A number of next-generation observatories and arrays will be commissioned in the coming years. These facilities will make important contributions to various areas of astronomy: exploring the secrets of the early universe, studying gravitational waves, determining the role of dark matter and dark energy in cosmic evolution, and directly mapping "Earth-like" exoplanets.
Unfortunately, this revolutionary development in astronomy could compete against another major project: the creation of megaconstellations. It is for this reason that the SKA Organization (SKAO), which oversees the International Square Kilometer Array (SKA), insists that corrective action be taken so that satellites do not interfere with their radio observations once they are operational.
The Square Kilometer Array, conceived in the 1990s, is an international collaboration aiming to build and operate the world's largest radio observatory. The project involved thousands of scientists and engineers from 15 countries and three Pathfinder facilities in Western Australia and South Africa – including the Australian Square Kilometer Array Pathfinder, the Murchison Widefield Array (MWA) and the MeerKAT Array.
Satellite constellations could be a serious source of interference for radio astronomy. Photo credit: ESA Science Office
Due to the fact that they use many of the same frequency ranges as radio telescopes, there have always been concerns about the potential for radio frequency interference (RFI) from satellites. In the past, due to the small number of (visible) satellites and their fixed position in geostationary orbit (GSO) around the earth, radio observatories were able to make observations in the same frequency ranges.
As of August 1, 2020, 2,787 satellites are in orbit around the earth. In the coming years, however, this number is expected to rise to 6,400. It is estimated that it could reach 100,000. This is expected to cause significant interference with radio telescopes, especially those like the SKA Mid telescope, which is about to be built in South Africa.
This telescope arrangement will consist of 197 radio dishes that scan the sky in the medium frequency range. Of particular concern are satellite signals in band 5b, which coincide with some receivers of the SKA Mid telescope. SKAO's low frequency telescope in Western Australia, which uses different antenna technologies, was not included in this analysis.
The SKAO recommendations came after doing a thorough analysis that quantified the impact of the proposed satellite constellations – like the Starlink constellation proposed by SpaceX – over the years to come. Overall, the SKAO analysis focused on three main categories. First, they looked at the potential for physical harm.
Frequency coverage of the SKA telescopes with zoom to SKA-Mid Band 5B, protected radio astronomy band and satellite downlink. Photo credit: SKAO
There is a concern with this category that intense radio signals from the satellites may directly illuminate the SKA's dishes. However, your study should rule out these concerns. Second, there is the potential for Instrument saturation, as very strong radio interference can saturate a receiving system to the point where all other signals are drowned out.
This leads to the loss of all data in the frequency band, making the receiver temporarily unusable. It is expected that saturation will occur for a short time during the first phase of the constellation mission (for 6,400 satellites). With 100,000 satellites, the saturation would be continuous without mitigation measures. Finally there is the scientific impact caused by large constellations.
Even with the smaller projected size of 6,400 satellites, the SKAO assumes that a continuous loss of sensitivity can be expected for all astronomical observations in the frequency range of satellite transmission. The effects will be most significant for studies of complex organic and extragalactic molecules, among other things. As Dr. Robert Braun, scientific director of SKA, stated in a statement from SKAO:
“There is tremendous scientific and public interest in identifying the origins of life beyond those found on Earth. One of the most promising ways to track it down elsewhere in our galaxy is to detect complex prebiotic molecules whose spectral signatures are concentrated between around 10 and 15 GHz. This is just one of many exciting scientific goals that depend on sensitive access to this frequency range. The prospect of losing sensitivity in this key frequency band is extremely worrying. "
Artistic conception of the Square Kilometer Array (SKA). Credit: SKA Organization
From this the analysts came to a number of conclusions. Among other things, the fact that satellites transmitting in the 5b band generate interference that can lead to a loss of sensitivity in this area. The further estimated time it takes astronomers to study certain objects in the sky in order to observe them clearly (also known as integration time) increases by 70%.
In short, radio observatories can only make just over half as many observations in this band. The analysis also showed that with significantly larger constellations (up to 100,000 satellites) the impact on the SCA would be much worse. In this case, they estimate that astronomy in the range of band 5b could experience up to 100% interference. Dr. Brown said:
"Loss of observation efficiency in addition to the expected large oversubscription of the telescope leads directly to a loss of science, and it is entirely possible that the most difficult experiments that would otherwise have been carried out will no longer be feasible under these circumstances."
One of the strategies they have identified is to ensure that the satellite transmitters do not direct their beams near the SKAO shells. This would require a simple software modification that instructs satellites to redirect their beams away from the telescope location. A measure that the SKAO insists on has no influence on the use, positioning or hardware of the constellations.
The Karoo Astronomy Advantage Area (KAAA), founded in 2007, is a fun-quiet area in the South Cape region of South Africa. Photo credit: SKAO
The analysts were certain that operators were already using this technique in accordance with international regulations to avoid disrupting telecommunications and television broadcasting when crossing paths with GSO satellites. If this strategy is followed, it could reduce potential interference with the SKA Mid telescope by a factor of 10 and reduce the extension of the integration time to only 7%.
While any disruption is regrettable, the SKA has indicated that some level of compromise is required. In the meantime, it will be possible to keep this disruption to a minimum, provided everyone involved follows the recommendations of the analysis. Said Prof. Philip Diamond, General Director of SKA:
“Thanks to our modeling work, the possible influence of satellite mega-constellations on the SKA is now known. We are building a multi-billion dollar state-of-the-art research facility funded by taxpayers around the world, and we need to protect and maximize its ability to bring innovation and new knowledge to humanity. "
Additionally, they pointed out that the Geographical Advantage of Astronomy Act (AGA) has already severely limited commercial incentives for companies looking to direct their rays at the SKA-Mid site. This measure, which was passed by the South African parliament in 2007, regulates how and where satellite operators can install ground-based infrastructure in the vicinity of the SKA-Mid location in order to establish a "radio-silent" zone.
Artist's impression of the SKA medium-frequency telescope in South Africa. Credit: SKA Organization
Speaking of damage control, these recommendations came just six months after Elon Musk announced that all future Starlink deployments would consist of “DarkSat” satellites. Aptly named, this design is based on a darkened phased array and parabolic antennas to reduce brightness by an estimated 55%. Musk's declared obligation to ensure that his constellation does not affect astronomy is what SKAO wants to hold on to!
Beyond the SKA, the astronomical community has raised concerns about how planned satellite constellations could disrupt next-generation optical and radio equipment such as the Vera C. Rubin Observatory (formerly the Large Synoptic Survey Telescope (LSST) and the Event Horizon Telescope (EHT)) – who received the first picture of a black hole!
In addition to the problem of radio interference, the prospect of thousands more satellites in orbit raises the problem of "space debris". Many Earth- and space-based strategies are currently in the works to address these issues. However, these will likely need to be sped up to ensure our skies and radio bands stay clear and unconstrained!
Further reading: SKA