We are all aware of sound pollution, yet we remain largely unaware or indifferent to the growing menace of light pollution. Sound pollution disrupts the natural environment, while light pollution intrudes upon natural darkness. At night, while we lie in bed, unwanted light trespasses into the room from streetlights, illuminated buildings, and glaring advertisements outside. This intrusion disturbs our sleep, undermines our health, and robs us of the restorative calm of night.
More profoundly, it obscures the stars, veiling the night sky from human gaze. Today, light pollution is no longer confined only to the Earth; it is encroaching on space itself. The surge in space exploration has led to the deployment of thousands of satellites in Earth’s outer orbits. Since 2018, Elon Musk’s SpaceX has launched nearly 1,700 Starlink satellites into low Earth orbit, with plans to add another 30,000 over the next decade. British company OneWeb has already placed about 150 satellites in orbit and intends to launch 6,000 more, while Amazon has announced plans for an additional 3,000.
If this unchecked proliferation continues, a time may come when true darkness will vanish from our skies. The first Starlink launch in May 2019 revealed streaks of light across the sky-a phenomenon unseen before. Astronomers now face obstructed views, as reflected light from these satellites interferes with observations. Mega-constellations – vast networks of satellites working in concert – reshape our collective vision of the stars. Unlike terrestrial pollutants, these satellites cannot be removed once deployed. It has been found that a single mega-constellation produces up to 19 parallel streaks across the sky. As they orbit Earth, sunlight reflects off their surfaces, scattering into the atmosphere, intensifying sky brightness.
This human-made ‘night glow’ complicates astronomical observations. Preliminary estimates suggest that this artificial ‘night glow’ has increased the brightness of night skies globally by about 10 per cent, compared with the natural skyglow measured in the 1960s. The consequences extend beyond human curiosity. Since the entire world – and indeed the Universe – is interconnected, no living creature escapes the effect of polluted skies. Migratory species are particularly affected by light pollution, which can result in them losing access to their migratory route.
Australia’s fauna has long faced this crisis since before the introduction of mega-constellations. With more skyglow and light pollution, positive outcomes for native fauna and migratory species diminish. Ironically, scientists have designed a method to harness light pollution for the benefit of humanity. The Sun is always present in the sky. But it remains hidden from us at nightfall when a portion of the Earth turns away from the sun’s rays. Could we devise a mechanism to use sunrays to deliver sunlight after sunset? One ambitious idea proposes reflecting sunlight to the Earth using mirrors mounted on satellites. A US start-up, Reflect Orbital, plans to produce the same by reflecting sunlight to the Earth even after sunset.
Reflect Orbital’s satellites would use mirrors to send light onto a patch of Earth, in the same way a watch face bounces sunlight to produce a spot of light on the wall. The reflected sunlight from the watch is intense when the distance of the wall is shorter; the spot becomes larger and dimmer as the distance to the wall increases. This is because the Sun is not a point of light but spans half a degree in angle in the sky. That’s why a beam of sunlight reflected off a flat mirror spreads out at an angle of half a degree. Even a curved mirror or a lens can’t focus the sunlight into a tighter spot due to the distance and the half-degree angle of the Sun in the sky.
To achieve meaningful illumination, reflectors must be much larger than a watch. Reflect Orbital’s satellites would orbit about 625 km above the ground and be equipped with mirrors 54 metres across. Scientists have calculated that for a satellite reflecting sunlight over a distance of roughly 800 km, considering that a 625km-high satellite would not always be directly overhead but beaming the sunlight at an angle, the illuminated patch of ground would be at least 7 km across. For a single 54 metre satellite, the estimated reflected sunlight is 15000 times fainter than the midday Sun, but this is still far brighter than the full moon. Sunlight at midday is approximately 400,000 times more intense than light from a full moon.
The Reflect Orbital tested its concept with a balloon experiment before moving into space. A flat square mirror, roughly 2.5 metres across, was carried on a hot air balloon. The mirror directed a beam of light down to solar panels and sensors. At a distance of 242 metres, the reflected beam delivered 516 watts of light per square metre, compared to 1000 watts per square metre with the midday sun. Although the amount is half of that the midday sun produced, it is enough to be useful. As mentioned before, if the satellites were 800 km from the area of interest on Earth, the reflector would need to be 6.5 km by 6.5 km, or 42 square kilometres across. Further experimentation with such a huge mirror using balloons is not a practical proposition. Reflect Orbital’s current plan aims to generate 200 watts per square metre, or 20 per cent of the midday sunlight.
If a single 54-metre satellite is 15,000 times fainter than the midday Sun, to produce 20 per cent of the midday Sun’s light, it would need 3,000 such satellites. That’s a lot of satellites needed to illuminate just one region. There are other critical issues also involving technical and ecological challenges. Companies are trying to figure out methods to reduce night glow. OneWeb decided to launch fewer satellites than proposed earlier to reduce overall pollution, and designed them to be stationed at a higher altitude, which enables them to cover a larger area with reduced skyglow.
Use of an antireflective coating on the surface is another option to reduce the reflected sunlight by about 50 per cent. Whether this bold project will remain simply speculative or eventually ‘see the light of day’ is uncertain. Yet, regardless of its outcome, it underscores humanity’s audacious attempts to use sunlight on demand, reshaping the natural rhythms of night and day.
(The writer, an author, was Editor-in-Chief of the journal Science and Culture for about two decades.)