Centauri Dreams

I’m still smarting about having to cancel my travel plans for the Tennessee Valley Interstellar Workshop in Chattanooga, particularly since the Tau Zero Foundation was one of the sponsors of the event. But fortunately, I do have people offering to write up the workshop for Centauri Dreams, so we’ll have some coverage and photos soon. Onward…

Hunting for Biosignatures

We only have two years before the James Webb Space Telescope is scheduled to launch. Assuming all goes well, JWST should help ease us into the era of biosignature detection, as we look for the characteristic signs of living organisms in the atmospheres of their worlds. But just how definitive are such signatures? A new paper from the University of Washington digs into potential false positives and offers specifics on the signatures that could fool us.

One way to study biosignatures is by transit spectroscopy, using data gathered from starlight as it passes through a planet’s atmosphere during a transit. This allows us to analyze the features of light that flag particular atmospheric constituents. Would a strong oxygen signature necessarily indicate life? You would think so, because the oxygen in Earth’s atmosphere, O₂, is unstable over geological timeframes and would gradually be depleted through reactions with volcanic gases and oxidation at the surface.

On our planet, then, oxygen needs a replenishing source, which is provided through photosynthesis as plants and algae tap sunlight for energy. But Edward Schwieterman and team argue that while abiotic oxygen will not build up to significant levels on Earth, it can certainly arise in a number of different planetary settings. That fact should give us pause.

Working under the aegis of the university’s Virtual Planet Laboratory, Schwieterman sees oxygen as a potential ‘biosignature impostor.’ The abiotic creation of oxygen, particularly around the kind of low mass stars that are likely to be our first targets for this kind of work, can happen when ultraviolet light from the star splits carbon dioxide molecules, allowing some of the oxygen atoms to form O₂. We get oxygen without any requirement for biology to sustain it.

Image: New research from the University of Washington-based Virtual Planetary Laboratory will help astronomers better identify and rule out “false positives” in the ongoing search for life. Shown is an artist’s rendering of Kepler 62E, about 1,200 light-years away in the constellation Lyra. Credit: NASA

Usefully, Schwieterman’s computer models show that this process will also produce detectable amounts of carbon monoxide. Thus the presence of carbon dioxide and carbon monoxide would raise doubts about the simultaneous detection of oxygen in that planetary atmosphere. Abiotic processes rather than life might well be the agent. We can use this to our advantage:

…in the cases presented here, the spectral discriminators against abiotic O₂/O₃ are more detectable with a hypothetical JWST observation than the O₂ or O₃ signatures themselves. In our example spectra, neither O₂ nor O₃ would be directly detectable with just 10 transits, but the abiotic discriminators CO/CO₂ and O₄ could be. This provides an opportunity to maximize the utility of observing time if the ultimate goal is to characterize planets where true biosignatures are obtainable. If spectral indicators for biosignature impostors are detected with reasonable confidence, the community may wish to reallocate the remaining time to other promising targets, rather than integrate further.

The mention of O₄ above is a reference to a second kind of biosignature impostor, where light from the primary star breaks down atmospheric water on the rocky planet under investigation. In this case, we get large amounts of oxygen as the hydrogen escapes, and might expect to find short-lived pairs of oxygen molecules that become O₄ molecules, producing a distinctive signature of their own. A higher percentage of oxygen is produced than Earth has ever had in its atmosphere, as Schwieterman notes:

“Certain O₄ features are potentially detectable in transit spectroscopy, and many more could be seen in reflected light. Seeing a large O₄ signature could tip you off that this atmosphere has far too much oxygen to be biologically produced.”

Detecting this particular ‘biosignature impostor’ is best done through direct imaging, while carbon dioxide and carbon monoxide signatures are more readily identified through transit spectroscopy. The paper looks at simulations of direct imaging spectra:

This simple test case demonstrates that if the high-O₂ atmospheres proposed by Luger & Barnes (2015) exist, the O₄ absorption band strength in those planetary spectra would rival or exceed that of the monomer O₂ bands. These spectra are qualitatively different than modern-Earth’s spectrum, even in the 0.3-1.0 µm range, with a different shape, broader O₂ features, and additional features from O₄. These are all signs of a much higher O₂ abundance than the Earth’s atmosphere – self-regulated by negative feedbacks – has ever achieved.

Schwieterman and team believe the first potentially habitable planets whose atmospheres we study will be orbiting M-class dwarf stars, the kind of planets most likely to show both kinds of biosignature impostors discussed above. Thus we’ll be faced with the need to interpret potential biosignatures that could be confusing without these criteria. Says Schwieterman:

“The potential discovery of life beyond our solar system is of such a huge magnitude and consequence, we really need to be sure we’ve got it right — that when we interpret the light from these exoplanets we know exactly what we’re looking for, and what could fool us.”

The paper is Schwieterman et al., “Identifying Planetary Biosignature Impostors: Spectral Features of CO and O₄ Resulting from Abiotic O₂/O₃ Production,” Astrophysical Journal Letters Vo.. 819, No. 1 (25 February 2016). Abstract / preprint. A University of Washington news release is also available.

When I first got interested in SETI, I naively assumed that we would get a detection fairly soon, and that we would detect not a directed beacon but simple background traffic in a remote civilization. I had no idea at the time how difficult it would be to pick up the kind of radio traffic we routinely generate on Earth from a distant star, and as a matter of fact, my interest in shortwave radio led me to assume that, just as I enjoyed the sport of DX — listening for distant signals — so SETI would simply be an offshoot of this, with a harder-to-get QSL card.

Some time in the mid-1980s I wrote a piece called “Where the Real DX Is” for Glenn Hauser’s Review of International Broadcasting, running through a list of the nearest stars and talking about SETI projects that had been tried up to then. I haven’t gone back to read that article in years and would probably find it an embarrassing chore. But it’s interesting to me that the idea of leakage radiation does have its place, particularly in the area of beamed power.

Power Beaming at Work

What I knew little of back in the 80’s was how beamed power — using high-power microwave beams but including millimeter-wave beams and lasers — might empower a space-based infrastructure. While television and radio signals are all but undetectable from another star (at least, for a civilization at our level of development), intense, focused beams to impart power to a spacecraft present a better target. The interesting question that James Benford (Microwave Sciences) and son Dominic (NASA GSFC) ask in the paper we began looking at yesterday is just how detectable power beaming would be.

This takes off on ideas Jim Benford discussed last year in these pages (see Seeing Alien Power Beaming), and both Jim and Dominic recently looked at the question in relation to SETI efforts on a very intriguing star indeed in Quantifying KIC 8462852 Power Beaming. After all, if a distant civilization were building something like a Dyson sphere or ‘swarm’ around a star, it would have to have constructed an infrastructure that might well involve power beaming technologies.

Image: A favorite image from a favorite artist, Rick Sternbach. Here we’re looking at an interstellar lightsail near an exotic world. The uses of beamed power in near and deep space are numerous, and many of them appear to be detectable from a SETI perspective. The image comes from a 1983 issue of Science Digest. Credit: Copyright Rick Sternbach.

The Benford paper runs through a number of beaming concepts ranging from orbit raising, a lower power application, to actual launch to orbit using microwave thermal thrusters. Various high-power beaming applications have been quantified for missions within the Solar System as well, using microwave or laser beams to boost craft to 100 to 200 kilometers per second. A mature infrastructure might involve high-speed unmanned supply craft being decelerated upon arrival by a beam system similar to the one that launched it. I mentioned yesterday that beaming from a solar power station in space to a planetary surface is not a likely observable. Here’s why, whether we’re talking about microwave bands or lasers in the optical range:

The beam must be carefully controlled to deliver power to the receiving rectifying antennas on the ground (Mankins 2014). Any side emissions are economic losses, therefore substantial measures would be taken to reduce side lobes to a minimum. Further, the first several sidelobes are absorbed in the ground. The remaining side lobes are dispersed in angle so that the power density in the far field will be very low. For the worked example in Mankins (Mankins 2014; Dickinson 2016), the back lobe is down 40 dB relative to the ? 1GW main beam.

And note this:

This is in contrast to power beaming transportation applications, in which the varying solid angle of the receiving spacecraft results in the main beam increasingly leaking around the edges of the vehicle being accelerated.

So that takes detection of one of the numerous power beaming scenarios off the table, but many others survive scrutiny. Have a look at this table from the paper, which draws on representative parameters for applications of power beaming presented earlier in the discussion.

Here we’re looking at observables in terms of slew rate (movement of the beam), the EIRP and the observation time. EIRP stands for Effective Isotropic Radiated Power, the measured radiated power in a single direction — it’s the product of the antenna gain times the power radiated. For useful background, the standard text is Jim Benford’s own High Power Microwaves (now in its third edition, CRC Press, 2016). A quick note on EIRP from the paper:

Spectral flux density, typically denoted in Janskys, is the power density divided by the bandwidth. While this is commonly used as the observed quantity in radio astronomy, we cannot know the bandwidth of an ETI transmitter. Consequently, in thinking about ETI power beaming mission [sic] we must deal with EIRP, not spectral flux density. Beaming power does not require or even necessarily benefit from narrow bandwidth; energy transference is what matters.

Ranking the Observables

The possibilities are numerous, and not all of them designed for deep space. Microwave thermal thrusters, for example, can be fed by a high-power microwave beam in a single-stage rocket with a flat aeroshell on the underside covered by microwave-absorbing heat exchangers. The system is efficient because the energy source remains on the ground — see the paper for further parameters on this as well as orbit raising, where ground-based energy sources lift a satellite to a higher orbit. As the table shows, orbit raising is significantly lower in power than launch from the surface.

But of course beaming applications deeper into the system are a part of a robust infrastructure based on beaming, and here we can look at interplanetary transfers by beam-driven sails, as well as starship concepts involving prolonged acceleration of an interstellar vehicle, like the Robert Forward concepts I mentioned yesterday. Given the power requirements for starships, such beams would be detectable with our current technology, but this would also require that the Earth fall within the bandwidth of the power beam.

These questions of detectability depend, of course, upon the level of technology of the civilization trying to make the detection. In our case, we have the interesting story of the star KIC 8462852 to give us some guidance. We have radio observations of this curious star that found no signs of power beaming in about 180 hours of observations. The idea was to look for incidental radiated power — leakage radiation — produced by an advanced civilization. The observations were made using the Allen Telescope Array in the 1-10 GHz range.

Refer back to the table above and the beaming applications there. These are the authors’ conclusions re the KIC 8462852 observations using the Allen Telescope Array:

• The 1 Hz channels could see all the applications, but they are not seen.
• Launch from a planetary surface into orbits is marginally detectable, at the threshold of the Allen Array for the 100 kHz observations, if at the frequencies observed. Orbit raising, which requires lower power, is not detectable.
• Interplanetary transfers by beam-driven sails should be detectable in their observations, but are not seen. This is for both the 1 Hz and for the 100 kHz observations.
• Starships launched by power beams with beamwidths that we happen to fall within (to other solar systems, not our own) would be detectable, but are not seen.

We also learn that the limited observation times and wavelength coverage mean that we have no firm conclusions about the star. More systematic studies, which would include observations at higher radio frequencies, would be called for to come up with a stronger result. I should also mention that an optical SETI attempt was made on KIC 8462852 at the Boquete Optical SETI Observatory in Panama. Given the detection limits of the equipment there and other issues discussed in the paper, none of the power beaming applications in consideration by the Benfords could have been detected at the Boquete site.

The authors’ conclusion:

*As discussed above, the beaming power levels are high and transient and easily dwarf any ETI civilization’s diffuse leakage to space (Sullivan 1978). Power beaming described here is larger than that necessary for beaming systems for communication: EIRP = 10¹⁸ W for a 1,000ly-range beacon (Benford, Benford & Benford 2010). SETI programs could explore a different part of parameter space by observations suitable to finding leakage from power beams.

We have no way of knowing whether any civilization exists outside of our own, nor can we know if such a civilization would use power beaming. But the virtues of beamed power for fast transportation within our own Solar System — and for lowering the cost to orbit, as well as raising orbits — give us good reason to continue to explore the use of this technology. In doing so, we should keep in mind that we are producing an observable signal that is well beyond those undetectable “I Love Lucy” episodes that are now 65 light years out. I think the Benfords are right when they argue that detectable radiation is a message in itself, whether or not we ever decide to impose a structured message into the beams that power a future infrastructure.

The paper is Benford & Benford, “Power Beaming Leakage Radiation as a SETI Observable,” submitted to The Astrophysical Journal and available as a preprint.

Thinking that we can understand the motivations of an extraterrestrial civilization seems like a fool’s gambit, but we have to try. The reason is obvious: We have exactly one technological society to work with — we’re all we have — and if we want to look for SETI signals, we have to interpolate as best we can. An alien culture, it is assumed, will do the same. This was the procedure outlined by Giuseppe Cocconi and Philip Morrison in their classic 1959 paper “Searching for Interstellar Communications,” that began the modern era of SETI.

If there are civilizations around stars like the Sun, the paper reasons, then some will be motivated to reach out elsewhere. From the paper:

To the beings of such a society, our Sun must appear as a likely site for the evolution of a new society. It is highly probable that for a long time they will have been expecting the development of science near the Sun. We shall assume that long ago they established a channel of communication that would one day become known to us, and that they look forward patiently to the answering signals from the Sun which would make known to them that a now society has entered the community of intelligence. What sort of a channel would it be?

What results is an explanation of the factors needed to overcome signal attenuation and the frequencies most likely to be used. Thus we get what Cocconi and Morrison called “a unique, objective standard of frequency, which must be known to every observer in the universe.” This is 1420 MHz, the 21 centimeter wavelength of neutral hydrogen. Thus we know where to look assuming a civilization is trying to make contact with us. The authors conclude “… the foregoing line of argument demonstrates that the presence of interstellar signals is entirely consistent with all we now know, and that if signals are present the means of detecting them is now at hand.”

The Hunt for SETI Observables

We’ve looked frequently in these pages at how SETI has changed since the Cocconi and Morrison days, with so-called ‘Dysonian SETI’ invoked as a way of looking for observable evidence in our astronomical data. The unusual star KIC 8462852 has elevated the method to wider attention because of the possibility that some kind of astroengineering could explain its unusual light curves (search the archives here for numerous articles on the star).

In any case, Dysonian SETI assumes, contra Morrison and Cocconi, no intent to contact anyone. We are simply looking for activity, albeit on a colossal scale. We must be the ones to figure out what that activity might be.

Which brings me to a new paper from James Benford and son Dominic (NASA GSFC). “Power Beaming Leakage as a SETI Observable,” submitted to The Astrophysical Journal, asks whether we might detect the use of power beaming to transfer energy and accelerate spacecraft within a distant solar system and, perhaps, beyond it if the technology is being used to accelerate starships. KIC 8462852 is also considered here because there have been two attempts to study it, one optical and one at radio frequencies, in the context of a SETI search.

As the paper notes:

The most observable leakage from an advanced civilization may well be from the use of power beaming to transfer energy and accelerate spacecraft, both within and beyond the star system where the civilization is located. In future, such applications may make the Earth’s radiation in the microwave, millimeter and visible/near-IR parts of the electromagnetic spectrum be very intense…. The power levels are high, focused, and transient and could easily dwarf any of our previous leakage to space. These are not SETI signals so much as leakage, a detectable aspect of advanced civilizations.

The kind of SETI observable the Benfords are examining is essentially leakage, but of a different kind than the widely repeated trope of picking up alien TV signals, just as extraterrestrial cultures are presumably now enjoying “I Love Lucy” from our signals. As the Benfords argue, the leakage of radio and television signals is essentially undetectable between stars not only due to the weakness of the signal but to its lack of coherence. Planetary radars are much more likely to be detectable, but signals like these are also extremely transient.

Intense beams of radiation being used to move power about in a distant solar system or accelerate spacecraft on long journeys should be more readily detected. James Benford argued as much in 2008, and more recently James Guillochon and Abraham Loeb have quantified the leakage to be expected from space propulsion-related beaming, showing that a beam being used to drive a large sail would be observable. The duo write: “…for a five-year survey with ~10 conjunctions per system, about 10 multiply-transiting, inhabited systems would need to be tracked to guarantee a detection” using our existing radio telescope infrastructure.

Image: Taken from the Guillochon and Loeb paper, this diagram shows a Mars mission using microwave beaming for propulsion. In the schematic, the dashed line represents the sailcraft. You can see the prospects for signal leakage here. Notice the inset showing the beam profile as it overlaps the sail of diameter D_s. Credit: Guillochon & Loeb.

But remember the notion we started with: If we are trying to understand what an extraterrestrial civilization might do, we have to look at the uses we ourselves would make of these technologies. We try to be as flexible as possible while acknowledging the fundamental gap in our knowledge of an alien culture, but we have to start somewhere. And this kind of reasoning takes us down an interesting path, as the Benfords note in their paper. For possessed of powerful beaming technologies, such a culture should be aware of SETI implications:

It has previously been noted that such leakage from other civilizations could be observable (Benford 2008). Guillochon & Loeb (2015) have quantified leakage from beaming for interplanetary space propulsion, its observables, and implications for SETI. Extraterrestrial Intelligence (ETI), having done the same thinking, could realize that they could be observed. Hence there may be a message on the power beam, delivered by modulating it in frequency, amplitude, polarization, phase, etc., and broadcast it for our receipt at little additional energy or cost. By observing leakage from power beams we may well find a message embedded on the beam.

That’s a fascinating notion in its own right. It’s a kind of METI signal sent out by an alien civilization that is, like a beacon, broadly targeted rather than aimed at a specific solar system. And it operates on the assumption that another culture might in the course of its SETI investigations notice high-powered, focused and transient beams and analyze them. Or put another way, if our own civilization has figured this out, our extraterrestrial counterparts working the SETI various concepts must surely have come up with the same implications.

Uses of the Beam

Detecting a message embedded in a power beam would, of course, be a breakthrough of historic proportions, but so would the simple detection of power-beaming itself — no message attached — which would signal the presence of an advanced civilization. One of the things recently looked at by the Allen Telescope Array was whether the intriguing star KIC 8462852 showed any signs of activity, and it turns out that the observations, while finding nothing, do allow us to set some limits on power-beaming in that system. KIC 8462852 became, in other words, a useful exercise even though the observations were short-lived. More tomorrow on this, and also see the Benfords’ Quantifying KIC 8462852 Power Beaming in these pages.

And just what might a technological civilization do with power-beaming methods that we could observe? One answer is obvious: We’ve been talking about beamed sails for well over ten years on Centauri Dreams, keying off Robert Forward’s fascination with the subject as it applied to interstellar missions. Here the requirements are enormous, with a space-based solar power station beaming to a 1000-kilometer sail in some of Forward’s missions, although subsequent work found ways to trim the sail down to the 1-10 kilometer range.

The idea of a power station orbiting in the inner part of the Solar System where the photon-flow is maximized has dual use, as using microwave beams to transport power to a planet’s surface is a major driver. As a SETI observable, power beaming to a planet is not a likely target. The paper notes that the beam would have to be tightly controlled, with side lobes maximally reduced. In contrast, power being beamed to a spacecraft should show increasing leakage as the craft is accelerated — the beam increasingly leaks around the edges of the accelerating vehicle — making transportation applications a realistic SETI observable.

The Benfords’ paper quantifies the various kinds of power beaming missions and applications and their observable parameters, looking at twenty concepts in terms of power radiated, duration and likely time for the radiation to repeat. More on this tomorrow as we continue looking at the paper and the question of how an advanced technology could use power beaming.

The Benfords paper is “Power Beaming Leakage Radiation as a SETI Observable,” submitted to The Astrophysical Journal and available as a preprint. The Guillochon and Loeb paper is “SETI via Leakage From Light Sails in Exoplanetary Systems,” The Astrophysical Journal 811, No. 2 (23 September 2015), with abstract here. Jim Benford also discusses the Guillochon and Loeb paper in Seeing Alien Power Beaming.