by Paul Gilster | Jul 25, 2019 | Astrobiology and SETI |
Breakthrough Listen has just announced a new optical SETI effort in partnership with the VERITAS Collaboration. The news took me by surprise, for VERITAS (Very Energetic Radiation Imaging Telescope Array System) generally deals in high-energy astrophysics, with a focus on gamma rays, which signal their presence through flashes of Cherenkov radiation when they strike the Earth’s atmosphere. Here, the array is being used to look for technosignatures, as Andrew Siemion (UC- Berkeley SETI Research Center) explains:
“Breakthrough Listen is already the most powerful, comprehensive, and intensive search yet undertaken for signs of intelligent life beyond Earth. Now, with the addition of VERITAS, we’re sensitive to an important new class of signals: fast optical pulses. Optical communication has already been used by NASA to transmit high definition images to Earth from the Moon, so there’s reason to believe that an advanced civilization might use a scaled-up version of this technology for interstellar communication.”
Image: View of the Fred Lawrence Whipple Observatory basecamp and the VERITAS array. Credit: VERITAS.
So the search for faint optical flashes that could signal the presence of an extraterrestrial civilization deepens, complementing the optical SETI work currently underway at Breakthrough Listen as well as its ongoing survey at radio frequencies. VERITAS brings four 12-meter telescopes located at the basecamp of the Fred Lawrence Whipple Observatory on Mount Hopkins in Arizona into the mix. This is quite an exoplanet venue: The observatory has facilities at different elevations, including exoplanet arrays for HAT (Hungarian-made Automated Telescope), the MEarth project and MINERVA, all three of these robotic.
In the Breakthrough Listen effort, VERITAS will be looking for pulsed optical beacons with durations as short as several nanoseconds, for at timescales like these, an artificial beacon could outshine any stars located in the same region of sky. All four telescopes will be used simultaneously, which should assist the effort in screening out false positive detections.
Although I hadn’t realized it until looking further into VERITAS, the array has already seen use in a search of Boyajian’s Star for such pulses (see Abeysekara et al., “A Search for Brief Optical Flashes Associated with the SETI Target KIC 8462852,” abstract here). You’ll recall that this star has received intense scrutiny because of its unusual pattern of dimming, which did not correspond to planetary transits and raised questions about the source of the lightcurve variations.
Now VERITAS goes to work on stars not already found on Breakthrough Listen’s primary star list. The numbers are striking: Breakthrough Listen calculates that if a laser delivering 500 terawatts in a pulse lasting a few nanoseconds were located at the same distance as Boyajian’s Star (an F3V-class object in Cygnus approximately 1470 light years away) and pointed in our direction, VERITAS would be able to detect it.
Most stars in the Breakthrough Listen target list, however, are considerably closer. Hence the VERITAS search will be sensitive to pulses a factor 100 – 10,000 times fainter still. Thus an array built with the purpose of studying very-high-energy gamma rays proves adaptable to a search for technosignatures, with UC-Santa Cruz physicist David Williams, one of the effort’s leaders, saying “It is impressive how well-suited the VERITAS telescopes are for this project.” Williams will work in collaboration with Jamie Holder (University of Delaware) and Andrew Siemion’s Breakthrough Listen team at UC-Berkeley’s SETI Research Center (BSRC).
While we’re on the topic of SETI, let me also call your attention to a new resource that Penn State’s Jason Wright and Alan Reyes have created. Go to the NASA ADS site and include in your search terms ‘bibgroup:SETI’. I just searched, for example, using ‘author:”maccone” bibgroup:SETI’ and came up with 56 hits. SETI has been short on bibliographical resources, so this is promising stuff. You’ll need to familiarize yourself with the search syntax, but it’s not at all difficult, and will reward those looking to firm up a citation or check on the status of a particular scientist’s work. Wright and Reyes have submitted a paper on the bibliography to JBIS. For more, see Towards a Comprehensive Bibliography for SETI.
Image: Penn State’s Jason Wright. Credit: PSU.
by Paul Gilster | Mar 28, 2019 | Deep Sky Astronomy & Telescopes |
VISTA (Visible and Infrared Survey Telescope for Astronomy) is a near-infrared instrument located at the European Southern Observatory’s Paranal site, and is by all accounts the world’s largest survey telescope, with extremely wide field of view and sensitive detectors. On the peak next to ESO’s Very Large Telescope (VLT), VISTA shares its exceptional viewing conditions using a 4.1-meter primary mirror and a three-tonne camera with 16 infrared detectors.
With its time devoted to six surveys ranging from complete southern sky coverage to small patches of sky looking for extremely faint objects, VISTA was bound to come up with interesting data, especially in the survey known as VVV, which stands for VISTA Variables in Via Láctea. Here, astronomers are homing in on regions that are obscured by dust in the bulge and southern Galactic disk, using pulsating RR Lyrae and Cepheid variables as distance indicators, with a focus on microlensing events, eclipsing binaries and pre-main sequence variable stars.
Image: VVV-WIT-07 in the centre of a star field. Credit: Saito et al.
This is fertile ground for discovery, but even so, the object known as VVV-WIT-07 is taking everyone a bit by surprise. We’re dealing with a star that reminds everyone of Boyajian’s Star (KIC 8462852), famed for its unusual dips in lightcurve. Were they signs of a Dyson Sphere under construction, or a natural phenomenon the likes of which we had never seen? Uneven rings of dust, dusty planetesimals or comets are still in contention. Now we have a star that is apparently even more extreme, one whose range in lightcurve variation is extraordinary. Viewed by the survey over a period of eight years, VVV-WIT-07 has been seem to dim first by a factor of 2, then by almost 80 percent. A week later, it was back to normal.
Here’s what the ESO blog had to say about the matter:
“The first observations showed nothing strange — simply a mild scatter in the brightness measurements, consistent with the observational uncertainties. However, in August-September 2011, just before the end of the observing season, the star dimmed by a factor of almost two! By June 2012, when we began re-observing it, the star’s brightness was nearly back to normal. But by mid-July, it had dimmed by almost 80%! Then it was back to its usual self in about a week. The data taken since then contain hints of additional drops in brightness, but nothing so dramatic.”
The WIT designation is fun — it stands for What Is This, and the VVV team is using it to describe any objects that do not apparently fit known classes of stellar variability. WIT may remind you of another appellation for Boyajian’s Star, which was the ‘WTF Star,’ its ambiguous acronym standing surely for ‘Where’s the Flux?’ Have a look at the lightcurve of VVV-WIT-07:
Image: Lightcurve of VVV-WIT-07 showing how it varied in brightness between 2010 and 2018. The insert shows an expanded view of the particularly dramatic dimming event that occurred in July 2012. Credit: Saito et al.
Is it possible we are looking at some kind of circumstellar disk with huge variations in it, a clumpy disk that blocks the star’s light in this highly irregular way? The odds on that seem long. Here is what the paper says:
Alternative scenarios for VVV-WIT-07 include a “dipper” T Tauri star with clumpy dust structures orbiting in the inner disk that transit our line of sight (e.g. Rodriguez et al. 2017), or even a long period, high-inclination X-ray binary. The deep, narrow eclipse delayed with respect to a broad and shallower dip is reminiscent of the morphology seen in high-inclination low-mass X-ray binaries (LMXB, e.g. Parmar et al. 1986; Baptista et al. 2002). However, LMXBs are restricted to orbital periods of less than a few days while high-mass x-ray binaries (HMXB) can be found at Porb up to hundreds of days (e.g. X1145-619 has Porb = 187.5 d, Watson et al. 1981). Moreover, in this scenario optical and IR spectra would be dominated by the mass-donor companion star, and should show rotationally-broadened hydrogen absorption lines at epochs of no mass ejection episodes, which is not the seen in the spectra of VVV-WIT-07.
So we have something that gives us echoes of Boyajian’s Star, and may also remind us of Mamajek’s Object (J1407), an interesting pre-main sequence K5 star with a ring system eclipsing it. Or something we haven’t yet identified, perhaps in the form of multiple objects, may be moving between us and the host star, even a dense family of comet-like objects. VIV-WIT-07 would be easy enough to explain if it were a binary, but the observations clearly rule that out.
VISTA has observed VIV-WIT-07 85 times already. Needless to say, it will be the subject of even more intense scrutiny. The paper also notes recent discoveries like OGLE LMC-ECL-11893, an eclipsing star consistent with a dense circumstellar dust disk structure, and PDS 110, an eclipsing system with likely transits by a companion with a circumstellar disc.
The paper is Saito et al., “VVV-WIT-07: another Boyajian’s star or a Mamajek’s object?” in process at Monthly Notices of the Royal Astronomical Society (preprint).
by Paul Gilster | Oct 15, 2018 | Deep Sky Astronomy & Telescopes |
These days we take in data at such a clip that a mission like New Horizons will generate papers for decades. The same holds true for our burgeoning databanks of astronomical objects observed from the ground. So it only makes sense that we begin to recover older datasets, in this case the abundant imagery — photographs, radio maps, telescopic observations — collected in the pre-digital archives of scientific journals. The citizen science project goes by the name Astronomy Rewind, and it’s actively resurrecting older images for comparison with new data.
Launched in 2017, Astronomy Rewind originally classified scans in three categories: 1) single images with coordinate axes; 2) multiple images with such axes; and 3) single or multiple images without such axes. On October 9, the next phase of the project launched, in which visitors to the site can use available coordinate axes or other arrows, captions and rulers to work out the precise location of each image on the sky and fix its angular scale and orientation.
Image: Astronomer E. E. Barnard photographed the Rho Ophiuchi nebula near the border of Scorpius in 1905 through a 10-inch refractor. When he published the image in the Astrophysical Journal five years later, he discussed the possibility — then fiercely debated — that bright nebulae are partially transparent and dark nebulae are opaque, hiding material farther away. Other researchers argued that dark nebulae are simply regions where stars and gas are absent. Credit: American Astronomical Society, NASA/SAO Astrophysics Data System, and WorldWide Telescope.
We have over a century of images to work with, some 30,000 at present drawn from American Astronomical Society journals the Astronomical Journal (AJ), Astrophysical Journal (ApJ), ApJ Letters, and the ApJ Supplement Series. These images were provided through the Astrophysics Data System (ADS), which draws on NASA funding and provides bibliographical and archival services at the Smithsonian Astrophysical Laboratory (SAO), which is part of the Harvard-Smithsonian Center for Astrophysics.
What’s next for the initial round of imagery is inclusion into the WorldWide Telescope. Originally a Microsoft project, the WWT is now managed by the American Astronomical Society, and serves as what the AAS calls a ‘virtual sky explorer that doubles as a portal to the peer-reviewed literature and to archival images from the world’s major observatories.’ 10,000 images (those with coordinate axes) are to be placed within the WWT within a few months, while volunteers proceed to identify where the remaining 20,000 images belong on the sky.
Image: Barnard’s photo has been placed on the sky in its proper position and orientation and is displayed in WorldWide Telescope (WWT) superimposed on a false-color background image from NASA’s Wide-field Infrared Survey Explorer (WISE). Credit: American Astronomical Society, NASA/SAO Astrophysics Data System, and WorldWide Telescope.
But these images are not the only ones arriving for inclusion into the growing database. Results from the related ADS All Sky Survey are also going into the WorldWide Telescope, along with a European image display tool called Aladin, developed at the Centre de Données astronomiques (CDS), Strasbourg Observatory, France. The software highlights the effectiveness of the concept, for with Aladin, users will be able to click on any image that originally appeared in one of the AAS journals and call up the corresponding research paper. Alyssa Goodman, one of the project’s leaders at the Harvard-Smithsonian Center for Astrophysics (CfA), comments:
“Without Astronomy Rewind, astronomers would be unlikely to make the effort to extract an image from an old article, place it on the sky, and find related images at other wavelengths for comparison. Once our revivified pictures are incorporated into WorldWide Telescope, which includes images and catalogs from across the electromagnetic spectrum, contextualization will take only seconds, making it easy to compare observations from a century ago with modern data to see how celestial objects have moved or changed.”
Image: In these two figures, Barnard’s photo has been made partially and fully transparent, respectively, to reveal it in context. In the visible-light photo, gas glows brightly while dust appears in silhouette. In infrared light, as seen by WISE, dust glows brightly where in visible light there was nothing but blackness. Barnard was right! Credit: American Astronomical Society, NASA/SAO Astrophysics Data System, and WorldWide Telescope. Credit: AAS.
As Centauri Dreams readers know, I’ve often enthused about the potential for citizen science projects both in terms of their effectiveness at identifying and cataloging astronomical phenomena as well as the opportunity they present for non-professionals to contribute to fields ranging from deep sky objects to exoplanets and our own Solar System. Astronomy Rewind is clearly keeping the momentum of such efforts going. As it moves into a more challenging phase of confirming the position, scale, and orientation of decades-old astronomical images, the project will offer help features run by astronomy graduate students.
Thus we revive work going back to the 19th Century and link to the work discussing it, with all journal images contextualized on the sky. That’s quite a goal, and it invariably reminds me of the debate over Boyajian’s Star (KIC 8462852, more familiarly known as Tabby’s Star), in which the question of long-term dimming was addressed by a study of 500,000 photographs in the archives of Harvard College Observatory, over a century’s worth of images being digitized through the Digital Access to a Sky Century@Harvard (DASCH) project.
Projects like these are massive in scope and their efforts constitute a heartening work in progress. Ultimately, every astronomical image available in any scientific journal or academic or observatory collection will be catalogued, giving us a way to study the sky over periods of time that are lengthy in comparison to a human lifetime but tiny at the astronomical scale. Nonetheless, KIC 8462852 showed us how an unexpected need to examine old data could propel a scientific debate and flesh out information about a newly discovered mystery.
by Paul Gilster | Jan 16, 2018 | Astrobiology and SETI |
Science fiction dealt with interstellar navigation issues early on. In fact, Clément Vidal’s new paper, discussed in these pages yesterday, notes a George O. Smith story called “Troubled Star,” which originally ran in a 1953 issue of Startling Stories and later emerged as a novel (Avalon Books, 1957). Smith is best remembered for a series of stories collected under the title Venus Equilateral, but the otherwise forgettable Troubled Star taps into the idea of using an interstellar navigation network, one that might include our own Sun.
The story includes this bit of dialogue between human and the alien being Scyth Radnor, the latter explaining why his civilization would like to turn our Sun into a variable star:
“We use the three-day variable to denote the galactic travel lanes. Very effective. We use the longer variable types for other things – dangerous places like cloud-drifts, or a dead sun that might be as deadly to a spacecraft as a shoal is to a seagoing vessel. It’s all very logical.”
“…you’re going to make a variable star out of Sol, just for this?”
Well, why not, in Scyth Radnor’s view — after all, what’s one star in a galaxy-spanning navigation network? From our point of view, distant pulsars make for less local disruption, and as we saw yesterday, navigation by X-ray millisecond pulsars is already undergoing testing.
Image: Our early experiments, described yesterday, explore how we might use millisecond X-ray pulsars (MSPs) to provide autonomous spacecraft navigation. Credit: Astrowatch.net.
Visualizing a Pulsar Navigation Network
Using millisecond X-ray pulsars (MSPs) for galaxy-spanning navigation raises more than a few questions, especially when we try to predict what an artificial pulsar navigation system might look like to outside observers. If we are willing to posit for a moment a Kardashev II-level civilization moving between stars at relativistic velocities, then we would make as one of our predictions that such a system would be suitable for navigation at such speeds. In following the predictive model of Vidal’s paper, we would then check through our voluminous pulsar data to see how such a prediction fares. The answer, in other words, is in our datasets, and demands analyzing the viability of pulsar navigation at high fractions of c.
To my knowledge, no one has yet done this, making Vidal’s paper a spur to such research. The key here is to make predictions to see which can be falsified. But a quick recap for those just coming in on the discussion. What Vidal (Universiteit Brussel, Belgium) offers is an examination of millisecond X-ray pulsars as navigational aids, of the sort we’re already beginning to exploit through experiments via NASA, Chinese efforts and studies at the European Space Agency.
Specifically, the idea here is to develop a methodology for studying cases where astrophysical phenomena may have as one proposed explanation an extraterrestrial technology. Vidal also wonders whether we might find SETI implications even if a fully natural network like this were simply put to work by civilizations more advanced than our own, using it as we might wish to do.
Image: From Vidal’s paper, Fig. 4. Caption: A three-dimensional position fix can be obtained by observing at least three pulsars. Given three well-chosen pulsars, there is only one unique set of pulses that solve the location of the spacecraft (SC). Figure adapted from Sheikh (2005, 200).
Vidal is hoping to make predictions that are testable against our accumulating data to assess the likelihood of natural and artificial explanations, with pulsars as the case in point. The paper examines the kinds of predictions we would want to weigh against available data in a program the author calls SETI-XNAV. Whatever conclusions it reaches, such a program would improve our knowledge of pulsars themselves and our techniques at using them, possibly leading to our augmenting existing resources like the Deep Space Network with XNAV capabilities.
The author’s approach assumes that subjecting decades of data to analysis will teach us much about pulsar navigation as well as future SETI efforts:
Scientifically, SETI-XNAV is a concrete ETI hypothesis to test. The data is here, the timing and navigation functionalities are here. Historically, the suspicion of artificial canals on Mars triggered space missions to Mars and developed knowledge about Mars. Similarly, the project to try to decipher any potentially meaningful information in pulsar’s signals… could lead to the development of tools and methods that can be used for any future candidate signal.
That, of course, would augment the SETI effort as we expand into Dysonian SETI and the examination of possible engineering as the explanation for enigimatic astrophysical observations. If we assume a galaxy with a completely natural navigational system of this power, then we can imagine other civilizations putting it to use. Thus MSPs are likely to be standards in timekeeping and navigation for all putative civilizations in the Milky Way.
The Landscape of Prediction
Millisecond pulsars account for perhaps 10% of known pulsars, and as I mentioned yesterday, they appear to be distributed isotropically in the galaxy, a contrast to the rest of the pulsar population, which appears more concentrated in the galactic disk. MSPs offer numerous advantages from a navigational standpoint given that, according to Vidal, they are more than 100,000 times more stable than normal pulsars. Timing noise, an irregularity found in normal pulsars, and so-called ‘glitches’ (abrupt changes in rotation speed) are less frequent in MSPs. The latter are also associated with lower velocities than the other 90 percent of the pulsar population.
From the standpoint of artificiality, Vidal breaks the possibility terrain down seven ways (this is drawn from the paper’s Figure 1):
0 – Natural. All pulsars are natural. We are just lucky they provide stable clocks and an accurate navigation system
1 – Pulsars as standards. All pulsars are natural, but ETIs use them for timing, positioning and navigation purposes. Communication is galacto-tagged and time-stamped with a pulsar standard
2 – Natural and alterable. Some ETIs have the technology and capability to jam, spoof or interfere with a natural pulsar positioning system
3 – Artificial MSXP for navigation. Only a few millisecond X-ray pulsars have been modified by ETI for galactic navigation and timing purposes
4 – Artificial MSXP for navigation and communication. Only a few MSXPs have been modified by ETI, for navigation, timing and communication purposes
5 – Artificial pulsars. All pulsars are artificial. ETI build them, even the new ones, by intentionally triggering supernovas
6 – Artificial pulsars for us. All pulsars are artificial. ETI build them and they are currently sending us Earth-specific messages
The point here is telling for Dysonian SETI in general. We have established pulsar formation models that seem to work. To establish a program of XNAV-SETI, examining our storehouse of pulsar data, we do not need to challenge it.
But as we have learned more about pulsars over the years, we have learned that there is no unified pulsar model that explains the variety we have seen among this population. We can look toward understanding what MSPs are doing by asking what new hypotheses explain this rich set of observations.
The wide range of Vidal’s seven scenarios makes his case straightforward: “…we do not necessarily need to contradict existing pulsar models to entertain the possibility that ETI might be involved.” The issue then becomes, Vidal adds, to make and validate new predictions.
Emergent Questions
Yesterday I mentioned a recent paper examining radio pulsars in a SETI context. It was Chennamangalam, Siemion, Lorimer & Werthimer, “Jumping the energetics queue: Modulation of pulsar signals by extraterrestrial civilizations,” New Astronomy Volume 34, January 2015, pp. 245-249 (abstract). The paper examines the possibility of pulsars as ‘naturally occurring radio transmitters’ onto whose emissions information has been encoded. Vidal likewise thinks about millisecond X-ray pulsars in the context of possible information content, noting that Carl Sagan pondered studying pulsar amplitude and polarization nulls as far back as 1973.
It might be argued that communications signals would likely be compressed, making decoding extremely problematic, but Vidal’s point here is that navigational systems differ in fundamental ways from communications systems. Navigational signals should be more regular and easier to process than highly modulated signals with communications intent. If we are looking for content grafted onto the navigational signal, we can bring to bear the entire SETI toolkit, perhaps examining pulsar data in light of delay-tolerant networking and discontinuities in connectivity.
We move back into the area of predictions. World clocks on Earth are regularly re-synchronized, just as the time on global positioning satellites is synchronized through methods Vidal discusses, using a control segment that communicates with a satellite segment. Can we observe anything like this in our pulsar data? The author frames the matter this way:
The fastest and most stable MSPs might constitute such a control segment, to which the other pulsars would synchronize. Concretely, we could look for time correction signals broadcasts (that exist in GNSSs [Global Navigation Satellite Systems]), or synchronization waves. For example, synchronization might occur first on pulsars nearest the putative control segment and then diffuse to further away pulsars. This could be investigated via rare MSP glitches, or other remarkable features, such as giant pulses in MSPs.
Synchronization between MSPs would be evidence for a distributed solution on an interstellar level.
Other questions to explore: Do we find that MSPs further away from the galactic plane are more powerful than those closer in, potentially designed for low-density regions of the galaxy? Is MSP distribution random or does it show a pattern fitting the needs of galactic navigation? Estimates of the number of MSPs needed to navigate the entire galaxy might be contrasted with astrophysical predictions of the MSP population, currently estimated to be between 30,000 and 200,000. This one, of course, is tricky: We can only derive a theoretical lower boundary.
How MSPs form and evolve is fruitful ground for inquiry, given that some scientists have argued that the most commonly cited scenario for MSP evolution does not produce the X-ray MSP population we see. It is hard to see how an MSP in a non-binary system can maintain its spin without degrading over time, making the single MSP ground for study. Thus another round of prediction is possible. Single MSPs, those without an energy source, may simply be non-working parts of the network. Do we see redundancy between single and binary MSP coverage, given that binary MSPs are likely more reliable over long time periods?
What Vidal calls SETI-XNAV makes a significant departure from conventional SETI in the sense that it is not localized around a single star, but rather involves a search for a distributed signal that exists in the form of a navigation system, one either established by extraterrestrial engineering or simply relying on a natural phenomenon to pursue its own activities. That we can begin to use millisecond X-ray pulsars as navigation standards implies that more advanced civilizations have done so. Thus SETI-XNAV as constructed in this paper intends to survey the testable predictions against which we can run our expanding dataset on pulsars.
…all pulsars could be perfectly natural, but we can reasonably expect that civilizations in the galaxy will use them as standards… By studying and using XNAV, we are also getting ready to receive and send messages to ETI in a galactically meaningful way. From now on, we might be able to decipher the first level of timing and positioning metadata in any galactic communication.
But I would also emphasize that making testable predictions about pulsar navigation also exercises our skills at analyzing future astrophysical data that may prove enigmatic. That, in and of itself, is a useful contribution in this era of KIC 8462852 and ‘Oumuamua.
The paper is “Pulsar positioning system: a quest for evidence of extraterrestrial engineering,” published online in the International Journal of Astrobiology 23 November 2017 (abstract / preprint). See also Vidal, “Millisecond Pulsars as Standards: Timing, Positioning and Communication,” Proceedings IAU Symposium No. 337, edited by P. Weltevrede, B. B. P. Perera, L. Levin Preston, and S. Sanidas. Jodrell Bank Observatory, UK (2017). Preprint available.
by Paul Gilster | Jan 15, 2018 | Astrobiology and SETI |
Pulsar navigation may be our solution to getting around not just the Solar System but the regions beyond it. For millisecond pulsars, a subset of the pulsar population, seem to offer positioning, navigation, and timing data, enabling autonomous navigation for any spacecraft that can properly receive and interpret their signals. The news that NASA’s SEXTANT experiment has proven successful gives weight to the idea. Station Explorer for X-ray Timing and Navigation Technology is all about developing X-ray navigation for future interplanetary travel.
At work here is NICER — Neutron-star Interior Composition Explorer — which has been deployed on the International Space Station since June as an external payload. NICER deploys 52 X-ray telescopes and silicon-drift detectors in the detection of the pulsing neutron stars called pulsars. Radiation from their magnetic fields sweeps the sky in ways that can be useful. A recent demonstration used four millisecond pulsar targets — J0218+4232, B1821-24, J0030+0451, and J0437-4715 — to track NICER within a 10-mile radius as it orbited the Earth.
X-ray Pulsar Navigation (XNAV) has become an active area of research, pursued not just at NASA but by Chinese satellite testing and by conceptual studies at the European Space Agency. Having barely left our own planet, we are far ahead of ourselves to talk about a galactic positioning system for future spacecraft, but there is reason to believe that the principles of pulsar navigation can be extended to make accurate deep space navigation a reality.
Pulsars as Navigational Matrix
The SEXTANT experiment dovetails with a new paper from Clément Vidal (Universiteit Brussel, Belgium), whose work falls into the broader context of recent studies of unusual astrophysical phenomena. The author of the ambitious The Beginning and the End (Springer, 2014), Vidal’s work has been the subject of several articles in these pages (see, for example, A Test Case for Astroengineering and related entries accessible in the archives). In this era of the enigmatic KIC 8462852 and the interstellar object ‘Oumuamua, we have begun to ask how to address possible extraterrestrial engineering within the confines of rigorous astrophysics.
Millisecond pulsars may offer a way to examine such questions, but it is important to point out at the outset the Vidal is not arguing that this type of pulsar is evidence of extraterrestrial engineering. What he is trying to do is ask a question with broader implications. How do we study unusual astrophysical phenomena in ways that include an extraterrestrial hypothesis? How, in fact, do we conclude when that hypothesis is remotely relevant? And are there ways to make observable and refutable predictions that would help us distinguish purely astrophysical phenomena from what Vidal calls ‘astrobiological’ phenomena that imply intelligence?
Image: An artist’s impression of an accreting X-ray millisecond pulsar. The inflowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit & copyright: NASA / Goddard Space Flight Center / Dana Berry.
We’ve seen in the analysis of KIC 8462852 how many hypotheses have been put forward to explain that star’s unusual light curves, with more and more attention now being paid to a natural explanation involving dust in the system. Vidal’s lengthy paper examines the question of millisecond pulsars being useful for navigation, as with our own civilization’s global navigation satellite systems, like the Global Positioning System (GPS) or the Russian GLONASS (GLObal NAvigation Satellite System).
If we can derive a navigational methodology out of astronomical objects found throughout the galaxy, it seems reasonable to believe that more advanced civilizations would have deduced the same facts and might be using a pulsar positioning system (PPS) in their own activities. Pulsar navigation might thus have SETI potential — might some future SETI candidate signal contain timing and positioning metadata? Might some astrophysical phenomena like pulsars be modified by advanced cultures for use as beacons?
And if we push the issue to its conclusion, is it conceivable that what we see as a pulsar navigation capability is the result of deliberate engineering on a vast scale, the sort of thing we’ve imagined the builders of Dyson spheres and Kardashev Type II civilizations engaging in? Vidal does not argue that this is the case, but calls instead for using pulsar navigation as a way into what he calls SETI-XNAV, a program of research that would use existing and future astronomical data to examine millisecond pulsars in the context of testable predictions.
Vidal sees this as a way to “join pulsar astrophysics, astrobiology and navigation science,” one whose benefits would include developing new methods to design more efficient global navigation satellite systems here on Earth even as we explore how to refine our early XNAV experiments. Not incidentally, we would also be examining our methods when, as seems inevitable, we are confronted with another case of an astrophysical object that raises questions about possible artificial origins.
Implications of Galactic Navigation
An ETI hypothesis has played around the idea of pulsars from the beginning, with a brief interest in extraterrestrial technologies leading to the objects being nicknamed ‘LGM stars,’ for ‘Little Green Men.’ But as Vidal explains, models explaining pulsar behavior are available that invoke nothing but natural processes. It’s fascinating to see that Italian astrophysicist Franco Pacini predicted pulsars based on his studies of neutron stars some months before their discovery was announced by Jocelyn Bell and Anthony Hewish in 1967. Vidal goes on to say:
Pacini’s and [Thomas] Gold’s models were the very first modeling attempts. Pulsar astronomy has immensely progressed since then, and pulsars display a phenomenology that requires much more advanced models (see the section Pulsar behavior). There is no single unified pulsar model that can explain all the variety of observations… nobody predicted that our Galaxy would host some pulsars with pulsations rivaling atomic clocks in stability, or that their distribution would make them useful for an out-of-the-spiral galactic navigation system.
It’s a system we’ve begun to explore because of the need for autonomous navigation, in which a spacecraft is capable of navigating without recourse to resources on Earth or in nearby space. Homing in on millisecond pulsars (MSPs) as a unique subset of the broader population of pulsars, Vidal asks what observable predictions we might make that could help us distinguish natural phenomena from artificial. Galactic distribution turns out to be one such marker.
The distribution of MSPs is isotropic, while normal pulsars appear to be concentrated in the galactic plane. Because they are formed in binary systems, this distribution of MSPs causes us to ask why there would be more binary star systems outside the galactic disk than in it.
Image: Figure 7 from the Vidal paper. Caption: The distribution of MSPs in Galactic coordinates, excluding those in globular clusters. Binary MSPs are shown by open circles. From Lyne & Graham-Smith (2012, 116). Credit: Clément Vidal.
Bear in mind that while pulsar navigation became an early topic, proposed as far back as 1974 by JPL’s George Downs, it was the proposal to use X-ray pulsars instead of radio pulsars (Chester and Butman, 1981) that demonstrated both improved accuracy and the ability to use the kind of small detectors that would be feasible for inclusion in a spacecraft payload.
The discovery of X-ray millisecond pulsars shortly thereafter illustrated the difference between ‘normal’ pulsars and MSPs (for more on this, see Duncan Lorimer’s “Binary and Millisecond Pulsars,” Living Reviews in Relativity December 2008, 11:8; abstract here). Although there is much to say about this issue, for now keep in mind the key difference noted above: MSPs accrete matter from a companion. They are generally found in binary systems.
Now we enter the realm of prediction. If there is a case to be made for MSPs as evidence of engineering, we would expect them to be distributed in ways that would appear non-random. We would expect few redundancies in their coverage areas, and in terms of their numbers, there should be enough for galactic navigation but not necessarily more. Moreover, we would expect artificial navigation sources like X-ray millisecond pulsars to beam preferentially in the galactic plane. If we do not find these things, the astrophysical model is supported.
What emerges in this paper is a series of such predictions that can be used to examine our growing data about pulsar, and in particular MSP, behavior. The data offer a rich enough hunting ground that we can look at such things as MSPs in globular clusters as opposed to elsewhere in the galaxy. We find that about half of MSPs appear in globular clusters, a fact that supports an astrophysical explanation, since stellar encounters are likely in such quarters and thus the formation of the binary star systems that produce MSPs in the first place is to be expected.
If MSPs are engineered objects, we would expect different properties between cluster MSPs and those in the disk. We should examine such questions as beaming direction, which an astrophysical explanation would find to be random. We would study as well whether pulsar beaming overlaps with other pulsar beaming within such clusters. Such a study under the SETI-XNAV rubric might help us uncover new binary MSPs, Vidal asserts, by modeling the coverage areas of MSPs and searching in places where coverage would be non-existent. The prediction would then be that we should find an MSP filling in the putative coverage gap.
Vidal’s paper offers numerous areas for such investigation. SETI-XNAV, he writes:
…draws on pulsar astronomy, as well as navigation and positioning science to make SETI predictions. This concrete project is grounded in a universal problem and needs: navigation. Decades of pulsar empirical data is available and I have proposed nine lines of inquiry to begin the endeavor… These include predictions regarding the spatial and power distribution of pulsars in the galaxy; their population; their evolutionary tracks; possible synchronization between pulsars; testing the navigability near the speed of light; decoding galactic coordinates; testing various directed panspermia hypotheses; as well as decoding metadata or more information in pulsar’s pulses.
My interest is in seeing how Vidal makes the distinction between astrophysical and astrobiological — in other words, as with KIC 8462852 and the interstellar object ‘Oumuamua, are we making progress as we begin to investigate under what some have called the ‘Dysonian’ SETI paradigm? That approach takes its name from the postulated Dyson spheres that have been the subject of early work and continue to be studied through projects like the Glimpsing Heat from Alien Technologies (G-HAT) program at Penn State (see Jason Wright’s Glimpsing Heat from Alien Technologies for more). These issues will grow in relevance as our observational tools hasten the pace of discovery.
More thoughts on all this in my next post. The paper is Vidal, “Pulsar positioning system: a quest for evidence of extraterrestrial engineering,” published online in the International Journal of Astrobiology 23 November 2017 (abstract / preprint). Also of interest: Chennamangalam, Siemion, Lorimer & Werthimer, “Jumping the energetics queue: Modulation of pulsar signals by extraterrestrial civilizations,” New Astronomy Volume 34, January 2015, pp. 245-249 (abstract).
