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).

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.

I’ve reminisced before about crossing Lake George in the Adirondacks in a small boat late one night some years back, when I saw the Milky with the greatest clarity I had ever experienced. Talk about dark skies! That view was not only breathtaking on its own, but it also raised the point about what we can see where. Ponder the cosmic optical background (COB), which sums up everything that has produced light over the history of the universe. The sum of light can be observed with even a small telescope, but the problem is to screen out local sources. No telescope is better placed to do just this than the Long Range Reconnaissance Imager (LORRI) aboard the New Horizons spacecraft.

Deep in the Kuiper Belt almost 60 AU from the Sun, the craft has a one-way light time of over eight hours (Voyager 1, by comparison, shows a one-way light time of almost 23 hours at 165 AU). It’s heartening that we’re continuing to keep the Voyagers alive even as the options slowly diminish, but New Horizons is still robust and returning data from numerous instruments. No telescope anywhere sees skies as dark as LORRI. That makes measurements of the COB as authoritative as anything we’re likely to get soon.

Image: Not my view from the Adirondacks but close. The Milky Way is gorgeous when unobscured by city lights. Credit: Derek Rowley.

The issue of background light came to the fore in 2021, when scientists at the National Science Foundation-funded NSF NOIRLab put data from New Horizons’ 20.8 cm telescope to work. That effort involved measuring the light found in a small group of images drawn from deep in the cosmos. It suggested a universe that was brighter than it should be, as if there were uncounted sources of light. Now we have further analysis of observations made with LORRI in 2023 supplemented by data from ESA’s Planck mission, which aids in calibrating the dust density in the chosen fields of view. We learn that contamination from the Milky Way can explain the anomaly.

The new paper from lead author Marc Postman (Space Telescope Science Institute) studies light from 16 different fields carefully chosen to minimize the background light of our own galaxy which, of course, surrounds us and compromises our view. This new work, rather than using archival data made for other purposes, explicitly uses LORRI to create images minimizing foreground light sources. The conclusion is evidently air-tight, as laid out by Postman:

At the outset of this work we posed the question: Is the COB intensity as expected from our census of faint galaxies, or does the Universe contain additional sources of light not yet recognized? With our present result, it appears that these diverse approaches are converging to a common answer. Galaxies are the greatly dominant and perhaps even complete source of the COB. There does remain some room for interesting qualifications and adjustments to this picture, but in broad outline it is the simplest explanation for what we see.

And let me throw in this bit from the conclusion of the paper because it adds an interesting dimension to the study:

If our present COB intensity is correct, however, it means that galaxy counts, VHE γ-ray extinction, and direct optical band measurements of the COB intensity have finally converged at an interesting level of precision. There is still room to adjust the galaxy counts slightly, or to allow for nondominant anomalous intensity sources.

In other words, to fully analyze the COB, the scientists have included VHE (very high energy) gamma ray extinction, meaning adjustments for the scattering of gamma rays as they travel to us. Although not visible at optical wavelengths, gamma rays can interact with the photons of the COB in ways that can be measured, as an adjustment to the rest of the COB data. That analysis complements the count of known galaxies and the optical band measurements to produce the conclusion now achieved.

I always find it interesting that there is both a deep satisfaction in solving a mystery and also a slight letdown, for let’s face it, odd things in the universe are fascinating, and let our imaginations run wild. In this case, however, the issue seems resolved.

I don’t have to mention to this audience how much good science continues to get done by having a fully functioning probe this deep in the Kuiper Belt. From New Horizons’ vantage point, there is little to no effect from zodiacal light, which is the result of sunlight scattering off interplanetary dust. The latter is a key factor in the brightness of the sky in the inner Solar System and has made previous attempts to measure the COB from the inner system challenging. We now look ahead to New Horizons’ search for other Kuiper Belt Objects to explore and try to learn whether there is a second belt of debris beyond the known one, and thus between it and the inner Oort Cloud.

We’ll doubtless continue to find things that challenge our assumptions as we press on, a reminder that a successor to New Horizons and the Voyagers is still a matter of debate both in terms of mission design and funding. As to the cosmic optical background, we give up the unlikely but highly interesting prospect that any significant levels of light come from sources unknown to us. As the paper concludes: “…the simplest hypothesis appears to provide the best explanation of what we see: the COB is the light from all the galaxies within our horizon..”

The paper is Postman et al., “New Synoptic Observations of the Cosmic Optical background with New Horizons,” The Astrophysical Journal Vol. 972, No. 1 (28 August 2024), 95 (full text). The 2021 paper is Lauer et al., “New Horizons Observations of the Cosmic Optical Background,” The Astrophysical Journal Vol. 906, No. 2 (11 January 2021), 77 (full text).