STARLINK O ISS: CÓMO DIFERENCIARLOS

Two different shows, the same question: "what is that?"

It's becoming more and more common to look up at dusk and see something that doesn't match what you expected: instead of a single bright point, a line of dots moving together in formation, one after another, like a moving string of pearls. That's not the ISS. It's almost certainly a Starlink satellite train. The confusion is common enough that it deserves a clear explanation.

What is a "Starlink train"?

Starlink is SpaceX's satellite internet network, made up of thousands of satellites in low Earth orbit. Every time SpaceX launches a new batch, between 20 and 60 satellites go up stacked together on the same rocket and are released as a cluster into a very low, tightly-packed orbit. For the first few days — sometimes weeks — after launch, they haven't yet spread out or climbed to their final operational orbit, so they travel almost in a line, at the same speed and in the same direction.

Seen from the ground, that freshly-released cluster produces the famous "Starlink train": a series of points of light — could be 20, 40, or more — crossing the sky in formation, one after another, at a very regular pace and spacing. It's a striking sight the first time you see it, and one of the reasons so many people search "line of lights in the sky" thinking they've spotted something strange.

How it differs from the ISS

Once you know what to look for, the difference is obvious:

• The ISS is a single point. One bright, golden-white, perfectly steady object crossing the sky alone, with no company.
• A Starlink train is several points in a row. Dozens of satellites, dimmer than the ISS, lined up and moving in formation, one after another at regular intervals.
• The brightness is different. The ISS is far brighter than any single Starlink satellite: its huge solar array surface reflects much more light than one small communications satellite.
• The formation changes over time. A Starlink train only looks like a "line" during its first days or weeks of life. Over time, each satellite raises its orbit independently until it reaches its operational altitude, and the group gradually spreads out until it no longer looks like a line and blends in with the rest of the satellites in the sky, essentially invisible to the naked eye one by one.

How often can you see each one

The ISS has predictable passes over every city: its orbit is stable and has been well known for over two decades, so any pass calculator — like ours — can tell you precisely what time and in which direction you'll see it this week. There are stretches of several weeks with no visible passes and stretches with bright passes every night, but it's always a figure that can be calculated in advance.

Starlink trains are far more unpredictable. Their visibility depends directly on whether there's been a recent launch and on which spreading-out phase that particular batch is in. SpaceX launches new satellite groups very frequently, so during active launch periods it's relatively easy to spot a train; at other times, it can be a while before you see one. Unlike the ISS, there's no fixed pass schedule: each train is a temporary phenomenon tied to a specific launch.

How to enjoy both shows

The good news is you don't have to choose — you can enjoy both. Use our visible pass calculator to find out exactly when the ISS passes over your location — time, direction, and maximum elevation — and go outside armed with that information. And if you also spot a line of dots moving in formation on some other night, now you know you're witnessing a recent Starlink launch in its spreading-out phase — a phenomenon just as fascinating, even if it has a completely different origin.

If you've never observed the ISS before and want a step-by-step guide — from why it shines so bright to how to photograph it with your phone — we have a complete ISS observation guide that pairs perfectly with everything explained here.

Frequently asked questions

How do I know if I'm looking at the ISS or Starlink?

Look at the number of dots. A single bright, steady point: that's the ISS. Several dots lined up moving together, one after another: that's a Starlink train. The ISS is also noticeably brighter than any individual Starlink satellite.

How often can you see Starlink trains?

It depends on SpaceX's recent launches. Right after a launch, during the first days or weeks, the train is highly visible; afterward the satellites spread out to their final orbit and stop looking like a formation. There's no fixed schedule like there is with the ISS — visibility is tied to launch activity.

Are Starlink satellites dangerous or a nuisance for astronomy?

They pose no danger to people on the ground. They do spark debate in the astronomical and scientific community, since their reflections can interfere with long-exposure observations and add to the growing congestion of objects in low Earth orbit.

Can I predict when a Starlink train will pass?

It's trickier than predicting an ISS pass, since it depends on when the most recent launch happened and what spreading-out phase that particular satellite batch is in. There are specialized websites and apps for tracking Starlink that publish forecasts based on SpaceX's most recent launches.