Low-Earth-Orbit Satellites Must Minimize Astronomical Interference

Ever since SpaceX began launching its low-earth-orbit [LEO] Starlink internet communications constellation in November, 2019, astronomers have been dismayed by the satellite swarm’s obstruction of their view of space from ground-based telescopes.

People have spotted long trains of the satellites when they are still bunched together soon after launch, which gives an idea of the challenge, but after they’ve spread out and are less obvious, they remain a problem for astronomers.

And it isn’t only the astronomers who are using visible light. Radio astronomy is also facing problems because of the signals these satellites transmit that can confound their observations of faint astronomical radio sources.

Starlink has launched more than 7,000 satellites, with 6,886 in orbit on September 5, 2024, and it is only the most visible of the planned satellite constellation networks, which are also being launched by Amazon’s Project Kuiper, Hughes’s Hughesnet, the Viasat network, Telesat’s Lightspeed, and Eutelsat’s OneWeb.

Project Kuiper plans an initial constellation of 3,236 satellites. The FCC’s approval of the network requires Amazon to launch half of those by 2026 and the whole network by 2029. Telesat’s network will have a planned 198 satellites.

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SpaceX boss Elon Musk told the Astro2020 conference that it is the company’s goal to make Starlink satellites invisible to the naked eye within a week from launch, as the satellites are moved into their final orbital locations. “If there’s any situation where we’re impeding science, we will take corrective action,” Musk pledged to the audience.

Responsible Space

OneWeb’s Responsible Space document describes measures the company takes with its satellites to ensure that no space debris is left in orbit after the satellites go out of service. It describes a high-reliability ability to de-orbit for the satellites, which will happen within five years of their end-of-life.

The satellites are specifically designed to burn up in the atmosphere on re-entry so that little-to-no debris reaches the ground. But potentially even better, they are designed with a grappling fixture that allows them to be captured, whether that would be for life-extending service or to be collected for return.

“If the constellation companies can invest more time and engineering capacity to do things like making their satellites less reflective to sunlight or operating their radios in different frequencies and/or avoiding any radio communications when passing over a radio telescope observatory, it could make a big difference for astronomers,” noted Virginia Tech research assistant professor Samantha Perry Kenyon, PhD. “The downsides to these mitigation strategies would be cost to the company and possible decreased performance of the network.”

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The problem is that in the midst of a land rush to grab market share in the nascent satellite data service market, the solutions aren’t necessarily obvious, according to Kenyon. “There are pros and cons to the constellation’s design on observatory impacts as well, which makes a clear optimal solution hard to determine for these companies.”

The problem, she explains, is that lower-flying satellites zoom past more quickly, presenting less opportunity to interfere. But that requires more satellites, which increases the chance of one flying past during observations. “Research is being done now to understand the pros and cons of constellation designs, as well as understanding how to dynamically change your network based on the geographical region a satellite is passing over to mitigate the effects on observatories,” she said. “The solution of coexistence is not a clear one.”

Paint it black

Nevertheless, companies are working on solutions. In January 2020, SpaceX tested DarkSat, a Starlink satellite with a menacing black paint job. They followed that in June 2020 with VisorSat, a more successful test of a satellite equipped with a black visor meant to shield it from reflecting sunlight. “It’s made of a special dark foam that’s extremely radio transparent, so as not to affect the phased array antennas. Looks a lot like a car sun visor,” Musk posted in explanation.

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However, Starlink found that the visors obstructed the laser links needed to expand coverage to more remote areas. Worse, they exacerbated aerodynamic drag, requiring more thruster propellant to stay in orbit. So, SpaceX discontinued the visors and pivoted to dielectric radio frequency-transparent mirror film that scatters the sunlight rather than reflecting it.

The company says that its new second-generation satellites employ film that is 10 times as effective at reducing observed brightness. SpaceX promises to sell the film on its website to other satellite operators seeking to reduce their satellites’ reflectivity.

Solar power

Satellites’ reflectivity is also caused by their solar panels, and SpaceX found that by changing the material behind the cells from white to a dark red that is designed to reduce reflectivity. The company acknowledges that this raises the temperature of the cells, reducing their performance.
SpaceX will also minimize the brightness of the solar panels by pointing the panels away from the sun while the satellites cross the zone in their orbit where they would be the brightest. This causes a further 25 percent reduction in available electric power, so the second-generation satellites are designed to accommodate that reduction.

While the satellite’s flat main body is wrapped in light-scattering film and the solar arrays are backed with a dark red background, all of the smaller and/or curved components are relying on a simple technique: they are painted black.

A related concern is that the proliferation of LEO satellites will lead to collisions between them that create debris fields of reflective material that further degrades the ability of terrestrial observatories to see their targets. “This increase in low-Earth-orbit traffic will lead to loss of astronomical data and diminish opportunities for ground-based discoveries as faint astrophysical signals become increasingly lost in the noise,” warns John C. Barentine, et. al. in the Nature Astronomy paper, Aggregate effects of proliferating low-Earth-orbit objects and implications for astronomical data lost in the noise.

Radio silence

To avoid radio interference with radio observatories such as the Very Large Array in New Mexico and the Green Bank Telescope in West Virginia, SpaceX and the National Radio Astronomy Observatory have conducted experiments aimed at learning whether the Starlink satellites can avoid blasting the observatories with unwanted radio interference. The work was documented in the paper Toward Spectrum Coexistence: First Demonstration of the Effectiveness of Boresight Avoidance Between the NRAO Green Bank Telescope and Starlink Satellites by Bang Nhan, et. al.

The researchers explained that “The Starlink system is capable of avoiding direct illumination of telescope sites with their adaptive tasking to place downlink beams far away. Nevertheless, even satellites operating in this mode can potentially present strong signals into the telescope's receiver system if they pass close to the telescope's main beam at the boresight.”

They continued to state that, “For additional protection, Starlink satellites can either momentarily redirect or completely disable their downlink channels while they pass within some minimum angular separation threshold from the telescope's boresight, methods that are referred to as ‘telescope boresight avoidance’.”

The results of the experiment were positive, according to the paper. “Preliminary analysis from these two experiments illustrate the feasibility of these avoidance methods to significantly reduce, if not eliminate, the negative impact of close-to-boresight satellite passages.”

Brightness mitigation best practices for satellite operators

As a result, SpaceX and the NRAO concluded an agreement [Brightness Mitigation Best Practices For Satellite Operators] to cooperate on preventing interference by the satellites. “These techniques are made possible by a real-time data sharing framework between radio astronomy observatories and Starlink that provides the Starlink network with a telescope’s planned observation schedule, including the telescope’s pointing direction (aka "boresight”) and its observed frequency band,” the agreement states. “With this information, the Starlink network can ensure that satellites passing near the boresight of a telescope dynamically redirect their beams away from the telescope.”

It is tricky because the Green Bank Telescope is located in rural West Virginia and the Very Large Array is in the New Mexico desert. Residents in both places rely on Starlink for their internet service. Just turning the satellites off anytime they are in those areas would deprive those customers of broadband internet access. “This boresight avoidance method protects the telescope’s observations while ensuring Starlink service remains uninterrupted for customers near the telescope,” SpaceX assures.

“SpaceX continues to work with a wide variety of astronomers, observatories, and astronomy-related groups to understand astronomer experiences, describe mitigations, and help minimize the effect of satellites on imagery,” the company said. “SpaceX also publishes very accurate state predictions for Starlink satellites, primarily for coordination with other satellite operators on collision avoidance, but also to enable highly sensitive ground telescopes to best schedule observations.”

While this takes effort and money, SpaceX says that its employees are kindred spirits with astronomers and they support their scientific work. “As a space exploration company, SpaceX is a strong supporter of astronomy and the scientific community. Science is at the heart of what we do every day, and the SpaceX team is passionate about our efforts to be responsible stewards of our shared space environment.”

However, Starlink isn’t the only mega-constellation operator, so it remains to be seen what commitment other companies will show to the goal of avoiding interference. “Starlink currently has the most satellites up in orbit, and thus are having a big impact, but other constellations will as well,” noted Kenyon.