Paul Gilster's Blog, page 10

A Gravitational Wave Surprise

I think gravitational wave astronomy is one of the most exciting breakthroughs we’re tracking on Centauri Dreams. The detection of black hole and neutron star mergers has been a reminder of the tough elasticity of spacetime itself, its interplay with massive objects that are accelerating. Ripples in the fabric of spacetime move outward from events of stupendous energy, which could include neutron star mergers with black holes or other neutron stars. Earth-based observing projects like LIGO (Laser Interferometer Gravitational-Wave Observatory), the European Virgo and KAGRA (Kamioka Gravitational Wave Detector) in Japan continue to track such mergers.

But there is another aspect of gravitational wave work that I’m only now becoming familiar with. It’s background noise. Just as ham radio operators deal with QRN, which is the natural hum and crackle of thunderstorms and solar events, so the gravitational wave astronomer has to filter out what is being called the astrophysical gravitational wave background, or AGWB, as the inevitable acronym would have it. Astronomers also have to consider GW signals associated with events in the early universe, stochastic background ‘static’ that could have originated, for example, in cosmic inflation or the creation of cosmic strings.

The AGWB is the background noise of countless astrophysical events, a ‘hum’ from all sources emitting gravitational waves in the universe. Recent work has been showing that this collective signal, primarily from black hole and binary neutron star mergers, is detectable by the technologies we’ll be deploying in the 2030s in the European Space Agency’s Laser Interferometer Space Antenna (LISA) mission. And it’s clear that for gravitational wave astronomy to proceed, we need to remove the AGWB to uncover underlying signals.

New work now makes the case that, surprisingly, we also have to reckon with the background noise of binary white dwarfs, although I see in the literature that scientists were delving into this as early as 2001 (citation below). In two recent papers, Dutch astronomers have developed models demonstrating that the background noise of white dwarfs would actually be stronger than that produced by black holes. Gijs Nelemans (Radboud University (Nijmegen, the Netherlands), who is working with the software and guidance mechanisms for the LISA mission, is a co-author on two papers on the subject. He sees white dwarf background noise as a way of studying stellar evolution on a galactic scale:

“With telescopes you can only study white dwarfs in our own Milky Way, but with LISA we can listen to white dwarfs from other galaxies. Moreover, in addition to the background noise of black holes and the noise of white dwarfs, perhaps other exotic processes from the early universe can be detected.”

Image: Dutch astronomer Gijs Nelemans. Credit: TechGelderland.

Nelemans has been developing the models described in the two recent papers with students Seppe Staelens and Sophie Hofman. Their work is significant given that until now, the LISA mission had not factored in a noisy white dwarf background problem. In a paper published in Astronomy & Astrophysics, the authors point out:

Given the amplitude of the WD component… it is expected that it can be very well measured by LISA. Furthermore, the relative amplitudes show that, if LISA detects an AGWB signal in the mHz regime, it is likely dominated by the WDs. This means that it is likely hard to make statements about the BH (and NS) population based on a measurement of the AGWB unless there is a way to disentangle the two, or to detect the high-frequency component of the AGWB above 40 mHz.

And in terms of the study of white dwarfs, the paper adds:

This offers an opportunity to study the WD binary population to much larger distances, while hampering the detection of the BH AGWB with missions such as LISA. The WD signal reaches a peak around 10 mHz and at higher frequencies the BH AGWB will become the dominant signal. The detectability of this transition by LISA and other mHz missions ought to be studied in detail.

Image: The LISA mission consists of a constellation of three identical spacecraft, flying in formation. They will orbit the Sun trailing the Earth, forming an equilateral triangle in space. Each side of the triangle will be 2.5 million km long (more than six times the Earth-Moon distance), and the spacecraft will exchange laser beams over this distance. This illustration shows two black holes merging and creating ripples in the fabric of spacetime. Some galaxies are visible in the background. In the foreground, the shape of a triangle is traced by shining red lines. It is meant to represent the position of the three LISA spacecraft and the laser beams that will travel between them. Credit: ESA.

This is indeed a unique kind of probe, because we’re talking about studying white dwarf evolution at high redshift in ways beyond the range of optical astronomy. Realize that only a small selection of gravitational wave sources can be detected with our current technologies. Millions of binaries in the Milky Way will simply merge into the stochastic foreground, a signal that is highly anisotropic (i.e., not uniform in all directions) while unresolved binary sources outside the galaxy produce a background signal that is profoundly isotropic, one that “encodes the combined information about the different source populations,” to quote the Hofman & Nelemans paper.

So we learn that filtering out white dwarf background mergers will be a major part of LISA’s investigations, but that the WD background is also a source of new information. LISA is to be the first dedicated space-based gravitational wave detector, involving three spacecraft in an equilateral triangle 2.5 million kilometers long in a heliocentric orbit. The European Space Agency hopes to launch LISA in 2035 on an Ariane 6.

The papers are Hofman & Nelemans, “On the uncertainty of the white dwarf astrophysical gravitational wave background,” accepted at Astronomy & Astrophysics (preprint); and Staelens & Nelemans, “Likelihood of white dwarf binaries to dominate the astrophysical gravitational wave background in the mHz band,” Astronomy & Astrophysics Vol. 683, A139 (March 2024). Full text. The 2001 paper is “Low-frequency gravitational waves from cosmological compact binaries,” Monthly Notices of the Royal Astronomical Society Vol. 324, Issue 4 (July 2001), pp. 797-810 (abstract).

Catches, Comets and Europa

If the public seems more interested in spaceflight as a vehicle for streaming TV dramas, the reality of both the Europa Clipper liftoff and the astounding ‘catch’ of SpaceX’s Starship booster may kindle a bit more interest in exploring nearby space. When I say ‘nearby,’ bear in mind that on this site the term refers to the entire Solar System, as we routinely discuss technologies that may one day make travel to far more distant targets possible. But to get there, we need public engagement, and who could fail to be thrilled by a returning space booster landing as if in a 1950’s SF movie?

Europa may itself offer another boost if Europa Clipper’s science return is anything like what it promises to be. Closing to 15 kilometers from the surface and making 49 passes over the icy ocean world, the spacecraft may give us further evidence that outer system moons can be venues for life. We also have the European Space Agency’s Jupiter Icy Moons Explorer (JUICE), which will study Europa, Callisto and, in a spectacular move, end up orbiting Ganymede for extended close-up observations.

Image: Europa Clipper begins its journey. Credit: SpaceX.

JUICE gets to Jupiter in July of 2031, while Europa Clipper starts its flybys in the same year, though arriving in 2030. As a measure of how tricky it can be to get to these destinations, both craft make flybys of other worlds, returning in fact to the Earth for some of these. Europa Clipper’s journey will be marked by gravity assists from Mars in February of 2025 and Earth in December 2026. JUICE has already performed one Earth/Moon flyby and will make a flyby of Venus (August, 2025) followed by two Earth flybys (September 2026 and January 2029). A long and winding road indeed!

Speaking of flybys, it’s interesting to note that we have two cometary appearances this month. Comet C/2023 A3 (Tsuchinshan-ATLAS) and C/2024 S1 (ATLAS) are both likely to be visible in October, with the latter closest to Earth on October 24 as it swings toward Sol where it will likely disintegrate. The former should make an appearance in the western sky just after sunset before growing fainter in the latter part of the month. C/2023 A3 appears to be an Oort Cloud object, or long period comet, with an orbital period of some 80,000 years. Short-period comets (Halley’s Comet is one of these) have much shorter orbits, with Halley’s showing up every 76 years.

I find the Oort Cloud a fascinating subject, for it’s based on deduction and not observation. Astronomer James Wray (Georgia Tech), writing in The Conversation, makes the point that while we can’t directly image this vast collection of comets, likely numbering in the hundreds of billions, we can estimate that it extends possibly as far as halfway to the Alpha Centauri system. That’s an intriguing thought, for it means our cometary cloud may intermingle with an equivalent cloud (if one exists) from the Centauri stars. The space covered by our first interstellar probes is not vacant, though the distances between individual objects would still be vast. On the other hand, if the theory that the Oort Cloud formed because of interactions with the giant planets, it’s possible that in the absence of such planets (still not demonstrated), Centauri A and B may not have formed such a cloud.

Wray makes the case that long-period comets are conceivably our greatest planetary threat, outranking near Earth asteroids in degree of danger since an incoming Oort object would likely not be spotted until well inside the planetary system, giving us little time to react. ‘Oumuamua, after all (not an Oort object) was discovered after its closest approach to Earth.

Cometary flybys of our Sun will always be cherished for their visual appeal as ices evaporate and a tail forms, and a collision course with Earth is a highly unlikely scenario, but it’s always best to consider the prospects. Wray puts it this way:

One way to prepare for these objects is to better understand their basic properties, including their size and composition. Toward this end, my colleagues and I work to characterize new long-period comets. The largest known one, Bernardinelli–Bernstein, discovered just three years ago, is roughly 75 miles (120 kilometers) across. Most known comets are much smaller, from one to a few miles, and some smaller ones are too faint for us to see. But newer telescopes are helping. In particular, the Rubin Observatory’s decade-long Legacy Survey of Space and Time, starting up in 2025, may double the list of known Oort Cloud comets, which now stands at about 4,500.

The European Space Agency’s Comet Interceptor mission, scheduled for launch later in this decade, should offer an option for intercepting an Oort Cloud comet when one appears, making it possible to learn more about these objects in terms of their composition and possible role in the delivery of volatiles to the inner system. Oort comets are tricky because their wide orbits mean gravitational influences from other stars can nudge one into a solar close pass without any prior warning. An incoming long-period comet, writes Wray, might offer mere weeks or days to prepare any defense measures we have in place. Even so, the odds of an impact are extremely low.

Image: A stunning return. The Starship booster comes home. Credit: SpaceX.

All this is by way of hoping public interest in space will be quickened both by recent mission successes, ongoing exploration of possible sources of life, and the appearance of the occasional comet. The startling SpaceX success with Starship’s ‘catch’ underlines that technological advances, like comets, can seem to come out of nowhere when we’re not paying attention. I’m thinking back to the science fiction I read as a kid and realizing that watching Starship’s booster descend was right out of Astounding Stories. Heinlein would have loved it, and indeed foreshadowed what unfolded on Sunday.

As SpaceX communications manager Dan Huot put it: “What we just saw, that looked like magic.”

Go Clipper

Is this not a beautiful sight? Europa Clipper sits atop a Falcon Heavy awaiting liftoff at launch complex 39A at Kennedy Space Center. Launch is set for 1206 EDT (1606 UTC) October 14. Clipper is the largest spacecraft NASA has ever built for a planetary mission, 30.5 meters tip to tip when its solar arrays are extended. Orbital operations at Jupiter are to begin in April of 2030, with the first of 49 Europa flybys occurring the following year. The closest flyby will take the spacecraft to within 25 kilometers of the surface. Go Europa Clipper!

Photo Credit: NASA.

In less than 24 hours, NASA's @EuropaClipper spacecraft is slated to launch from @NASAKennedy in Florida aboard a @SpaceX Falcon Heavy rocket.

Tune in at 2pm PT / 5pm ET as experts discuss the prelaunch status of the mission. https://t.co/Nq36BeKieX

— NASA JPL (@NASAJPL) October 13, 2024

Is Dark Energy Truly a Constant?

In a tantalizing article in The Conversation, Robert Nichol (University of Surrey) offers a look at where new physics might just be emerging in conjunction with the study of dark energy. Nichol is an astronomer and cosmologist deeply experienced in the kind of huge astronomical surveys that help us study mind-boggling questions like how much of the universe is made up of matter, dark matter or dark energy. We’ve assumed we had a pretty good idea of their proportions but a few issues do arise.

One of them seems particularly intriguing. Nichol’s article asks whether dark energy, regarded as a constant, may not actually vary over time. That’s quite a thought. The consensus over a universe made up of normal matter (5 percent), dark matter (25 percent) and dark energy (70 percent) came together early in our century, with dark matter taking the role of the cosmological constant Einstein once considered. Although he came to reject the idea, Einstein would doubtless take great interest in the work of observational cosmologists like Nichol, who keep refining the numbers to reduce errors.

Image: The University of Surrey’s Nichol. Credit: University of Portsmouth.

At the heart of the investigation is the Dark Energy Survey, an international effort involving some 400 scientists in seven countries. The survey’s latest numbers, Nichol reports, are that 31.5 percent of the universe is either dark or normal matter, with an error on the measurement of a scant 3 percent. The question of how almost 70 percent of the universe could be in the form of something we can’t see, and something that is indeed not associated in any way with matter, is what Nichol calls “the biggest challenge to physics, even after 20 years of intense study.”

Remember that we first learned of the acceleration of the universe by studying Type Ia supernova (SNeIa) explosions. These occur in binary systems when a white dwarf star begins drawing off material from its companion, usually a red giant. Reaching the Chandrasekhar limit (approximately 1.4 times the mass of the Sun), the white dwarf releases vast amounts of energy, forming a ‘standard candle’ for cosmologists because the luminosity of these events is completely predictable. In other words, supernovae like these have an intrinsic brightness that can be compared to what is observed, making their distance measurable. Plug in the observed redshift and astronomers can use supernovae to make measurements on the rate of the universe’s expansion.

Image: The Hubble Ultra Deep Field, a view of nearly 10,000 galaxies, a reminder of the stunning scope of cosmological studies. The snapshot includes galaxies of various ages, sizes, shapes, and colours. The smallest, reddest galaxies, about 100, may be among the most distant known, existing when the universe was just 800 million years old. The nearest galaxies – the larger, brighter, well-defined spirals and ellipticals – thrived about 1 billion years ago, when the cosmos was 13 billion years old. This image required 800 exposures taken over the course of 400 Hubble orbits around Earth. The total amount of exposure time was 11.3 days, taken between Sept. 24, 2003 and Jan. 16, 2004. Credit: NASA, ESA, and S. Beckwith (STScI) and the HUDF Team.

The Dark Energy Survey has now reported results on such supernovae over a decade of study which included thousands of such events. The paper makes for fascinating reading. Titled “The Dark Energy Survey: Cosmology Results With ∼1500 New High-redshift Type Ia Supernovae Using The Full 5-year Dataset” (citation below), it significantly adds to the number of observed supernovae. There is just a hint here of flexibility in the direction of a variable dark energy. Let me quote the paper:

The standard Flat-ΛCDM cosmological model is a good fit to our data. When fitting DES-SN5YR alone and allowing for a time-varying dark energy we do see a slight preference for a dark energy equation of state that becomes greater (closer to zero) with time (wa < 0) but this is only at the ∼ 2σ level, and Bayesian Evidence ratios do not strongly prefer the Flat-w0waCDM cosmology.

Untangling: The standard Flat-ΛCDM model is the current description of cosmological structure and evolution, using cold dark matter (CDM) and a cosmological constant (Λ). “Flat’ means that the total energy density of the universe equals the critical density (i.e., a flat universe that continues to expand but at ever slower rates). Again, the cosmological constant is what we associate with dark energy and use to explain the accelerating expansion of the universe. And as the paper makes clear, the DES data fit the existing model, but it’s interesting that a dark energy that varies with time is not ruled out, even if the evidence for this is only enough to hint at the possibility.

Now it gets more intriguing. Nichols points out that a second probe looking at Baryon Acoustic Oscillations (BAO), which are “relics of predictable sound waves in the plasma…of the early universe, before the CMB [cosmic microwave background],” likewise hints at the possibility of dark energy that varies with time. This work is being done with the Dark Energy Spectroscopic Instrument (DESI), which has taken position as the successor to the Sloan Digital Sky Survey (SDSS), which had focused on measuring galactic redshifts.

The DESI results are indeed provocative, especially when seen in light of the supernovae results. From the paper on that work (citation below):

…combining any two of the DESI BAO, CMB or SN data sets shows some level of departure from the ΛCDM model. Relaxing the assumption of a spatially flat geometry through varying ΩK [the curvature density parameter] marginally increases the uncertainties but does not change the overall picture. It remains important to thoroughly examine unaccounted-for sources of systematic uncertainties or inconsistencies between the different datasets that might be contributing to these results. Nevertheless, these findings provide a tantalizing suggestion of deviations from the standard cosmological model that motivate continued study and highlight the potential of DESI and other Stage-IV surveys to pin down the nature of dark energy. (italics mine)

As Nichol puts it in his article:

In particular, when DESI analyses the combination of its BAO results with the final DES SNeIa data, the significance of a time-varying dark energy increases to 3.9 sigma (a measure of how unusual a set of data is if a hypothesis is true) – only 0.6% chance of being a statistical fluke.

Most of us would take such odds, but scientists have been hurt before by systematic errors within their data that can mimic such statistical certainty. Particle physicists therefore demand a discovery standard of 5 sigma for any claims of new physics – or less than a one in a million chance of being wrong!

As scientists will say: “Extraordinary claims require extraordinary evidence.”

Indeed. If we do learn that dark energy varies over time, that would mean that there is less of it now than in the past. We would also need to reconsider our notions about the ultimate fate of the universe depending on this new variable. What a time for physics, when the European Southern Observatory is getting ready to start another massive redshift survey and the European Space Agency’s Euclid mission, launched in 2023, is engaged in its own compilation of redshift data. And then there’s the Vera Rubin Observatory in Chile, which will one day soon be adding its own results to the mix. And then there is the quantum question. Adds Nichol:

According to quantum mechanics, empty space isn’t really empty, with particles popping in and out of existence creating something we call “vacuum energy”. Ironically, predictions of this vacuum energy do not agree with our cosmological observations by many orders of magnitude.

So we’re likely to be learning a great deal more in short order, for the Dark Energy Survey continues to compile its own data, and combining these with the above sources should give us a pretty good handle on the question of a variable dark energy. It’s intriguing to think that we may pin down why current dark energy studies are at variance with quantum mechanics. This is new physics of the kind that should make for Nobel Prizes down the road whatever the outcome of the combined data studies. Cosmology is in Nichol’s view likely entering a ‘new era of cosmological discovery.’

The Nichol article is “Dark energy: could the mysterious force seen as constant actually vary over cosmic time?” in The Conversation 10 October 2024 (full text). The DES paper is DES Collaboration, “The Dark Energy Survey: Cosmology Results With ~1500 New High-redshift Type Ia Supernovae Using The Full 5-year Dataset,” Astrophysical Journal Letters Vol. 973, No. 1 (1 October 2024) L14 (full text). The paper on the BAO measurements is DESI Collaboration et al., “DESI 2024 VI: Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations” (abstract) and available in full text as a preprint.

Planetary Defense: Good News from the Taurids

Evidently discovered by French astronomer Pierre Méchain in 1786, Comet Encke was the first periodic comet to be found after Halley’s Comet. It was named after Johann Franz Encke, who first calculated its orbit. It comes into play this morning because it is considered the source of at least part of the Taurid meteor shower, which is the subject of new work out of the University of Maryland that has implications for our thinking about asteroid and comet mitigation.

Image: This is an image of short-period comet Encke obtained by Jim Scotti on 1994 January 5 while using the 0.91-meter Spacewatch Telescope on Kitt Peak. The image is 9.18 arcminutes square with north on the right and east at top. The integration time is 150 seconds. Credit: NASA.

The Taurids show up in October and November as Earth encounters this stream of debris in an area of its orbit thought to conceal possibly dangerous asteroids. The American Astronomical Society’s Division of Planetary Sciences annual meeting was the occasion for the announcement of the work earlier this week, as noted by Quanzhi Ye at UMD, who summarized the finding:

“We took advantage of a rare opportunity when this swarm of asteroids passed closer to Earth, allowing us to more efficiently search for objects that could pose a threat to our planet. Our findings suggest that the risk of being hit by a large asteroid in the Taurid swarm is much lower than we believed, which is great news for planetary defense.”

The UMD team, working with colleagues at the University of Western Ontario and the University of Washington, Seattle and Poolesville High School in Maryland, used data from the Zwicky Transient Facility telescope, a widefield astronomical survey at Palomar Observatory in California. The idea was to search for objects at least a kilometer in diameter left behind by a much larger source.

The result is heartening, as Ye explains:

“Judging from our findings, the parent object that originally created the swarm was probably closer to 10 kilometers in diameter rather than a massive 100-kilometer object. While we still need to be vigilant about asteroid impacts, we can probably sleep better knowing these results.”

Image: An image of the Taurid meteor shower taken in 2015 by Czech amateur Martin Popek, who produced this striking composite recording fireballs occurring roughly once an hour from the direction of Taurus. Credit: Martin Popek.

Sky surveys like those conducted at the Zwicky Transient Facility track potentially dangerous near-Earth objects, and the ZTF will be used to conduct follow-up studies on the Taurids in coming years. The unusually dusty Comet Encke is relatively large for a short-period comet, with a nucleus of 4.8 kilometers, and it is believed to have experienced significant and likely ongoing periods of fragmentation.

Each new result charting potential danger zones for our world is useful as we work out the likelihood of possible future impacts. While that hunt continues, so too does the effort to learn more about changing the orbit of a potential impactor, as witness the Double Asteroid Redirection Test (DART), a NASA mission that impacted the asteroid moon Dimorphos in 2022 and clearly disrupted the object. The European Space Agency’s Hera mission, launched on October 7, will assess the DART results when it arrives in two years (see A spaceship punched an asteroid — we’re about to learn what came next in the latest issue of Nature for more on this).

The original orbit of Dimorphos was oblate but became much more stretched out (prolate) after the collision with DART. The impact shortened the period of the asteroid’s orbit around its primary by 33 minutes. So we’re learning about at least one way to nudge an asteroid orbit, with other techniques still on the table for future study. Asteroid mitigation will drive near-Earth space technologies forward and move deeper into the system as we add to our catalog of potential impactors, one of which may eventually pose a threat significant enough to prompt action.

Third Time’s a Charm: A Planet at Barnard’s Star

If you follow the fortunes of the stars closest to us, you know that Barnard’s Star has always excited interest, both because of its proximity to our system (about six light years) but also because of the early work on the star performed by Peter Van de Kamp at Sproul Observatory (Swarthmore College). That work, which ran until the early 1970s, initially appeared to show a Jupiter-class planet at the star but the results were later explained as instrumentation errors in Van de Kamp’s equipment.

It was a cautionary tale, but credit the astronomer for working tirelessly using astrometry to attempt to validate a conclusion we now take for granted: There are planets around other stars. In 2018 we seemed to have a solid detection of a much different planet candidate via Guillem Anglada-Escudé (Queen Mary University, London) and Ignasi Ribas (Institute of Space Studies of Catalonia and the Institute of Space Sciences, CSIC in Spain), indicating a super-Earth of 3.3 Earth masses in an orbit near Barnard Star’s snowline (see A Super-Earth Orbiting Barnard’s Star for that coverage), but no confirmation followed.

Indeed, we may have been looking at stellar activity in this second detection rather than a planet, according to a new paper announcing the discovery of a planet below Earth mass at the star. On the 2018 work, the paper notes that “ESPRESSO data does not support the existence of the 233 d candidate planet.” See Paul Robertson’s A very stealthy alias: the impostor planet of Barnard’s star for a detailed look at the detection and the stellar activity explanation.

But this new announcement of a Barnard’s Star planet looks to be solid. Lead author Jonay González Hernández (Instituto de Astrofísica de Canarias) and team, working at the European Southern Observatory’s Very Large Telescope (VLT) made the find with the help of ESPRESSO (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations), the successor to the highly successful HARPS spectrograph, capable of teasing out the wobble induced in the star by a planet.

We now have a low-mass planet, as confirmed by HARPS at the La Silla Observatory, HARPS-N (on La Palma, Canary Islands) and CARMENES at the Calar Alto Observatory, Spain. Twenty times closer to Barnard’s Star than Mercury is to the Sun, the planet orbits in 3.15 Earth days and has a surface temperature around 400 K. The planet is about half the mass of Venus, or three times the mass of Mars. Says Hernández:

“Barnard b is one of the lowest-mass exoplanets known and one of the few known with a mass less than that of Earth. But the planet is too close to the host star, closer than the habitable zone. Even if the star is about 2500 degrees cooler than our Sun, it is too hot there to maintain liquid water on the surface.”

T

Image: This stunning panorama shows the Milky Way galaxy arching above the platform of ESO’s Very Large Telescope (VLT) on Cerro Paranal, Chile, where the work on the new Barnard’s Star discovery was performed. At 2635 metres above sea level, Paranal Observatory is one of the very best astronomical observing sites in the world and is the flagship facility for European ground-based astronomy. The extent of our galaxy’s cloudy and dusty structure can be seen in remarkable detail as a dim glowing band across the observation deck. Credit: ESO.

Indeed, Barnard’s Star b (which I see is being referred to simply as Barnard b) may not be the only planet here. The paper makes note of three other candidates currently under investigation using ESPRESSO. Here we have to be careful. The radial velocity data show several signals at periods less than 10 days: The paper reports periods of 3.15 d, 4.12 d, 2.34 d and 6.74 d, sorted by strength of the signals. The researchers cannot confirm these signals at this point, but are able to model a system that fits the data. Let me go a bit into the weeds here. From the paper:

[The modeled system] would correspond to a system of four sub-Earth mass planets with mp sin i = 0.32, 0.31, 0.22 and 0.17 M⊕. All candidate planetary orbits would be located inner to the habitable zone of the star, with orbital semi-major axes between 0.019 AU and 0.038 AU. Thus all the candidate planets would be irradiated more than the Earth with incident fluxes between 2.4 S ⊕ to 10.1 S ⊕, and their equilibrium temperatures, assuming albedo of 0.3, would be in between 440 K of the inner planet to the 310 K of the outer planet.

Let’s untangle this (this is how I learn things). The four potential planets that emerge from this model are described by mp sin i, which helps us determine a minimum mass (mp) for a planet. What is at stake here is the inclination angle (i) of the planet’s orbit as viewed from Earth, but because we cannot see such planets, we can go from an edge-on orbit (sin close to 1) to a face-on orbit, where sin i is small and the mass of the planet is much higher. So the numbers above refer to minimum masses that could be higher depending on how the system is tilted to our point of view. If these other worlds exist, they’re all too close to the star to fit the liquid water habitable zone. Indeed, the S value in the quote refers to solar flux, which in the case of the hypothetical planets would be 2.4 to 10.1 times the stellar radiation that Earth receives from the Sun.

In any case, the authors are careful to add that confirming an actual four-planet system at Barnard’s Star would take many more observations using ESPRESSO:

These observations would need to be done with sufficient cadence to sample these planet periods as well as with enough baseline to be able to properly model the activity of the star, in particular, those activity signals associated with the stellar rotation.

So the hunt continues, encouraged by the one newly confirmed planet, as we scour this and other nearby red dwarfs for evidence of small rocky worlds. We can look ahead to ANDES, the ArmazoNes high Dispersion Echelle Spectrograph, which will be used in conjunction with the European Southern Observatory’s Extremely Large Telescope, a 39-meter instrument that will be the largest visible and infrared light telescope in the world. Located at Cerro Armazones in Chile’s Atacama Desert, the telescope should see first light as soon as 2028.

The paper is Hernández et al., “A sub-Earth-mass planet orbiting Barnard’s star,” Astronomy & Astrophysics Volume 690 (October 2024). Full text.

We Are the Music: Reflections on Galactic Immensity

While I’m immersed in the mechanics of exoplanet detection and speculation about the worlds uncovered by Kepler, TESS and soon, the Roman Space Telescope (not to mention what’s coming with Extremely Large Telescopes), I’m daunted by a single fact. We keep producing great art showing what exoplanets in their multitudes look like, but we can’t actually see them. Or I should say that the few visual images we have captured thus far are less than satisfying blobs of light marking hot young worlds.

Please don’t interpret this as in any way downplaying the heroic work of scientists like Anne-Marie Lagrange (LESIA, Observatoire de Paris) on Beta Pictoris b and all the effort that has gone into producing the 70 or so images of exoplanets thus far found. I’m actually just pointing out how difficult seeing an exoplanet close up would be, for the goal of interstellar flight that animates our discussions remains hugely elusive. The work continues, and who knows, maybe in a century we’ll get a close-up of Proxima Centauri b. Until then, I need periodically to return to deep sky objects to refresh the part of me that needs sensual imagery (and also accounts for my love of Monet).

Herewith some images that would challenge even the greatest of the Impressionists to equal. We’re looking at the Milky Way with new eyes thanks to two related projects, the VISTA Variables in the Vía Láctea (VVV) survey and the companion VVV eXtended (VVVX) survey. Roberto Saito (Universidade Federal de Santa Catarina, Brazil) is lead author of the paper introducing this work, which includes close to 100 co-authors. VISTA is the European Southern Observatory’s Visible and Infrared Survey Telescope for Astronomy, run out of the Paranal Observatory in Chile. Its great tool is the infrared camera VIRCAM, which opens up areas otherwise hidden by dust and gas.

Image: This collage highlights a small selection of regions of the Milky Way imaged as part of the most detailed infrared map ever of our galaxy. Here we see, from left to right and top to bottom: NGC 3576, NGC 6357, Messier 17, NGC 6188, Messier 22 and NGC 3603. All of them are clouds of gas and dust where stars are forming, except Messier 22, which is a very dense group of old stars. The images were captured with ESO’s Visible and Infrared Survey Telescope for Astronomy (VISTA) and its infrared camera VIRCAM. The gigantic map to which these images belong contains 1.5 billion objects. The data were gathered over the course of 13 years as part of the VISTA Variables in the Vía Láctea (VVV) survey and its companion project, the VVV eXtended survey (VVVX). Credit: ESO/VVVX survey.

We’re dealing with some 200,000 images covering an area of sky that is equivalent to 8600 full moons, according to ESO, and 10 times more objects than released by the same team in 2012, based on observations that began two years earlier and ended early in 2023. Working over that timeframe allowed scientists to chart brightness changes and movement that can be useful in calculating distances on this huge scale.

Image: This image shows the regions of the Milky Way mapped by the VISTA Variables in the Vía Láctea (VVV) survey and its companion project, the VVV eXtended survey (VVVX). The total area covered is equivalent to 8600 full moons. The Milky Way comprises a central bulge — a dense, bright and puffed-up conglomeration of stars — and a flat disc around it. Red squares mark the central areas of our galaxy originally covered by VVV and later re-observed by VVVX: most of the bulge and part of the disc at one side of it. The other squares indicate regions observed only as part of the extended VVVX survey: even more regions of the disc at both sides (yellow and green), areas of the disc above and below the plane of the galaxy (dark blue) and above and below the bulge (light blue). The numbers indicate the galactic longitude and latitude, which astronomers use to chart objects in our galaxy. The names of various constellations are also shown. Credit: ESO/VVVX survey.

The twin surveys have already spawned more than 300 scientific papers while producing a dataset too large to release as a single image, although the processed data and objects catalog can be found at the ESO Science Portal. More than 4000 hours of observation went into the work, and while the twin projects cover about 4 percent of the celestial sphere, the region covered contains the majority of the Milky Way’s stars and the largest concentration of gas and dust in the galaxy.

Clearly, a survey like this will be useful for observations from future instruments like the Vera Rubin Observatory, which will deploy an 8.4-meter mirror and the largest camera ever built for astronomy and astrophysics in a deep survey of the southern hemisphere at optical wavelengths. Instruments like the James Webb Space Telescope are obviously able to home in on objects with much higher resolution but cannot be used for broad area surveys of this kind. Next generation ground-based instruments will use the survey in compiling their target lists, and eventually the Roman Space Telescope will be able to produce deep infrared images of large regions with higher resolution.

As the paper notes:

…there are many more applications of this ESO Public Survey for the community to exploit for future studies of Galactic structure, stellar populations, variable stars, star clusters of all ages, among other exciting research areas, from stellar and (exo)planetary astrophysics to extragalactic studies. The image processing, data analysis and scientific exploitation will continue for the next few years, with many discoveries yet to come. The VVVX Survey will also be combined with future facilities to boost its scientific outcome in unpredictable ways: we are sure that this survey will remain a goldmine for MW studies for a long time.

But I fall back on sheer aesthetics this morning. As witness starbirth:

Image: A new view of NGC 3603 (left) and NGC 3576 (right), two stunning nebulas imaged with ESO’s Visible and Infrared Survey Telescope for Astronomy (VISTA). This infrared image peers through the dust in these nebulas, revealing details hidden in optical images. NGC 3603 and NGC 3576 are 22,000 and 9,000 lightyears away from us, respectively. Inside these extended clouds of dust and gas, new stars are born, gradually changing the shapes of the nebulas via intense radiation and powerful winds of charged particles. Given their proximity, astronomers have the opportunity to study the intense star formation process that is as common in other galaxies but harder to observe due to the vast distances. The two nebulas were catalogued by John Frederick William Herschel in 1834 during a trip to South Africa, where he wanted to compile stars, nebulas and other objects in the sky of the southern hemisphere. This catalogue was then expanded by John Louis Emil Dreyer in 1888 into the New General Catalogue, hence the NGC identifier in these and other astronomical objects. Credit: ESO/VVVX survey,

And a nebula inset into a riotous field of stars:

Image: This image shows a detailed infrared view of Messier 17, also known as the Omega Nebula or Swan Nebula, a stellar nursery located about 5500 light-years away in the constellation Sagittarius. This image is part of a record-breaking infrared map of the Milky Way containing more than 1.5 billion objects. ESO’s VISTA ― the Visible and Infrared Survey Telescope for Astronomy ― captured the images with its infrared camera VIRCAM. The data were gathered as part of the VISTA Variables in the Vía Láctea (VVV) survey and its companion project, the VVV eXtended survey (VVVX). Credit: ESO/VVVX survey.

The vistas opening up with our new technologies inspire a deep sense of humility. We are within and a part of what we are observing, which forces us continually to look with new eyes. I think of Carl Sagan’s frequent admonition that we are made of star-stuff. Or as T. S. Eliot put it in the “The Dry Salvages” (from Four Quartets):

For most of us, there is only the unattended
Moment, the moment in and out of time,
The distraction fit, lost in a shaft of sunlight,
The wild thyme unseen, or the winter lightning
Or the waterfall, or music heard so deeply
That it is not heard at all, but you are the music
While the music lasts.

We are the music. The immense VISTA data-trove will advance further discovery while igniting and shaping our imagination. Perspective frames the seasoned mind.

The paper is Saito et al, “The VISTA Variables in the Vía Láctea extended (VVVX) ESO public survey: Completion of the observations and legacy,” Astronomy & Astrophysics Vol. 689, A148 (September 2024). Full text.

Habitability around F-class Stars

Some years back I read a science fiction story in which the planet where the action took place orbited an F-class star. That was sufficiently odd to get my attention, and I began to pay attention to these stars, which represent on the order of 3 percent of all stars in the galaxy. Stars like our G-class Sun weigh in at about 7 percent, while the vast majority of stars are M-dwarfs, still our best chances for life detection because of the advantages they offer to our observing technologies, including deep transits and lower stellar brightness for direct imaging purposes.

F-stars are intriguing despite the fact that they tend to be somewhat larger than the Sun (up to 1.4 times its mass) and also hotter (temperatures in the range of 6200-7200 K). Back in 2014, I looked at the work of Manfred Cuntz (University of Texas at Arlington), who had performed a study examining radiation levels in these stars and the damage that DNA would experience with an F-star in the sky at various stages of stellar evolution. We’re dealing here with a shorter life expectancy than the Sun, usually reckoned in the range of 2-8 billion years on the main sequence depending on mass.

We’re also dealing with a larger habitable zone, a width 1.5 to 4 times greater than in the case of the Sun, again depending on the mass of the star and the climate models used to calculate the HZ. So there are advantages, for in the 2014 work, Cuntz and team found that the outer regions of the HZ experience tolerable levels of UV radiation. Now Cuntz has pushed the F-star work forward with a new paper, working with lead author Shaan Patel, a UTA grad student, and colleague Nevin Weinberg. The new work embarks on a statistical analysis of planet-hosting F-class stars drawn from data in the NASA Exoplanet Archive, which is a resource I don’t link to often enough. Says Cuntz:

“F-type stars are usually considered the high-luminosity end of stars with a serious prospect for allowing an environment for planets favorable for life. However, those stars are often ignored by the scientific community. Although F-type stars have a shorter lifetime than our Sun, they have a wider HZ. In short, F-type stars are not hopeless in the context of astrobiology.”

Image: The habitable zone as visualized around different types of star. Credit: NASA.

206 planetary systems emerge from the investigation, of which 18 offer a planet in the liquid water habitable zone for at least part of its orbit. The authors break these worlds down into categories based on the amount of time each spends in the HZ. It’s worth noting that all the currently known planets in the habitable zone of F stars are Jupiter-class worlds, so what we are thinking about here in terms of astrobiology is habitable moons, about which interesting new work continues to emerge. I also assume we’ll be finding terrestrial-class worlds around these stars with deeper investigation.

The exo-Jupiter 38 Virginis (HD 111998) is noteworthy for spending the entirety of its orbit in the habitable zone, which most of these worlds do not. Now things get intriguing. There are reasons for including planets whose orbital eccentricity allows only partial passage through the HZ, drawing on previous research (citation below) on atmospheric conditions for Earth-class planets in extremely elliptical orbits. That 2002 study found that despite large variations in surface temperature, long-term climate depended on the average stellar flux over the entire orbit, meaning that planets not in but near the HZ may still be potentially habitable, at least for extremophiles.

And we can possibly extend our definition of habitable zone. From the paper:

As part of our study, we also consider cushions for both HZ limits. This approach is informed by previous studies given by Abe et al. (2011) and Wordsworth et al. (2013). The former work deals with climate simulations for “land planets” (i.e., desert worlds with limited surface water), which based on those models have a significantly extended inner HZ limit than planets with abundant surface water (akin to Earth). Moreover, Wordsworth et al. (2013) continued to explore the outer limit of HZs by considering the impact of CO2, including CO2 clouds. They found that in their models the outer HZ is notably extended, commensurate to the Martian orbit in the solar system.

Image: This is Figure 10 from the paper. Caption: Depiction of all 18 systems that spend at least part of their time within their respective HZs. Empty markers in panel (c) represent actual planetary mass values as opposed to minimum mass values, which are represented by filled in markers. Credit: Patel et al.

Consider that the lowest-mass planet currently in a habitable zone in all these systems has an estimated mass 143 times Earth and you’ll agree with the need to probe further into potentially habitable exomoons, about which we know next to nothing. Overall, with projects like the Habitable Worlds Observatory on the table, we should consider F-class stars as targets for deeper study. As lead author Patel says, “In future studies, our work may serve to investigate the existence of Earth-mass planets and also habitable exomoons hosted by exo-Jupiters in F-type systems.”

The paper is Patel et al., “Statistics and Habitability of F-type Star–Planet Systems,” The Astrophysical Journal Supplement Series Vol. 274, No. 1 (12 September 2024), 20 (full text). The paper on habitability in eccentric orbits is Williams & Pollard, “Earth-like worlds on eccentric orbits: excursions beyond the habitable zone,” International Journal of Astrobiology Vol. 1, Issue 1 (January, 2002), 61-68 (abstract).

The Long Afternoon of Earth

Every time I mention a Brian Aldiss novel, I have to be careful to check the original title against the one published in the US. The terrific novel Non-Stop (1958) became Starship in the States, rather reducing the suspense of decoding its strange setting. Hothouse (1962) became The Long Afternoon of Earth when abridged in the US following serialization in The Magazine of Fantasy & Science Fiction. I much prefer the poetic US title with its air of brooding fin de siècle decline as Aldiss imagines our deep, deep future.

Imagine an Earth orbiting a Sun far hotter than it is today, a world where our planet is now tidally locked to that Sun, which Aldiss describes as “paralyzing half the heaven.” The planet is choked with vegetation so dense and rapidly evolving that humans are on the edge of extinction, living within a continent-spanning tree. The memory of reading all this always stays with me when I think about distant futures, which by most accounts involve an ever-hotter Sun and the eventual collapse of our biosphere.

Image: The dust jacket of the first edition of Brian Aldiss’ novel Hothouse.

Indeed, warming over the next billion years will inevitably affect the carbon-silicate cycle. Its regulation of atmospheric carbon dioxide is a process that takes CO2 all the way from rainfall through ocean sediments, their subduction into the mantle and the eventual return of CO2 to the atmosphere by means of volcanism. Scientists have thought that the warming Sun will cause CO2 to be drawn out of the atmosphere at rates sufficient to starve out land plants, spelling an end to habitability. That long afternoon of Earth, though, may be longer than we have hitherto assumed.

A new study now questions not only whether CO2 starvation is the greatest threat but also manages to extend the lifetime of a habitable Earth far beyond the generally cited one billion years. The scientists involved apply ‘global mean models,’ which help to analyze how vegetation affects the carbon cycle. Lead author Robert Graham (University of Chicago), working with colleagues at Israel’s Weizmann Institute of Science, is attempting to better understand the mechanisms of plant extinction. Their new constraints on silicate weathering push the conclusion that the terrestrial biosphere will eventually succumb to temperatures near runaway greenhouse conditions. The biosphere dies from simple overheating rather than CO2 starvation.

The implications are intriguing and offer fodder for a new generation of science fiction writers working far-future themes. For in the authors’ models, the lifespan of our biosphere may be almost twice as long as has been previously expected. Decreases in plant productivity act to slow and eventually (if only temporarily) reverse the future decrease in CO2 as the Sun continues to brighten.

Here’s the crux of the matter: Rocks undergo weathering as CO2 laden rainwater carrying carbonic acid reacts with silicate minerals, part of the complicated process of sequestering CO2 in the oceans. The authors’ models show that if this process of silicate weathering is only weakly dependent on temperature – so that even large temperature changes have comparatively little effect – or strongly CO2 dependent, then “…progressive decreases in plant productivity can slow, halt, and even temporarily reverse the expected future decrease in CO2 as insolation continues to increase.”

From the paper:

Although this compromises the ability of the silicate weathering feedback to slow the warming of the Earth induced by higher insolation, it can also delay or prevent CO2 starvation of land plants, allowing the continued existence of a complex land biosphere until the surface temperature becomes too hot. In this regime, contrary to previous results, expected future decreases in CO2 outgassing and increases in land area would result in longer lifespans for the biosphere by delaying the point when land plants overheat.

How much heat can plants take? The paper cites a grass called Dichanthelium lanuginosum that grows in geothermal settings (with the aid of a symbiotic relationship with a fungus) as holding the record for survival, at temperatures as high as 338 K. The authors take this as the upper temperature limit for plants, adding this:

Importantly, with a revised thermotolerance limit for vascular land plants of 338 K, these results imply that the biotic feedback on weathering may allow complex land life to persist up to the moist or runaway greenhouse transition on Earth (and potentially Earth-like exoplanets). (Italics mine)

The long afternoon of Earth indeed. The authors point out that the adaptation of land plants (Aldiss’ continent-spanning tree, for example) could push their extinction to even later dates, limited perhaps by the eventual loss of Earth’s oceans.

…an important implication of our work is that the factors controlling Earth’s transitions into exotic hot climate states could be a primary control on the lifespan of the complex biosphere, motivating further study of the moist and runaway greenhouse transitions with 3D models. Generalizing to exoplanets, this suggests that the inner edge of the “complex life habitable zone” may be coterminous with the inner edge of the classical circumstellar habitable zone, with relevance for where exoplanet astronomers might expect to find plant biosignatures like the “vegetation red edge” (Seager et al. 2005).

The paper is Graham, Halevy & Abbot, “Substantial extension of the lifetime of the terrestrial biosphere,” accepted at Planetary Science Journal (preprint).

Beamed Propulsion and Planetary Security

Power beaming to accelerate a ‘lightsail’ has been pondered since the days when Robert Forward became intrigued with nascent laser technologies. The Breakthrough Starshot concept has been to use a laser array to drive a fleet of tiny payloads to a nearby star, most likely Proxima Centauri. It’s significant that a crucial early decision was to place the laser array that would drive such craft on the Earth’s surface rather than in space. You would think that a space-based installation would have powerful advantages, but two immediate issues drove the choice, the first being political.

The politics of laser beaming can be complicated. I’m reminded of the obligations involved in what is known as the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies (let’s just call it the Outer Space Treaty), spurred by a paper from Adam Hibberd that has just popped up on arXiv. The treaty, which comes out of the United Nations Office for Space Affairs, emerged decades ago and has 115 signatories globally.

Here’s the bit relevant for today’s discussion, as quoted by Hibberd (Institute for Interstellar Studies, London):

States Parties to the Treaty undertake not to place in orbit around the earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction, install such weapons on celestial bodies, or station such weapons in outer space in any other manner. The moon and other celestial bodies shall be used by all States Parties to the Treaty exclusively for peaceful purposes. The establishment of military bases, installations and fortifications, the testing of any type of weapons and the conduct of military manoeuvres on celestial bodies shall be forbidden. The use of military personnel for scientific research or for any other peaceful purposes shall not be prohibited. The use of any equipment or facility necessary for peaceful exploration of the moon and other celestial bodies shall also not be prohibited.

So we’re ruling out weaponry in orbit or elsewhere in space. Would that prohibit building an enormous laser array designed for space exploration? Hibberd believes a space laser would be permitted if its intention were for space exploration or planetary defense, but you can see the problem: Power beaming at this magnitude can clearly be converted into a weapon in the wrong hands. And what a weapon. A 10 km X 10 km installation as considered in Philip Lubin’s DE-STAR 4 concept generates 70 GW beams. You can do a lot with that beyond pushing a craft to deep space or taking an Earth-threatening asteroid apart.

Build the array on Earth and the political entanglements do not vanish but perhaps become manageable as attention shifts to how to avoid accidentally hitting commercial airliners and the like, including the effects on wildlife and the environment.

Image: Pushing a lightsail with beamed energy is a feasible concept capable of being scaled for a wide variety of missions. But where do we put the beamer? Credit: Philip Lubin / UC-Santa Barbara.

The second factor in the early Starshot discussions was time. Although now slowed down as its team looks at near-term applications for the technologies thus far examined, Starshot was initially ramping up for a deployment by mid-century. That’s pretty ambitious, and we wouldn’t have a space option that could develop the beamer if that stretchiest-of-all-stretch goals actually became a prerequisite.

So if we ease the schedule and assume we have the rest of the century or more to play with, we can again examine laser facilities off-planet. Moreover, Starshot is just one beamer concept, and we can back away from its specifics to consider an overall laser infrastructure. Hibberd’s choice is the DE-STAR framework (Directed Energy Systems for Targeting of Asteroids and Exploration) developed by Philip Lubin at UC-Santa Barbara and first described in a 2012 on planetary defense. The concept has appeared in numerous papers since, especially 2016’s “A Roadmap to Interstellar Flight.”

If the development of these ideas intrigues you, let me recommend Jim Benford’s A Photon Beam Propulsion Timeline, published here in 2016, as well as Philip Lubin’s DE-STAR and Breakthrough Starshot: A Short History, also from these pages.

What Hibberd is about in his new paper is to work out how far away various categories of laser systems would have to be to ensure the safety of our planet. This leads to a sequence of calculations defining different safe distances depending on the size of the installation. The DE-STAR concept is modular, a square phased array of lasers where each upgrade indicates a power of base 10 expansion to the array in meters. In other words, while DE-STAR 0 is 1 meter to the side, DE-STAR 1 goes to 10 meters to the side, and so on. Here’s the chart Hibberd presents for the system (Table 1 in his paper).

Keep scaling up and you achieve arrays of stupendous size, and in fact an early news release from UC-Santa Barbara described a DE-STAR 6 as a propulsion system for a 10-ton interstellar craft. It’s hard to imagine the 1,000 kilometer array this would involve, although I’m sure Robert Forward would have enjoyed the idea.

So taking Lubin’s DE-STAR as the conceptual model (and sticking with the more achievable lower end of the DE-STAR scale), how can we lower the risks of this kind of array being used as a weapon? And that translates into: Where can we put an array so that even its largest iterations are too far from Earth to cause concern?

Hibberd’s calculations involve determining the minimum level of flux generated by an individual 1 meter aperture laser element (this is DE-STAR 0) – “the unphased flux of any DE-STAR n laser system” – and using as the theoretical minimum safe distance from Earth a value on the order of 10 percent of the solar constant at Earth, meaning the average electromagnetic radiation per unit area received at the surface. The solar constant value is 1361 watts per square meter (W/m²); Hibberd pares it down to a maximum allowed flux of 100 W/m² and proceeds accordingly.

Now the problems of a space-based installation become strikingly apparent, for the calculations show that DE-STAR 1 (10 m X 10 m) would need to be positioned outside cis-lunar space to ensure these standards, and even further away (beyond the Earth-Moon Lagrange 2 point) for ultraviolet wavelengths (λ ≲ 350nm). That takes us out 450,000 kilometers from Earth. However, a position at the Sun-Earth L2 Lagrange location would be safe for a DE-STAR 1 array.

The numbers add up, and we have to take account of stability. The Sun/Earth Lagrange 4 and 5 points would allow a DE-STAR 2 laser installation to remain at a fixed location without on-board propulsion. DE-STAR 3 would have to be positioned beyond the asteroid belt, or even beyond Jupiter if we take ultraviolet wavelengths into account. The enormous DE-STAR 4 level array would need to be placed as far as 70 AU away.

All this assumes we are working with an array on direct line of sight with the Earth, but this does not have to be the case. Let me quote Hibberd on this, as it’s rather interesting:

Two such locations are the Earth/Moon Lagrange 2 point (on a line from the Earth to the Moon, extending beyond the Moon by ∼ 61, 000 km) and the Sun/Earth Lagrange 3 point (at 1 au from the Sun and diametrically opposite the Earth as it orbits the Sun). In both cases, the instability of these points will result in the DE-STAR wandering away and potentially becoming visible from Earth, so an on-board propulsion would be needed to prevent this. One solution would be to use the push-back from the lasers to provide a means of corrective propulsion. However it would appear a DE-STAR’s placement at either of these points is not an entirely satisfactory solution to the problem.

So we can operate with on-board propulsion to achieve no direct line-of-sight to Earth, but the orbital instabilities involved make this problematic. Achieving the goal of a maximum safe flux at Earth isn’t easy, and we’re forced to place even DE-STAR 2 arrays at least 1 AU from the Sun at the Sun/Earth Lagrange 4 or 5 positions to achieve stable orbits. DE-STAR 3 demands movement beyond the asteroid belt at a minimum. DE-STAR levels beyond this will require new strategies for safety.

Back to the original surmise. Even if we had the technology to build a DE-STAR array in space in the near future, safety constraints dictate that it be placed at large distances from the Earth, making it necessary to have first developed an infrastructure within the Solar System that could support a project like this. As opposed to one-off missions from Earth launching before such an infrastructure is in place, we’ll need to have the ability to move freely at distances that ensure safety, unless other means of planetary protection can be ensured. Hibberd doesn’t speculate as to what these might be, but somewhere down the line we’re going to need solutions for this conundrum.

The paper is Hibberd, “Minimum Safe Distances for DE-STAR Space Lasers,” available as a preprint. Philip Lubin’s “A Roadmap to Interstellar Flight” appeared in Journal of the British Interplanetary Society 69, 40-72 (2016). Full text.