Starlink gets a lot of bad press from astronomers, so I want to leave this here to balance it out. It's written by Casey Handmer, an (ex) NASA astrophysicist. Source: https://caseyhandmer.wordpress.com/2021/11/17/science-upside...

Sorry for the wall of text, but I think it's worth reading:

Starlink will ultimately be a network of tens of thousands of satellites connecting to hundreds of millions of user terminals located all over the Earth. Its radio encoding scheme adapts the signal rate to measured atmospheric opacity along the signal line of sight across 10 different frequency bands in real time. Collectively, the system measures trillions of baselines of Earth’s entire atmosphere every day. This data, fed into standard tomography algorithms such as those used by medical CT imagers, can resolve essentially all weather structure in the atmosphere. No more careful scrutiny of remote weather station pressure gauge measurements. No more reliance on single mission oxygen emission line broadening. Instead, complete real time resolution of the present state of the entire atmosphere, a gift for weather prediction and climate study.

Starlink satellites are equipped with perhaps the most versatile software defined radios ever put into mass production. Each antenna allows the formation of multiple beams at multiple frequencies in both send and receive. With sufficiently accurate position, navigation and timing (PNT) data from GPS satellites, Starlink satellites could perform fully 3D synthetic aperture radar (SAR) of the Earth’s surface, with enough bandwidth to downlink this treasure trove of data. Precise ocean height measurements. Precise land height measurements. Surface reflectivity. Crop health and hydration. Seismology and accumulation of strain across faults. City surveying. Traffic measurements in real time. Aircraft tracking for air traffic control. Wildlife study. Ocean surface wind measurements. Search and rescue. Capella has produced extraordinary radar images with a single satellite. Now imagine the resolving power with birds from horizon to horizon.

Starlink SAR is great for Earth observation, but the same principle can be applied looking outwards. Starlink is a network of thousands of software defined radios with highly precise PNT information and high speed data connections. It is practically begging to be integrated into a world-sized radio telescope. With 13000 km of baseline (trivially extendable with a handful of GTO Starlink launches) and the ability to point in any desired direction simultaneously, Starlink could capture practically holographic levels of detail about the local radio environment. Literally orders of magnitude better resolution than ground-based antennas like the Very Large Array. Cheaper than repairing Arecibo and independent of Earth’s rotation. Potentially capable of resolving exoplanets.

There’s no reason to do only passive radio astronomy. Starlink can exploit its exceptional resolving power and onboard amplifiers to perform active planetary radar, for examination of close-flying asteroids and transmission of radio signals to distant missions in support of the Deep Space Network. As of November 2021, all Starlink satellites are flying with lasercoms so in principle the DSN application could also support laser, as well as radio, communication with distant probes. No need to build even larger dishes than the 70 m monsters. The potential to greatly increase our data rates from distant probes.

And while Starlink can derive PNT from the GPS constellation, it need not depend on it forever. High capacity radio encoding schemes such as QAM4092 and the 5G standard contain zero-epoch synchronization data, meaning that any radio capable of receiving Starlink handshake signals is able to obtain approximate pseudorange information. What Starlink’s onboard clocks may lack in atomic clock-enabled nanosecond stability, they make up in sheer quantity of connections and publicly available information about their orbital ephemerides. Already a group from OSU has demonstrated <10 m accuracy, while a group based at UT Austin is developing a related method for robust PNT estimation using Starlink hardware. It seems likely to me that Starlink could support global navigation with few to no software changes and no hardware changes, improving the resilience of satellite navigation especially in a case where the relatively small GPS constellation is disabled. I won’t go into vast detail, but GNSS signals are not only used for pizza delivery, but also support a vast array of Earth science objectives, including the monitoring of tectonic drift.

Starlink has received its fair share of criticism, drawn perhaps by its overwhelming scale and potential impacts to ground-based astronomy. But Starlink can also be the single greatest scientific instrument ever built, a hyperspectral radio eye the size of the Earth, capable of decoding information about the Earth and the universe that is right up against the limits of physics.