How radio astronomy is affected by satellite mega-constellations

Light pollution is becoming a growing problem for radio astronomers who want to scour the early Universe.

An illustration of the SKAO radio telescopes
Published: February 23, 2022 at 9:30 am
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While radio astronomers are used to handling some radio interference from terrestrial sources, and a few air or space borne transmitters, the new megaconstellations of internet access satellites – which number in the thousands – are a much greater problem.

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It’s a complicated issue and it helps to start off with a bit of background on the subject, to understand the impact on the relatively young science of radio astronomy.

It's a field that, after all, will have the greatest chance of discovering advanced extraterrestrial civilisations.

Milky Way bubbles MeerKAT telescope, South African Radio Astronomy Observatory (SARAO), 5 September 2019 Credit: Oxford, SARAO
Milky Way bubbles captured by the MeerKAT telescope and the South African Radio Astronomy Observatory (SARAO), 5 September 2019 Credit: Oxford, SARAO

Radio astronomy is protected by international law, through the ITU (International Telecommunications Union) and its Radio Regulations.

These Radio Regulations define certain ‘silent’ frequency bands, where transmitters are not allowed to operate.

Although this was adequate for the early years of this new branch of astronomy, the Universe ‘speaks’ in a very broad range of frequencies.

Modern radio astronomy needs to often ‘listen’ in bands assigned to transmitting users.

The antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) in the Chilean Andes. Credit: ESO/C. Malin
The antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescopes in the Chilean Andes. Credit: ESO/C. Malin

This is possible by building telescopes in very remote and protected areas of the planet where the use of the radio spectrum is minimal.

In these ‘Radio Quiet Zones’, all radio transmitters are banned. Indeed, mobile phones, laptops and even smart watches are not allowed.

The difficulty is that, even in Radio Quiet Zones, we can’t escape air- and space-borne transmitters and have to cope with them.

Why use radio astronomy?

Jupiter captured in radio waves. Credit: ALMA (ESO/NAOJ/NRAO), I. de Pater et al.; NRAO/AUI NSF, S. Dagnello
Jupiter captured in radio waves. Credit: ALMA (ESO/NAOJ/NRAO), I. de Pater et al.; NRAO/AUI NSF, S. Dagnello

Radio observations of the Universe can inform us about a vast variety of processes, in galaxy, star and planet evolution.

So choosing one example to illustrate the impact of the new radio interference sources does not do justice to the scale of science that Square Kilometre Array (SKA) radio telescopes can do, but here goes.

An image of a protoplanetary disc around star HL Tauri. The dark rings could indicate newly-forming planets in orbit, pushing aside dust as they go. Credit: ALMA (ESO/NAOJ/NRAO)
An image of a protoplanetary disc around star HL Tauri, captured by the ALMA radio telescope. The dark rings could indicate newly-forming planets in orbit, pushing aside dust as they go. Credit: ALMA (ESO/NAOJ/NRAO)

The Universe is mostly empty, but gas is relatively common, and gas molecules emit radio signals which are unique to them.

Those interested in astronomy will probably be familiar with the concept of redshift, where light emitted by something moving quickly has its frequency shifted.

Displacement applies to radio signals as well as optical light.

A NASA image showing how wavelengths of light are stretched - or redshifted - as the Universe expands. Credit: NASA/JPL-Caltech//R. Hurt (Caltech-IPAC)
A NASA image showing how wavelengths of light are stretched - or redshifted - as the Universe expands. Credit: NASA/JPL-Caltech//R. Hurt (Caltech-IPAC)

This means that the further away – hence the further back in time – we look for a particular molecule, the lower its frequency will be.

So what was a single frequency of interest has become a range of frequencies.

For molecules that are particularly important for star evolution, we want to be able to visualise their distribution over time/distance and effectively create a map of the evolution of the Universe over time.

A view of the Crab Nebula captured by the VLA (radio) in red; Spitzer Space Telescope (infrared) in yellow; Hubble Space Telescope (visible) in green; XMM-Newton (ultraviolet) in blue; and Chandra X-ray Observatory (X-ray) in purple.
A view of the Crab Nebula captured by the VLA (radio) in red; Spitzer Space Telescope (infrared) in yellow; Hubble Space Telescope (visible) in green; XMM-Newton (ultraviolet) in blue; and Chandra X-ray Observatory (X-ray) in purple.

One good example is carbon monoxide (CO).

For such molecules, in distant galaxies where the SKA has the power to look, the frequency is found in a region where the satellite mega-constellations are now transmitting strongly.

We are becoming blind to certain times in the past (and therefore stages of stellar evolution).

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It's extremely worrying, which is why we want to raise more awareness about the issue.

Authors

Tim stevenson SKAO engineer
Tim StevensonSpace engineer

Tim Stevenson is the SKA Chief System Engineer. He has over 40 years experience as an engineer in space and ground-based astronomy and space science.

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