Dark skies

Light pollution affects every region on Earth, including at the poles and even above us in low-Earth orbit. That isn’t news. But the extent of the problem, laid out in our Focus on dark skies, is startling and should turn us all into activists.

Low Earth orbits are increasingly congested, impacting astronomical observations and dark skies. Globally coordinated regulatory policies and mitigation strategies are among urgent next steps to protect this shared environment and intangible heritage.

Unfettered access to dark night skies is rapidly diminishing, due to light pollution and satellite constellation tracks. Scientists should do more to stand up to ‘big light’ and ‘big space’ and preserve this natural resource.

Each space launch is assessed for various risks, but not its wider impacts. This Perspective shows how the aggregate effects of space launches, plus the attendant rise of space debris, affect the darkness of our night sky now and in the future.

Harnessing the power of citizen science and machine learning, this study takes in 20 years of Hubble Space Telescope images, of which 2.7% show satellite streaks, and predicts that this fraction will increase by up to an order of magnitude in the next decade.

Models for night sky brightness are used to characterize sites for astronomical observatories, but in the presence of artificial light pollution, certain assumptions regarding aerosol shapes mean that the estimates are systematically underestimated, particularly at low altitudes.

Although space debris has been a problem for decades, recent investment in active debris clearing shows a wider commitment to sustainable space development.

Light pollution is a growing problem affecting the night sky. This Review takes a comprehensive look at the factors contributing to elevated brightness at night, the measurement techniques and the ongoing assessment of sky quality.

Each uncontrolled rocket body in orbit poses a low casualty risk on reentry. But the cumulative risk is unacceptable and disproportionately borne by the Global South. Spacefaring states must stop exporting these risks and plan for safer reentries.

Crowded with satellites and debris, the orbital space around the Earth should be formally recognized as an ecosystem—like the ocean and the atmosphere—to ensure sustainable development and protection from irreversible damage.

Historical under-regulation of the Earth’s atmosphere and the orbital space around it have brought the astronomy and space communities to a critical point at which action is needed to move towards a sustainable future.

Small satellites are revolutionizing the way we access space for commerce and science, including astronomy and planetary science. Yet, like many breakthrough technologies, it has a double-edge.

In the context of near-Earth space becoming increasingly privatized and crowded due to the launch of satellite constellations, space must be viewed as an ancestral global commons that contains the heritage and future of humanity’s scientific and cultural practices.

The number of small satellites has grown hugely in the past decade, from tens of satellites per year in the mid-2010s to a projection of tens of thousands in orbit by the mid-2020s. This presents both problems and opportunities for observational astronomy. Small satellites offer complementary cost-effective capabilities to both ground-based astronomy and larger space missions. Compared with ground-based astronomy, these advantages are not just in the accessibility of wavelength ranges where the Earth’s atmosphere is opaque, but also in stable, high-precision photometry, long-term monitoring and improved areal coverage. Astronomy has a long history of new observational parameter spaces leading to major discoveries. Here we discuss the potential for small satellites to explore new parameter spaces in astrophysics, drawing on examples from current and proposed missions, and spanning a wide range of science goals from binary stars, exoplanets and Solar System science to the early Universe and fundamental physics.

Earth’s exosphere is set to become increasingly crowded, with tens of thousands of commercial telecommunication satellites planned in the next few years. We need to ensure that technological and socio-economic advancements will not imperil scientific progress and humanity’s access to dark skies.

The historic launch of the first several hundred out of 12,000 planned Starlink satellites heralds the arrival of the era of ultra-large satellite constellations. If it will bring new opportunities or insurmountable challenges to astronomy will probably depend on whether you are conducting your observations in space or from the surface of the Earth.