When I take people out for a stargazing session on a dark night, they’re always surprised to find out how many human-made satellites are visible as they pass overhead. In fact, if you stay out for an hour or two, you can easily spot a dozen or more making their way across the sky! I understand the confusion, though. Satellites seem so far away—in space—that you’d think they’d be almost invisible and only rarely seen.
But this is misleading: most of them are a lot closer than you might think. Many are also big and shiny, which makes them brighter—and although it may be night where you are on the ground, a satellite above you can still bask in full sunlight and merrily reflect all those solar photons. All this makes most satellites fairly easy to spot.
There are presently more than 7,500 active satellites around Earth. They range from military “birds” with classified capabilities to far-seeing astronomical observatories to run-of-the-mill commercial spacecraft that provide weather forecasting, communications and other consumer services. And thousands more are inactive. Many of these defunct satellites are spent rocket boosters that still circle our planet long after they lofted their orbital payloads.
Some satellites are small, such as toaster-sized CubeSats that could easily fit on a desktop, while others are huge, such as the school-bus-sized Hubble Space Telescope. Bigger ones tend to be brighter because they offer more surface area to reflect sunlight.
If you’ve ever watched the sun set on a partly cloudy day, you may have noticed that it can still directly illuminate clouds even after it has set. Because clouds are at a higher altitude than you are, for a short time they can still see the sun over the horizon and thus gain “extra” moments of daylight. Satellites are even higher up, so they can be in full sunlight hours after the sun has set on the planet below.
How high up are they? There’s no easily definable upper limit to Earth’s atmosphere, which becomes more diffuse with height, but most space aficionados accept that space starts at 100 kilometers above Earth’s surface (though a strong argument can be made for it starting at just 80 km). This boundary, called the Kármán line, is where the air becomes so thin that its effects on large orbiting objects are negligible. To be safe and prevent atmospheric drag, though, most satellites orbit at least 160 km above the ground. Anything between that altitude and about 2,000 km above Earth’s surface is considered “low-Earth orbit.”
That means satellites passing overhead are pretty close to you. You could drive 160 km in less than two hours—just not straight up!
In truth, most satellites will be farther away than that. It depends on the size of their orbit and where they happen to be in that orbit relative to your location on the ground. If you see one directly overhead, at the zenith, it’s as close to you as it can be. When it’s more toward the horizon, even a low-Earth orbit satellite can be more than 1,000 km away from you and therefore much fainter.
Variations in structural geometry can also change a satellite’s ground-based visibility. Some have reflective panels that can catch the sunlight at just the right angle to act as a mirror and briefly beam that light directly at you. This is called a flare, and it can cause a satellite to rapidly and dramatically increase in brightness before dimming just as quickly. Many times, I’ve seen this effect make a humdrum orbital bird suddenly outshine Venus! It’s quite a spectacle.
Another eye-catching phenomenon arises when a satellite tumbles, rolling end-over-end as it orbits. Spent rocket boosters are the typical culprits here. When you see the shorter end of the tumbling booster, there’s less surface area to reflect sunlight. But as it spins, it then presents its longer side to you. This makes the booster appear to slowly pulsate; it gets brighter and dimmer on a regular cycle as it tumbles. That’s a pretty amazing thing to see. In long-exposure photographs, it looks like a dashed line against the stars.
The speed a satellite moves across the sky depends on how high it orbits. Low-altitude satellites move at about the same apparent speed as an airplane. Don’t be fooled, though: typical velocities in low-Earth orbit are still about 28,000 kilometers per hour, which is a couple of dozen times faster than a passenger jet! This similarity in apparent speeds makes it easy to mistake a satellite for an airplane, in fact. But one defining way to tell them apart is that a satellite’s brightness tends to be steady (or slowly changing), while an airplane’s collision avoidance lights flash quickly. Also, when a plane is sufficiently close, you can distinguish the red and green navigation lights on its wingtips.
The best example of a bright satellite that’s fun to spot is the International Space Station (ISS). At more than 100 meters long, it’s the largest structure in orbit, and it can get quite bright as it serenely and smoothly slides across the sky. And as you observe it, remember: there are human beings living in that point of light! That never fails to awe me.
Some satellites are in a much higher orbit and move far more slowly. They tend to be dimmer, too, making them tougher to spot. The most notable and numerous of these, called geosynchronous satellites, orbit Earth at a height of about 36,000 km, which gives them an orbital period exactly equal to Earth’s rotation. To us on the ground, they appear to be nailed to the sky—unmoving (or moving very slowly) as the stars rise and set. These are usually best spotted in time-lapse videos because they don’t move relative to the horizon even as much as stars do.
You might also see several satellites bunched together in a row, moving along the same line as they cross the sky like train cars on a track: these are likely freshly deployed SpaceX Starlink satellites following a shared trajectory before entering their assigned orbital slots. It’s an astonishing sight, even though they’re the bane of modern astronomy.
Sometimes after a recent spacecraft launch to the ISS, you can see a dimmer “star” trailing the station; that’s the resupply ship (which occasionally carries astronauts, too) catching up to the ISS in preparation for a rendezvous.
Every now and then, you might even witness an eastward-bound satellite dim, turn red and then fade from view far above your horizon. Congratulations; you’ve just seen the sun set on it! As a satellite moves east, the sun appears to set from its point of view. Just as sunlight appears to redden and fade when it is viewed from Earth’s surface (attenuated by its passage through a swath of our planet’s atmosphere) it also momentarily casts ruddy hues on the satellite before plunging it into orbital darkness—a surreal sight!
So how can you watch these phenomena for yourself? There are apps and websites aplenty to track and tell you which satellites are visible from your location, as well as when and where in the sky to look for them. I typically use Heavens Above, but an Internet search will find you several more. Just enter your location as accurately as possible in your satellite tracker (you can often select it from a map), and away you go.
I hope you can spot a few on the next clear, moonless night. It’s a thrill to watch them, knowing that you’re seeing something that we humans sent aloft. And who knows? If it’s an Earth-observing bird, it might even be looking right back at you.
