By () and ()
A proposal for a new constellation of satellites in Earth’s orbit has astronomers very worried. Unlike common satellites that reflect sunlight and have sunlight as an undesirable byproduct, the North American startup’s satellites would produce light pollution on purpose.
The company promises to provide “sunlight on demand” with mirrors that reflect the Sun’s light back to Earth, enabling solar power plants.
The company plans to start with an 18-meter test satellite called Earendil-1, which the company requested for . It would eventually be followed by around 4,000 satellites in orbit by 2030.
But how serious would this light pollution be? And perhaps even more so, can Reflect Orbital’s satellites actually work as advertised?
Reflecting sunlight
In the same way that you can reflect sunlight off a watch face to produce a point of light, Reflect Orbital’s satellites would use mirrors to project light onto an area of Earth.
But the scale involved is very different. Reflect Orbital’s satellites would orbit about 625 km above the ground and have mirrors with .
When you reflect sunlight from your clock onto a nearby wall, the spot of light can be very bright. But if you reflect it off a distant wall, the dot becomes larger – and fainter.
This is because the Sun is not a point of light, but covers half a degree of angle in the sky. This means that over large distances, a beam of sunlight reflected from a flat mirror spreads out at an angle of half a degree.
What does this mean in practice? Let’s consider a satellite reflecting sunlight at a distance of approximately 800 km – because a satellite at 625 km high will not always be directly overhead, but reflecting sunlight at an angle. The illuminated area of the ground would be at least 7 km in diameter.
Even a curved mirror or lens cannot concentrate sunlight to a narrower point due to the Sun’s distance and half-degree angle in the sky.
Would this reflected sunlight be bright or dim? Well, for a single 54-meter satellite, it would be 15,000 times fainter than the midday Sun, but still much brighter than the full Moon.
The balloon test
Last year, Reflect Orbital founder Ben Nowack, summarizing a test of the “last thing to build before moving to space”. It was a reflector carried by a hot air balloon.
In the test, a flat, square mirror about 2.5 meters in diameter directs a beam of light towards solar panels and sensors. In one case, the team measured 516 watts of light per square meter while the balloon was 242 meters away.
For comparison, the midday Sun produces about 1,000 watts per square meter. So 516 watts per square meter is about half that, which is enough to be useful.
But let’s scale up the balloon test into space. As we noted earlier, if the satellites were 800 km from the area of interest, the reflector would need to be 6.5 km by 6.5 km – 42 square kilometers. It’s not practical to build such a giant reflector, so the balloon test has some limitations.
What is Reflect Orbital planning to do?
Reflect Orbital’s plan is “simple satellites in the right constellation illuminating existing solar farms”. And your goal is just –20% of the midday sunlight.
Can smaller satellites meet this goal? If a single 54-meter satellite is 15,000 times fainter than the midday Sun, it would take 3,000 of them to reach 20% of the midday Sun’s intensity. That’s a huge amount of satellites to illuminate a region.
Another question: satellites at an altitude of 625 km move at 7.5 km per second. Therefore, a satellite will be within 1,000 km of a given location for no more than 3.5 minutes.
This means that 3,000 satellites would provide a few minutes of illumination. To provide even an hour would require thousands more.
But Reflect Orbital doesn’t lack ambition. In an interview, Noback suggested 600 km high orbits. That’s more than the large pieces of space debris known in orbit combined.
This vast constellation, however, would only provide 20% of midday sunlight to no more than 80 locations at once, based on our calculations above. In practice, even fewer places would be illuminated due to cloudy weather.
Furthermore, given their altitude, the satellites could only provide illumination to most locations around dusk and dawn, when mirrors in low Earth orbit would be bathed in sunlight. Aware of this, Reflect Orbital plans for their constellation to circle the Earth in order to keep them continually in sunlight.
Bright lights
So are mirror satellites a practical means of producing affordable solar power at night? Probably not. But could they produce it? For sure.
In the early evening, it doesn’t take long before we can spot satellites and space junk – and they’re not deliberately designed to be bright. With , even if just the test satellite works as intended, it will sometimes appear much brighter than the full Moon.
Such a constellation of mirrors would be devastating for astronomy and dangerous for astronomers. For someone looking through a telescope, the surface of each mirror could be almost as bright as the surface of the Sun, causing a risk of .
Light pollution will also harm everyone’s ability to see the Universe, and light pollution is known to affect .
While Reflect Orbital aims to illuminate specific locations, the satellites’ beams would also scan the Earth as they move from one location to another. The night sky could be lit up with flashes of light brighter than the Moon.
The company did not respond to the The Conversation about these concerns within the deadline for publishing this text. However, you plan to redirect sunlight in ways “brief, predictable and targeted”avoiding observatories and sharing the location of satellites so scientists can plan their work.
Consequences would be dire
It is not yet known whether the Reflect Orbital project will move forward. The company may launch a test satellite, but it still has a long way to go before it can reach 250,000 huge mirrors constantly orbiting the Earth to keep a few solar plants running for a few extra hours a day.
Still, it is a project to be followed. The consequences of success for astronomers — and anyone else who enjoys a dark night sky — would be dire.
* and are astronomy professors at and , respectively.
This text has been republished under a Creative Commons license. Read the original text.
