Planned satellite constellations may swamp future orbiting telescopes

On Wednesday, three NASA astronomers released an analysis showing that several planned orbital telescopes would see their images criss-crossed by planned satellite constellations, such as a fully expanded Starlink and its competitors. While the impact of these constellations on ground-based has been widely considered, orbital hardware was thought to be relatively immune from their interference. But the planned expansion of constellations, coupled with some of the features of upcoming missions, will mean that at least one proposed observatory will see an average of nearly 100 satellite tracks in every exposure.

Making matters worse, some of the planned measures meant to minimize the impact on ground-based telescopes will make things worse for those in orbit.

Constellations vs. astronomy

Satellite constellations are a relatively new threat to astronomy; prior to the drop in launch costs driven by SpaceX’s reusable rockets, the largest constellations in orbit consisted of a few dozen satellites. But the rapid growth of the Starlink system caused problems for ground-based astronomy that are not easy to solve.

Unfortunately, even if we had an infinite budget, we couldn’t just solve this by increasing our reliance on space-based hardware. While orbital satellites may be above some of the problem-causing constellations, enough of the new hardware is orbiting at altitudes where they can interfere with observations. A check of the image archive of the Hubble Space Telescope, for example, shows that over four percent of recent images contain a satellite track, a significant increase from earlier in the century.

(There are some space-based telescopes that aren’t orbiting the Earth, like the James Webb Space Telescope, that will remain worry-free. But these require expensive launches and are too far from Earth for the sort of regular servicing that something like the Hubble has received.)

And the problem will only get worse, according to three astronomers at NASA’s Ames Research Center in California (Alejandro Borlaff, Pamela Marcum, and Steve Howell). Based on filings made with the Federal Communications Commission, they found that the current total of satellites represents only 3 percent of what will be in orbit a decade from now if everybody’s planned launches take place.

To estimate the impact that this massive population of satellites might have on astronomy, the three researchers focused on several key orbiting observatories. One is the Hubble. Another is the recently launched SPHEREx, which will perform an all-sky survey in the infrared. The Chinese are developing a telescope called Xuntian that will operate in conjunction with their orbiting space station, and the ESA is preparing a mission called ARRAKIHS meant to characterize the dark matter halos of nearby galaxies.

Lots of tracks

The impact of satellites on observations depends on many factors. Many satellites have constant infrared and radio emissions and thus always have the potential to interfere with imaging at those wavelengths. They can also reflect sunlight, but they’re most likely to do that when they’re near the horizon (meaning what someone on the satellite would see as the dawn or dusk edges of the Earth). While it’s possible to prioritize observations that avoid the horizon, that becomes difficult when longer exposures are required. Surveys meant to identify asteroids that cross Earth’s orbit will always need to image near the horizon.

Another key factor is the altitude of the observatory. Something like Xuntian, which requires an orbit that takes it to a space station, will necessarily be at a relatively low altitude and therefore below more of the constellations. Something like SPHEREx, a smaller satellite that operates independently, can potentially be lifted to a higher orbit.

So the risk the constellations pose to observatories can vary greatly based on where your observatory is, what wavelengths it’s sensitive to, and what you’re doing with it. That’s why Borlaff, Marcum, and Howell looked at several very different pieces of hardware, although they did limit their analysis to interference from satellites that would be sunlit as they traversed across the observatory’s field of view.