A mixture of space junk and a growing array of functional satellites like SpaceX’s Starlink have astronomers concerned about possible orbital material interfering with observations. This is justifiable, given that researchers are currently debating whether one of the observations represents one of the most distant supernovae ever observed or if it represents a spent Russian booster.
Obviously, this mess is a huge problem for ground-based observatories, which are underneath everything in orbit. But many observatories, including the Hubble Space Telescope, are in low Earth orbit, which puts them under many satellites. And a new survey of Hubble images shows that it is capturing an increasing number of satellite tracks in its images. So far, this hasn’t seriously hurt its science, but it clearly shows that orbiting observatories aren’t immune to these problems.
The work came from a citizen science project, The Hubble Asteroid Hunter, which organized volunteers to search for asteroid tracks left in long-exposure Hubble observations. If an asteroid passes through Hubble’s field of view during this exposure, it can leave a short streak in the resulting image. But participants began to notice that some of the lines they were seeing crossed Hubble’s entire field of view in a single image (the project maintains a forum where volunteers can discuss their work).
Nothing as distant as an asteroid can move fast enough to leave long traces. So the only realistic explanation is something much closer: a satellite.
This informal identification of satellites was not comprehensive enough to provide us with reliable statistics on their numbers. But it did give the researchers a dataset that was enough to train an AI system to identify paths in a much larger database of images. (They had already trained two and confirmed that they gave consistent results.) Once trained, the AI was put into the full database of images from Hubble’s two cameras, the Advanced Camera for Surveys and the Wide Field Camera 3. Those with exposures long enough for the satellite to cross Hubble’s entire field of view.
Not surprisingly, a lot of space crossings have been identified. The worrying thing is the trend. Between 2002 and 2005, 2.8 percent of the longer exposures taken by Hubble contained a satellite track. By 2018-2021, that fraction had risen to 4.3 percent. The Wide Field Camera 3, which was not active throughout the study period, also experienced a significant rise. Depending on the camera, the rise during this period was 60 to 70 percent.
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The researchers note that the tracks often appear at lower wavelengths; Satellites are less likely to be visible in ultraviolet light. Satellites also appeared more often when Hubble was pointed along the equator. The researchers suggest that this is an indication that most of the tracks are left by satellites in geostationary orbits, which are often located along the equator.
Fortunately, most of the massive constellations that are put into orbit are lower than Hubble’s height, so adding them didn’t really affect those numbers. But Hubble’s orbit has been slowly decaying over time, so it may eventually end up falling into the region where these constellations are located before its instruments begin to fail. The researchers also note that many other observatories are in near-Earth orbits, and so can suffer from similar problems.
Nature Astronomy, 2023. DOI: 10.1038 / s41550-023-01903-3 (about DOIs).
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