Satellites Overwhelm Space Telescopes With “Railroad” Tracks

There’s bad news to report on the clear skies front. A new paper, available as a preprint on arXiv from researchers at NASA’s Ames Research Center in California, reports that 73.3 percent of images the agency’s new SPHEREx space telescope collected between May and September of last year were contaminated by at least one artificial satellite trail. And it’s only going to get worse from here.

Unfortunately this doesn’t come as a surprise. SPHEREx (short for the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer) was designed to map the entire sky in near-infrared light. That means it would require long exposure times and cover a very large chunk of the visible sky at any one time. Both requirements are a recipe for interruption from orbiting satellites.

Typically this type of light pollution is primarily associated with ground telescopes. But SPHEREx is an orbital satellite about 700 kilometers above the Earth’s surface. Apparently even that wasn’t enough to escape from the light trails. On average there were 2.18 trails per exposure, most of which are concentrated in an “X” pattern that mimics the orbital paths of the satellite megaconstellations.

There appears to be no easy way to handle this interruption, either. SPHEREx uses an automated “sample up-the-ramp” algorithm to protect itself from stray cosmic rays. When a sudden energy blast from one of those rays hits a pixel, the system halts data collection on that pixel to prevent saturation. But commercial satellites are now so bright that they are triggering this system without the help of any stray cosmic rays.

Satellite Trails Ruin SPHEREx Images

The resultant images have what the authors describe as “railroad” tracks, where the blinding center of the trail is scrubbed out but parallel lines running alongside it are permanently etched into the science imagery. As a result, the images lose the photometric data of anything hidden beneath the rails.

As if that news wasn’t bad enough, SPHEREx isn’t the only one suffering from this fate. In 2023, a different team led by Sandor Kruk, an astronomer at the European Space Agency, published a study in Nature Astronomy that found the fraction of Hubble images crossed by satellites rose from 2.8 percent in the early 2000s to 5.9 percent in 2021. Admittedly, Hubble doesn’t take shots as wide as those taken by SPHEREx, but the fact that one of the most venerable space telescopes still operating is suffering from the same problem is not a good sign.

Satellite designers have tried various efforts to mitigate this problem, including dark coatings or specialized visors to reduce their optical brightness. But newer systems with large antennas are eliminating any potential benefit of darker coatings and cementing them as some of the brightest objects in the sky.

And it’s only getting worse. Recent filings with the U.S. Federal Communications Commission have been made to approve up to 2 million satellites in low Earth orbit, compared to the 20,000 or so currently in orbit. If those are approved and launched, simulations from the new paper forecast that 100 percent of SPHEREx’s images would be polluted by a satellite trail, with the average image having 189 trails in it.

Needless to say, that is catastrophic for observational platforms below or in the orbital plane where those satellite megaconstellations exist. Various groups have been ringing the alarm bell about this potential catastrophe for years at this point, but there has been little movement on an international agreement to do anything to address the problem.