antennas, but the noise problem affects each antenna individually, which physics dictates will always be sensitive to a broad range of directions and frequencies. Unlike optical images, the effect is not a localised streak, but a complex effect across the whole map, which can be hard to recognise and remove — it is like trying to listen to very quiet music in a noisy room. A radio astronomy antenna is sensitive to a range of directions typically a few degrees across — the ‘main beam’ — but also has reduced sensitivity in very different directions — the “sidelobes”. Likewise the satellite antenna emits most of its power in a main beam, but also some in sidelobes. The worst effects, which can potentially damage sensitive electronic receiver systems, are for an alignment between the astronomical and satellite main beams — this rules out radio observations close to GSO targets, and should be avoided even for fast-moving LEO satellites. Sidelobe–sidelobe alignments are much harder to avoid however, as there may be tens or hundreds of LEO satellites in the sky at any one time, and they are all moving quickly across the sky. The net effect is extremely hard to calculate, but a simulation by the Square Kilometre Array (SKA) project suggests that once the mature Starlink population is in orbit, every observation in the relevant bands will on average take 70% longer.
International regulation of the use of the radio spectrum designates some protected frequency bands for radio astronomy. This approach was originally a great success. However the protected bands were chosen many decades ago when receiver systems were intrinsically narrow-band. Most modern radio astronomy is carried out with state-of-the-art broadband systems, which allow the detection of much weaker natural signals. As a consequence, protection of radio astronomy now relies on geographical radio quiet zones, which some nations provide and some do not. Where available, this zoning can protect against terrestrial interference, but not against satellite interference. While such interference was dominated by a small number of slowly moving GSO satellites, this was acceptable, but the new LEO constellations could lead to very serious issues. The new systems inevitably overlap with satellite communication bands. Furthermore, volume manufacture and deployment of large numbers of relatively low-cost satellites is likely to increase the chance of sideband leakage into protected bands.
Like the spatial interference issue, the assignment of protected bands sets a precedent, as frequency interference is implicitly recognised as an environmental effect. Recognising that the issues should be subject to environmental laws such as NEPA is the logical next step as the problems get much worse.
Space astronomy
Some spacecraft used for astronomy are placed at very large distances from the Earth, and are not affected by LEO satellites. Many however, like the Hubble Space Telescope (HST), are in LEO, and can certainly suffer from streaking. Occasionally a satellite may pass relatively close by (< 100 km) in which case the streak caused is an extremely bright out of focus stripe, obliterating a significant fraction of the image. An example is shown in Fig. 5. A recent study [25] showed that, depending on the instrument and observational parameters being used, between 2% and 8% of HST images were affected by satellite streaks, but also that the frequency was changing with time, reflecting the growth of the LEO satellite population. Our 2030-era population indicates that by the end of the decade a third of HST
