by Paul Gilster | Jan 13, 2023 | Exoplanetary Science |
LHS 475b, a planet whose diameter is all but identical to Earth’s, makes news not so much because of what it is but because of what it tells us about studying the atmospheres of small rocky worlds. Credit for the confirmation of this planet goes to the NIRSpec (Near-Infrared Spectrograph) instrument aboard the James Webb Space Telescope, and LHS 475b marks the telescope’s first exoplanet catch. Data from the Transiting Exoplanet Survey Satellite (TESS) were sufficient to point scientists toward this system for a closer look. JWST confirmed the planet after only two transits.
Based on this detection, the Webb telescope is going to live up to expectations about its capabilities in exoplanet work. NIRSpec is a European Space Agency contribution to the JWST mission, and a major one, as the instrument’s multi-object spectroscopy mode is able to obtain spectra of up to 100 objects simultaneously, a capability that maximizes JWST observing time. No other spectrograph in space can do this, but NIRSpec deploys a so-called ‘micro-shutter’ subsystem developed for the instrument by NASA GSFC. Think of tiny windows with shutters, each measuring 100 by 200 microns.
The shutters operate as a magnetic field is applied and can be controlled individually. Murzy Jhabvala is chief engineer of Goddard’s Instrument Technology and Systems Division:
“To build a telescope that can peer farther than Hubble can, we needed brand new technology. We’ve worked on this design for over six years, opening and closing the tiny shutters tens of thousands of times in order to perfect the technology.”
LHS 475b is a rocky world that orbits a red dwarf some 40 light years out in Octans. The data from NIRSpec, taken on 31 August 2022, could not be clearer, as the image below shows.
Image: This graphic shows the change in relative brightness of the star-planet system at LHS 475 spanning three hours. The spectrum shows that the brightness of the system remains steady until the planet begins to transit the star. It then decreases, representing when the planet is directly in front of the star. The brightness increases again when the planet is no longer blocking the star, at which point it levels out.] Credit: NASA, ESA, CSA, L. Hustak (STScI), K. Stevenson, J. Lustig-Yaeger, E. May (Johns Hopkins University Applied Physics Laboratory), G. Fu (Johns Hopkins University), and S. Moran (University of Arizona).
Transmission spectroscopy – applying the NIRSpec capabilities to the spectrum of starlight passing through the planet’s atmosphere during ingress and egress from a transit – should give us the opportunity to analyze the components of that atmosphere, assuming one is present. That work is ongoing but promising, as the scientists in a team led by Kevin Stevenson and Jacob Lustig-Yaeger (Johns Hopkins University Applied Physics Laboratory) pursue their investigation. JWST’s sensitivity to a range of molecules is clear, and the researchers have been able to rule out a methane-dominated atmosphere, while a compact envelope made up entirely of carbon dioxide remains a possibility. Additional spectra will be taken this summer.
Image: The graphic shows the transmission spectrum of the rocky exoplanet LHS 475b. The data points are plotted as white circles with grey error bars on a graph of the amount of light blocked in percent on the vertical axis versus wavelength of light in microns on the horizontal axis. A straight green line represents a best-fit model. A curvy red line represents a methane model, and a slightly less curvy purple line represents a carbon dioxide model.] Credit: NASA, ESA, CSA, L. Hustak (STScI), K. Stevenson, J. Lustig-Yaeger, E. May (Johns Hopkins University Applied Physics Laboratory), G. Fu (Johns Hopkins University), and S. Moran (University of Arizona).
Given that LHS 475b orbits its star in two days, it’s no surprise to learn that the planet is considerably warmer than Earth even though it orbits an M-dwarf. We may well be looking at a Venus analogue, if an atmosphere does turn out to be present.
Bear in mind as JWST pushes into this area that M-dwarfs are prone to flares, especially in their earlier stages of development, and thus raise the question of whether a thick and detectable atmosphere can survive. LHS 475b may help us find out. The signs are promising, as the paper notes:
…our non-detection of starspot crossings during transit and the lack of stellar contamination in the transmission spectrum are promising signs in this initial reconnaissance of LHS 475b. These findings indicate that additional transit observations of LHS 475b with JWST are likely to tighten the constraints on a possible atmosphere. A third transit of LHS 475b is scheduled as part of this program (GO 1981) in 2023. An alternative path to break the degeneracy between a cloudy planet and an airless body is to obtain thermal emission measurements of LHS 475b during secondary eclipse because an airless body is expected to be several hundred Kelvin hotter than a cloudy world and will therefore produce large and detectable eclipse depths at JWST’s MIRI wavelengths… Our findings only skim the surface of what is possible with JWST.
The paper is Lustig-Yaeger et al., “A JWST transmission spectrum of a nearby Earth-sized exoplanet,” in process at Nature Astronomy (preprint).
by Paul Gilster | Dec 7, 2022 | Missions |
Some topics just take off on their own. Several days ago, I began working on a piece about Europa Clipper’s latest news, the installation of the reaction wheels that orient the craft for data return to Earth and science studies at target. But data return is one thing for spacecraft working at radio frequencies within the Solar System, and another for much more distant craft, perhaps in interstellar space, using laser methods.
So spacecraft orientation in the Solar System triggered my recent interest in the problem of laser pointing beyond the heliosphere, which is acute for long-haul spacecraft like Interstellar Probe, a concept we’ve recently examined. Because unlike radio methods, laser communications involve an extremely tight, focused beam. Get far enough from the Sun and that beam will have to be exquisitely precise in its placement.
So let’s take a quick look at Europa Clipper’s methods for orienting itself in space, and Voyager’s as well, and then move on to how Interstellar Probe intends to get its signal back to Earth. NASA has just announced that engineers have installed four reaction wheels aboard Europa Clipper, to provide orientation for the transmission of data and the operation of its instruments as it studies the Jovian moon. The wheels are slow to have their effect, with 90 minutes being needed to rotate Europa Clipper 180 degrees, but they run usefully on electrical power from the spacecraft’s solar arrays rather than relying on fuel that would have to be carried for its thrusters.
Image: All four of the reaction wheels installed onto NASA’s Europa Clipper are visible in this photo, which was shot from underneath the main body of the spacecraft while it is being assembled at the agency’s Jet Propulsion Laboratory in Southern California. The spacecraft is set to launch in October 2024 and will head toward Jupiter’s moon Europa, where it will collect science observations while flying by the icy moon dozens of times. During its journey through deep space and its flybys of Europa, the spacecraft’s reaction wheels rotate the orbiter so its antennas can communicate with Earth and so its science instruments, including cameras, can stay oriented. Two feet wide and made of steel, aluminum, and titanium, the wheels spin rapidly to create a force that causes the orbiter to rotate in the opposite direction. The wheels will run on electricity provided by the spacecraft’s vast solar arrays. NASA/JPL-Caltech.
Interstellar Pointing Accuracy
How do reaction wheels fit into missions much further out? In our recent look at Interstellar Probe, the NASA design study out of the Johns Hopkins University Applied Physics Laboratory (JHU/APL), I mentioned problems with pointing accuracy when it came to a hypothetical laser communications system aboard. The team working on Interstellar Probe (IP) chose not to go with a laser comms system, opting instead for X-band communications (or conceivably Ka-band), because as principal investigator Ralph McNutt told me, several problems arose when trying to point such a tight communications signal at Earth from the ultimate mission target: 1000 AU.
IP, remember, has 1000 AU as a design specification – the idea is to produce a craft that, upon reaching this distance, would still be able to transmit its findings back to Earth, but whether this distance can be achieved within the cited 50 year time frame is another matter. Wherever the distance of the craft is 50 years after launch, though, the design calls for it to be able to communicate with Earth. We can still talk to the Voyagers, but that brings up the issue of the best method to make the connection.
Both Voyagers are a long way from home, but nothing like 1000 AU, with Voyager 1 at 158 AU and Voyager 2 at 131 AU from the Sun. The craft are equipped with six sets of thrusters to control pitch, yaw and roll, allowing the orientation with Earth needed for radio communications (Voyager transmits at either 2.3 GHz or 8.4 GHz). But what about those reaction wheels we just looked at with Europa Clipper, which allow three-axis attitude control without using attitude control thrusters or other external sources of torque? Here we run into a technology with a history that is problematic for going beyond the Solar System or, indeed, extending a mission closer to home. Just how problematic we learned all too clearly with the Kepler mission.
For reaction wheels are all too prone to failure over time. The hugely successful exoplanet observatory found itself derailed in May of 2013, when the second of its reaction wheels failed (the first had given out the previous July). Operating something like a gyroscope, the reaction wheels were designed to spin up in one direction so as to move the spacecraft in the other, thus allowing data return from the rich star field Kepler was studying. Kepler had four reaction wheels and needed three to function properly. With only two wheels operational, the spacecraft quickly went into safe mode.
The problem, likely the result of something as mundane as issues with ball bearings, is hardly confined to a single mission, and although the Kepler team was able to mount a successful K2 extended mission, the larger question extends to any long-term mission relying on this technology. Reaction wheels were a problem on NASA’s Far Ultraviolet Spectroscopic Explorer in 2001 and complicated the Japanese Hayabusa mission in 2004 and 2005. The DAWN mission had two reaction wheel failures during the course of its operations. A NASA mission called Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) suffered a reaction wheel failure in 2007.
So by the time Kepler was close to launch, the question of reaction wheels was much in the air. We should keep in mind that the reaction wheel failures occurred despite extensive precautions taken by the mission controllers, who sent the Kepler reaction wheels back to the manufacturer, Ithaco Space Systems in Ithaca, NY, removing them from the spacecraft in 2008 and replacing the ball bearings before the 2009 launch. It became clear with the reaction wheel failures Kepler sustained that the technology was vulnerable, although it did function up to the end of the spacecraft’s primary mission.
Based on experience, the technology shows a shelf-life on the order of a decade, which is why the Interstellar Probe team had to reject the reaction wheel concept for laser pointing. Remember that IP is envisioned as a fully operational spacecraft for 50 years, able to return data from well beyond the heliosphere at that time. As McNutt pointed out in an email, the usable laser beam size at the Earth, based on a 2003 NIAC study, was approximately Earth’s own diameter. Let me quote Dr. McNutt on this:
“With a downlink per week from 1000 au that lasted ~8 hours for that concept, one would have to point the beam ahead, so that the Earth would be “under it” when the laser train of light signals arrived. It also meant that we needed an onboard clock good to a few minutes after 50 years at worst and a good ephemeris on board to tell where to point in the first place. These start at least heading toward some of the performance of Gravity Probe B… but one needs these accuracies to hold for ~50 years.”
This gets complicated indeed. From a 2002 paper on optical and microwave communications for an interstellar explorer craft operating as far as 1000 AU (McNutt was a co-author here, working on a study that fed directly into the current Interstellar Probe design), note the possible errors that must be foreseen:
These include trajectory knowledge derived from an onboard clock and ephemerides to track the receiving station and downlink platform so that the spacecraft-to-earth line-of-sight orientation is known sufficiently accurately within the total spacecraft pointing error budget. In order to maintain the transmitter boresight accurately a high-precision star tracker is also needed, which must be aligned very accurately with respect to the laser antenna. Alignment errors between the transmitter and star tracker can be minimized by using the same optical system for the star tracker and laser transmitter and compensating any residual dynamic errors in real-time. This must be accomplished subject to various spacecraft perturbations, such as propellant bursts, or solar radiation induced moments. To also avoid significant beam loss when coupling into the receiver near Earth, the beam shape should be controlled, i.e., be a diffraction-limited single mode beam as well.
X-band radio communications, as considered by the Interstellar Probe team at JHU/APL, thus emerges as the better option considering that a mission coming out of the upcoming heliophysics decadal would be launching in the 2030s, with the recent analysis from Pontus Brandt et al. noting that “Although, optical laser communication offers high data rates, it imposes an unrealistic pointing requirement on the mission architectures under study.”
What to do? From the Brandt et al. paper (my additions are in italics):
The conclusion following significant analysis was that the implementation with the largest practical monolithic HGA [High Gain Antenna] with the corresponding lower transmission frequency to deal with a larger pointing dead-band. This corresponds to a 5-m diameter HGA at X-band for Options 1 and 2 and a smaller, 2-m HGA at Ka-band for Option 3 [here the options refer to the mass of the spacecraft]. The corresponding guidance and control system is based upon thrusters and must provide the required HGA pointing as commensurate with spacecraft science needs.
I checked in with Ralph McNutt again while working on this post on the question of how IP would orient the spacecraft. He confirmed that attitude control thrusters would be the method, and went on to note that, at flight-tested status (TRL 9), control authority of ~0.25° with thrusters is possible; we also have much experience with the technology.
Dr. McNutt passed our discussion along to JHU/APL’s Gabe Rogers, who has extensive experience on the matter not only with the Interstellar Probe concept but through flight experience with NASA’s Van Allen Probes. Dr. Rogers likened IP’s attitude control to Pioneer 10 and 11 more than Voyager, saying that IP would be primarily spin-stabilized rather than, like Voyager, 3-axis stabilized. The Pioneers carried six hydrazine thrusters, two of which maintained the spin rate, while two controlled forward thrust and two controlled attitude.
As to reaction wheels, they turn out to be both a lifetime and a power issue, ruling them out. Both scientists added that surviving launch vibration and acceleration is a factor, as are changes in moments of inertia as fuel is burned for guidance and control.
Says McNutt:
“One way of dealing with this (looks good on paper) is actively moving masses around to compensate for pointing issues – but then one has to worry about the lifetime of mechanisms. Galileo actually had motors to control the boom deployments of its two RTGs to control the moments of inertia of the spinning section (a different “issue”). Of course, Galileo is also the poster child of what can happen if deployment mechanisms fail on a $1B + spacecraft – in that case the HGA deployment. The LECP [Low-Energy Charged Particle] stepper motors on Voyager have gone through over 7 million steps – but that was not the “plan” or “design.”
What counts is the result. Will engineers fifty years after launch be able to download meaningful scientific data from a craft like Interstellar Probe? The question frames the entire discussion as we move toward interstellar space. Rogers adds:
“We can always mitigate risk, but we have to think very carefully about the best, most reliable way to recover the science data requested. Sometimes simpler is better. The key is to get the most bits down to the ground. I would rather have a 1000 bit per second data rate that would work 8 hours per day than a 3000 bps data rate that worked 2 hours per day. X-band is also less susceptible to rain in Spain falling mainly on the plains.”
Indeed, and with RF as opposed to laser, we have less concern about where the clouds are. So the current thinking about using X-band resolves issues beyond pointing accuracy. Bear in mind that we are talking about a spacecraft deliberately crafted to be operational for 50 years or more, a seemingly daunting challenge in what McNutt calls ‘longevity by design,’ but every indication is that longevity can be achieved, as the Voyagers remind us despite their not being built for the task.
And while I had never heard of the Oxford Electric Bell before this correspondence, I’ve learned in these discussions that it was set up in 1840 and has evidently run ever since its construction. So we’ve been producing long-lived technologies for some time. Now we incorporate them intentionally into our spacecraft to move beyond the heliosphere.
As to Europa Clipper’s reaction wheels, they fit the timeframe of the mission, considering we have a decade to work with, from 2024 launch to end of operations (presumed in 2034). But aware of the previous problems posed by reaction wheels, Europa Clipper’s engineers have installed four rather than three to provide a backup, and we can hope that knowledge hard-gained through missions like Kepler will afford an even longer lifetime for the steel, aluminum, and titanium wheels aboard Clipper.
Image: Engineers install 2-foot-wide reaction wheels onto the main body of NASA’s Europa Clipper spacecraft at the agency’s Jet Propulsion Laboratory. The orbiter is in its assembly, test, and launch operations phase in preparation for a 2024 launch. Credits: NASA/JPL-Caltech.
Many thanks to Ralph McNutt and Gabe Rogers for their help with this article. The study on optical communications I referenced above is Boone et al., “Optical and microwave communications system conceptual design for a realistic interstellar explorer,” Proc. SPIE 4821, Free-Space Laser Communication and Laser Imaging II, (9 December 2002). Abstract. The Brandt paper on IP is “Interstellar Probe: Humanity’s exploration of the Galaxy Begins,” Acta Astronautica Volume 199 (October 2022), pages 364-373 (full text). For broader context, be aware as well of Rogers et al., “Dynamic Challenges of Long Flexible Booms on a Spinning Outer Heliospheric Spacecraft,” published in 2021 IEEE Aerospace Conference (full text).
by Paul Gilster | Jun 10, 2022 | Outer Solar System |
I get an eerie feeling when I look at spacecraft before they launch (not that I get many opportunities to do that, at least in person). But seeing the Spirit and Opportunity rovers on the ground at JPL just before their shipment to Florida was an experience that has stayed with me, as I pondered how something built by human hands would soon be exploring another world. I suppose the people who do these things at the Johns Hopkins Applied Physics Laboratory and the Jet Propulsion Laboratory itself get used to the feeling. For me, though, the old-fashioned ‘sense of wonder’ kicks in long and hard, as it did when Europa Clipper arrived recently at JPL.
Not that the spacecraft is by any means complete, but its main body has been delivered to the Pasadena site, where it will see final assembly and testing over a two-year period. Here I fall back on the specs to note that this is the largest NASA spacecraft ever designed for exploration of another planet. It’s about the size of an SUV when stowed for launch, but we know from the James Webb Space Telescope how large these things can become when fully deployed. In Europa Clipper’s case, the recently delivered main body is 3 meters tall and 1.5 meters wide. Extending the solar arrays and other deployable equipment takes it up to basketball court size.
Image: The main body of NASA’s Europa Clipper spacecraft has been delivered to the agency’s Jet Propulsion Laboratory in Southern California, where, over the next two years, engineers and technicians will finish assembling the craft by hand before testing it to make sure it can withstand the journey to Jupiter’s icy moon Europa. Here it is being unwrapped in a main clean room at JPL, as engineers and technicians inspect it just after delivery in early June 2022. Credit: NASA.
Eight antennas are involved, powered by a radio frequency subsystem that will service a high-gain antenna measuring three meters wide, and as JPL notes in a recent update, the electrical wires and connectors collectively called the ‘harness’ themselves weigh 68 kilograms. Stretch all that wiring out and you get 640 meters, taking us twice around a football field. The main body will include a fuel tank and an oxidizer tank connecting to an array of 24 engines. Tim Larson is JPL deputy project manager for Europa Clipper:
“Our engines are dual purpose. We use them for big maneuvers, including when we approach Jupiter and need a large burn to be captured in Jupiter’s orbit. But they’re also designed for smaller maneuvers to manage the attitude of the spacecraft and to fine tune the precision flybys of Europa and other solar system bodies along the way.”
So what is arriving, or has arrived at JPL, is a spacecraft in pieces, its main body now joining key instruments like E-THEMIS, a thermal emission imaging system developed at Arizona State, and Europa-UVS, the mission’s ultraviolet spectrograph. E-THEMIS is an infrared camera that should give us insights into temperatures on the Jovian moon, and hence offer information about its geological activity. Given that we’re interested in finding places where liquid water is close to the surface, the data from this instrument should be extremely valuable during the spacecraft’s nearly fifty close passes.
The theory here is that as Europa’s surface cools after local sunset, the areas of the most solid ice will retain heat longer than areas with a looser, more granular texture. E-THEMIS will be able to map cooling rates across the surface. The infrared camera works in three heat-sensitive bands, and the warmer regions it should see may be the result of liquid water close to the surface, or possible impacts or convection activity. Not surprisingly, E-THEMIS lead project engineer Greg Mehall points to the radiation environment in Jupiter space as one of the team’s biggest issues:
“The extreme radiation environment at Europa gave far more design challenges for the ASU engineering team than on any previous instrument we’ve developed. We had to use dense shielding materials, such as copper-tungsten alloys, to provide the necessary protection from the expected radiation. And to ensure that E-THEMIS will survive during the mission, we also carried out radiation tests on the instrument’s electronic components and materials.”
Image: The thermal imager will use infrared light to distinguish warmer regions on Europa’s surface, where liquid water may be near the surface or might have erupted onto the surface. The thermal imager will also characterize surface texture to help scientists understand the small-scale properties of Europa’s surface. In the image above, we’re seeing a diurnal temperature color image from the first light test of Europa Clipper’s thermal imager (called E-THEMIS), taken from the rooftop of the Interdisciplinary Science and Technology Building 4 on the Tempe Campus of Arizona State University (ASU). The top image was acquired at 12:40 PM, the middle at 4:40 PM, and the bottom image at 6:20 PM (after sunset). Temperatures are approximations during this testing phase. Credit: ASU.
As to the Europa-UVS instrument, this ultraviolet spectrograph will search for water vapor plumes and study the composition of both the surface and the tenuous atmosphere as it uses an optical grating to spread and analyze light, identifying basic molecules like hydrogen, oxygen, hydroxide and carbon dioxide.
The spacecraft’s visible light imaging system (EIS) is going to upgrade those well-studied images from the Galileo mission enormously. The plan is to map 90 percent of the moon’s surface at 100 meters per pixel, which is six times more of Europa’s surface than Galileo, and at five times better resolution. And when Europa Clipper swings close to Europa during a flyby, it will produce images with a resolution fully 100 times better than Galileo. The Europa Imaging System includes both wide- and narrow-angle cameras, each with an eight-megapixel sensor. Both of these cameras will produce stereoscopic images and include the needed filters to acquire color images.
All told, the spacecraft’s nine science instruments should be able to extract information about the depth and salinity of the ocean under the ice and, crucially, the thickness of the ice crust (I can imagine wagers on that issue going around in certain quarters). Gathering information about the moon’s surface and interior should further illuminate the issue of plumes from the ocean below that may break through the ice.
Assembly, test and launch is a two year phase that, by the end of this year, should see assembly of most of the flight hardware and the remaining science instruments. Kudos to JHU/APL, which has just delivered a flight system that is the largest ever built by engineers and technicians there. Now we look toward bolting on the radio frequency module, radiation monitors, power converters, the propulsion electronics and those hundreds of meters of wiring. Not to mention the electronics vault that must stand up to hard radiation.
The full instrument package will include an imaging spectrometer, ice-penetrating radar, a magnetometer, a plasma instrument, a mass spectrometer and a dust analyzer. Only two years and four months before launch onto a six-year journey of 2.9 billion kilometers. Europa Clipper isn’t a life-finder, but it does have the capability of detecting whether the moon’s ocean really does allow for the possibility of life to develop. It’s our first reconnaissance of Europa since the 1990s. What surprises will it reveal?
Bear in mind, too, that we still have ESA’s JUICE (JUpiter ICy moons Explorer) in the offing, with launch planned for 2023. I note with interest that on June 19, Europa will occult a distant star, which should be useful in tweaking our knowledge of the moon’s orbit before the arrival of both missions. Destined to end its life as a Ganymede orbiter, JUICE will make only two close passes of Europa, but its period of operations will coincide with part of Europa Clipper’s numerous flybys of the moon.
by Paul Gilster | May 20, 2022 | Astrobiology and SETI |
What can we say about the possible appearance and spread of civilizations in the Milky Way? There are many ways of approaching the question, but in today’s essay, Dave Moore focuses on a recent paper from Robin Hanson and colleagues, one that has broad implications for SETI. A regular contributor to Centauri Dreams, Dave was born and raised in New Zealand, spent time in Australia, and now runs a small business in Klamath Falls, Oregon. He adds: “As a child, I was fascinated by the exploration of space and science fiction. Arthur C. Clarke, who embodied both, was one of my childhood heroes. But growing up in New Zealand in the ‘60s, such things had little relevance to life, although they did lead me to get a degree in biology and chemistry.” Discovering like-minded people in California, he expanded his interest in SETI and began attending conferences on the subject. In 2011, he published a paper in JBIS, which you can read about in Lost in Time and Lost in Space.
by Dave Moore
I consider the paper “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare,” by Robin Hanson, Daniel Martin, Calvin McCarter, and Jonathan Paulson, a significant advance in addressing the Fermi Paradox. To explain exactly why, I need to go into its background.
Introduction and History
In our discussions and theories about SETI, the Fermi paradox hangs over them all like a sword of Damocles, ready to fall and cut our assumptions to pieces with the simple question, where are the aliens? There is no reason not to suppose that Earth-like planets could not have formed billions of years before Earth did and that exosolar technological civilizations (ETCs) could not have arisen billions of years ago and spread throughout the galaxy. So why then don’t we see them? And why haven’t they visited us, given the vast expanse of time that has gone by?
Numerous papers and suggestions have tried to address this conundrum, usually ascribing it to some form of alien behavior, or that the principle of mediocrity doesn’t apply, and intelligent life is a very rare fluke.
The weakness of the behavioral arguments is they assume universal alien behaviors, but given the immense differences we expect from aliens—they will be at least as diverse as life on Earth—why would they all have the same motivation? It only takes one ETC with the urge to expand, and diffusion scenarios show that it’s quite plausible for an expansive ETC to spread across the galaxy in a fraction (tens of millions of years) of the time in which planets could have given rise to ETCs (billions of years).
And there is not much evidence that the principle of mediocrity doesn’t apply. Our knowledge of exosolar planets shows that while Earth as a type of planet may be uncommon, it doesn’t look vanishingly rare, and we cannot exclude from the evidence we have that other types of planets cannot give rise to intelligent life.
Also, modest growth rates can produce Kardeshev III levels of energy consumption in the order of tens of thousands of years, which in cosmological terms is a blink of the eye.
In 2010, I wrote a paper for JBIS modeling the temporal dispersion of ETCs. By combining this with other information, in particular diffusion models looking at the spread of civilizations across the galaxy, it was apparent that it was just not possible for spreading ETCs to occur with any frequency at all if they lasted longer than about 20,000 years. Longer than that and at some time in Earth’s history, they would have visited/colonized us by now. So, it looks like we are the first technological civilization in our galaxy. This may be disappointing for SETI, but there are other galaxies out there—at least as many as there are stars in our galaxy.
My paper was a very basic attempt to deduce the distribution of ETCs from the fact we haven’t observed any yet. Robin Hanson et al’s paper, however, is a major advance in this area as it builds a universe-wide quantitative framework to frame this lack of observational evidence and produces some significant conclusions.
It starts with the work done by S. Jay Olsen. In 2015, Olson began to bring out a series of papers assuming the expansion of ETCs and modeling their distributions. He reduced all the parameters of ETC distribution down to two: (?), the rate at which civilizations appeared over time, and (v) their expansion rate, which was assumed to be similar for all civilizations as ultimately all rocketry is governed by the same laws of physics. Olsen varied these two parameters and calculated the results for the following: the ETC-saturated fraction of the universe, the expected number and angular size of their visible domains, the probability that at least one domain is visible, and finally the total expected fraction of the sky eclipsed by expanding ETCs.
In 2018, Hanson et al took Olsen’s approach but incorporated the idea of bringing in the Hard Steps Power Law into modeling the appearance rate of ETCs, which they felt was more accurate and predictive than the rate-over-time models Olsen used.
The Hard Steps Power Law
The Hard Steps power law was first introduced in 1953 to model the appearance of cancer cells. To become cancerous an individual cell must undergo a number of specific mutations (hard steps i.e. improbable steps) in a certain order. The average time for each mutation is longer than a human lifetime, but we have a lot of cells in our body, so 40% of us develop cancer, the result of a series of improbabilities in a given cell.
If you think of all the planets in a galaxy that life can evolve on as cells and the ones that an ETC arises on being cancerous, you get the idea. The Hard Steps model is a power law, so the chances of an outcome happening in a given period of time is the inverse of the chance of a step happening (its hardness) to the power of the number of steps. Therefor the chance of anything happening in a given time goes down very rapidly with the number of hard steps required.
In Earth’s case, the given period of time is about 5.5 billion years, the time from Earth’s origin until the time that a runaway greenhouse sets in about a billion years from now.
The Number of Hard Steps in our Evolution
In 1983 Brandon Carter was looking into how likely it was for intelligent life to arise on Earth, and he thought that due to the limitations on the time available this could be modeled as a hard step problem. To quote:
This means that some of the essential steps (such as the development of eukaryotes) in the evolution process leading to the ultimate emergence of intelligent life would have been hard, in the sense of being against the odds in the available time, so that they are unlikely to have been achieved in most of the earth-like planets that may one day be discovered in nearby extra-solar systems.
Carter estimated that the number of hard steps it took to reach our technological civilization was six: biogenesis, the evolution of bacteria, eukaryotes, combogenisis [sex], metazoans, and intelligence. This, he concluded, seemed the best fit for the amount of time that had taken for us to evolve. There has been much discussion and examination of the number of hard steps in the literature, but the idea has held up fairly well so Hanson et al varied the number of hard steps around six as one of their model variables.
The Paper
The Hanson paper starts out by dividing ETCs into two categories: loud aliens and quiet aliens. To quote:
Loud (or “expansive”) aliens expand fast, last long, and make visible changes to their volumes. Quiet aliens fail to meet at least one of these criteria. As quiet aliens are harder to see, we are forced to accept uncertain estimates of their density, via methods like the Drake equation. Loud aliens, by contrast, are far more noticeable if they exist at any substantial density.
The paper then puts aside the quiet aliens as they are, with our current technology, difficult to find and focuses on the loud ones and, in a manner similar to Olsen, runs models but with the following three variables:
i) The number of hard steps required for an ETC to arise.
ii) The conversion rate of a quiet ETC into a loud, i.e. visible, one.
iii) The expansion speed of a civilization.
In their models, (like the one illustrated below) a civilization arises. At some point, it converts into an expansive civilization and spreads out until it abuts a neighbor at which point it stops. Further civilizations in the volume that is controlled are prevented from happening. Results showing alien civilizations that are visible from our point of view are discarded, narrowing the range of these variables. (Note: time runs forward going down the page.)
Results
In a typical run with parameters resulting in them not being visible to us, expansive civilizations now control 40-50% of the universe, and they will finish up controlling something like a million galaxies when we meet one of them in 200 million year’s time. (Note, this paradoxical result is due to the speed of light. They control 40-50% of the universe now, but the electromagnetic radiation from their distant galaxies has yet to reach us.)
From these models, three main outcomes become apparent:
Our Early Appearance
The Hard Step model itself contains two main parameters, number of steps and the time in which they must be concluded in. By varying these parameters, Hanson et al showed that, unless one assumes fewer than two hard steps (life and technological civilizations evolve easily) and a very restrictive limit on planet habitability lifetimes, then the only way to account for a lack of visible civilizations is to assume we have appeared very early in the history of civilizations arising in the universe. (In keeping with the metaphor, we’re a childhood cancer.)
All scenarios that show a higher number of hard steps than this greatly favor a later arrival time of ETCs, so an intelligent life form producing a technological civilization is at this stage of the universe is a low probability event.
Chances of other civilizations in our galaxy
Another result coming from their models is that the higher the chance of an expansive civilization evolving from a quiet civilization, the less the chance there is of there being any ETCs aside from us in our galaxy. To summarize their findings: assuming a generous million year average duration for a quiet civilization to become expansive, very low transition chances (p) are needed to estimate that even one other civilization was ever active anywhere along our past light cone (p < 10?3), or existed in our galaxy (p < 10?4), or is now active in our galaxy (p < 10?7).
For SETI to be successful, there needs to be a loud ETC close by, and for one to be close by, the conversion rate of quiet civilizations to expansive, loud ones must be in the order of one per billion. This is not a good result pointing to SETI searches being productive.
Speed of expansion
The other variable used in the models is the speed of expansion. Under most assumptions, expansive civilizations cover significant portions of the sky. However, when taking into account the speed of light, the further distant these civilizations are, the earlier they must form for us to see them. One of the results of this relativistic model is that the slower civilizations expand on average, the more likely we are to see them.
This can be demonstrated with the above diagram. The orange portion of the diagram shows the origin and expansion of an ETC at a significant proportion of the speed of light. We—by looking out into space are also looking back in time—can only see what is in our light cone (that which is below the red line), so we see the origin of our aliens (say one billion years ago) and their initial spread up to about half that age. After which, the emissions from their spreading civilization have not yet had time to reach us.
The tan triangle represents the area in space from which an ETC spreading at the same rate as the orange aliens would already have arrived at our planet (in which case we would either not exist or we would know about it), so we can assume that there were no expansive aliens having originated in this portion of time and space.
If we make the spread rate a smaller proportion of the speed of light, then this has the effect of making both the orange and tan triangles narrower along the space axis. The size of the tan exclusion area becomes smaller, and the green area, which is the area that can contain observable alien civilizations that haven’t reached us yet, becomes bigger.
You’ll also notice that the narrower orange triangle of the expansive ETC crosses out of out of our light cone at an earlier age, so we’d only see evidence of their civilization from an earlier time.
The authors note that the models rely on us being able to detect the boundaries between expansive civilizations and unoccupied space. If the civilizations are out there, but are invisible to our current instruments, then a much broader variety of distributions is possible.
Conclusions
We have always examined the evolution of life of Earth for clues as to the distribution alien life. What is important about this paper is that it connects the two in a quantitative way.
There are a lot of assumptions build into this paper (some of which I find questionable); however, it does give us a framework to examine them and test them, so it’s a good basis for further work.
To quote Hanson et al:
New scenarios can be invented and the observable consequences calculated immediately. We also introduce correlations between these quantities that are obtained by eliminating dependence on ? [appearance rate], e.g. we can express the probability of seeing at least one domain as a function of v [expansion velocity] and the currently life-saturated fraction of the universe based on the fact we haven’t see or have encountered any.
I would point out a conclusion the authors didn’t note. If we have arisen at an improbably early time, then there should be lots of places (planets, moons) with life at some step in their evolution, so while SETI searches don’t look promising from the conclusions of this paper, the search for signs of exosolar life may be productive.
This paper has given us a new framework for SETI. Its parameters are somewhat tangential to the Drake Equation’s, and its approach is to basically work the equation backwards: if N=0 (number of civilizations we can communicate with in the Drake equation, number of civilizations we can observe in this paper), then what is the range in values for fi (fraction of planets where life develops intelligence), fc (fraction of civilizations that can communicate/are potentially observable) and (L) length of time they survive. The big difference is that this paper factors in the temporal distribution of civilizations arising, which is not something the Drake Equation addressed. The Drake equation, for something that was jotted down before a meeting 61 years ago, has had a remarkably good run, but we may be seeing a time where it gets supplanted.
References
Robin Hanson, Daniel Martin, Calvin McCarter, Jonathan Paulson, “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare,” The Astrophysical Journal, 922, (2) (2021)
Thomas W. Hair, “Temporal dispersion of the emergence of intelligence: an inter-arrival time analysis,” International Journal of Astrobiology 10 (2): 131–135 (2011)
David Moore, “Lost in Time and Lost in Space: The Consequences of Temporal Dispersion for Exosolar Technological Civilizations,” Journal of the British Interplanetary Society, 63 (8): 294-302 (2010)
Brandon Carter, “Five- or Six-Step Scenario for Evolution?” International Journal of Astrobiology, 7 (2) (2008)
S.J. Olson, “Expanding cosmological civilizations on the back of an envelope,” arXiv preprint arXiv:1805.06329 (2018)
by Paul Gilster | Feb 9, 2022 | Culture and Society |
Anyone who ever had the pleasure of talking to Freeman Dyson knows that he was a gracious man deeply committed to helping others. My own all too few exchanges with him were on the phone or via email, but he always gave of his time no matter how busy his schedule. In the article below, Colin Warn offers an example, one I asked him for permission to publish so as to preserve these Dysonian nuggets for a wider audience. Colin is an Associate Propulsion Component Engineer at Maxar, with a Bachelor of Science in mechanical engineering from Washington State University. His research interests dip into in everything from electric spacecraft propulsion to small satellite development, machine learning and machine vision applications for microrobotics. Thus far in his young career, he has published two papers on the topics of nuclear gas core rockets and interstellar braking mechanisms in the Journal of the British Interplanetary Society. He tells me that when he’s not working on interstellar research, he can be found teaching music production classes or practicing martial arts.
by Colin Warn
Three years ago, I decided to make a switch from being a part time dance music ghost producer to study something that would help advance humanity’s knowledge of the stars. Eventually, I decided that something would be mechanical engineering, a switch which was in no small part due to space podcasts that introduced me to cool technologies such as Nuclear Pulsed Propulsion (NPP): Rockets propelled by small mini-nuclear explosions. The man behind this technology? Freeman Dyson.
Dyson worked on Project Orion for four years, deeply involved in studies that produced the world’s first and only prototype spacecraft powered by NPP. Due to the 1963 Partial Test Ban Treaty, which he supported, humanity’s best bet for interstellar travel was filed away. Yet, something about the audacity of this project resonated with me decades later when I uncovered it, especially when I found out that Dyson was in charge of this project despite not being a PhD.
So, as a bright-eyed and optimistic freshman entering his first year of college, figuring that out of anyone in the world he would have the best insights on what technology would lead humanity to the stars, I decided to send him this email:
Hello Professor Dyson,
My name is Colin Warn, and I’m a freshman pursuing a degree in mechanical engineering/physics.
Had a few questions for you regarding how I should structure my career path. My ambitions are to work on interstellar propulsion technologies, and I figured you might know a thing or two about the skill set required.
If you have the time, here’s what I’d love to hear your opinion on:
1. What research/internships would you suggest I focus on as an undergraduate to learn the skills that will be needed for working on advanced propulsion technologies? Especially in my freshman and sophomore years?
2. For my initial undergrad years, would you suggest that I focus more on taking physics or engineering courses initially?
Thank you so much in advance for your time. Been reading the book your son wrote about Orion. Let’s just say the reactions I’ve been getting from my friends when I tell them what I’m reading about is already quite fun to observe.
Regards,
-Colin
I sent it to his Princeton email, as I’ve sent many emails in the past to fairly high-caliber people, without a hope of getting anything in return.
Two days later, I woke up to find this email in my inbox.
Dear Colin Warn,
I will try to answer your two questions and then go on to more general remarks.
1. So far as I know, the only techniques for interstellar propulsion that are likely to be cost-effective are laser-propelled sails and microwave-propelled sails. Yuri Milner has put some real money into his Starshot project using a high-powered laser beam. Bob Forward many years ago proposed the Starwisp spacecraft using a microwave beam. Either way, the power of the beam has to be tens of Megawatts for a miniature instrument payload of the order of a gram, or tens of Terawatts for a human payload of the order of a ton. My conclusion, the manned mission is not feasible for the next century, the instrument mission might be feasible.
For the instrument mission, the propulsion system is the easy part, and the miniaturization of the payload is the difficult part. Therefore, you should aim to join a group working on miniaturization of instruments, optical sensors, transmitters and receivers, navigation and information handling systems. These are all the technologies that were developed to make cell-phones and surveillance drones. An interstellar mission is basically a glorified surveillance drone. You should go where the action is in the development of micro-drones. I do not know where that is. Probably a commercial business attached to a technical university.
2. For undergraduate courses, I would prefer engineering to physics. Some general background in physics is necessary, but specialized physics courses are not. More important is computer science, applied mathematics, electrical engineering and optics, chemistry of optical and electronic materials, microchip engineering. I would add some courses in molecular biology and neurology, with the possibility in mind that these sciences may be the basis for big future advances in miniaturization. We still have a lot to learn by studying how Nature does miniaturization in living cells and brains.
This email contained more detailed insights to my questions than I could have ever hoped for. Then to top it all off, he still had one more piece of advice for me to a question I hadn’t even asked.
General remarks. In my own career I never made long-range plans. I would advise you not to stick to plans. Always be prepared to grab at unexpected opportunities as they arise.Be prepared to switch fields whenever you have the chance to work with somebody who is doing exciting stuff. My daughter Esther, who is a successful venture capitalist running her own business, puts at the bottom of every E-mail her motto, “Always make new mistakes”. That is a good rule if you want to have an interesting life.
With all good wishes for you and your career, yours sincerely,
Freeman Dyson.
Upon reading this email in 2018, I promised myself that one day I’d put myself in a position to thank him in person. Sadly I’ll never get the opportunity. I discovered watching an old YouTube video featuring him that he died in February of 2020.
So this article is my way of saying thank you to him. For creating literal star-shot projects to inspire a new generation. For being someone who always questioned the status quo. But most of all, for still being down to earth enough to email some amazingly insightful answers to a freshman’s cold-email. I hope one day I’m in a position where I can pass on the favor.
