New Study Reassesses Habitability of Exoplanets Around Multiple Star Systems

An artist's concept of Kepler-47, which was the first ever planetary system to be discovered orbiting a binary star. A new research that was based on data taken with NASA's Hubble Space Telescope provides stringest limits to the potential habitablity of exoplanets in several such systems. Image Credit: NASA/JPL-Caltech/T. Pyle

An artist’s concept of Kepler-47, which was the first-ever planetary system to be discovered orbiting a binary star. A new research that was based on data taken with NASA’s Hubble Space Telescope provides stringest limits to the potential habitablity of exoplanets in several such systems. Image Credit: NASA/JPL-Caltech/T. Pyle

One defining scientific revolution of our generation is the discovery of thousands of exoplanets around other stars, which has transformed our view of the Solar System from being the only one in existence in a vast and immense Universe, to being just one between millions or even billions in our home galaxy alone. This plurality of worlds has forced scientists and non-scientists alike to ask the next big question: How many of them harbor planets that could sustain life? In the absence of hard evidence, this topic has been the subject of a multitude of theoretical studies throughout the years, with many of them often reaching a variety of different conclusions. A new research based on data from NASA’s Hubble Space Telescope offers a new insight into this fascinating subject, by presenting evidence that many of the extrasolar worlds that have been previously deemed as being potentially habitable, might actually not fit the bill.

In the search for alien worlds beyond our Solar System, no other observatory has had such a huge impact than that of NASA’s Kepler space telescope. Having already entered its sixth year of successful operations, Kepler forever changed the field of exoplanetary research with its ground-breaking discoveries of thousands of candidate and confirmed exoplanets to date. Extrapolating on this treasure trove of data, astronomers have reached the conclusion that our Milky Way galaxy alone most probably harbors hundreds of billions more, with a large fraction of them being potentially habitable. In turn, the concept of exoplanet habitability itself is intrinsically linked with the notion of the “habitable zone,” which defines the region around a star within which conditions would be just right for any existing exoplanets to exhibit life-friendly conditions, mainly mild temperatures that would allow for the presence of liquid water. In our own Solar System, the Earth is right in the middle of the Sun’s habitable zone, which has allowed for the emergence and evolution of life as we know it. But what about planets around other stars? Could we find alien worlds that share life-friendly conditions similar to Earth? A series of past AmericaSpace articles has explored such a possibility for a varying set of exoplanets, including terrestrial-type worlds around single and multiple red dwarf star systems, nearby terrestrial exoplanets more than 10 billion years-old, as well as hypothesised “super-habitable” worlds far from the habitable zone of their host stars.

A diagram showing the extend of the habitable zone for different types of stars. The light blue region depicts the “conventional” habitable zone for planets with nitrogen, carbon dioxide and water-rich atmospheres. The yellow region shows the habitable zone as extended inward for dry planets, as dry as 1% relative humidity. The outer darker blue region shows the outer extension of the habitable zone for hydrogen-rich atmospheres and can extend even out to free-floating planets with no host star. The planets of our Solar System are shown with images. Known exoplanets are represented with the purple-grey dots. Image Credit: Sara Seager/Science Vol. 340 no. 6132

A diagram showing the extend of the habitable zone for different types of stars. The light blue region depicts the “conventional” habitable zone for planets with nitrogen, carbon dioxide and water-rich atmospheres. The yellow region shows the habitable zone as extended inward for dry planets, as dry as 1% relative humidity. The outer darker blue region shows the outer extension of the habitable zone for hydrogen-rich atmospheres and can extend even out to free-floating planets with no host star. The planets of our Solar System are shown with images. Known exoplanets are represented with the purple-grey dots. Image Credit: Sara Seager/Science Vol. 340 no. 6132

One key characteristic of habitable zones is that they are not fixed in place, but they are very dynamic in nature, constantly being shaped by a host of other factors like stellar flux and stellar evolution, the mass and size of the exoplanets themselves, as well as their atmospheric conditions. For instance, the Sun is at present 30 percent brighter than what it was during the early history of the Solar System, causing the latter’s habitable zone to move outward with time. In a few billion years when the Sun’s luminosity will have increased further still, the inner region of the habitable zone will have moved beyond the current orbit of Mars, leaving our own planet uninhabitable.

But what about double and multiple star systems? Contrary to the Sun, which is a solitary star, the bulk of our galaxy’s stellar population resides in such systems. It was previously thought that the existence of planets around such systems was an unlikely scenario, because the multiple gravitational interactions of the closely packed stars in these systems were believed to be a great inhibitor to planetary formation. Yet, as the Kepler space telescope has shown in recent years, exoplanets are the norm not only for single stars like the Sun, but for multiple ones as well. Nearly half of the more than 4,000 exoplanet candidates detected by Kepler to date have been found inside such binary and multi-star systems, while various studies based on these findings by Kepler have argued that nearly half of all the galaxy’s total of binary and multi-star systems must indeed harbor exoplanets. If so, what are the chances of finding any potentially habitable worlds around those stars? In recent years, a growing list of exoplanets discovered inside such systems have been shown to lie inside their stars’ habitable zone, like Kepler-16b inside the Kepler-16 binary star system and the planets of the Gliese 667 triple star system—just to name a few.

A defining factor that can affect the prospects of exoplanet habitability inside multiple star systems is the nature of the planetary orbits themselves. If the stars in such systems form close pairs, they would create one common habitable zone for all orbiting planets, whereas if the companion stars have wide separations, each one would have its own habitable zone which in turn would be affected by the properties of their neighboring stellar companions, like mass, size, and luminosity. For these reasons, many scientists have argued that the prospects of habitability inside such systems is probably very low.

A new study, recently published at The Astrophysical Journal, comes to further reassess the potential habitability for a sample of exoplanets inside multiple star systems, by providing evidence that some of these alien worlds which were previously thought to harbor conditions that were favorable for life are in fact much more inhospitable. More specifically, the study’s research team, which was led by Kimberly Cartier, a graduate student at the Pennsylvania State University’s Department of Astronomy & Astrophysics, conducted a detailed photometric analysis and high-resolution imaging of a total of 22 Kepler exoplanet candidates with NASA’s Hubble Space Telescope, in order to better determine the properties of the exoplanet candidates and their respective host stars. The researchers focused on three of the most promising planetary systems in their sample: Kepler-296, KOI-2626, and KOI-3049. The latter two are multiple red dwarf systems comprised of three and two stellar companions respectively and harbor one exoplanet candidate each, while Kepler-296, which harbors a total of five exoplanets (two confirmed and three candidates), was originally thought to be a single red dwarf star.

The analysis by Cartier’s team, with the help of Hubble’s superior observing capabilities, showed that Kepler-296 is a close binary system of two red dwarf stars instead that were unresolved from the Kepler space telescope. With the help of previously gathered data from Kepler, as well as from the results of simulations and computational analysis, the researchers determined that due to the binary nature of the Kepler-296 system, the exoplanet Kepler-296 d (the third planet in the system), which was previously thought to lie inside the system’s habitable zone, was actually much farther in. In contrast, the system’s other two outer confirmed exoplanets (Kepler-296 e and f) were repositioned inside the habitable zone, due to the binary system’s recalculated stellar flux levels. “[Previous studies] had determined that Kepler-296 d was in the Habitable Zone of the assumed single star,” write the researchers in their study. “Using our stellar solutions for Kepler-296, Kepler-296 d is not habitable around either star, and in fact falls significantly interior to the Habitable Zone of either star. The outermost planet in the system (Kepler-296 f) now falls comfortably within the Habitable Zones of both the primary and the secondary stars. Kepler-296 e also falls just barely interior to the Habitable Zone of the secondary, but the uncertainty on the effective stellar flux at that planet makes it another likely habitable candidate.”

An artistic representation of all the known exoplanets as of April 2015 (ranked from closest to farthest from Earth), which exhibit any potential to support life as we know it. Earth, Mars, Jupiter, and Neptune are shown for scale on the right. Image Credit: Planetary Habitability Laboratory (PHL)/University of Puerto Rico @Arecibo

An artistic representation of all the known exoplanets as of April 2015 (ranked from closest to farthest from Earth), which exhibit any potential to support life as we know it. Earth, Mars, Jupiter, and Neptune are shown for scale on the right. Image Credit: Planetary Habitability Laboratory (PHL)/University of Puerto Rico @Arecibo

Another important factor for habitability, besides an exoplanet’s location in respect to its star’s habitable zone, is its size. Astronomers theorise that exoplanets with a radius up to 1.6 times that of our home planet have most probably retained a rocky composition, which renders them potentially habitable. If on the other hand they are bigger than this upper limit, they most probably belong in the mini-Neptune class of gaseous alien worlds. The main reason for classifying the exoplanets of Kepler-296 as potentially habitable in the first place was due to the fact that previous studies had estimated that their sizes were slightly larger than Earth’s. Yet the recent study by Cartier’s team, aided by Hubble’s high-resolution imaging, have readjusted upward the sizes of the Kepler-296 exoplanets, as well as those of the other two systems, KOI-2626 and KOI-3049. “Habitable planets in the canonical sense must not only have the capability for liquid water on the surface, but also have a solid surface on which that water can exist,” write the researchers. “In short, the planets must be rocky and not gaseous. Using radial velocity measurements coupled with Doppler spectroscopy, high-resolution imaging, and asteroseismology, [previous studies] measured the radii and masses for 65 planet candidates and concluded that only planets with radii less than 1.5 Earth radii are compatible with purely rocky compositions. Planets larger than that must have a larger fraction of low-density material, e.g. hydrogen, helium and water. Our updated planetary radii indicate that none of our potentially habitable planets (Kepler-296Af, Kepler-296Bf, Kepler-296Be, KOI-2626 A.01, KOI-2626 B.01, and KOI-2626 C.01) are small enough to have purely rocky compositions and thus are not habitable in the canonical sense.”

Even though these findings may seem disappointing, Cartier’s team acknowledges the fact that their results are the product of a limited data set and that future observations could help further reassess the habitability in the stellar systems examined, as well as in other similar ones in the galaxy. “We cannot exclude the possibility of a very massive yet rocky planet like Kepler-10c as we lack radial velocity measurements needed to calculate the planetary masses and densities directly,” conclude the researchers. “Even if Kepler-296Af, Kepler-296Bf, Kepler-296Be, KOI-2626 A.01, KOI-2626 B.01, and KOI-2626 C.01 remain too large to be rocky, the possibility of habitable exomoons would remain.”

Exoplanetary research is an ever-changing and dynamic field of study, one where past assumptions are replaced with new ones as more detailed data become available with time. It’s worthy to keep in mind that our understanding of exoplanetary formation and evolution advances with each new finding, as the last 20 years of exoplanetary discovery have demonstrated. Since the time of the first discoveries of giant gaseous hot-Jupiters, we have progressed to the point where we can detect and characterise terrestrial alien worlds, some of which have been found to be even smaller than Earth. In similar fashion, our continuing search of distant worlds around other suns will eventually allow us to make the long-awaited first discoveries of far-away, alien pale blue dots, somewhere inside the vast expanses of the Milky Way.

 

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39 comments to New Study Reassesses Habitability of Exoplanets Around Multiple Star Systems

  • Excellent article, Leonidas! For quite some time now I have voiced my concerns in print and on-line about the potential habitability of a lot of extrasolar planets being overstated in the media. This has especially been the case with planets with radii greater than 1.5 times that of Earth which, as we have learned over the last year or so, are most likely mini-Neptunes. While there may be only a few truly potentially habitable planets currently known (e.g. Kepler 62f, Kepler 186f and Kepler 442b), thankfully there are many more candidates in the pipeline including Earth-size planets in Earth-like orbits around Sun-like stars. The coming years promise to be exciting ones for the study of Earth-like extrasolar planets.

  • I would guess–I am certainly no expert–that the larger the radius the greater the gravitational force. So unless human explorers are hitting the weights like maniacs on the way to one of these larger-than-earth planets, wouldn’t they have trouble living there? I wonder how adaptable humans are to such an environment? A lot of unk-unk’s.

    • Jim, you need to be careful about the interpretation of the term “habitable planet”. The scientific definition of a habitable planet has historically been a world whose surface conditions can support liquid water and possibly life. Surface gravity or the planet’s ability to support human life or any lifeforms from Earth are not part of the definition. In fact, because of the carbonate-silicate cycle that is expected to act as a global thermostat to maintain a world’s temperatures in the habitable range, the vast majority of “habitable planets” will have atmospheric CO2 levels that would be lethal to humans and any other animal life from Earth. In addition, given that there are many forms of autotrophism including types of photosynthesis that do not produce O2, it is highly unlikely that “habitable planets” as they are defined by science will have sufficient O2 to support humans either. This would not be a problem for any native lifeforms whose biochemistry evolved in these high-CO2/low-O2 environments which would thrive. In fact, the archetypical “habitable planet”, Earth, would have been incapable of supporting human life for the first three or so billion years of its history even though life thrived here.

      In the end, only a very tiny fraction of “habitable planets” would be expected to have atmospheric conditions that could support humans.

    • Gary Church

      “-the larger the radius the greater the gravitational force.”

      Much larger icy moons in our solar system have less gravity than our rocky Moon. Depends on mass, not size.

      “I wonder how adaptable humans are to such an environment?”

      We evolved in Earth gravity and may acclimate to small differences but Gerard K. O’Neill came to the conclusion that larger differences were irreconcilable and artificial mega-structures are the answer to colonizing the solar system (not Mars). And also the logical precursor to any kind of generation ship that would voyage for centuries to other stars at some percentage of the speed of light.

      In regards to living on the low gravity of our Moon, the idea of “sleeper trains”, circular underground habitats providing one gravity for half the day- and the other half spent working in lunar factories, has been proposed. This is not really practical for large populations but for a lunar factory workforce might be necessary.

      • Gary Church

        I personally do not believe in any miracle solutions for star travel- no warp or hyper drives, no stargates. But I also do not believe the absence of these violations of the laws of physics is any problem either- we can still do star travel. H-bombs work quite well and are certainly powerful enough to shove supertanker size masses a step at a time to some percentage of the speed of light. Star travel will consist of extremely long voyages and the critical technology is freezing people. Though it does not appear to be that difficult a technical challenge, especially compared to faster-than-light travel, if for some reason freezing does not work then that leaves the “generation ship.” The third middle of the road option is artificial wombs utilizing frozen sperm and ovum and presumably robot parents. For now mega-projects like space exploration and colonization seem unlikely but that could change very quickly.

        I would add that finding “habitable” planets may not be as important as finding systems with a large asteroid and comet population with which to construct artificial hollow moons (Bernal Spheres). When the catastrophes that are prevalent on our world are surveyed there is not much doubt that any world close to Earth-like will have the same earthquakes, volcanoes, and asteroid and comet impacts. In addition if it is a “young” world and requires life being introduced there will be problems and if it is already a living world the indigenous organisms, including pathogens, may pose insurmountable obstacles to colonization.

      • “In regards to living on the low gravity of our Moon, the idea of “sleeper trains”, circular underground habitats providing one gravity for half the day- and the other half spent working in lunar factories, has been proposed.”

        Hmmmm… I had never heard of this idea before but it makes sense. It could also be used by workers on other low-gravity yet resource-rich locales like Mars where it would be even easier to implement since the difference between Earth and Mars surface gravity is even less.

        • guest

          Actually the lower the gravity, the easier it is to build and operate such circular centrifugal habitats. So Mars would not be easier to implement. The main problem with Mars is a lack of solar energy. The Moon has long nights but mirror-sats, Ehricke had a term for them but I forgot- and a variety of solutions are available- and the resources on the Moon in the form of silicon, titanium, water ice, and the volatiles trapped in that ice are only a couple days away- not years.

          • guest

            Yes, this is Gary, but I am in an airport and using a free computer.

          • Hmmmm… I’ll have to give some more thought to the physics involved but since you have read the studies on this interesting concept and I have not, I have to trust you on this. As far as the argument about the power source, the amount of solar energy incident on the surface of Mars is still on the order of half that of the Moon (not that big of a difference and certainly better than any target in the asteroid belt or beyond) and that is not a show stopper by any means. In addition, there is nothing that says this concept could not employ a different energy source such as nuclear.

            • Gary Church

              Too much gravity to land on easily, not enough to stay healthy, not enough solar, too far away…..All showstoppers for colonization or industrial projects.

              • Well, if Mars does not have enough solar energy for colonization and industrial projects (and the point of whether Mars’ solar flux around half that of the Moon is enough is arguable), then by you standards all plans for colonization and industrial projects in the asteroid belt and beyond are impossible is well. Personally, I don’t like this rather arbitrary limitation that only solar power can be used in space-related projects. What’s wrong with using nuclear power sources on Mars and beyond?

                • Gary Church

                  Colonization in my view is the large-scale migration of millions of people into space. As I have stated Andrew, I consider O’Neill’s conclusion that natural bodies are unsuitable for colonization and that leaves mega-structures.

                  Such structures will have to be fabricated from lunar materials using huge amounts of energy to refine and create the alloy stocks in the millions of tons necessary. In my view the decades of commercial air travel type transportation of millions of colonists from Earth to cislunar space will not use conventional rockets- they will use single stage to orbit beam propelled craft as envisioned by NASA researcher Kevin Parkin. This will probably require solar power satellites beaming energy down to ascending transports as a “second stage.” The materials infrastructure to refine silicon and titanium lunar resources will certainly require massive solar energy. Such a public works project is not going to happen anywhere except cislunar space.

                  As you can see in this form of colonization and the space-related projects supporting it the need for the more easily accessible lunar resources and solar energy on a massive scale is not arbitrary.

                  • I’ll ask again: What’s wrong with using nuclear power sources for industrial projects on Mars and beyond?

                    • Gary Church

                      Ever been to a nuclear power plant Andrew?

                    • Funny, for a person who has no problem with nuclear-pulse propulsion technology and the huge amounts of radiation it would generate, you apparently have some unspoken issue with nuclear power? How do you expect to power those missions to Ceres, the icy moons of the outer solar system and the stars beyond that you talk about without nuclear power once the nuclear-pulse engines shut down? Seems that beyond building cities out of lunar materials, your vision of our future in space is shortsighted in addition to being arbitrarily narrow.

                      And to answer your question, yes, I’ve been to a nuclear power plant and various types of nuclear research facilities including working at facilities with particle accelerators.

                    • Gary Church

                      “-your vision of our future in space is shortsighted in addition to being arbitrarily narrow.”

                      As usual, you are turning this into another toxic debate filled with veiled insults and demeaning language. And any reply in kind I make will be deleted. So what’s the point?

                    • You are correct… I am taking this debate too far to be productive and I do apologize.

              • “Too much gravity to land on easily…”

                I’ve seen you use this phrase a couple of times before and I find that while it is catchy, it is not factual nor even logical. If Mars has too much gravity to land on easily, then Earth with its higher gravity would be even more difficult to land on. Considering that flying craft of all sorts land on the surface of Earth every day without an unduly huge accident rate,then logically it is a false statement.

                Venus also has a surface gravity than Mars and every spacecraft that survived launch and the cruise to Venus to attempt a landing were successful except for three – Venera 4, 5, and 6. They were crushed during descent because the Venusian atmosphere proved denser than expected. Gravity played no direct role in these failures.

                Also, there is no factual basis of for this statement when it comes to Mars missions. There have been a dozen attempts to actually land on Mars (not counting craft that succumb to launch accidents and survived to actually attempt a landing). Of those 12, 7 landed successfully and 5 did not. Of those 5 failures: Mars 2 was lost due to a navigation failure, Mars 7 suffered a control system failure, Mars Polar Lander had a hardware/software design flaw and the causes of the failures of Mars 5 and Beagle were never determined. None of the failures were related to Mars having a higher surface gravity than the Moon (which also experienced its share of landing attempt failures having noting directly to do with gravity).

                • Gary Church

                  “-no factual basis of for this statement-

                  It is factual as well as catchy. We have a thick atmosphere to aerobrake and fly airplanes in for returning to and landing on Earth. Mars has a tenuous atmosphere that does not facilitate easy aerobraking or aerodynamic landings making it a far more difficult and less effective option than it is for Earth. Venus? Nobody is ever going to live in a Venusian cloud city. That is just click-bait.

                  An extremely poor argument to use for supporting Mars development.

                  • Who said I was supporting any near term development of Mars? Or that I like the idea of Venusian cloud cities? These diversions have nothing to do with your oft repeated phrase “Too much gravity to land on easily…” having no basis in fact. Even you admit it since you now claim that the atmosphere plays a role as well… fair enough. But then again, the Moon has no atmosphere which makes landing there more difficult: no atmosphere, no aerobraking or parachutes with a resulting increase in mass for an all-propulsive landing. And of course you have failed to address the fact that none of the Mars landing failures had anything directly to do with Mars having “Too much gravity to land on easily”.

                    • Gary Church

                      Mars 0.376 g

                      Moon 0.165 g

                    • Mars – delta-v from atmospheric braking: up to 6,000 m/s or more

                      Moon – delta-v from atmospheric braking: 0 m/s

                      In my book, the greater gravity of Mars is more than made up for by its atmosphere to help slow down an approaching spacecraft. And even you admit that Earth’s atmosphere helps in landings despite having six-times the gravity of the Moon (thus easily discrediting your oversimplified claim of “Too much gravity to land on easily”).

                      Thanks for helping to prove my point!

                    • Gary Church

                      Funny, seems that you have some short-sighted and arbitrarily narrow vision concerning building nuclear power plants on Mars. The huge amounts of water necessary for coolant, steam turbines, and very large facilities required, even without the massive safety containment domes and other massive structures, mean that thousands of tons of construction equipment and workers would have to be transported, landed, fed, sheltered, etc. etc.

                      “-the greater gravity of Mars is more than made up for by its atmosphere to help slow down an approaching spacecraft.”

                      You wish. It is not “more than made up for.” It is a far more complicated maneuver than a powered descent and adds weight and risk to the lander. The speed of a lunar lander after gradually slowing (the Earth’s gravity slows it down) while approaching the Moon and the speed of a Mars craft are….not really comparable- yet you are comparing them to try and make a point? There is no free lunch.

                    • Like so much of your practical information on space systems, your working knowledge about space-based nuclear power systems is just plain wrong. Not all nuclear reactors use water as coolant or as the working medium for energy conversion. Nuclear reactors made for space applications have used a liquid lithium-based or sodium-potassium alloys to cool their reactors and transfer the heat to solid state thermoelectric converters (similar in principal to those used in RTGs but many time more efficient because of the higher temperatures) or high efficiency converters like stirling engines (more efficient especially for higher power levels but potentially less reliable TE converters). The US flew a SNAP 10A reactor in 1965 in a test flight of this hardware in 1965. The whole unit had a mass of 435 kg and generated over 0.5 kW of power. The old USSR flew dozens of RORSATS with operational nuclear reactors culminating in the Topaz II which weighed in at 1,100 kg to produce 5 kW of electrical power and were rated for up to five years of operation using 12 kg of fuel. Off and on for decades the US has examined and done component-level development work on space-based nuclear reactors capable of producing megawatts of electrical power from packages with masses of a few metric tons.

                      If you actually bothered to study nuclear power technology for space applications you would know that there is no need for huge amounts of water to use as a coolant, no steam turbines, and no need for very large facilities, no massive safety containment domes and no other massive structures, no need for thousands of tons of construction equipment and no workers would have to be transported, landed, fed, sheltered, etc. The requirements for Earth-based commercial power generation simply do not necessarily apply to a space-based application.

                      And you still haven’t answered my question: How do you expect to power those missions to Ceres, the icy moons of the outer solar system (basically, anywhere else except Mars) and the stars beyond that you talk about so often without nuclear power once the nuclear-pulse engines shut down? You have discounted nuclear power even though it is obvious you have absolutely no working knowledge of space-based nuclear power systems technology.

                    • Repeating what I said above, I am taking this debate too far to be productive and I do apologize.

            • Joe

              There is another point to be considered.

              Most of what is known about the human interface to rotation rates/radius for using centrifugal forces to simulate gravity comes from testing done in the 1960’s. The test set up was similar to what is proposed for these “trains”. It was predicted, based on that testing, that the maximum rotation rate tolerable (without causing motion sickness problems) was two RPM.

              Several years ago I was sent to a human factors conference where Dr. Joseph P. Kerwin was also in attendance. I was able to have an extended discussion with him on this subject. He said that his and subsequent Skylab Crews performed experiments that convinced him that, once adapted to micro-gravity, the crews became “lead heads” much less susceptible to motion sickness. He felt that rotation rates as high as five RPM would be acceptable (and that limit was due to other factors). That would (in microgravity) reduce the needed spin radius for any given simulated gravity by a factor of 6.25.

              No way of knowing how this would play out in various G levels between 0 and 1, but it is not unreasonable to speculate that the lower the surface gravity the higher the allowable spin rate and the lower the required spin radius for any desired simulated gravity.

              • Gary Church

                It is very seldom mentioned that Gemini 11 tested tether-generated artificial gravity in 1966. It only generated a tiny fraction of a “G” but….it did work.

                For small space craft the tether system is by far the best solution. For dampening out oscillation there is the option of more than one tether (a “system”) with small loads moving along the length of these cables. Zubrin mentioned that zero gravity debilitation is really not that difficult a problem to solve using a tether many years ago in his writings.

                It speaks volumes that even the mass penalty for a couple cables (and the extra structural strength of the structures under load) is not entertained in any of the present proposals; except for that ridiculous centrifuge depicted in the powerpoint NautilusX that keeps making the rounds. What this indicates is one of two dirty little secrets that NASA and NewSpace advocates will just not discuss- that first inconvenient truth being that chemical propulsion is essentially useless for interplanetary travel.

                The second elephant in the room is the mass of cosmic radiation shielding required that makes the penalty for a tether system trivial- and makes even nuclear thermal rockets nowhere near powerful and efficient enough. All that is left is nuclear pulse (bombs). I find it interesting the idea of bomb propulsion has been ruthlessly mocked and ridiculed in the past but suddenly nobody is making jokes about it anymore. The closer SLS gets the clearer the the grim realities of space travel become.

                For a large torus like a von Braun wheel the water-shield massing thousands of tons will dampen out any oscillation. Such wheels would have such a large radius they would be formed more like hula-hoops than the classic doughnut depictions- probably looking distinctly frisbee-like with inside of the hoop covered by a load-bearing skin supporting solar panels, heat exchangers, antenna, cargo areas- etc.

              • Joe, I seem to recall that there had been plans to include a variable-G lab module on the ISS at some point. Too bad it never materialized. The one thing that the ISS is ideal for is investigations of the effects of extended spaceflight on human crees.

                • Joe

                  Yes, in the original plans there was a Centrifuge Facility Module.

                  It would not have rotated itself, but would have contained a centrifuge internal to the module.

                  I do no remember the exact diameter, but it was close to the 15 foot diameter of the module.

                  It was to be used for testing various G levels and rotation rates on both plants and animals.

                  It would have been limited, but on the other hand it would have been infinitely more than we have now.

                  It got canceled due to lack of funds, though there were (perhaps paranoid) rumors among the working troops that the Clinton Administration opposed any research that could be seen as supporting Human Deep Space Missions.

      • john hare

        I vaguely remember a trains article with it being mentioned that they were activity trains. Sleep was more comfortable in stationary low gee, since bed rest is sometimes used as an analog for zero gee medical research. Dining, showers, gyms and such that required effort were on the trains. Low gee shifts were considered comparable to one gee office workers. It seems that trains capable of handling 20% or so of the population at a time was considered sufficient.

        Vague memory and no references, so a large grain of salt.

        • Joe

          The basic Sleeper Train concept was proposed by Krafft Ehricke for use on the Moon.

          You can read more about it (and a lot of other interesting things) if you buy this book:

          http://www.amazon.com/Krafft-Ehrickes-Extraterrestrial-Imperative-Apogee/dp/1894959914

          Note: I will not make a penny off you buying it.

          • john hare

            I’m cheap, possibly as a side effect of being self employed. Did he mention the activity train or was that a vague memory of some other reference on my part? I’m guessing the second as I don’t remember reading his book.

            • Joe

              I just moved recently, so it is still boxed up.

              But as I remember it was really sort of both. There would be a minimum amount of time that workers would spend on the train in off hours to stay sufficiently conditioned for eventual Earth return.

          • Gary Church

            I actually came up with it on my own- as almost anyone would who had seen or read about pilot centrifuges or rode amusement park rides and thought about the problem long enough. That Ehricke and others also came up with it goes all the way back to the late 1930’s when much of the original theorizing took place. It was not known if consciousness could be maintained for any length of time so many science fiction stories featured tether or torus generated artificial gravity even at this early date.

            It is just basic engineering the less gravity the easier to spin a torus.

            http://scifi2feature.tumblr.com/post/91279472270/astounding-stories-february-1937-at-the

  • ken anthony

    One thing we may not get any data on for a while is habitable moons of exoplanets.

    Otherwise, you also have other parameters besides as in the rare earth hypothesis.

    The Fermi paradox is a real problem that can’t just be brushed away because it’s based on sublight travel. Even another single intelligent species in our galaxy should be here unless we’re the first which is supposed to be highly unlikely.

    Somebody’s got some ‘splaining to do? It is great to live at a time when we’re finding thousands of planets.

    • Gary Church

      The most likely solution to the Fermi Paradox is a succession of natural filters- such as a lack of gas giants to soak up impacts that would normally keep sterilizing a planet like ours, or the absence of a magnetic field for radiation protection, and several other factors having to do with amount of gravity and the elements common on Earth- all resulting in intelligent life never evolving. The very few salmon swimming upstream that make it as far as we do may inevitably self-destruct when they develop a sufficient level of technology. Intelligent life may just be too stupid to survive. My best bet for extinction is an engineered pathogen escaping from a not-too-sophisticated lab in some backwater.

      I really get a kick out of how far the obnoxious naysayers go in denying that anything could possibly kill off the human race. We are gods in their estimation. I hold the opposite view. Which is why I consider space exploration to be a critical security issue. Unfortunately the DOD spends a mind-boggling amount of money on everything but insurance policies like off-world survival colonies. Thus, my statement about being too stupid to survive.

  • Future generations will look back at our present exploration of exoplanets and call it the “golden age” where mankind began the intelligent analysis of data from the then state-of-the-art technologies. But I still maintain that it will be microbial life that will be found long before any evidence of “intelligent” life is discoveted. The exoplanets to date represent an infinitely tiny fraction of what’s out there and, as Leonidas states on the concluding paragraph, our continuing search WILL allow us to find those habitable planets. It’s only a matter of time and technology.

    • Tracy the Troll

      Tom,

      “Future generations will look back at our present exploration of exoplanets and call it the “golden age” where mankind began the intelligent analysis of data from the then state-of-the-art technologies”

      Now if we could just use the same type of analysis on Earth than perhaps there might be species out there that thinks Human Beings are Intelligent Life…At least we can hope.