Satellites keep space images of the photobombing. Astronomers need an Ars Technica fix


Zoom in / A Hubble image of a pair of colliding galaxies, with a satellite trail running through them.

Space Telescope Science Institute; NASA

Crowds of satellites whiz across the field of view of the Hubble Space Telescopes, leaving what appear to be scratches on space photos and hampering the work of scientists. Swarming swarms of these satellites, which reflect sunlight and mimic astronomical objects, threaten to gradually transform the night sky and affect the way astronomy can be done.

We see these satellite trails in Hubble data, and really all astronomical data, and they’re a bit of a bummer, said David Stark, an astronomer at the Space Telescope Science Institute in Baltimore, speaking at the American Astronomical Society conference last week. in Albuquerque. , New Mexico. In fact, he said, his team used a new sensing method to measure the speed of satellite trails doubling. But Stark was presenting his team’s idea for a Band-Aid fix: a new software described in a recent report that is five to ten times more sensitive at finding tracks than older software and thus masking them. He’s especially good at finding satellite tracks that can be missed by eye, he said.

It’s standard procedure for astronomers to try to clean images of artifacts, such as the effects of cosmic rays hitting Hubble’s camera detectors or diffraction spikes, which make bright stars look like crosses. Occasionally, a pesky nearby star in the Milky Way might get in the way of a distant object. The new technique, called Median Radon Transform, looks at every linear path through an image at every possible angle. When a particular path aligns with a satellite track, the system detects a deviation from average flux or brightness at a particular wavelength in a pixel measured across areas of apparently empty sky. It can also detect short streaks, but they need to be slightly brighter to be identified, as they cover fewer pixels.

This software then allows astronomers to mask satellite tracks, so that affected pixels are ignored in their data analysis. It’s like eavesdropping on some poorly printed pages in a book, so you skip them while you study the rest of the volume.

But it’s best not to lose those pages. When there are multiple exposures of the same field, an astronomer can use additional software tools to completely remove the line from the final combined image. That part of the sky will then appear as it should, although the signal-to-noise ratio in those pixel lines will be lower than if the satellite had never passed the telescope that day. Stark and his team have packaged their code in a standard software package called acstools which they maintain.

However, this fix has a major limitation: It’s designed for Hubble, which orbits 332 miles above Earth, and is less plagued by satellite streaks than ground-based observatories. Terrestrial optical telescopes with wide field imaging, which often do not accept multiple exposures, will suffer much more. There have already been a few cases of photobombing images of satellites taken by telescopes at the Cerro Tololo Inter-American Observatory in Chile and the Lowell Observatory in Arizona, for example.

The problem will be much, much worse for the long-awaited National Science Foundation-funded Vera Rubin Observatory, which is being assembled in the Chilean Andes and will begin taking images next year. Its incredibly sensitive camera will detect faint, changing objects, like a star going supernova or a near-Earth asteroid, and the telescope will automatically send alerts to astronomers when it spots such things. But the Rubin collaboration expressed concern about the possibility of false alarms from reflected light from satellites or orbiting space junk and warned that as many as 30 percent of its images could be affected by satellite streaks. For example, a glare of sunlight on a small piece of insulation emitted by a satellite might appear in a telescope image as a shining star. Unless an astronomer can also measure the spectrum of light, they could be deceived, says John Barentine, a Tucson, Ariz., astronomer who recently wrote a study of light pollution from objects in low Earth orbit.

A second problem is that the number of satellite series is growing. Stark tested the software on 20 years of data from Hubbles Advanced Camera for Surveys. Although the satellite trails have not changed in brightness, their speed has approximately doubled. The team found traces every three to four hours of Hubble data acquired in 2002. But in 2022, satellites have been bombarding Hubble every one to two hours. That means 5 percent of images taken 20 years ago were affected, and now it’s about 10 percent.

The rate will certainly continue to rise, says Sandor Kruk, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Munich, Germany. Expect more streaks in the images as time goes on. It goes in proportion to the number of satellites you have above the observatory, says Kruk, lead author of a recent study that used crowdsourced classifications and machine learning to plot satellite tracks in images.

Kruk and his colleagues have observed a lower rate, which has dropped from about 2.5 to 5 percent over the past two decades. They found that this trend increased rapidly starting around 2018, around the same time that companies began deploying satellite mega-constellations, connecting hundreds or thousands of them into networks. (Stark and Kruk attribute their studies to different percentages using different measurement techniques.)

These mega-constellations have clear advantages for their operators. Satellites are smaller and therefore cheaper to manufacture and launch, and network services are less vulnerable to disruption, for example from space weather or anti-satellite weapons. SpaceX’s Starlink makes up the largest satellite network by far, with about 4,000 in orbit, and plans to increase that to 42,000. The OneWeb constellation has more than 600 satellites, but they are located in a higher orbit, reducing the effect on astronomical observations. And Amazon is set to launch its Kuiper project this summer, launching its first broadband service delivery satellites on the maiden flight of United Launch Alliances’ Vulcan Centaur rocket. The company plans to populate that constellation with more than 3,000 satellites.

SpaceX and a few other companies have been testing possible solutions, such as covering a satellite in a thin film to darken it so it reflects less light or adding a visor to reflect light away from Earth. These limited efforts fell far short of the International Astronomical Union’s luminosity targets, and some of these projects have caused problems for the satellites themselves, heating them too much or interfering with inter-satellite communication.

A concept by NASA to work with its commercial partners to lift Hubble to a higher orbit could inadvertently mitigate the photobombing problem. Atmospheric drag gradually brought the spacecraft closer to Earth. Moving it back is intended to extend its life, but it would also move it away from a small fraction of passing satellites.

None of this will solve the problems for ground-based observers, who have to peer through the entire atmosphere, including all satellite orbits. And Barentine fears that, although the companies have not yet found technological solutions, they have not slowed down the rapid pace of the satellite launch. People in industry have this unerring faith in innovation, he says, and my response was: The history of science and technology and the environment is littered with cases where we’ve rushed headlong into a technology we didn’t understand. and that has created many negative side effects problems.

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