Starlink: low-earth orbit satellites could ruin radio astronomy

The grow­ing num­ber of satel­lites being launched into Earth’s low orbit is severe­ly dis­rupt­ing astro­nom­i­cal obser­va­tions and, as a result, space stud­ies as a whole. This is the lat­est warn­ing from researchers at the Paris Obser­va­to­ry and the Nançay Radio Astron­o­my Obser­va­to­ry. With­out effec­tive mit­i­ga­tion strate­gies, par­tic­u­lar­ly through col­lab­o­ra­tion between astronomers and satel­lite oper­a­tors, the con­se­quences for radio astron­o­my could be dis­as­trous, they say.

It has long been known that sig­nals emit­ted by satel­lites inter­fere with astro­nom­i­cal obser­va­tions across dif­fer­ent wave­lengths. In opti­cal astron­o­my, satel­lites orbit­ing the Earth reflect sun­light, cre­at­ing bright streaks, or flares, in tele­scope images. In radio astron­o­my, glob­al nav­i­ga­tion satel­lite sys­tem (GNSS) satel­lites can con­tribute to radio inter­fer­ence, prob­a­bly through so-called ‘out-of-band’ emissions.

Xing Zhang. In our study, we noticed that SpaceX’s sec­ond-gen­er­a­tion Star­link satel­lites are emit­ting very intense out-of-band emis­sions (with flux den­si­ties exceed­ing 500 Jan­skys, Jy) that light up the night sky in the radio fre­quen­cy range. These emis­sions are start­ing to com­pli­cate our astro­nom­i­cal observations.

Indeed,Starlink aims to pro­vide glob­al Inter­net cov­er­age with no delay between com­mu­ni­ca­tions. This requires satel­lites orbit­ing at low alti­tude – typ­i­cal­ly about 500 kilo­me­ters – so that the sig­nal trav­el time between Earth and a satel­lite and back is very short, on the order of tens of mil­lisec­onds. The prob­lem with low-alti­tude satel­lites, how­ev­er, is that they only see a small por­tion of the Earth at a time. This means that many satel­lites must be simul­ta­ne­ous­ly put into orbit. Their num­ber is esti­mat­ed at over 42,000 – equiv­a­lent to the num­ber of square degrees of the whole sky. On aver­age, this trans­lates to one satel­lite per square degree; a colos­sal num­ber. Until now, we had some­thing like a few thou­sand satel­lites in oper­a­tion at any giv­en time. But this num­ber is grow­ing rapid­ly, and Star­link is not the only play­er in this field: oth­er poten­tial large-scale con­stel­la­tions, such as OneWeb and Ama­zon’s Kuiper project, are also expect­ed to con­tribute to the grow­ing con­ges­tion in low Earth orbit.

This con­ges­tion could quick­ly lead to what is known as the Kessler syn­drome: there will be so much debris in orbit that sub­se­quent launch­es will become less safe. The Earth will then become a kind of prison sur­round­ed by debris pol­lut­ing not only the opti­cal wave­length range, but in the radio band too.

This is direct­ly impact­ing the work we do at Nançay, which involves search­ing for sig­nals from tar­get­ed exo­plan­ets. Instead of these, we recent­ly observed some objects mov­ing across our radio images. We com­pared their tra­jec­to­ries with those of air­craft and satel­lites to find out what they were. It turned out that they were Star­link satellites.

Philippe Zar­ka. Nor­mal­ly, these satel­lites com­mu­ni­cate at very high fre­quen­cies, of above 12 giga­hertz, to send data. In prin­ci­ple, only radio astronomers work­ing in this fre­quen­cy band should be direct­ly affect­ed. But the prob­lem is that the elec­tron­ic com­po­nents on board these satel­lites seem to pro­duce their own radi­a­tion, which is out­side of this band.

XZ.  At Nançay, we want­ed to test whether we could detect the satel­lites in our images and if there was any­thing in the 10 to 85-mega­hertz range. We did this using obser­va­tions from Nen­u­FAR, a net­work of low-fre­quen­cy radio tele­scopes opti­mized for these frequencies.

In 2023, we did­n’t detect any­thing. But a year lat­er, we start­ed see­ing unusu­al sig­nals on our images, so we decid­ed to track the Star­link satel­lites and com­pare the mov­ing objects on our images with these sig­nals. We then realised what was hap­pen­ing: while the first gen­er­a­tion of Star­link satel­lites also emit in radio bands, these emis­sions were not as bright at the fre­quen­cies we employ. The sec­ond-gen­er­a­tion emis­sions, on the oth­er hand, were much brighter, which explained the unusu­al sig­nals and the source of the mov­ing objects in our images.

There were two main ver­sions of Star­link satel­lites: the ear­ly V0.9 to V1.5 mod­els and the more recent V2 Mini satel­lites. It has actu­al­ly been since the mass launch of the Mini V2 (around 40 satel­lites per week start­ing in 2023) that we have begun to observe a dra­mat­ic increase in light signals.

XZ. Indeed, their rep­re­sen­ta­tives have begun par­tic­i­pat­ing in con­fer­ences on radio inter­fer­ence, which shows they are open to dia­logue with the sci­en­tif­ic com­mu­ni­ty. Some mit­i­ga­tion strate­gies have been test­ed, such as mak­ing satel­lites ‘invis­i­ble’ by tem­porar­i­ly dis­abling cer­tain onboard sys­tems when fly­ing over radio obser­va­to­ries. How­ev­er, these approach­es are cur­rent­ly only real­is­tic for large instal­la­tions able to nego­ti­ate spe­cial agreements.

Some satel­lite oper­a­tors appear to be seek­ing to reduce out-of-band emis­sions in future ver­sions of their satel­lite sys­tems. While this would be a step in the right direc­tion, these efforts have not yet been con­firmed or imple­ment­ed in cur­rent fleets. In the mean­time, many hun­dreds of satel­lites will con­tin­ue to oper­ate in low orbit until they deor­bit, a process that typ­i­cal­ly takes between five and sev­en years. This pos­es a prob­lem in itself, as one can imag­ine the quan­ti­ty of debris gen­er­at­ed when they do fall back to Earth.

The radio emis­sions from these satel­lites are 10 mil­lion times more intense than those emit­ted by the faintest celes­tial objects observ­able by LOFAR. They exceed the lev­els set by the Inter­na­tion­al Telecom­mu­ni­ca­tion Union in the 150.5 to 153 MHz band allo­cat­ed to radio astron­o­my, but since they are for­mal­ly clas­si­fied as ‘unin­ten­tion­al’ elec­tro­mag­net­ic pol­lu­tion, cur­rent leg­is­la­tion does not cov­er this infringement.

PZ. If these emis­sions were in a band that Star­link uses on pur­pose, only then would it be a legal issue. There are reg­u­lar inter­na­tion­al con­fer­ences that define by mutu­al agree­ment how the dif­fer­ent fre­quen­cies of the elec­tro­mag­net­ic spec­trum will be exploit­ed. Some bands are reserved for defense, for exam­ple, while oth­ers are used for telecom­mu­ni­ca­tions, tele­vi­sion and radio. In the bands reserved for radio astron­o­my, no one is allowed to emit. How­ev­er, it turns out that when this emis­sion is unin­ten­tion­al, that is, acci­den­tal, you are pro­tect­ed by law. This is obvi­ous­ly a loop­hole in the reg­u­la­tions. You could say that it is like inten­tion­al or unin­ten­tion­al homi­cide: these satel­lites are inad­ver­tent­ly ‘killing’ cer­tain radio frequencies.

XZ. Per­haps these out-of-band trans­mis­sions were sim­ply not antic­i­pat­ed dur­ing the devel­op­ment of these satel­lites. It is also pos­si­ble that no thor­ough emis­sion test­ing was car­ried out before launch. We just don’t know.

PZ. We agree that test­ing is no easy task. To obtain the entire spec­trum of a satel­lite’s elec­tro­mag­net­ic emis­sions, you have to put it in a Fara­day cage and mea­sure all emit­ted fre­quen­cies. Since Star­link satel­lites are quite large (sev­er­al meters in diam­e­ter), a very large Fara­day cage is required, which is obvi­ous­ly expen­sive to build. When we and oth­er radio astronomers pub­lish our find­ings, we are, in a sense, work­ing indi­rect­ly for Star­link and oth­er satel­lite com­pa­nies, as we are essen­tial­ly doing their test­ing for them. How­ev­er, such pre­lim­i­nary test­ing should be manda­to­ry and includ­ed in reg­u­la­tions: no one should be allowed to launch objects into space with­out exhaus­tive pri­or analyses.

By pub­lish­ing our results, we want to make astronomers aware that if they see any form of con­t­a­m­i­na­tion in their radio images, they know where it comes from. Radio astron­o­my is pro­gress­ing at a fast pace across all fre­quen­cies, and there is a lot of research these days on ‘tran­sient’ events. These are phe­nom­e­na that appear only for a short peri­od of time and include cat­a­clysmic events and explo­sions, fast radio bursts and pul­sars, to name but a few.

We can make an anal­o­gy with opti­cal astron­o­my here: when you are look­ing for stars or galax­ies whose posi­tions are fixed in the sky and streaks of light from satel­lites appear in the image, this is a prob­lem, but you can ‘com­pen­sate’ for these trails. You can­not con­fuse them with the image of the object being observed. But, if you are observ­ing tran­si­to­ry sig­nals, that is, sig­nals emit­ted by vari­able objects, you may of course con­fuse some satel­lite sig­nals with the object you are observ­ing. The same applies to radio astron­o­my images.

PZ. Star­link satel­lites prob­a­bly emit their radio pol­lu­tion con­tin­u­ous­ly, but since they only cross your field of vision for just a few sec­onds, you observe an event that actu­al­ly lasts for a very short time when you look at cer­tain objects. You there­fore need to be extreme­ly cau­tious and com­pare all the emis­sions you detect with those of satel­lites to elim­i­nate them.

XZ. Ini­tial­ly, we thought we would­n’t have any prob­lems with polarised emis­sions like those emit­ted by exo­plan­ets, because inter­fer­ence is main­ly unpo­larised. How­ev­er, Star­link inter­fer­ence is also polarised, which pos­es a real prob­lem and com­pli­cates the study of the tran­sient radio sky. With Nen­u­FAR, no mat­ter where you look in the sky, you typ­i­cal­ly have a Star­link cross­ing your radio beam every 15 min­utes. It makes it com­pli­cat­ed for us, and the sit­u­a­tion will only get worse in the future if noth­ing is done.

PZ. For me, this is one of the most seri­ous prob­lems with Star­link and oth­er such satel­lites. Peo­ple had already lost their sense of won­der of the sky and the stars because of light pol­lu­tion in cities, but they could always go to the coun­try­side or to a desert­ed place to redis­cov­er that mag­ic. With Star­link, there are trails of light all over the night sky. You could say that Star­link is steal­ing the dark sky from human­i­ty. We no longer have a vir­gin sky. This, I would say, is a giant hold-up, and the whole of human­i­ty is the victim.

Interview by Isabelle Dumé