For the general public, the expression “light pollution” evokes this overflow of urban lighting that makes many stars invisible. For astronomers, the expression has an additional meaning: it also designates the reflections of sunlight produced by spacecraft, which defile the images taken by telescopes. In a study published on March 2 by Nature Astronomyan international team has, for the first time, quantified this particular pollution for the most famous of astronomical equipment, the Hubble Space Telescope (HST).
The work of the American and European space agencies, Hubble was launched in 1990 and it sails today in a so-called “low” orbit, 538 kilometers from the ground. Satellites moving in higher orbits are therefore likely to leave streaks on the images with a long exposure time (eleven or thirty-five minutes) taken by its instruments, streaks analogous to the luminous bands made by the headlights of a car. on a photo with a long exposure time. To verify this, the authors of the study explored the archives of the HST with an algorithm specially trained for this using machine learning techniques.
More than 150,000 images recorded between 2002 and 2021 by two of Hubble’s instruments have thus been sifted through. Result: 2.7% of the images contained at least one trail left by a satellite. This is an average over the entire period. When we look in detail, we realize that the figures increase over time, with in particular the arrival of mega-constellations of satellites (Starlink, OneWeb). Thus, for the first instrument, we went from 2.8% of polluted images over the period 2002-2005 to 4.3% for the 2018-2021 segment. For the second instrument, starting from 1.2% of photos affected between 2009 and 2012, we arrive at 2% for the period 2018-2021.
Loss of 1% of observations
The authors of the study acknowledge that “the fraction of HST images traversed by satellites is currently small, with negligible impact to science”. Astronomer at the European Southern Observatory (ESO), Belgian Olivier Hainaut has been working for some years on the issue of light pollution left by satellites and adds that terrestrial installations are less affected than Hubble, which observes 24 hours a day. -four : “For terrestrial telescopes, we lose around 1% of observations during the first and last hour of the night. » The rest of the time, the satellites passing over the observatories are immersed in the Earth’s shadow and therefore do not reflect sunlight.
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