by Paul Gilster | Jan 14, 2013 | Advanced Rocketry |
Tim Folger and Les Johnson (NASA MSFC) stood last summer in front of a nuclear rocket at Marshall Space Flight Center in Huntsville, Alabama. Johnson’s work in advanced propulsion concepts is well known to Centauri Dreams readers, but what he was talking to Folger about in an article for National Geographic was an older technology. NERVA, once conceived as part of the propulsion package that would send astronauts to Mars, had in its day the mantle of the next logical step beyond chemical propulsion. A snip from the story:
Johnson looks wistfully at the 40,000-pound engine in front of us… “If we’re going to send people to Mars, this should be considered again,” Johnson says. “You would only need half the propellant of a conventional rocket.” NASA is now designing a conventional rocket to replace the Saturn V, which was retired in 1973, not long after the last manned moon landing. It hasn’t decided where the new rocket will go. The NERVA project ended in 1973 too, without a flight test. Since then, during the space shuttle era, humans haven’t ventured more than 400 miles from Earth.
I’m looking forward to getting back to Huntsville and seeing Les, as well as a number of other friends in the interstellar community, at the 2nd Tennessee Valley Interstellar Workshop, coming up this February, where it may be that NERVA will have a place in the discussion of how we go about building a system-spanning civilization. You’ll want to give Folger’s article a look for comments not only from Les but Freeman Dyson and Andreas Tziolas (from the Icarus team), as well as Elon Musk, the 100 Year Starship’s Mae Jemison, and NASA’s Mason Peck.
Image: NERVA nuclear rocket being tested. (Smithsonian Institution Photo No. 75-13750).
In fact, there are a number of issues presented here that I’ll want to get back to later, but I can’t cover the rest of the story today. I’m all but out the door for a brief but intense period of Tau Zero work that will leave me no time to keep up regular posts here or even to moderate comments. More about this later, and more about Folger’s essay as well, and please bear with me through the temporary slowdown. Things should get back to normal by mid-day Thursday.
Speaking of NERVA, though, I’ll leave you with an interesting petition Gregory Benford alerted me to with regard to the development of nuclear thermal rockets, one that calls for an effort to:
Harness the full intellectual and industrial strength of our universities, national laboratories and private enterprise to rapidly develop and deploy a nuclear thermal rocket (NTR) adaptable to both manned and un-manned space missions. A NTR (which would only operate in outer space) will jump-start our manned space exploration program by reducing inner solar system flight times from months to weeks. This is not new technology; NTRs were tested in the 1960s (President Kennedy was a guest at one test). The physics and engineering are sound. In addition to inspiring young Americans to careers in science, technology, engineering and mathematics, a working NTR will herald a speedy and economical expansion of the human presence in the cosmos.
Going significantly beyond the Moon demands advances in propulsion of the kind that nuclear thermal rockets can deliver. Getting NERVA concepts out of mothballs and updating them with modern materials are necessary steps as we push out into the Solar System.
by Paul Gilster | Jan 11, 2013 | Exoplanetary Science |
Yesterday’s post on exomoons and their possibilities as abodes for life leads naturally to new work from René Heller (Leibniz Institute for Astrophysics, Potsdam) and Rory Barnes (University of Washington). We’re finding planets much larger and more massive than Earth in the habitable zone, as the recent findings of the Planet Hunters project attest. What can we say about the habitability of any large moons these planets may have? In their paper, Heller and Barnes look at the issues that separate exomoon habitability from habitability on an exoplanet itself.
If Earth-sized satellites of giant planets exist, they may have certain advantages over terrestrial planets in the same orbit, depending on the host star. We know that M-class dwarfs are by far the most common kind of star in the galaxy, and that habitable zone planets around one of these will probably be tidally locked, with one hemisphere permanently facing the star and the other in permanent darkness. Extreme weather conditions would result, creating severe limitations on the size of any habitable regions. But an Earth-mass exomoon around a gas giant will be locked not to the star but to the planet, a configuration that could stabilize the climate and prevent the dark side atmosphere from freezing out and the bright side atmosphere from evaporating.
Image: Artist’s conception of two extrasolar moons orbiting a giant gaseous planet. (Credits: R. Heller, AIP)
The stability of the atmosphere remains an issue, though, dependent on the size of the moon, the type of atmosphere and the intensity of incoming radiation. The researchers believe a nitrogen-dominated atmosphere would be stripped away by ionizing extreme ultraviolet radiation (EUV) in some habitable zone situations. This is interesting so let me quote the paper:
If Titan would be moved from its roughly 10AU orbit around the Sun to a distance of 1AU (AU being an astronomical unit, i.e. the average distance between the Sun and the Earth), then it would receive about 100 times more EUV radiation, leading to a rapid loss of its atmosphere due to the moon’s smaller mass, compared to the Earth. For an Earth-mass moon at 1AU from the Sun, EUV radiation would need to be less than seven times the Sun’s present-day EUV emission to allow for a long-term stability of a nitrogen atmosphere. CO2 provides substantial cooling of an atmosphere by infrared radiation, thereby counteracting thermal expansion and protecting an atmosphere’s nitrogen inventory.
For that matter, how massive does a moon need to be to create the magnetic shield that can sustain a long-lived atmosphere, and to drive tectonic activity and support a carbon-silicate cycle? The authors peg the figure at somewhere above 0.25 Earth masses, a figure that can be adjusted depending on the moon’s composition and structure. As I mentioned yesterday, the ongoing Hunt for Exomoons with Kepler (HEK) project pegs 0.2 Earth masses as the minimum size for an exomoon detection using current technology.
How large exomoons form is a question we’ve been kicking around here in the comments. If a gas giant is assumed to have formed well beyond the snow line and then migrated to its position in the habitable zone, would it bring with it a massive enough moon to sustain habitability? One recent paper (Canup and Ward, 2006) has shown that moons formed in the circum-planetary disk of a gas giant will have masses in the range of 10-4 times that of the planet’s mass. The paper runs through formation and capture scenarios and notes the work of Takanori Sasaki (Tokyo Institute of Technology), who suggests the formation of Earth-mass moons is indeed possible. We’re left with evolving ideas in what is clearly an active area of research.
Tidal heating is also a key factor in exomoon habitability, capable of causing intense magmatism and resurfacing on the moon’s surface in close orbits and enough volcanic activity to render the moon uninhabitable. But Heller and Barnes see scenarios where a moon becomes habitable only because of tidal heating, such as when the host planet orbits outside the outer edge of the habitable zone. For that matter, tidal heating can drive plate tectonics. Here the authors point to Europa, where tidal effect provides the heat to sustain a subsurface ocean of liquid water.
A theoretical model emerges, estimating the minimum distance a moon can be from its host planet and still allow habitability. The authors call the inner part of the circum-planetary habitable zone the ‘habitable edge.’ Moons inside the edge can run into runaway greenhouse effects because of tidal heating, while those outside the habitable edge are habitable by definition:
Similar to the circumstellar habitable zone of extrasolar planets… we conclude that more massive exomoons may have somewhat wider habitable zones around their host planets – of which the inner boundary is defined by the habitable edge and the outer boundary by Hill stability – than do less massive satellites. In future investigations it will be necessary to include simulations of the moons’ putative atmospheres and their responses to irradiation and tidal heating. Thus, our irradiation plus tidal heating model should be coupled to an energy balance or global climate model to allow for more realistic descriptions of exomoon habitability.
The minimum distance model for a planet to orbit and still be habitable should come in handy if we do find some candidate moons through the HEK project. “This concept will allow future astronomers to evaluate the habitability of extrasolar moons. There is a habitable zone for exomoons, it’s just a little different than the habitable zone for exoplanets,” Barnes said. But getting spectroscopic biosignatures in the atmospheres of any detected exomoons awaits next-generation space telescopes, so for now we’re left to use orbital configurations and studies of exomoon composition to assess potential habitability. One upcoming positive is the 2022 launch of ESA’s Jupiter Icy Moons Explorer mission, which should offer insights into the massive Galilean moons from tidal effects to surface chemistry and useful data on their interiors.
The paper is Heller and Barnes, “Exomoon habitability constrained by illumination and tidal heating,” accepted by Astrobiology (preprint). A news release from the Leibniz Institute for Astrophysics is available.
by Paul Gilster | Jan 10, 2013 | Exoplanetary Science |
Because the sky is full of surprises, we can’t afford to be too doctrinaire about what tomorrow’s discovery might be. After all, ‘hot Jupiters’ were considered wildly unlikely by all but a few, and even here in the Solar System, probes like our Voyagers have turned up one startling thing after another — volcanoes on Io were predicted just before Voyager arrived, but who thought we’d actually see them in the act of erupting? So I don’t think we can rule out the idea of habitable moons around a gas giant in the habitable zone, but there are reasons to question how numerous they would be.
We’ve had this discussion before on Centauri Dreams, and while I love the idea of a huge ‘Jupiter’ hanging in the sky of a verdant, life-bearing planet, there are some factors that argue against it, as reader FrankH pointed out recently. One problem is that moons around a gas giant will probably be made largely of ice and rock, because the planet itself would have formed beyond the snow line and migrated into the habitable zone. A Mars-sized moon is going to melt and, given its low escape velocity, will gradually lose its atmosphere in these warmer regions.
We could imagine capture scenarios as a migrating gas giant moves into the warm inner system, but it’s hard to see that as a frequent occurrence. The key question for me would be what factors govern the formation of gas giant moons in the first place, and what is the likelihood of finding moons much larger than Mars? David Kipping’s continuing exomoon work has suggested we could detect a moon of approximately 0.2 Earth masses with existing technology, but this is far larger than Ganymede, and we have no analog in our own Solar System.
Image: An artist’s rendition of a sunset view from the perspective of an imagined Earth-like moon orbiting the giant planet, PH2 b. The scene is spectacular, but how likely is it that gas giants would have moons beyond Mars size? The answer to the question awaits further work in exomoon detection. Credit: H. Giguere, M. Giguere/Yale University.
So we continue the hunt and the speculation. All this comes to mind because of the discovery of PH2 b-a, the second planet to be confirmed by the Planet Hunters project. The volunteers of Planet Hunters come from all walks of life and use their computers to analyze public domain data from the Kepler mission. The idea behind the project was that humans, with their unique gift of pattern recognition, might see things in light curves that Kepler’s algorithms had missed. Debra Fischer (Yale University), lead scientist for Planet Hunters, champions the idea, particularly in light of the most recent findings, which include a number of other candidates:
“We are seeing the emergence of a new era in the Planet Hunters project where our volunteers seem to be at least as efficient as the computer algorithms at finding planets orbiting at habitable zone distances from the host stars. Now, the hunt is not just targeting any old exoplanet – volunteers are homing in on habitable worlds.”
The estimated surface temperature on PH2 b-a is 46 degrees Celsius, so we are indeed in the habitable zone but without any evidence of moons circling the gas giant. Work with the HIRES spectrograph and NIRC2 adaptive optics system on the Keck telescopes on Mauna Kea has confirmed the existence of the planet, which had been detected by volunteers examining Kepler lightcurves. All told, Planet Hunters identifies in the paper on this work 43 new discoveries, most of which have orbital periods above 100 days. The findings increase the number of gas giant planet candidates with orbital periods over 100 days and radii between Neptune and Jupiter by thirty percent. Nine candidates are members of multi-planet systems. And note this:
Among these new candidates, twenty appear to orbit at distances where the temperature at the top of the atmosphere would be consistent with temperatures in habitable zones. Most of these habitable zone planet candidates have radii comparable to or larger than Neptune; however, one candidate (KIC 4947556) has a radius of 2.60±0.08 R? and may be a SuperEarth or mini-Neptune.
Whether or not the imagined habitable moons exist, this is outstanding work and a tribute to the power of ‘citizen science’ in identifying candidates that the Kepler automatic detection and validation pipeline overlooked. The paper on the latest detections runs through the project’s previous candidates and the confirmed planet PH1 b, an interesting world in a 137-day circumbinary orbit around an eclipsing binary in a quadruple star system. The paper adds that Planet Hunters volunteers are most effective at detecting transit candidates with radii larger than 4 Earth radii, while smaller worlds are best retrieved through mathematical algorithms.
For more, see Ji Wang et al., “Planet Hunters. V. A Confirmed Jupiter-Size Planet in the Habitable Zone and 42 Planet Candidates from the Kepler Archive Data,” submitted to The Astrophysical Journal (preprint).
by Paul Gilster | Jan 9, 2013 | Deep Sky Astronomy & Telescopes |
The American Astronomical Society’s meeting in Long Beach is going to occupy us for several days, and not always with exoplanet news. Brown dwarfs, those other recent entrants into the gallery of research targets, continue to make waves as we learn more about their nature and distribution. The hope of finding a brown dwarf closer than Alpha Centauri has faded and recent work has emphasized that there may be fewer of these objects than thought — WISE data point to one brown dwarf for every six stars. But habitable planets around brown dwarfs are not inconceivable, and in any case we are continuing to build the census of nearby objects.
The latest from AAS offers up what could be considered a probe of brown dwarf ‘weather.’ If the idea of weather on a star seems odd, consider that the cooler brown dwarfs are far closer to gas giants than stars, unable to trigger hydrogen fusion and gradually cooling as they age. That means cloud patterns form and huge storms plow through the various atmospheric layers. At AAS, Daniel Apai (University of Arizona) presented the results of work on the brown dwarf 2MASSJ22282889-431026, which he conducted with a team led by the university’s Esther Buenzli. The results are useful not just for brown dwarf study but planetary atmospheres as well.
Using the Hubble and Spitzer space telescopes simultaneously, the researchers found that every ninety minutes the light from the star varied as it rotated. Because they were looking at the object at different wavelengths, they were able to see that the timing of the brightness change depended on wavelength. Some infrared wavelengths emerge from deep within the star, while others are blocked by water vapor and methane at higher altitudes. What we’re getting, in other words, is a look at layers of material being carried around the brown dwarf in likely storms.
But these aren’t your usual clouds, according to Mark Marley (NASA Ames), a co-author on the paper:
“Unlike the water clouds of Earth or the ammonia clouds of Jupiter, clouds on brown dwarfs are composed of hot grains of sand, liquid drops of iron, and other exotic compounds. So this large atmospheric disturbance found by Spitzer and Hubble gives a new meaning to the concept of extreme weather.”
The brown dwarf in question is cool in stellar terms but still hot enough — 600 to 700 degrees Celsius — to produce clouds like those Marley describes. The light variations offer the researchers a chance to understand the brown dwarf’s weather in the vertical dimension. Another scientist involved in the work is Adam Showman, also at the University of Arizona, who notes: “The data suggest regions on the brown dwarf where the weather is cloudy and rich in silicate vapor deep in the atmosphere coincide with balmier, drier conditions at higher altitudes — and vice versa.”
Image: This graph shows the brightness variations of the brown dwarf named 2MASSJ22282889-431026 measured simultaneously by both NASA’s Hubble and Spitzer space telescopes. As the object rotates every 1.4 hours, its emitted light periodically brightens and dims. Surprisingly, the timing, or phase, of the variations in brightness changes when measured at different wavelengths of infrared light. Spitzer and Hubble’s wavelengths probe different layers in the atmosphere of the brown dwarf. The phase shifts indicate complex clouds or weather patterns that change with altitude. Credit: NASA/JPL-Caltech.
A way forward in brown dwarf studies is suggested in the paper:
We have measured longitudinal variability throughout the near-infrared in a T6 dwarf and found an unusual correlation of light curve phase with the pressure probed by a given wavelength, which suggests a complex horizontal and vertical atmospheric structure. Our observations should provide an incentive to drive the development of higher-dimensional atmospheric models in order to gain a deeper understanding of dynamical and radiative processes in brown dwarf and exoplanet atmospheres.
Hubble and Spitzer are thus giving us the ability to probe a brown dwarf atmosphere in ways that, according to Apai, are not dissimilar to how doctors probe the body with medical imaging techniques. The unexpected offset between the different layers of atmospheric material tells us that a feature we might see on the surface of a brown dwarf may have shifted as we push into the inner layers of the object. That’s a sign of wind-driven clouds in constant motion, a warmer version of features like Jupiter’s Great Red Spot on an object not quite planet, not quite star.
The paper is Buenzli et al., “Vertical Atmospheric Structure in a Variable Brown Dwarf: Pressure-dependent Phase Shifts in Simultaneous Hubble Space Telescope-Spitzer Light Curves,” Astrophysical Journal Letters 760 (2012), L31 (preprint).
by Paul Gilster | Jan 8, 2013 | Exoplanetary Science |
Plenty of interesting news is coming out of the American Astronomical Society meeting in Long Beach CA, enough that I’ll want to spread our look at it out over the next few days. I want to start with Geoff Marcy’s investigations with grad student Erik Petigura at UC-Berkeley, the two working in tandem with Andrew Howard (University of Hawaii) on the question of Earth-sized planets and their distribution in the galaxy. But I can’t help noting before I begin how science fictional all these exoplanets are starting to seem as each day brings a new paper or announcement.
For me, science fiction has always been as much about landscape as it is about science, and exoplanets are the ultimate exercise in imagining exotic places. When exoplanet announcements were still new and we had only a small catalog of these worlds, I would find myself pondering each and thinking about what it would be like to orbit one, or stand on it. Now we’re getting hard data on potentially habitable places that evoke the compelling artwork on the covers of SF magazines I’ve read over the years. People gripe about not having flying cars or humans in the outer system, but to me the future I’ve lived into is every bit as provocative as the one that used to be portrayed in the pages of Astounding or Galaxy.
I spent yesterday afternoon thinking about the upcoming presentation of Marcy’s team at the AAS. They’ve created a new structure within which to analyze Kepler photometry for transiting planets, applying their algorithm to a sample of 12,000 G and K-class stars that were chosen because they are among the most photometrically quiet of Kepler’s targets, and thus best suited for the detection of small planets. The work focused on close-in planets with orbits between 5 and 50 days, Earth-sized worlds that are at the margin of detectability within the Kepler data.
The results show that about 17 percent of all Sun-like stars have planets in the 1-2 Earth diameter range orbiting close to their host stars — here we’re talking about distances within about 0.25 AU, which places these worlds well inside the orbit of Mercury. The team also extrapolates from its results that the fraction of stars having planets of Earth size or a bit bigger orbiting in Earth-like orbits may be as high as 50 percent. Both findings follow from the researchers’ analysis of how often planets of a particular size appear, as Andrew Howard notes:
“Our key result is that the frequency of planets increases as you go to smaller sizes, but it doesn’t increase all the way to Earth-size planets — it stays at a constant level below twice the diameter of Earth.”
In other words, there are more small planets than large ones, with perhaps one percent of stars having planets the size of Jupiter, while 10 percent have planets the size of Neptune. But Petigura’s work goes farther than this, suggesting that the increase in planets as size decreases stops when we get down to planets of about twice Earth’s diameter. The numbers then remain the same until we reach planets the size of the Earth, beyond which this analysis ends. That’s a finding that leaves plenty of room for Earth-like worlds in abundance throughout the galaxy.
Petigura’s work was clearly key for the project because it was he who developed TERRA (Transiting ExoEarth Robust Reduction Algorithm), a program through which the UC-Berkeley team fed 12 quarters of Kepler data. Petigura wanted to find out how many Earth-sized planets Kepler was missing, their faint signals lost in the ‘noise’ of a transit lightcurve. Measuring the fraction of planets Kepler wasn’t seeing, the team was able to extract 119 Earth-like worlds ranging from six times Earth’s diameter all the way down to a planet the size of Mars. Thirty-seven of the team’s planets had not been identified in the previous Kepler work.
Image: The fraction of Sun-like stars having planets of different sizes, orbiting within 1/4 of the Earth-Sun distance (0.25 AU) of the host star. The graph shows that planets as small as Earth (far left) are relatively common compared to planets 8.0x the size of Earth (similar to Jupiter). For example, 7.9% of Sun-like stars harbor a planet with a size of 1.0-1.4 times the size of Earth, orbiting inward of 1/4 the Earth-Sun distance (closer than Mercury’s distance from the Sun). There are increasing numbers of planets from 8x the size of Earth down to 2.8x Earth. Remarkably, the number of planets smaller than 2.8x Earth is approximately constant with planet size, down to the size of our Earth. The gray indicates the planets discovered in this study, and the orange represents the correction applied to account for planets the TERRA software would miss statistically, typically about 20%. Credit: Image by Erik Petigura and Geoff Marcy, UC Berkeley, and Andrew Howard, Institute for Astronomy, University of Hawaii.
This study takes us another step along the way to a major exoplanet goal. Getting a sound estimate of the fraction of Sun-like stars bearing Earth-sized planets in the habitable zone — eta Earth — is tantalizingly close, and the paper on this work points out that its 12,000 stars will be among the sample from which this estimate is drawn. The Berkeley team homes in on close-in planets as a way of studying planet frequency in relation to size, the assumption being that estimates of habitable zone ‘Earths’ will tighten as we get further data. From the paper:
Our key result is the plateau of planet occurrence for the size range 1-2.8 RE for planets having periods 5-50 days. With 8 years of total photometry in an extended Kepler mission (compared to 3 years here), the computational machinery of TERRA— including its light curve de-trending, transit search, and completeness calibration—will enable a measurement of [eta Earth] for habitable zone orbits with extended mission photometry.
All in all, the team estimates that uncertainties in detection mean that Kepler misses about one in four big Earth-sized worlds, a figure this work now corrects. Planets larger than Earth in this category may often turn out to be more like Neptune than Earth, with a rocky core surrounded by helium and hydrogen — planets like these may, in close orbits, be water worlds with vast oceans and no exposed land surface. But the UC-Berkeley work gives us plenty of reason to think that rocky worlds like Earth within stellar habitable zones are going to be anything but rare.
