A combination of space junk and a growing constellation of functional satellites like SpaceX’s Starlink has astronomers concerned about the potential for orbital materials to interfere with observations. And rightly so, given that scientists are currently arguing over whether a sighting represents one of the most distant supernovae ever observed or a spent Russian booster.
This mess is obviously a big problem for ground-based observatories, which sit underneath everything in orbit. But several observatories, including the Hubble Space Telescope, sit in low Earth orbit, placing them below many satellites. And a new study of Hubble images shows that it is capturing an increasing number of satellite tracks in its images. So far this has not seriously compromised its science, but it clearly shows that orbiting observatories are not immune to these problems.
The work came from a citizen science project, the Hubble Asteroid Hunter, which organized volunteers to search for the tracks asteroids left in long-exposure Hubble observations. If an asteroid happens to pass through Hubble’s field of view during this exposure, it may leave a short streak in the resulting image. But participants began to notice that some of the streaks they saw crossed Hubble’s entire field of view during a single image (the project maintains a forum where volunteers can discuss their work).
Anything as distant as an asteroid can’t move fast enough to leave trails that long. So the only realistic explanation is something much closer: a satellite.
This informal identification of satellites was not thorough enough to give us reliable statistics on their numbers. But it gave the researchers a data set sufficient to train an AI system to identify tracks in a much larger database of images. (They actually trained two and confirmed that they produced consistent results.) Once trained, the AI was set loose on the full database of images from two Hubble cameras, the Advanced Camera for Surveys and the Wide Field Camera 3. The images were limited to those with a long enough exposure for a satellite to fully cross Hubble’s field of view.
Not surprisingly, lots of satellite crossings were identified. The worrying thing is the trend. Between 2002-2005, 2.8 percent of the longer exposures taken by Hubble contained a satellite track. In 2018-2021, that share rose to 4.3 percent. Wide Field Camera 3, which was not active throughout the study period, also experienced a significant increase. Depending on the camera, the increase during this period was 60 to 70 percent.
Fits into the constellations
The researchers note that the tracks appear more often at lower wavelengths; the satellites are less likely to be visible in the UV. 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 mega-constellations being orbited are below Hubble’s altitude, so their addition hasn’t really affected these 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 present before its instruments begin to fail. The researchers behind the work also note that several other observatories are in near-Earth orbit and therefore may suffer from similar problems.
Nature Astronomy, 2023. DOI: 10.1038/s41550-023-01903-3 (About DOIs).