[intro ].

Imagine you're an astronomer. Months ago, you requested time on one of the world's most advanced telescopes, and now, it's finally here.

Tonight is your night. You set everything up, careful to put in all the right parameters, and spend hours taking shots of the sky. But later, when you review the data, you're devastated.

Bright spots and streaks of light ruined tons of your photos. All that work was for nothing. It's an astronomer's nightmare, and a scenario a lot of people are worried will become more and more common.

Because over the next few years, t here could be more than 26,000 new satellites orbiting Earth, compared to the 5500 or so up there right now. So the European Southern Observatory, or ESO, decided to figure out what this means for astronomers. In a study released last week, which was accepted for publication in the journal Astronomy and Astrophysics, they looked at the possible impact of these new satellites on different types of observations.

Turns out that in most cases, they won't interfere as much as you might expect. But for some research, they could be a big problem. The surge of satellites comes from an idea multiple companies are pursuing: a globally-accessible internet.

Satellite-based internet has been around since the ‘90s. But the satellites used for it are so high above Earth that it takes more than half a second for a signal to get from you to somewhere else and back. That might not seem like long, but think about how annoying a half-second lag is during a video call.

It's just not practical. These new satellites are designed for a much lower orbit, though, bringing the delay down to around a 30th or 40th of a second. That's similar to what you might get on a decent regular broadband connection.

And for people in remote or underserved areas, that type of internet access could be a game-changer — especially if companies keep costs low. So there are advantages to these satellites. But astronomers have been worried about the downsides.

See, satellites are shiny. They have to be — if they absorbed most of the sunlight that hit them, they'd overheat fast. They also move across the sky much faster than, say, a star.

So if you're taking a long-exposure photo, where the shutter stays open for a while, you could end up with a bright streak against the background of stars. Even worse, if the streak is bright enough, the light could overwhelm the sensors and ruin the photo entirely. But the new ESO study shows that in many cases, this might not be a huge concern.

The most important factor is timing. Generally, you can only see satellites overhead during astronomical twilight, when the Sun is 12 to 18 degrees below the horizon, and for an hour or so at the start and end of the night. For the rest of the night, most satellites are in Earth's shadow.

No reflections, no problem. So the issue is only within a certain time frame. And even then, for many types of observations, the satellites would only cause problems for a small fraction of data.

Based on how the satellites would be distributed and how visible they'd be, the researchers concluded that for short and medium exposures — meaning, 100 seconds or less — there wouldn't be much interference. At most, about 0.5% of observations could be ruined, and only during twilight. And even for long exposures, if you're only looking at a narrow field of the sky, you're probably fine.

For wide-field imaging, though, things could be much worse. The team estimates that around 1 to 5% of exposures would be ruined at the start and end of the night, with higher numbers during twilight. And if you have a really big telescope looking at a very wide section of the sky?

You're in trouble. For example, at the Vera C. Rubin Observatory, a powerful wide-field telescope that's under construction in Chile, around a third of the exposures would be ruined during the beginning and end of the night, and about half during twilight.

The only time that would be unaffected is the middle of winter. Thankfully, the team did have suggestions for how to mitigate the problem — although they won't work for everyone. The easiest move would be to just… not collect data during part of the night.

Or, if you know where the satellites are, you could schedule your observations for a time when your part of the sky will be satellite-free. But that isn't always practical. So another option would be to keep track of where the satellites are at any given time, and plan accordingly.

For example — if you know a satellite will cross the telescope's field of view, you can close the shutter while it passes, then reopen it. But for that to work, you would need to have up-to-date information on the locations and trajectories of every satellite, which the companies that launched them would have to provide. The team also pointed out that they only studied how the satellites affect observations that use light in the visible and part of the infrared range.

They plan to publish other papers about the impacts on observations in other parts of the electromagnetic spectrum, like radio waves. But whatever those impacts are, these satellites aren't going anywhere. Many have already been launched, and the idea of a global internet won't just disappear.

Still, if scientists and the companies launching these satellites work together, having thousands of new lights in the sky may not be as bad as it seems. Thanks for watching this episode of SciShow Space News! We make an episode like this every Friday, where we dive into the latest discoveries, but we couldn't do it without our patrons on Patreon.

They keep these episodes free and open to everyone. And also, they're just a great and thoughtful community. If you like SciShow and want to help us make more of it, you can go to patreon.com/scishow. [ outro ].