The presentation will detail the proposed N8 (Night) mission – Global Environmental Effects of Artificial Nighttime Lighting. It was originally proposed as Earth Explorer 11 to ESA (European Space Agency).
At any moment, one half of the Earth’s surface is experiencing daytime and one half nighttime. Most Earth surface observation missions focus on the first half. The remote sensing N8 mission would focus on the half that is experiencing nighttime and on the quantification of the global environmental effects of artificial nighttime.
The natural light regime of a substantial, and growing, proportion of Earth’s surface is being eroded as a consequence of the direct illumination and sky brightening that result from the introduction of artificial light sources at night. These sources are associated with human settlements and activities, including public, private business and residential areas, and associated land, water, and air transport infrastructure. By enabling activities that are largely or entirely independent of natural light, artificial lighting of the nighttime has brought enormous benefits to humankind, and has shaped societies in dramatic ways. However, it is predicted to have significant impacts on human health and well-being and the natural environment, given that natural biological systems are organized foremost by light, and particularly by daily and seasonal cycles of light and dark, and that there have been no natural analogues, at any timescale, to the extent, nature, distribution, or timing of spread of artificial lighting.
A large body of observational and experimental studies - most of it arising in just the last few years - has illustrated that these adverse effects on human health and the natural environment occur, and has begun to characterize their sensitivity to the form of the lighting. However, to date, it has not been possible to map and evaluate the associated biological risks and opportunities in the way that has been achieved for other anthropogenic pressures on the environment, that enables the impacts of artificial light to be incorporated into local, national, and international strategies and policies for addressing these pressures. This is because the globally consistent characterization of the spectral-spatial-temporal dynamics of artificial nighttime lighting has been inadequate, and has had to rely entirely on remote sensing systems that were not explicitly designed to measure lighting in the most appropriate ways.
The N8 mission would resolve this challenge. Its core objective is to enable the creation and update of a validated model of spectral-spatial-temporal variation in nighttime artificial lighting and thence of the human health and environmental effects that it causes. This requires characterization of how much artificial outdoor light or radiation is emitted (intensity), in what form (spectral wavelength λ, source light type), where (spatial directions d,α, light distribution), and when (time t, light use). The acquisition of this information requires global, frequent, high-resolution, multi-spectral, multi-angular optical remote sensing nighttime low-light (NTL) data at multiple local times providing a unique view of the activities of humans on Earth’s surface.
The dynamics of outdoor nighttime artificial lighting and the global dynamic maps of its influences on a wide range of parameters will be used to answer key research questions on human health and environmental effects. Different forms, occurrences, and timings of light emissions result in different influences. Key parameters will be deliverables of relevance to scientific researchers in diverse disciplines. For human health (e.g. sleep quality, obesity, breast and prostate cancer risk) the α-optic radiances in the five human photoreceptor bands and the photopic and scotopic bands are of importance. The melatonin suppression index and circadian stimulus index are measures for one of the key drivers of biological rhythms in a wide array of organisms, whose disruption can have major health and disease implications, and its production is highly responsive to light spectrum, intensity, and timing. For animals (e.g. physiology, behavior, life histories from reproduction to mortality, abundance, and distribution, ecosystem function) the radiances in photometric bands of ecological interest by taxonomic group of focal interest (e.g. moths, sea turtles, birds, bats) are of importance to the resultant influences of artificial nighttime lighting. This is also true for plants, with light receptors being key to determining the timing of many activities (e.g. germination, growth, flowering), with the additional potential to affect levels of photosynthesis (as measured by the induced photosynthesis index). In order to enable appropriate changes in policies to reduce impacts of artificial nighttime lighting on the global environment, it is important also to determine the nature of the sources from which it has been produced. This includes the lighting technology, shielding and temporal usage. Estimations are feasible of associated levels of energy consumption and carbon dioxide emissions, enabling fuller evaluation of costs and benefits of patterns of usage of artificial nighttime lighting and of interactions with other environmental changes (e.g. atmospheric pollution). Most Earth observing missions monitor the effects of climate change. Here, the causes are addressed and this will support strategies to combat climate change. Scientific methods with which to analyze remote sensing data for the different applications mentioned are now feasible and extendable by applying artificial intelligence and data fusion techniques beside multivariate statistics. Extending beyond the immediate focus, data on artificial nighttime lighting and its short- and long-term variation have been found to be valuable in understanding patterns of human density, urbanization, economy size and the occurrence of disasters and conflicts.
Despite their inadequacies (e.g. in spectral sensitivity, geometry of acquisition, daily timing) for determining the impacts on human health and well-being and the natural environment, the data on artificial nighttime lighting that have previously been collected from satellite platforms provide valuable historical information on how this lighting has varied spatially and changed through time. The data especially from the Day-Night Band (DNB) of the VIIRS instruments (operated since 2011), will be cross-calibrated with that from this mission enhancing its value.
The N8 mission would acquire all populated land surfaces at night to achieve the objectives. These areas will be observed at least once every 90 days, at least for 12 almost equally distributed local times (to consider short-term changes in nights in the same season), and for at least 3 years (to consider long-term changes between nights of different seasons). This will be achieved in the repeat orbit of 214 orbits in 15 days with a drift of 480 sec/day (12 hours/90 days sufficient by considering ascending and descending orbits) by one satellite with a swath of 284 km (Field-of-View of 20.5°). Acquisitions will be performed in 144+2/3 orbits close to nadir viewing (to consider vertical light emissions) and in twice 35+2/3 orbits close below limb viewing (to consider horizontal light emission) from two directions close to orthogonal to each other (limiting occlusion in both close below limb views, e.g. consider straight streets with high buildings). In the visible and near infrared (VIS/NIR) one panchromatic band at ≤ 10 m (to detect single street lamps having a common distance of ≥ 25 m) and seven multi-spectral broad and narrow bands (specific to nighttime artificial light sources, to consider the lighting characteristics) at ≤ 20 m will be achieved at nadir view and in the longwave infrared (LWIR) two spectral bands at ≤ 100 m (to consider temperatures and atmosphere). The three multi-angular acquisitions of an area is one of the major differences to all proposed NTL so far, besides the acquisitions at multiple local times. Because of low-light conditions, e.g. for PAN a radiometric range 5×10-8 (detection limit) to 8×10-4 (saturation) Wm-2sr-1nm-1 is required, time delayed integration (TDI) detectors will be applied resulting with state-of-the-art optics in a Signal-to-Noise Ratio ≥ 10 at reference radiance 5×10-7 Wm-2sr-1nm-1. This requires a highly stable platform with precise yaw steering. Finally, view and access to the products at various processing levels will be provided through the official Copernicus digital platform services to fulfil user demands for modelling and mapping. To obtain required Bottom-of-Atmosphere observations based on Top-of-Atmosphere measurements this implies an accurate consideration of the nocturnal atmosphere which is a supported research topic on its own. The sustained and quality-controlled observations of the proposed N8 mission would revolutionize understanding of artificial nighttime lighting and its human and environmental impacts.
The review of the proposal by ESA highlights the important novel aspects. They represent long-standing observational gaps and address some urgent scientific and societal questions of the Living Planet Challenges. Adaptions of the originally proposed N8 mission will focus on more specific research questions allowing to pare down the N8 mission.