Centauri Dreams
Imagining and Planning Interstellar Exploration
Ion Propulsion for the Nearby Interstellar Medium
When scientists began seriously looking at beaming concepts for interstellar missions, sails were the primary focus. The obvious advantage was that a large sail need carry no propellant. Here I’m thinking about the early work on laser beaming by Robert Forward, and shortly thereafter George Marx. Forward’s first published work on laser sails came during his tenure at Hughes Aircraft Company, having begun as an internal memo within the firm, and later appearing in Missiles and Rockets. Theodore Maiman was working on lasers at Hughes Research Laboratories back then, and the concept of wedding laser beaming with a sail fired Forward’s imagination.
The rest is history, and we’ve looked at many of Forward’s sail concepts over the years on Centauri Dreams. But notice how beaming weaves its way through the scientific literature on interstellar flight, later being applied in situations that wed it with technologies other than sails.
Thus Al Jackson and Daniel Whitmire, who in 1977 considered laser beaming in terms of Robert Bussard’s famous interstellar ramjet concept. A key problem, lighting proton-proton fusion at low speeds during early acceleration, could be solved by beaming energy to the departing craft by laser.
Image: Physicist A. A. Jackson, originator of the laser-powered ramjet concept.
In other words, a laser beam originating in the Solar System powers up reaction mass until the ramjet reaches about 0.14 c. The Bussard craft then switches over to full interstellar mode as it climbs toward relativistic velocities. Jackson and Whitmire would go on in the following year to confront the problem that a ramscoop produced enough drag to nullify the concept. A second design emerged, using a space-based laser to power up a starship that used no ramscoop but carried its own reaction mass onboard.
The beauty of the laser-powered rocket is that it can accelerate into the laser beam as well as away from it, since the beam provides energy but is not used to impart momentum, as in Forward’s thinking about sails. In the paper, huge lasers are involved, up to 10 kilometers in diameter, with a diffraction limited range of 500 AU.
But note this: As far back as 1967, John Bloomer had proposed using an external energy source on a departing spacecraft, but focusing the beam not on a departing fusion rocket but one carrying an electrical propulsion system bound for Alpha Centauri. So we have been considering electric propulsion wed with lasers as far back as the Apollo era.
Now we can swing our focus back around to the paper by Angelo Genovese and Nadim Maraqten that was presented at the recent IAC meeting in Paris. Here we are looking not at full-scale missions to another star, but the necessary precursors that we’ll want to fly in the relatively near-term to explore the interstellar medium just outside the Solar System. The problem is getting there in a reasonable amount of time.
As we saw in the last post, electric propulsion has a rich history, but taking it into deep space involves concepts that are, in comparison with laser sail proposals, largely unexplored. A brief moment of appreciation, though, for the ever prescient Konstantin Tsiolkovsky, who sometimes seems to have pondered almost every notion we discuss here a century ago. Genovese and Maraqten found this quote from 1922:
“We may have a case when, in addition to the energy of ejected material, we also have an influx of energy from the outside. This influx may be supplied from Earth during motion of the craft in the form of radiant energy of some wavelength.”
Tsiolkovsky wouldn’t have known about lasers, of course, but the gist of the case is here. Angelo Genovese took the laser-powered electric propulsion concept to Chattanooga in 2016 when the Interstellar Research Group (then called the Tennessee Valley Interstellar Workshop) met there for a symposium. Out of this talk emerged EPIC, the Electric Propulsion Interstellar Clipper, shown in the image below, which is Figure 9 in the current paper. Here we have a monochromatic PV collector working with incoming laser photons to convert needed electric power for 50,000 s ion thrusters.
Image: An imaginative look at laser electric propulsion for a near-term mission, in this case a journey to the hypothesized Planet 9. Credit: Angelo Genovese/Nembo Buldrini.
Do notice that by ‘interstellar’ we are referring to a mission to the nearby interstellar medium rather than a mission to another star. Stepping stones are important.
Genovese and Maraqten also note John Brophy’s work at NASA’s Innovative Advanced Concepts office that delves into what Brophy considers “A Breakthrough Propulsion Architecture for Interstellar Precursor Missions.” Here Brophy works with a 2-kilometer diameter laser array beaming power across the Solar System to a 110 meter diameter photovoltaic array to feed an ion propulsion system with an ISP of 40,000 seconds. That gets a payload to 550 AU in a scant 13 years, an interesting distance as this is where gravitational lensing gets exploitable. Can we go faster and farther?
Image: John Brophy’s work at NIAC examines laser electric propulsion as a means of moving far beyond the heliosphere, all the way out to where the Sun’s gravitational lens begins to produce useful scientific results. Credit: John Brophy.
An advanced mission to 1000 AU emerged in a study Genovese performed for the Initiative for Interstellar Studies back in 2014. Here the author had considered nuclear methods for powering the craft, with reactor specific mass of 5 kg/kWe. Genovese’s calculations showed that such a craft could reach this distance in 35 years, moving at 150 km/s. This saddles us, of course, with the nuclear reactor needed for power aboard the spacecraft. In the current paper, he and co-author Maraqten ramp up the concept:
The TAU mission could greatly profit from the LEP concept. Instead of a huge nuclear reactor with a mass of 12.5 tons (1-MWe class with a specific mass of 12.5 kg/kWe), we could have a large monochromatic PV collector with 50% efficiency and a specific mass of just 1 kg/kWe… This allows us to use a more advanced ion propulsion system based on 50,000s ion thrusters. The much higher specific impulse allows a substantial reduction in propellant mass from 40 tons to 10 tons, leading to a TAU initial mass of just 23 tons instead of 62 tons. The final burnout speed is 240 km/s (50 AU/yr), 1000 AU are reached in just 25 years (Genovese, 2016 [26]).
In fact, the authors rank electric propulsion possibilities this way:
- Present EP performance involves ISP in the range of 7000 s, which can deliver a fairly near-term 200 AU mission with a cruise time in the range of 25 years.
- Advanced EP concepts with ISP of 28,000 s draw on an onboard nuclear reactor, and produce a mission to 1000 AU with a trip time of 35 years. The authors consider this ‘mid-term development.’
- In terms of long-term possibilities, very advanced EP concepts with ISP of 40,000 s can be powered by a 400 MW space laser array, giving us a 1000 AU mission with a trip time of 25 years.
So here we have a way to cluster technologies in the service of an interstellar precursor mission that operates well within the lifetime of the scientists and engineers who are working on the project. I mention this latter fact because it always comes up in discussions, although I don’t really see why. Many of the team currently working on Breakthrough Starshot, for example, would not see the launch of the first probes toward a target like Proxima Centauri even if the most optimistic scenarios for the project were realized. We don’t do these things for our ourselves. We do them for the future.
The Maraqten & Genovese paper is “Advanced Electric Propulsion Concepts for Fast Missions to the Outer Solar System and Beyond,” 73rd International Astronautical Congress (IAC), Paris, France, 18-22 September 2022 (available here). The laser rocket paper is Jackson and Whitmire, “Laser Powered Interstellar Rocket,” Journal of the British Interplanetary Society, Vol. 31 (1978), pp.335-337. The Bloomer paper is “The Alpha Centauri Probe,” in Proceedings of the 17th International Astronautical Congress (Propulsion and Re-entry), Gordon and Breach. Philadelphia (1967), pp. 225-232.
Ion Propulsion: The Stuhlinger Factor
How helpful can electric propulsion become as we plan missions into the local interstellar medium? We can think about this in terms of the Voyager probes, which remain our only operational craft beyond the heliosphere. Voyager 1 moved beyond the heliopause in 2012, which means 35 years between launch and heliosphere exit. But as Nadim Maraqten (Universität Stuttgart) noted in a presentation at the recent International Astronautical Congress, reaching truly unperturbed interstellar space involves getting to 200 AU. We’d like to move faster than Voyager, but how?
Working with Angelo Genovese (Initiative for Interstellar Studies), Maraqten offers up a useful analysis of electric propulsion, calling it one of the most promising existing propulsion technologies, along with various sail concepts. In fact, the two modes have been coupled in some recent studies, about which more as we proceed. The authors believe that the specific impulse of an EP spacecraft must exceed 5000 seconds to make interstellar precursor missions viable in a timeframe of 25-30 years, acknowledging that this ramps up the power needed to reach the desired delta-v.
Electric propulsion is a method of ionizing a propellant and subsequently accelerating it via electric or magnetic fields or a combination of the two. The promise of these technologies is great, for we can achieve higher exhaust velocities by far with electric methods than through any form of conventional chemical propulsion. We’ve seen that promise fulfilled in missions like DAWN, which in 2015 became the first spacecraft to orbit two destinations beyond Earth, having reached Ceres after previously exploring Vesta. We can use electric methods to reduce propellant mass or achieve, over time, higher velocities. [Addendum: Thanks to several readers who noticed that I had reversed the order of Vesta and Ceres in the DAWN mission above. I’ve fixed the mistake.]
Image: 6 kW Hall thruster in operation at the NASA Jet Propulsion Laboratory
Unlike chemical propulsion, electric concepts have a relatively recent history, having appeared in Robert Goddard’s famous notebooks as early as 1906. In fact, Goddard’s 1917 patent shows us the first example of an electrostatic ion accelerator useful for propulsion, even if he worked at a time when our understanding of ions was incomplete, so that he considered the problem as one of moving electrons instead. Konstantin Tsiolkovsky had also conceived the idea and wrote about it in 1911, this from the man who produced the Tsiolkovsky rocket equation in 1903 (although Robert Goddard would independently derive it in 1912, and so would Hermann Oberth about a decade later).
As Maraqten and Genovese point out, Hermann Oberth wound up devoting an entire chapter (and indeed, the final one) of his 1929 book Wege zur Raumschiffahrt (Ways to Spaceflight) to what he describes as an ‘electric spaceship.’ That caught the attention of Wernher von Braun, and via him Ernst Stuhlinger, who conceived of using these methods rather than chemical propulsion to make von Braun’s idea of an expedition to Mars a reality. It had been von Braun’s idea to use chemical propulsion with a nitric acid/hydrazine propellant, as depicted in a famous series on space exploration that ran in Collier’s from 1952-1954.
But Stuhlinger thought he could bring the mass of the spacecraft down by two-thirds while expelling ions and electrons to achieve far higher exhaust velocity. It was he who introduced the idea of nuclear-electric propulsion, by replacing a power system based on solar energy with a nuclear reactor, thus moving us from SEP (Solar Electric Propulsion) to NEP (Nuclear Electric Propulsion). Let me quote Maraqten and Genovese on this:
Stuhlinger immersed himself in electric propulsion theory, and in 1954 he presented a paper at the 5th International Astronautical Congress in Vienna entitled, “Possibilities of Electrical Space Ship Propulsion”, where he conceived the first Mars expedition using solar-electric propulsion [4]. The spacecraft design he proposed, which he nicknamed the “Sun Ship”, had a cluster of 2000 ion thrusters using caesium or rubidium as propellant. He calculated that the total mass of the “Sun Ship” would be just 280 tons instead of the 820 tons necessary for a chemical-propulsion spaceship for the same Mars mission. In 1955 he published: “Electrical Propulsion System for Space Ships with Nuclear Source” in the Journal of Astronautics, where he replaced the solar-electric power system with a nuclear reactor (Nuclear Electric Propulsion – NEP). In 1964 Stuhlinger published the first systematic analysis of electric propulsion systems: “Ion Propulsion for Space Flight” [3], while the physics of electric propulsion thrusters was first described comprehensively in a book by Robert Jahn in 1968 [5].
In 1957, the Walt Disney television program ‘Mars and Beyond’ (shown in the series ‘Tomorrowland’) featured the fleet of ten nuclear-electric powered spacecraft that Stuhlinger envisioned for the journey. As you can see in the image below, this is an unusual design, a vehicle that became known as an ‘umbrella ship.’ I’ve quoted him before on this, but let me run the passage again. It’s from Stuhlinger’s 1955 paper “Electrical Propulsion System for Space Ships with Nuclear Power Source”:
A propulsion system for space ships is described which produces thrust by expelling ions and electrons instead of combustion gases. Equations are derived from the optimum mass ratio, power, and driving voltage of a ship with given payload, travel time, and initial acceleration. A nuclear reactor provides the primary power for a turbo-electric generator; the electric power then accelerates the ions. Cesium is the best propellant available because of its high atomic mass and its low ionization energy. A space ship with 150 tons payload and an initial acceleration of 0.67 x 10-4 G, traveling to Mars and back in a total travel time of about 2 years, would have a takeoff mass of 730 tons.
Image: Ernst Stuhlinger’s Umbrella Ship, built around ion propulsion. Notice the size of the radiator, which disperses heat from the reactor at the end of the boom. The source for this concept was a Stuhlinger paper called “Electrical Propulsion System for Space Ships with Nuclear Power Source,” which ran in the Journal of the Astronautical Sciences 2, no. Pt. 1 in 1955, pp. 149-152. Credit: Winchell Chung.
While I’ve only talked about Stuhlinger’s work on electric propulsion here, his contribution to space sciences was extensive, ranging from a staging system crucial to Explorer 1 (this involved his pushing a button at the precise time required, hence his nickname as ‘the man with the golden finger’), to his work as director of the Marshall Space Flight Center Science Laboratory, which involved an active role in plans for lunar exploration.
For his contributions to electric propulsion, the Electric Rocket Propulsion Society renamed its award for outstanding achievement as the Stuhlinger Medal after his death. In terms of his visibility to the public, those interested in space advocacy will know about his letter to Sister Mary Jucunda, a nun based in Zambia, which laid out to a profound skeptic the rationale for pursuing missions to far destinations at a time of global crisis.
Image: In the above photo, taken at the Walt Disney Studios in California, Wernher von Braun (right) and Ernst Stuhlinger are shown discussing the technology behind nuclear-electric spaceships designed to undertake the mission to the planet Mars. As a part of the Disney ‘Tomorrowland’ series on the exploration of space, the nuclear-electric vehicles were shown in the program “Mars and Beyond,” which first aired in December 1957. Credit: NASA MSFC.
In the next post, I want to look at the deep space applications that Maraqten and Genovese considered in their IAC presentation.
For more details on Stuhlinger’s Mars ship, see Adam Crowl’s Stuhlinger Mars Ship Paper, and the followup I wrote in these pages back in 2015, Ernst Stuhlinger: Ion Propulsion to Mars. The Maraqten & Genovese paper is “Advanced Electric Propulsion Concepts for Fast Missions to the Outer Solar System and Beyond,” 73rd International Astronautical Congress (IAC), Paris, France, 18-22 September 2022 (available here). Ernst Stuhlinger’s paper on nuclear-electric propulsion is “Electrical Propulsion System for Space Ships with Nuclear Source,” appearing in the Journal of Astronautics Vol. 2, June 1955, p. 149, and available in manuscript form here. For more background on electric propulsion, see Choueiri, E., Y., “A Critical History of Electric Propulsion: The First 50 Years (1906-1956),” Journal of Propulsion and Power, vol. 20, pp. 193-203, 2004.
M-Dwarfs: The Asteroid Problem
I hadn’t intended to return to habitability around red dwarf stars quite this soon, but on Saturday I read a new paper from Anna Childs (Northwestern University) and Mario Livio (STScI), the gist of which is that a potential challenge to life on such worlds is the lack of stable asteroid belts. This would affect the ability to deliver asteroids to a planetary surface in the late stages of planet formation. I’m interested in this because it points to different planetary system architectures around M-dwarfs than we’re likely to find around other classes of star. What do observations show so far?
You’ll recall that last week we looked at M-dwarf planet habitability in the context of water delivery, again involving the question of early impacts. In that paper, Tadahiro Kimura and Masahiro Ikoma found a separate mechanism to produce the needed water enrichment, while Childs and Livio, working with Rebecca Martin (UNLV) ponder a different question. Their concern is that red dwarf planets would lack the kind of late impacts that produced a reducing atmosphere on Earth. On our planet, via the reaction of the iron core of impactors with water in the oceans, hydrogen would have been released as the iron oxidized, making an atmosphere in which simple organic molecules could emerge.
If we do need this kind of impact to affect the atmosphere to produce life (and this is a big ‘if’), we have a problem with M-dwarfs, for delivering asteroids seems to require a giant planet outside the radius of the snowline to produce a stable asteroid belt.
Depending on the size of the M-dwarf, the snowline radius is found from roughly 0.2 to 1.2 AU, close enough that radial velocity surveys are likely to detect giant planets near but outside this distance. The transit method around such small stars is likewise productive, but we find no such giant planets in those M-dwarf systems where we currently have discovered probable habitable zone planets:
The Kepler detection limit is at orbital periods near 200 days due to the criterion that three transits need to be observed in order for a planet to be confirmed (Bryson et al. 2020). However, in the case of low signal-to-noise observations, two observed transits may suffice, which allows longer-period orbits to be detected. This was the case for Kepler-421 b, which has an orbital period of 704 days (Kipping et al. 2014). Furthermore, any undetected exterior giant planets would likely raise a detectable transit timing variation (TTV) signal on the inner planets (Agol et al. 2004). For these reasons, while the observations could be missing long-period giant planets, the lack of giant planets around low-mass stars that are not too far from the snow line is likely real.
Image: A gas giant in orbit around a red dwarf star. How common is this scenario? We know that such planets can exist, but so far have never detected a gas giant outside the snowline around a system with a planet in the habitable zone. Credit: NASA, ESA and G. Bacon (STScI).
In the search for stable asteroid belts, what we are looking for is a giant planet beyond the snowline, with the asteroid belt inside its orbit, as well as an inner terrestrial system of planets. None of the currently observed planets in the habitable zone around M-dwarfs shows a giant planet in the right position to produce an asteroid belt. Which is not to say that such planets do not exist around M-dwarfs, but that we do not yet find any in systems where habitable zone planets occur. Let me quote the paper again:
By analyzing data from the Exoplanet Archive, we found that there are observed giant planets outside of the snow line radius around M dwarfs, and in fact the distribution peaks there. This, combined with observations of warm dust belts, suggests that asteroid belt formation may still be possible around M dwarfs. However, we found that in addition to a lower occurrence rate of giant planets around M dwarf stars, multiplanet systems that contain a giant planet are also less common around M dwarfs than around G-type stars. Lastly, we found a lack of hot and warm Jupiters around M dwarfs, relative to the K-, G-, and F-type stars, potentially indicating that giant planet formation and/or evolution does take separate pathways around M dwarfs.
Image: This is Figure 2 from the paper. Caption: Locations of the giant planets, r, normalized by the snow-line radius in the system, vs. the stellar mass, M?. The point sizes in the top plot are proportional to m?. Red dots indicate planets around M dwarf stars and blue dots indicate planets around FGK-type stars. The point sizes in the legend correspond to Jupiter-mass planets. The bottom plot shows normalized histograms of the giant planet locations for both single planet and multiplanet systems. The location of the snow line is marked by a black dashed vertical line. Credit: Childs et al.
The issues raised in this paper all point to how little we can say with confidence at this point. Are asteroid impacts really necessary for life to emerge? The question would quickly be resolved by finding biosignatures on an M-dwarf planet without a gas giant in the system, presuming no asteroid belt had formed by other methods. As one with a deep curiosity about M-dwarf planetary possibilities, I find this work intriguing because it points to different architectures around red dwarfs than other stars. It’s a difference we’ll explore as we begin to fill in the blanks by evaluating M-dwarf planets for early biosignature searches.
The paper is Childs et al., “Life on Exoplanets in the Habitable Zone of M Dwarfs?,” Astrophysical Journal Letters Vol. 937, No. 2 (4 October 2022), L42 (full text).
M-Dwarf Habitable Planets: The Water Factor
Small M-dwarf stars, the most common type of star in the galaxy, are likely to be the primary target for our early investigations of habitable planets. The small size of these stars and the significant transit depth this allows when an Earth-mass planet crosses their surface as seen from Earth mean that atmospheric analysis by ground- and space-based telescopes should be feasible via transmission spectroscopy. Recent studies have shown that the James Webb Space Telescope has the precision to at least partially characterize the atmospheres of Earth-class planets around some M-dwarfs.
Soon-to-be commissioned ground-based extremely large telescopes will likewise play a role as we examine nearby transiting systems. But M-dwarfs make challenging homes for life, if indeed it can exist there. In addition to flare activity, we also have to reckon with the presence of water. Too much of it could suppress weathering in the geochemical carbon cycle, but too little does not allow for the development of a temperate climate. Thus new work on water content in such systems is welcome.
For purposes of reference, Earth’s seawater accounts for 0.023% of the planet’s total mass. According to Tadahiro Kimura, a doctoral student at the University of Tokyo, and Masahiro Ikoma (National Astronomical Observatory of Japan), a number of models suggest that terrestrial planets around M-dwarfs would have either too much water or no water at all. Are habitable planets around such stars, then, a celestial rarity?
In a new paper in Nature Astronomy, the authors argue that there is a mechanism beyond the infall of icy planetesimals that can produce water as a young planet accumulates its atmosphere. It involves interactions between the hydrogen-rich atmosphere, drawn from the protoplanetary disk, and the magma ocean that would be present from impacts during the early days of planet formation. Water is accumulated through the chemical reaction between atmospheric hydrogen and the oxides found in the surface magma – a magma ‘ocean’ – of the young planet. From the paper:
…water can be secondarily produced in a primordial atmosphere of nebular origin through reaction of atmospheric hydrogen with oxidising minerals from the magma ocean, which is formed because of the atmospheric blanketing effect[8], thereby enriching the primordial atmosphere with water. By assuming effective water production, we recently showed that nearly-Earthmass planets can acquire sufficient amounts of water for their atmospheric vapour to survive in harsh UV environments around pre-main-sequence M stars [9]. The results suggest that including this water production process significantly affects the predicted water amount distribution of exoplanets in the habitable zone around M dwarfs.
Image: Probability distribution of seawater mass fractions for planets of Earth-like mass (0.3-3 times Earth mass) located in the habitable zone around M-type stars (0.3 solar masses). Green is the result of calculations following the conventional model and considering only the acquisition of water-bearing rocks. Orange is the result when the model of the present study is used and the effect of water production in the primordial atmosphere is taken into account. The dotted line is the present-day seawater amount on the Earth. Credit: National Astronomical Observatory of Japan.
In this scenario, the amount of water present depends on how the planet forms. The authors have created a planetary population synthesis model that tracks the mass and orbital evolution of planets in formation, including among other things the structure of the protoplanetary disk, potential orbital migration, instabilities in multi-planet systems and the effects of water production in the primordial atmosphere. The model, which refines that presented in an earlier paper by the same researchers, allows the calculation of the amount of water that should be produced through the atmosphere/magma interaction.
The range of water outcomes is wide, but if we narrow it to planets with seawater mass fractions similar to Earth, most of this water is found to come through atmosphere/magma interaction rather than by incoming impacts by comets and other water-bearing objects. And it turns out that a few percent of planets with a radius between 0.7 and 1.3 times that of Earth produce the right amount of water to sustain temperate climates. Let me quote the paper on this – note that in the passage below, HZ-NEMP refers to nearly-Earth-mass planets in the habitable zone:
The HZ-NEMPs of 0.7–1.3 R?… have lost their hydrogen atmospheres completely, ending up with rocky planets covered with oceans. It turns out that those planets are diverse in water content and do include planets with Earth-like water content. Several climate studies argue the amounts of seawater appropriate for temperate climates, considering the effects of seafloor weathering, high-pressure ice, water cycling and heterogeneous surface water distribution… According to those studies, the appropriate seawater amount ranges from ?0.1 to 100 times that of the Earth.
Clearly, target selection for exoplanet habitability would benefit from being able to exclude planets that are unlikely to be habitable, which according to this paper would include habitable zone worlds with radii > 1.3R? that have deep oceans with high-pressure ice, and planets with ocean mass fractions greater than 100 times that of Earth. The authors believe that we should be able to identify such worlds if planetary mass and radius can be measured within ? 20% and 5% accuracy respectively. Having eliminated these, we turn to planets in the 0.7 to 1.3R? range. The authors refer to them as ‘water-poor,’ in comparison to their larger cousins, but they still can have seawater fractions similar to that of Earth:
…the HZ-NEMPs with appropriate amounts of seawater for habitability are estimated to account for ?5% of the “water-poor rocky planets” orbiting 0.3M M dwarfs. This frequency becomes higher for larger stellar mass, and around 0.5M stars, for example, more than 10% of the water-poor rocky planets are expected to have the appropriate amounts of seawater.
So 5% to 10% of the M-dwarf exoplanets in the appropriate size range (< 1.3R?) have the fraction of water needed for habitability. The paper makes this prediction: Survey missions like TESS and the upcoming PLATO should detect approximately 100 Earth-sized planets in the habitable zone around M-dwarfs. 5 to 10 of these, according to this model, are likely to be planets with oceans and temperate climates, a sharp contrast to earlier studies indicating such worlds should not exist.
The paper is Kimura & Ikoma, “Predicted diversity in water content of terrestrial exoplanets orbiting M dwarfs,” Nature Astronomy 29 September 2022 (abstract / preprint). The authors’ earlier paper on water enrichment is Kimura & Ikoma, “Formation of aqua planets with water of nebular origin: effects of water enrichment on the structure and mass of captured atmospheres of terrestrial planets,” Monthly Notices of the Royal Astronomical Society 496, 3755 (2020) (abstract).
Great Winds from the Sky
Do we need to justify pushing our limits? Doing so is at the very heart of the urge to explore, which is embedded in our species. Recently, while doing some research on Amelia Earhart, I ran across a post on Maria Popova’s extraordinary site The Marginalian, one that examines the realm of action within the context of the human spirit. Back in 2016, Popova was looking at Walter Lippmann (1889-1974), the famed journalist and commentator, who not long after Earhart’s fatal flight into the Pacific discussed the extent of her achievement and the reasons she had flown.
Here’s a passage from Lippmann’s New York Herald Tribune column, written on July 8, 1937, just six days after the aviator and her navigator, Fred Noonan, disappeared somewhere near Howland Island between Hawaii and Australia. Lippmann asks whether such ventures must be justified by a utilitarian purpose and concludes that what is at stake here transcends simple utility and speaks to the deepest motivations of our explorations. It is a belief in a goal and the willingness to risk all. Practicality carries little weight among those who actually do the deed:
“The best things of mankind are as useless as Amelia Earhart’s adventure. They are the things that are undertaken not for some definite, measurable result, but because someone, not counting the costs or calculating the consequences, is moved by curiosity, the love of excellence, a point of honor, the compulsion to invent or to make or to understand. In such persons mankind overcomes the inertia which would keep it earthbound forever in its habitual ways. They have in them the free and useless energy with which alone men surpass themselves.
Such energy cannot be planned and managed and made purposeful, or weighted by the standards of utility or judged by its social consequences. It is wild and it is free. But all the heroes, the saints, the seers, the explorers and the creators partake of it. They do not know what they discover. They do not know where their impulse is taking them. They can give no account in advance of where they are going or explain completely where they have been. They have been possessed for a time with an extraordinary passion which is unintelligible in ordinary terms.
No preconceived theory fits them. No material purpose actuates them. They do the useless, brave, noble, the divinely foolish and the very wisest things that are done by man. And what they prove to themselves and to others is that man is no mere creature of his habits, no mere automaton in his routine, no mere cog in the collective machine, but that in the dust of which he is made there is also fire, lighted now and then by great winds from the sky.”
Image: Amelia Earhart’s Lockheed Electra 10E. During its modification, the aircraft had most of the cabin windows blanked out and had specially fitted fuselage fuel tanks. The round RDF loop antenna can be seen above the cockpit. This image was taken at Luke Field in Hawaii on March 20, 1937. Earhart’s final flight in this aircraft took place on July 2, 1937, taking off from Lae, New Guinea. Credit: Wikimedia Commons. Scanned from Lockheed Aircraft since 1913, by René Francillon. Photo credit USAF.
Lippmann’s tribute is a gorgeous piece of writing, available in The Essential Lippmann (Random House, 1963). Naturally, it makes me think of other flyers who rode those same winds, people like Antoine de Saint-Exupéry and Beryl Markham, who in 1936 was the first to dare a solo non-stop flight across the Atlantic from east to west. As I’ve recently re-read Markham’s elegant West With the Night (1942), she as well as Earhart has been on my mind. What a shame that Earhart didn’t live to pen a memoir as powerful, but perhaps Lippmann in some small way did it for her.
Juno: First Image from Europa
Juno’s close pass of Europa on September 29 (1036 UTC) took it within 352 kilometers of the icy moon, marking the third close pass in history below 500 kilometers. The encounter saw the spacecraft come within a single kilometer of Galileo’s 351 kilometers from the surface back in January of 2000, and it provided the opportunity for Juno to use its JunoCam to home in on a region north of Europa’s equator. Note the high relief of terrain along the terminator, with its ridges and troughs starkly evident.
Image: The complex, ice-covered surface of Jupiter’s moon Europa was captured by NASA’s Juno spacecraft during a flyby on Sept. 29, 2022. At closest approach, the spacecraft came within a distance of about 352 kilometers. Credit: NASA/JPL-Caltech/SWRI/MSSS.
This first image from JunoCam captures features at the region called Annwn Regio, and was collected in the two-hour window available to Juno as it moved past Europa at 23.6 kilometers per second. What we hope to gain from analysis of the data should be high resolution images at approximately 1 kilometer per pixel, along with data on the ice shell covering the moon’s ocean, along with a good deal more about its surface composition, its internal structure and tenuous ionosphere. Says Candy Hansen, a Juno co-investigator (Planetary Science Institute, Tucson):
“The science team will be comparing the full set of images obtained by Juno with images from previous missions, looking to see if Europa’s surface features have changed over the past two decades. The JunoCam images will fill in the current geologic map, replacing existing low-resolution coverage of the area.”
In other words, more JunoCam imagery to come, all useful to the upcoming Europa Clipper and JUICE missions. In particular, data from the spacecraft’s Microwave Radiometer should fill in our understanding of variations in Europa’s ice beneath the crust, and possibly point to regions where liquid water may be captured in subsurface pockets.
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