s. Magsail-beam interaction remains an aspect of this concept that needs further study, probably by simulations. This promising method for interstellar travel should receive further attention.

References

1. G.A. Landis, “Optics and Materials Considerations for Laser-Propelled Lightsail,” IAA-89-664, 1989.

2. G. Nordley and A. J. Crowl, “Mass Beam Propulsion, An Overview”, JBIS 68, pp. 153-166, 2015.

3. Alan Mole, “One Kilogram Interstellar Colony Mission”, JBIS, 66, pp.381-387, 2013.

4. J, Benford, “Beam-Driven Sails and Divergence of Neutral Particle Beams” JBIS 70, pg. 449-452, 2017.

5. Report to the APS of the study on science and technology of directed energy weapons, Rev. Mod. Phys 59, number 3, part II, pg. 80,1987.

6. G. D. Nordley, “Relativistic Particle Beams for Interstellar Propulsion,” JBIS 46, pp 145-150,1993

7. Andrews, D. G. and R. M. Zubrin, “Magnetic Sails and Interstellar Travel”, JBIS 43, pp. 265-272, 1990

8. J. T. Kare, “High-acceleration Micro-scale Laser Sails for Interstellar Propulsion,” Final Report NIAC RG#07600-070, 2002.
www.niac.usra.edu/files/studies/final_report/597Kare.pdf. Accessed 03 Dec 2018.

9. K. L. G. Parkin, “The Breakthrough Starshot System Model”, Acta Astronautica 152, 370-384, 2018.

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A Closer Look at Ultima Thule

“We think we are looking at the most primitive object ever imaged by a spacecraft,” said Jeff Moore (NASA Ames) at today’s Ultima Thule press conference. Moore, New Horizons geology and geophysics lead, went on to describe the process of innumerable particles growing into nodes amidst growing low velocity collision and interaction. We are truly looking at primordial materials with Ultima Thule, which is now revealed as a contact binary. Have a look.

Image: This image taken by the Long-Range Reconnaissance Imager (LORRI) is the most detailed of Ultima Thule returned so far by the New Horizons spacecraft. It was taken at 5:01 Universal Time on January 1, 2019, just 30 minutes before closest approach from a range of 18,000 miles (28,000 kilometers), with an original scale of 730 feet (140 meters) per pixel. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Bear in mind that New Horizons was working with a Sun 1,900 times fainter than a sunny day on Earth, as mission principal investigator Alan Stern reminded the audience when he unveiled the image above. “It’s a snowman, not a bowling pin,” joked Stern as the image was displayed. Bear in mind as well that these early images are just the beginning. The mission team has now downloaded less than 1 percent of the data available on the spacecraft’s solid state recorders.

One of Jeff Moore’s slides:

And here’s the slide Moore showed to illustrate the process of accretion:

Putting these two lobes together would, Moore said, be gentle enough that “…if you were in a car collision at this speed you wouldn’t bother to fill out the insurance forms.” These are high-Sun images, meaning we see little shadow, but the Sun angle will change as we move into later views at higher resolution. Even so, note the absence of obvious impact craters, and the mottled suggestions of hills and ridges. Also note the brightness of the ‘neck’ between the lobes.

Image: The first color image of Ultima Thule, taken at a distance of 85,000 miles (137,000 kilometers) at 4:08 Universal Time on January 1, 2019, highlights its reddish surface. At left is an enhanced color image taken by the Multispectral Visible Imaging Camera (MVIC), produced by combining the near infrared, red and blue channels. The center image taken by the Long-Range Reconnaissance Imager (LORRI) has a higher spatial resolution than MVIC by approximately a factor of five. At right, the color has been overlaid onto the LORRI image to show the color uniformity of the Ultima and Thule lobes. Note the reduced red coloring at the neck of the object. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Ultima Thule’s rotation period is currently pegged at 15 hours, plus or minus an hour. The object turns out to be red, as expected. As to reflectivity, deputy project scientist Kathy Olkin (SwRI) pointed out that the brightest areas reflect about 13 percent of incident sunlight, the darkest areas only 6 percent. Ultima Thule is, in other words, very dark, as dark as potting soil, Olkin added, with significant variation across the surface.

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OSIRIS-REx: Orbital Operations at Bennu

Sometimes one mission crowds out another in the news cycle, which is what has happened recently with OSIRIS-REx. The study of asteroid Bennu, significant in so many ways, continues with the welcome news that OSIRIS-REx is now in orbit, making Bennu the smallest object ever to be orbited by a spacecraft. That milestone was achieved at 1943 UTC on December 31, which in addition to the upcoming New Year’s celebration was also deep into the countdown for New Horizons’ epic flyby of MU69, the Kuiper Belt object widely known as Ultima Thule.

Image credit: Heather Roper/University of Arizona.

I suppose the classic case of mission eclipse was the Voyager flyby of Uranus, which occurred on January 24, 1986. I was flying commercial students in a weekend course four days later in Frederick, MD and anxious to hear everything I could about the flyby, its images and their analysis, but mid-morning between flights I learned about the Challenger explosion, and the news for days, weeks, was filled with little else. Now, of course, we can study the striking images of Uranus’ rings and the tortured moon Miranda, putting them in the great context of Voyager exploration, but for a time the story was mute.

OSIRIS-REx has a long period of mapping and sampling ahead of it, with the sample site selection gearing up, and we’ll have plenty to say about it in coming weeks. Ponder that the spacecraft orbits Bennu from a distance just 1.75 kilometers from its center, a tighter value even than Rosetta’s, when it orbited 7 kilometers from the center of comet 67P/Churyumov-Gerasimenko.

From OSIRIS-REx flight dynamics system manager Mike Moreau (NASA GSFC):

“Our orbit design is highly dependent on Bennu’s physical properties, such as its mass and gravity field, which we didn’t know before we arrived. Up until now, we had to account for a wide variety of possible scenarios in our computer simulations to make sure we could safely navigate the spacecraft so close to Bennu. As the team learned more about the asteroid, we incorporated new information to hone in on the final orbit design.”

Using 3-D models of Bennu’s terrain created from OSIRIS-REx’s recent global imaging and mapping campaign, the mission team intensifies its navigation survey, and will analyze changes in the spacecraft’s orbit to study the minute gravitational pull of the object, which should tighten up existing models of not just the gravity field, but Bennu’s thermal properties and spin rate. By the summer of 2020, controllers will be ready for the spacecraft to touch the surface for sampling operations, with the sample scheduled for return to earth in September of 2023.

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