VIIRS-DNB data might be able to tell us. We modeled the effect of adding reflective materials to roadway surfaces, commonly referred to as “cool pavements”. These materials have a paint-like consistency and are applied directly atop existing street pavements. There is some evidence that they work, lowering air temperatures in treated areas by up to a few degrees Celsius. Based on field measurements of the cool pavement materials applied to some streets in Phoenix, Arizona, U.S., we calculated the expected change in nighttime brightness of the roadway surfaces at between about 2-6%. Although the materials appear bright to the eye, much of the radiation they reflect is in the infrared part of the spectrum, some of which we perceive as heat.

To test this model, we looked at VIIRS-DNB measurements of the nighttime brightness of Phoenix made over more than a decade, including the time when the City of Phoenix applied cool pavement materials in certain neighborhoods. We compared the measurements to those of nearby neighborhoods whose streets did not receive the pavement treatments. The results were recently published in the Journal of Quantitative Spectroscopy and Radiative Transfer. 

What we found was surprising and a little disappointing: we couldn’t tell whether application of the cool pavement treatments made any difference in the satellite data. We saw unexpectedly high variations in the brightnesses of all of the neighborhoods from one month to the next. Those variations were so large that the brightness changes due to the cool pavement treatments our model predicted were lost in the noise. In the end, we couldn’t rule either in or our real changes that might affect the brightness of the night sky over Phoenix.

All remote sensing measurements are obtained in imperfect conditions. We can control these circumstances to some extent; for example, the effect of moonlight on nighttime lights observations can be limited by acquiring images only when the Moon is below the local horizon. Some influences, like weather, can’t be completely avoided. Long-term observations can help even out the extremes in order to get at the true underlying trends.

But some of the difficulties we experience are due to satellite designs that are less than ideal for this kind of application. Facilities like the VIIRS-DNB are almost afterthoughts, added to satellites for reasons other than studying night lights. While they provide a trove of information about the distribution of artificial light at night on our planet, they’re not really built specifically for that reason. 
Consequently, some of their characteristics fall short of what we might want. A prime example of this has to do with the “blindness” of the VIIRS-DNB to blue light. The instrument is only sensitive to light with wavelengths of between about 500 and 900 nanometers (one-millionth of a meter). But a lot of modern LED lighting emits considerable light just below the 500-nanometer cutoff. 
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This presented a problem for light pollution researchers from the very beginning when the first copy of the VIIRS instrument was launched in 2012. It was around that time that white LED light was beginning to replace earlier lighting technologies. As lighting modernization projects proceeded, VIIRS-DNB observations saw changes that could easily be misinterpreted. Cities seemed to be getting darker in the satellite images, whereas measurements on the ground clearly indicated that they were no darker than before the lighting retrofits; see the example below. We can account for effects like this to some extent, yet we are left to guess the exact amount of light that the VIIRS-DNB simply doesn’t see.

In June 1969, the Cuyahoga River, bisecting Cleveland, Ohio, on its way to Lake Erie, caught fire. The level of flammable material in the river, resulting from dumping of industrial waste into the river and its tributaries, rose to the point where it could ignite. A Time magazine photographer was there to witness the event. The resulting photo spread in the magazine horrified Americans and galvanized the nascent environmental movement in the U.S. A short three years later the Clean Water Act became law and the health of American waterways began improving.

But, as the U.S. National Park Service tells the tale, the fire was not particularly shocking to locals because it was only one of many such instances historically. “The river had burned more than 10 times over the previous century. The first newspaper coverage focused on the damage, not the fact that the river had burned. At the time, people largely saw the river as a part of industrial infrastructure. In that light, a river fire seemed more normal. It is when we view a river as a natural system that a fire seems out of place." The notion that industrial pollution of rivers was an inevitable consequence of modernity and progress was gradually replaced by refusal to accept outcomes like rivers catching fire. Faced with a parallel situation involving light pollution, is a similar reorientation of thinking the best way forward?

In an earlier post here, we explored environmental law in the United States context particularly, noting how effective it was during the past half century in substantially reducing environmental pollution. We argued that this represented a kind of road map leading toward comprehensive management of a pollutant (artificial light at night) fouling a valuable resource (natural nighttime darkness).

We also previously wrote about Dark Skies and the "Rights Of Nature". The argument in part was that novel legal theories (at least in the West) that confer some legal rights on natural systems may be a way to pursue protection of nighttime darkness. But the idea has yet to gain significant traction. It relies on an assumption that the night sky is a kind of commons owned by no one, in tension with legal guarantees of private property rights.

With University of San Francisco professors Dana Zartner and Aparna Venkatesan, we recently reviewed what's known about this and related subjects. In our paper, we landed on three broad legal approaches to protecting dark night skies. In addition, we considered individual rights and community rights descending from concepts such as the right to a healthy environment and the rights of future generations to inherit from us a planet where humans can continue to live. 

In the end, we concluded, there are "needed cultural shifts in how we think about the impacts of increasing light pollution and what we are losing with decreasing darkness. ...Creative use of newer legal strategies may address this environmental justice issue and support the need to protect darkness." In short, we need to rethink the issue entirely and try new methods because the old ones aren’t working.

At some level the reason why there has been no meaningful breakthrough in advancing legal protections for dark night skies has to do with the way the sky is treated in our culture and in our law, which centers human needs and desires in our interactions with the natural world. The Western legal tradition places humans at the top of a created hierarchy, below which are the elements of the natural environment. In the Christian worldview humans were given dominion over the natural world and laws were created to govern the affairs of people; it's therefore no surprise that law historically favored human priorities over environmental protection.

But that began to change during the 20th century. Environmental law began to emerge after a seismic shift took place in thinking about how humans interact with the environment. Although humans have modified the environment for millennia, the scale of that modification tended to be local. Only recently has technology developed to the point that the effects were large-scale or even global. 

Industrialization began to yield environmental harms as early as the 18th century. This accelerated in the postwar years and by the late 1950s environmental destruction itself was industrialized. Science gradually showed how humans were modifying the natural environment in global and ultimately negative ways.

Environmental law was a reaction to the changes that became increasingly evident during the last century. But those laws mainly functioned only at the national level, even if most countries began adopting a canon of similar laws. Environmental pollution respects no political boundaries, so approaches that focus on single jurisdictions are usually inadequate to solve significant problems that result. 

Few international efforts have achieved success, although those that have suggested ways forward: witness the wild success of the Montreal Protocol (1987) that ended industrial fluorocarbon production in order to heal the ozone hole over Antarctica. While limited in scope, Montreal proved that major environmental goals could be achieved if the world committed itself to them. 

A turning point of swords was the recognition that human activity is inseparable from the environment. The U.S. National Environmental Policy Act of 1969 (NEPA) was one of the first laws to introduce the use of the term "human environment". NEPA defined this term "comprehensively" as "the natural and physical environment and the relationship of present and future generations with that environment." It flows from the the NEPA statement of legislative intent, which reads in part "to declare a national policy which will encourage productive and enjoyable harmony between man and his environment; to promote efforts which will prevent or eliminate damage to the environment and biosphere and stimulate the health and welfare of man; [and] to enrich the understanding of the ecological systems and natural resources important to the Nation."

Three years later the UN held its first global environmental conference, the "United Nations Conference on the Human Environment", resulting in the Stockholm Declaration (also here) that led to the establishment of the United Nations Environment Programme (UNEP). This began an ongoing dialogue among nations exploring links between economic growth, environmental pollution, and the well-being of humanity. It also started the process by which the world began reacting to the threat of global climate change.

The human environment is a coupled system: humans impact the environment, and the environment impacts humans. For most of the existence of our species, it was a largely closed system bounded by the atmosphere. If the atmosphere is used as a defining boundary, then even before space travel the human environment extended to the Moon. To the extent that such discoveries come as a surprise, it’s because they challenge our preconceived ideas about how the world is composed and structured.

Space travel changed that by establishing a human presence in outer space, even if only in the form of the artifacts produced by human hands. In the decades, since the launch of the first artificial satellite, orbital space has become an increasingly congested volume around our planet. In many respects, the condition of low-Earth orbit (LEO) is fully determined by human activities there.

In 2023 we wrote here about the effect of satellites on the appearance of the night sky, asking whether the rapid proliferation of satellites in LEO was worth worrying about. The answer at the time was that we didn't really know, given uncertainties about the future development of space. There are some encouraging signs involving voluntary actions taken by commercial space companies, but there are still considerable hazards presented by effects such as the so-called "Kessler Syndrome" of runaway space debris generation. The prospect of space warfare also looms on the horizon, most recently brought into focus by the November 2021 test of a destructive anti-satellite weapon by the Russian Federation. In short, space is an increasingly dangerous place.

Now we're also worried about a potential feedback cycle between the space and terrestrial environments from the spacecraft life cycle. That is to say, the entire process of building, launching, operating, and ultimately de-orbiting spacecraft has its own environmental footprint. A warming climate plus water vapor and black carbon soot emitted into low- to mid-altitudes could increase the prevalence of clouds. Rocket launches are punching "holes" in the ionosphere. Metals deposited in the upper atmosphere during spacecraft re-entry have unknown effects on the Earth's energy budget. All of this probably impacts the visibility of the night sky and comes on top of an alarming increase in terrestrial skyglow in recent years.

Yet existing legal mechanisms, continue to treat the earth and space like they were fundamentally different and decoupled environments unto themselves. That view is increasingly untenable in a world where the connection between those two spaces is demonstrably stronger than ever. Between them is the sky that represents humanity's portal to the stars, views of which inspired untold generations of people to reach for them. As threats to those views now come from both above and below, the need to change how we think of Earth, sky and space as elements of a single human environment is more important than ever.

If humans were to reach the conclusion that the night sky is “environment”, we might begin to treat it differently as a matter of law. That view finds synergy with the related contention that the night sky is “culture” unique to no particular society. In coming to these realizations, we might further decide that there is value in this resource that is diminished by its pollution. For now, we haven’t yet decided that a clean river is what we want — much less acknowledged that the polluted river we already have is on fire. 

​A visit to an astronomical observatory at night is a magical experience. On a far mountaintop, the starry vault overhead, they are like temples to the study of the cosmos. Telescopes silently scan the heavens all night, shutting their eyes before the first light of dawn. But with increasing frequency, these distant sites are under threat from artificial light at night. What were once thought to be the most defensible locations are less so now. Some suggest that a new management paradigm is the only way to save them.

Astronomical discovery depends on access to faint cosmic light. It is the one physical attribute of the universe directly accessible to science over very large distances. Information about physical conditions in faraway stars and galaxies is encoded in the light we collect with telescopes. Absent this fact, we would know very little about outer space. Because the intensity of light decreases with the distance to its source, these signals are very faint when they reach Earth. Telescopes with massive mirrors collect this light in enough quantity to make sense of the information it brings.

Yet it is now often the case that cosmic light must compete with light from artificial sources on the ground. This began as early as the 18th century, when astronomical observatories were still situated in cities. The installation of gas lighting systems, later replaced by electric ones, began brightening the night sky. Astronomers moved out of cities and took their telescopes with them to the countryside. Many such locations, especially in Europe were unfavorable for observations. Bad weather got in the way, and turbulence in the atmosphere made for poor-quality images. They then moved telescopes to the summits of mountains far from cities. In the 20th century, they started launching them into space. All this increased the difficulty and cost of collecting astronomical data.

Well into the last century, many important observatories in and near cities continued their work. Some sought to hold back the rising tide of light pollution with public policy interventions. Lowell Observatory, in Arizona, U.S., pioneered this approach in the late 1950s. It convinced the council of the city of Flagstaff, its home since the 1890s, to enact what became known as the "Searchlight Law". [1] It aimed to reduce the impact of the use of searchlights for advertising purposes on the observatory's work. But this could only go so far. Flagstaff's population now over four times that in 1960, and the observatory has moved its operations out of town.

Rising night sky brightness over observatories means that scientists need more time and money to achieve the same science outcomes. Consider a case in which the brightness of the night sky at an otherwise unpolluted observatory site doubles. The exposure time for a given telescope and camera needed to reach some science goal then doubles. [2] Given the dollar cost associated with operating a modern observatory, the cost to achieve that goal also doubles. Light pollution thus not only threatens to slow the pace of discovery, but it also makes that discovery more expensive.

There are some recent successes. In 1998, the Chilean National Congress enacted the "Norma de Emisión para la Regulación de la Contaminación Lumínica" ("Emission Standard for the Regulation of Light Pollution”). It required a few basic restrictions on the operation of outdoor lighting in three northern provinces of the country. The goal was to protect observatories in the mountains above the Atacama Desert. Successive updates to the 'Norma Lumínica' have proven rather successful. Yet even the most remote observatory sites there are now threatened. Light can travel hundreds of kilometers through the atmosphere, fouling the night sky far from where it is emitted. Now, no observatory is entirely safe from light pollution. [3] 

Astronomers began confronting this problem in a systematic way over a half-century ago. In 1973, the International Astronomical Union (IAU) established a 'commission' tasked with identification and protection of existing and potential observatory sites. At its 1976 General Assembly, the IAU adopted this statement of concern:

The IAU notes with alarm the increasing levels of interference with astronomical observations resulting from artificial illumination of the night sky, radio emission, atmospheric pollution, and the operation of aircraft above observatory sites. The IAU therefore urgently requests that the responsible civic authorities take action to preserve existing and planned observatories from such interference. To this end, the IAU undertakes to provide through Commission 50 information on acceptable levels of interference and possible means of control.

​It issued guidelines for what it considered 'acceptable' levels of light-pollution interference in 1977. By the following year, the issue attracted the attention of the International Commission on Illumination (CIE). As the international authority on light and illumination, it took note of the increasing problem faced by observatories. It adopted a statement acknowledging the problems caused by uncontrolled outdoor lighting near the best observatory sites.  Further, it urged authorities to take all possible action to protect these sites.

The IAU and CIE joined forces in 1980, releasing the joint publication "Guidelines for minimizing urban sky glow near astronomical observatories". [4] Among its main findings, the document recommended that:

The increase in sky brightness at 45° elevation due to artificial light scattered from clear sky should not exceed 10 per cent of the lowest natural level in any part of the spectrum between wavelengths 300 and 1000 nm.

A lot has changed in 45 years. For one thing, we know more about by how much the natural night sky varies in brightness. [5] That's true on timescales ranging from minutes to years. For example, the 1980 CIE-IAU report presumed a typical night sky brightness at unpolluted sites that is only found sometimes near the minimum of the 11-year solar activity cycle. At the cycle's maximum the brightness can be over 50% brighter, even in the absence of light pollution. Natural sources of light in the night sky varying from one night to the next can yield even bigger changes.

​The bottom line in all this is that the natural night sky itself is a dynamic system "alive with its own light," as the American astronomer Al Grauer says. The night sky, apart from stars and other sources, is not a pure black due to the absence of light. It varies in brightness and color like a noisy audio signal varies in intensity and tone. To continue the analogy, light pollution is like a loud sound atop this noise, with a pure timbre that stays mostly constant from night to night. In the midst of this racket, astronomers are trying to sense a mere whisper.

In 2019, astronomers reacted to a new threat from above: the light of thousands of new satellites launched into orbit around the Earth. They held an international conference in 2021 on the emerging idea of preserving "dark and quiet skies" (D&QS). The report of this conference make a series of recommendations for protecting observatory sites from encroaching light pollution. Some of these are technical in nature, while others leverage the power of public policy to regulate outdoor lighting installations.

The current IAU view, echoing the D&QS report, is:

Present-day professional observatories are located in remote, high-altitude locations; a key selection criterion is the actual sky darkness being as close to the natural background as possible. These sites have an artificial light contamination significantly below the 10% limit recommended by the IAU in 1979. Hence, this limit is not appropriate for the protection of modern professional astronomical sites.
Its new guidance is to "keep the total contribution to skyglow from ALAN substantially below the 10% dark site limit defined by the IAU" (emphasis in the original). At the same time, it recognizes that there is no "one size fits all" approach:

The core of the recommendations is that each major site has a unique limit that should not be exceeded by growing ALAN. It requires each observatory to know what its ALAN contribution is and the rate at which it is currently growing, quantities that can be easily measured.

We can monitor the light situation near observatories from the ground and space, but this involves making long-term observations. Changing the trajectory of that situation requires the commitment not only of governments, but also of the public they serve. There are various proposals for how to do this. We previously wrote about one such idea: the active management of so-called "lightsheds" near observatories.

Another approach is to incentivize reducing light pollution through means such as cap-and-trade schemes. But this still requires treating observatory sites as a special kind of reservation. "For example," the IAU writes,

if an observatory has a current ALAN growth rate of 0.04% per year, which is to be brought to zero within five to eight years, then the ALAN contribution will be less than 0.5% for the foreseeable future. The condition that the ALAN growth rate must be brought to zero and reversed at a site that now has an extremely low artificial contribution sets strong constraints; there will be no way to accommodate a new major artificial light source within these rules, as there is no offset for any sources that could be reduced.

There is now movement in the astronomical community to revise the existing 1980 standard with new information gleaned during the intervening decades. It is also informed by radical changes in the way the way the outdoor world is lit at night since the introduction of LED technology. The high directionality of LED and the ease by which active controls manipulate its light is a game-changer. And non-white light sources, such as amber LED, are more available than ever before. These sources emit light in a limited range of colors, leaving much of the rest of the spectrum dark.

​Astronomers are optimistic that these factors enable a tightening of the guidance that governs outdoor lighting. Over time, it could make a real difference for the wellbeing of the world's observatories. It may also extend their useful lifetimes. Controlling light pollution in this way could lead to a renaissance of science's "cathedrals of the night". 

The news reports are at first ominous. Purple glows appear over towns. Some headlines are even comical: "Tomato factory lights mistaken for 'lovely aurora'". But they share a common origin. To extend growing seasons and further the agriculture industry, more food crops are grown in greenhouses. Artificial lighting used at night aims to increase yields. And that light is having an unwanted effect on neighbors and the night sky. Yet some are betting the farm that new technologies and better regulations can solve this problem. 

Greenhouse farming is on the rise around the world. In the U.S. alone, its economic output was almost $6 billion in 2024. Rising at an annual rate of almost 9% per year, analysts expect it to top $11 billion by 2030. Industry experts make a case for investing further into this sector. Last year Agritechture wrote: "The agricultural landscape in the United States is undergoing a transformative shift, driven by supply chain weaknesses identified during the pandemic, the changes in our climate making it harder for field farmers to grow consistently, and the increasing demand from consumers for more sustainable and locally sourced produce. Greenhouses offer a viable solution to meet this shift in consciousness by providing a controlled environment for year-round cultivation."

With year-round cultivation comes an effort to grow plants faster by lengthening the growing day. Illuminating plants during overnight hours can make them bigger and more productive. "Horticultural lighting" can also reduce the time from seed to crop by accelerating growth. Rapid improvements in lighting technology better support year-round growing seasons.

In particular, the arrival of LED lighting was as much of a game-changer for agriculture as in other sectors. Earlier lamps consumed a lot of electricity and radiated much of their power in the form of heat. Besides being much more energy-efficient, LED has remarkable color characteristics. Its properties are selectable to give plants only the light they need for photosynthesis, explaining the pink or purple colors reported in news stories.

Given the challenge of providing adequate food for the global population, no one doubts the value of greenhouse farming. Food insecurity is a serious threat to economic development. According to the World Bank, "food security continues to be at alarming levels in most low-income countries." That is particularly true in Africa, where famines and rising food prices put millions at risk.
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At the same time — and ironically in part due to light pollution — populations of pollinating insects are in decline. Thus far, machines cannot replace the "ecosystem services" pollinators provide for free. These factors combine to create distinct threats to humans through potential disruptions in the global food supply. 

Their clear glass walls and roofs that make greenhouse ideal during the day is the source of a problem at night if owners use light to extend the growing day. Even with good lighting design and best-in-class lighting products, greenhouses are the source of "obtrusive" light. Light scatters and reflects from glass, interior surfaces, and plants themselves. Some of that light emerges from the greenhouses sideways and can cause light trespass. Light leaving from transparent roof panels travels unimpeded into the night sky. This causes the strange glows reported in news stories. And it can have ecological consequences of its own. For example, one recent study found that greenhouse lighting is harmful to songbirds.

What can be done about this? The fundamental solution involves keeping light contained within the building at night. That's an obvious challenge for structures whose very nature is to let outside (sun)light in. To address that particular aspect, some have experimented with so-called 'smart glass'. This method uses exotic materials that are alternately transparent or opaque to visible light. The state of transparency changes when, for example, an electric current is applied to a glass panel. But these materials are rather expensive, and their opacity is usually not enough to keep all interior light contained.

​A lower-tech solution involves a much lower-tech approach: close the blinds. Roof-mounted machinery deploys physical window coverings at dusk and retracts them at dawn. As shown in the example below, these can be an effective mitigation for much of the light that would otherwise escape the greenhouse. Yet even this trick isn't inexpensive, and it involves moving parts subject to wear and tear.

These mitigations can be combined with local regulations to enforce changes. The simplest laws impose a lighting "curfew" time, after which greenhouse owners must switch lights off each night. That may be most fair to people who live near greenhouses to reduce the impact of lighting. Of course, growers may take exception to such policies as unfairly limiting their operations. Zoning restrictions can help put some distance between greenhouses and their neighbors. And some jurisdictions may choose to prohibit greenhouses entirely within their territories.

​But regulation is challenging to put in place correctly and consistently. It often fails to completely address the problem. Like other examples of conflicting land uses, satisfying all those involved may be impossible. The benefits to society that greenhouse farming represents must be weighed against its social costs. In the ideal case, workable solutions respect the rights and wishes of all stakeholders.

Modern greenhouse lighting is here to stay, and by all accounts will only be more important to the global economy in the future. Yet there are clear challenges to enabling its future development while reducing its effect on the nighttime environment.

The controllability of LED could be the key to solving this problem, alongside other tech. But good old-fashioned "shutting the blinds" is the best approach. It presents an added expense to operators, which could as easily be counted alongside other cost of doing business. In that sense it's not unlike complying with building codes or workplace safety laws. And it's up to each jurisdiction to decide how much (and how best) to regulate.

Recent experience suggests that jurisdictions should get out ahead of this issue before it lands on their doorstep. Often the first sign of a new greenhouse locals notice is the nighttime glow. Reactionary efforts at regulation have a habit of proceeding rather poorly. Proactive attention to the problem can reduce that tendency.

There may be also some lessons learned here about things like land-use zoning. They may point the way to how to deal with other outdoor lighting challenges from 'speciality' lighting applications. Because of its associated effects, greenhouse farming might be subject to geographic restrictions. Regulators should, however, be mindful of the fact that light pollution can drift far from its sources. Other interventions may still be necessary to avoid conflicts.

​Can the future of greenhouse farming remain bright without compromising the dark? Any serious discussion of that question must consider both lighting design and regulation. It must further weigh its economic benefits against potential social and environmental harms. As in many such instances, the ideal solutions find the right balance between the needs of both people and planet.

It sounds like something out of a bad dream. You're outside in the early evening, enjoying the night air and contemplating the stars. All of a sudden, a glowing message drift across the sky: EAT AT JOE'S. It has all the appeal of a commercial message in a nature reserve. Yet such a scenario may only be a few years away.

​In a previous post here, we asked whether we should worry about satellite light pollution. At that time we argued there was little reason for concern. Yet it's easy to conceive of a future in which it is a problem. We made that argument based on certain realities about shortcomings in how we govern human activities in outer space. "Space advertising" is in even earlier stages of development, but the same anxieties apply. The difference is that for space advertising, there is still time to get out ahead of the situation.

​The uses and occupation of orbital space near the Earth have transformed since 2019. In the time since, the number and pace of launches of satellites into orbit around our planet more than doubled compared to their Cold War peak. An important difference compared to decades ago is that private actors conduct most of these operations. This brings activities motivated by profit rather than, say, geopolitical superiority.

Yet long before the arrival of this new use regime, some eyed outer space as the next frontier of product marketing.  For some, it was enough to engage in product placement on rockets or spacecraft. Others saw space as the medium of advertisement itself.

One notable episode in 1993 involved a proposed orbiting billboard made of Mylar that would appear about half the size of the full moon in the sky. Outcry from astronomers and environmental groups led to an act of the U.S. Congress that prohibited such activity if launched from U.S. territory.

​The 1993 law defined obtrusive space advertising as "advertising in outer space that is capable of being recognized by a human being on the surface of the Earth without the aid of a telescope or other technological device." Congress worded it so to draw a distinction with a more passive form of advertising such as corporate logos on launch vehicles. But in enacting the prohibition, Congress made clear that the U.S. would not abide "obtrusive" commercial messages in space. 

Besides the potential to annoy billions of people, obtrusive space advertising poses a threat to astronomy. To be bright enough for people on the ground to see it, ads in space must be bright. Any such object wandering through the field of view of a telescope would obliterate faint cosmic light beyond. These object would also be bright at infrared wavelengths.

Even if the lights switched off while flying over observatories, dark objects would block starlight for several seconds. There are already concerns that much smaller conventional satellites will have similar effects on astronomical observations.

At the same time, it seems inevitable that someone will try this. The lure of passive income from space advertising is great. It was once the case that the high cost of launching objects into orbit was a disincentive to potential space advertisers. But the recent mass-commercialization of launch services caused per-kilogram launch costs to plummet.

​Another factor is a shift in government attitudes toward commercial activities in space. From its founding, the U.S. National Aeronautics and Space Administration (NASA) faced restrictions on even appearing to promote commercial products or services. That stance is shifting. For example, in 2018 the NASA administrator formed a committee to investigate changing the policy.

Nothing in international law now governs this variety of space advertising. The founding document of international space law, the 1967 Outer Space Treaty (OST), is silent on this topic. It does not prohibit commercial activity in space. In fact, it refers to the "use of outer space", which many construe to include both commercialism and exploitation.

But it doesn't render such activities limitless. Its Article IX calls for "due regard" among spacefaring nations toward "the corresponding interests of all other States Parties to the Treaty". And it considers the possibility that some of those activities could drive conflict. Any activity that "would cause potentially harmful interference with activities of other States Parties in the peaceful exploration and use of outer space" requires consultations between countries causing and receiving the interference.

The U.S. prohibition on obtrusive space advertising also faces civil liberties challenges. Some assert that the First Amendment to the U.S. Constitution precludes such laws. In their 1995 law review article "People Do Read Large Ads: The Law of Advertising from Outer Space", Don Tomlinson and Robert Wiley came to different conclusions on this point. Tomlinson argued "Ban Without Reservation", while Wiley promoted "Regulate with Reservations".

Still, the world often looks to the United States to lead on international policy matters. The Outer Space Treaty led to the establishment of the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS). It serves as an international forum for diplomatic discussions of matters flowing from implementation of the OST. At its annual meetings (and those of its subcommittees), national delegations debate various concerns.

​Because COPUOS operates on a consensus basis, the pace of legal conventions over the decades has been very slow. The U.S. and its allies exert an outsized influence in the international community. To that extent they have led efforts to recognize norms of behavior outside the realm of binding, 'black-letter' law.

The American Astronomical Society (U.S.) and Royal Astronomical Society (U.K.) recently asked for exactly that. Both are associations of astronomers that advocate for their members on both national and international matters. In the past year, both issued statements calling for their national delegations to promote discussing obtrusive space advertising at COPUOS. That led to media coverage (e.g., here, here and here) that raised more public attention to the issue.

A particular episode spurred them on. As the AAS statement notes, "Avant-Space Systems LLC, a private entity incorporated in the Russian Federation, recently launched a prototype cubesat intended to demonstrate the feasibility of this technology." Avant-Space launched a 3U cubesat in April 2024 to test objects that would make glowing message formations in space. The successful effort made the prospect of near-term advertising missions palpable.

The AAS statement calls out two specific aspects of the OST. First, it declares that astronomy is a kind of Article I "use" of outer space. And second, as a result Article IX dictates that it is entitled to "due regard" from launching States to limit harmful interference.

But it goes further. Because "no known mitigation of such harmful interference enables the peaceful co-existence of obtrusive space advertising alongside astronomy consistent with Article IX", it argues that States should simply prohibit launch of such payloads from their territories. That's exactly what U.S. law already demands. AAS thus asks the U.S. COPUOS delegation to promote the same policy worldwide.

The RAS takes a similar stance, noting that the Society "opposes all space-based advertising, recognising it as detrimental to the science of astronomy and to our shared heritage of the night sky". It references the 2007 Declaration in Defence of the Night Sky and the Right to Starlight, a nonbinding statement supported in part by the United Nations Educational, Scientific and Cultural Organization (UNESCO). That document called an unpolluted night sky "an inalienable right of humankind equivalent to all other environmental, social, and cultural rights."

It also builds on moves the international astronomical community made almost a quarter-century ago. In its 2001 COPUOS statement, the International Astronomical Union (IAU) referenced the 1993 U.S. obtrusive space advertising prohibition. It called on COPUOS to encourage member States to "adopt similar legislation on obtrusive space advertising, so that this activity is regulated by all space-faring nations."

​This month, the COPUOS Science and Technology Subcommittee will hold its annual meeting in Vienna, Austria. At that meeting, it will hear again from the IAU: "[T]here is no means of mitigating the potential harm to astronomy of obtrusive space advertising.  It represents the ultimate light trespass because its purpose is to be visible as widely as possible on the surface of the Earth.  The IAU urges COPUOS national delegations to consider a prohibition on this technology that creates the risk of unwanted messaging and disruption of the dark night sky."

No one yet knows whether COPUOS will take this advice to heart. Reading the diplomatic tea leaves is often difficult. But this effort may well have the majority of public sentiment on its side. As effective as advertising is, many people tire of being targets of a nonstop, 24-hour-a-day marketing regime. Especially given the proliferation of electronic media and mobile devices, their waking hours are often bombarded with commercial messages. And for some, the notion of advertisements brightening the night sky beyond the pale.

The world faces an imminent choice. It can allow this kind of activity in space, or it can prevent it from ever taking root. If it chooses the former, experience shows that it will be difficult to put the proverbial genie back in the bottle. Of course, a rogue actor could (and may well) launch an space advertisement in violation of domestic law. Some argue against the very notion of law in space to begin with; others note that the OST enforcement regime has no effect at the international level. It remains an evolving geopolitical matter.

The night sky is one of the few remaining aspects of the natural world that all humans share. Our increasingly connected world has blurred other boundaries, but the sky remains a kind of public commons. Whether the world will assert proper control over that commons remains to be seen. But if we lose that shared sense of ownership by allowing narrow commercial interests to take it over, we will have lost something much more significant: a rare medium that draws us together more than it pushes us apart.