What was that? The ISS and bright satellites

6 min read

A bright, steadily moving point of light crosses the evening sky. It doesn't blink, it makes no sound, and then after a few minutes it simply disappears. Sightings like these regularly lead to UFO reports, even though the explanation is most often the International Space Station, orbiting about 400 km above our heads, or another bright satellite.

Why does a satellite shine?

Satellites have no light of their own – they reflect sunlight, like the Moon. The trick is the timing: after sunset it is already dark down here, but a few hundred kilometers up the satellite is still lit by the Sun. That is why they are best seen after dusk and before dawn, in a window of one to two hours.

The International Space Station (ISS) is the brightest of them. It orbits at about 400 km, circles the Earth roughly every 90 minutes, at a speed of about 8 km per second. It is about 109 meters from end to end – the size of a soccer field – and its huge solar arrays reflect light like mirrors. During the best passes it rivals Venus, meaning it can be the brightest point in the sky after the Moon.

Our own calculation: how long can the ISS be seen over Budapest?

Sky Glitch calculated when the space station, passing overhead at about 410 km, still receives sunlight. The geometry is simple: as long as the Sun is no more than about 19.9 degrees below the horizon, the ISS at the zenith is still lit by the Sun. If the Sun sinks lower than that, the station enters the Earth's shadow and "goes out."

In Budapest, this works out as follows by month (on the 15th of the month, local time):

April: about 2 hours 10 minutes (sunset 19:33, the ISS is lit until 21:45)
May: about 2 hours 50 minutes (20:14 → 23:07)
June: all night – around the summer solstice the ISS never goes fully into shadow
July: about 3 hours 25 minutes (20:37 → 00:02)
August: about 2 hours 25 minutes (19:56 → 22:19)

Jan: 16:19 – 18:23
Feb: 17:07 – 19:03
Mar: 17:49 – 19:46
Apr: 19:33 – 21:45
May: 20:14 – 23:07
Jun: 20:42 – all night
Jul: 20:37 – 00:02
Aug: 19:56 – 22:19
Sep: 18:56 – 20:57
Oct: 17:56 – 19:52
Nov: 16:07 – 18:08
Dec: 15:53 – 17:59

The most interesting season is summer. In mid-June in Budapest, after midnight the Sun sinks only about 19 degrees below the horizon – just less than what would be needed to darken the space station. That is why, around the summer solstice, the ISS can be bathed in sunlight all night, and can cross the sky as a brilliant point even at one in the morning. Many summer "midnight UFOs" are actually this.

In winter, however, the situation is reversed: after eight in the evening, the ISS passing overhead is definitely in shadow. So a point of light in the dead of a December night is almost certainly not the space station. (Near the horizon, in the direction of the setting Sun, satellites can remain lit somewhat longer.)

Two more numbers we calculated: the ISS passing overhead covers about 1 degree of sky per second – roughly the width of two full moons. A pass through the zenith, from 10 degrees above the horizon to 10 degrees, lasts about 6 minutes.

What does it look like?

Brightness: a very bright star, often brighter than any of them.
Motion: steady, in a straight line, neither accelerating nor turning. It rises from the west and crosses the sky in a few minutes.
Color: white or slightly yellowish, steady light.
No blinking: no red-green navigation lights, which are characteristic of airplanes.
No sound: complete silence, no contrail.
Sudden disappearance: in the middle of the sky it fades, sometimes with a reddish tint, and disappears. At that point it has entered the Earth's shadow, meaning the "Sun has set" for the satellite too.

Why is it misleading?

The sudden disappearance is the most common "evidence" mentioned by people who report sightings. Yet this is precisely the most characteristic feature of a satellite.
Because of the silence and the lack of blinking it can't be an airplane – so many people jump straight to the mysterious explanation.
Estimating size and distance is nearly impossible at night. A point of light 400 km away can just as easily seem to be "hovering above the rooftop."
Because of the unusual brightness many people don't believe they are seeing a human-made object.

Flares: when a satellite "explodes" for a moment

In the 2000s, Iridium flares were the best known. The mirror-like antennas of the first-generation Iridium satellites reflected the Sun toward the observer for a moment. When that happened, the point of light flashed brighter than even Venus for a few seconds, then faded. The last member of the original fleet was retired in December 2019, so these flares have now ended.

A similar brief flare can still be caused today by the solar panel or antenna of any satellite. If a stationary "star" brightens for a few seconds and then disappears, there's a good chance that's what you saw.

The new bright giants: BlueBird

AST SpaceMobile's BlueBird satellites provide mobile phone internet service, and to do so they unfold a huge antenna of about 64 square meters. According to a 2023 Nature study, the prototype (BlueWalker 3) rivaled the brightest stars. The company plans more than a hundred such satellites, which is why astronomers are increasingly pointing out that these, too, appear as conspicuous points moving like "new stars."

How can you check, and what should you do?

Note the time to the minute, and the location.
Watch the direction: where it came from, where it was heading, how high it passed.
Note how long it was visible, and how it disappeared – did it suddenly fade, or did it drop below the horizon?
Check a prediction tool: NASA's Spot the Station site, Heavens-Above, or the ISS Detector / ISS Spotter app will show you when the ISS or another bright satellite passed over your location.
Compare it with the season: in summer the ISS can be visible late in the evening, even after midnight; in winter only shortly after sunset (see the table above).
If none of these fit, send us the description or the photo, and we'll look into it.

Quick identifiers

Steady, straight-line motion without blinking → almost certainly a satellite.
Crosses the sky in about 5–6 minutes and fades in the middle of the sky → the ISS or another bright satellite.
A strong flash lasting a few seconds, then nothing → reflection from a solar panel or antenna.
Dead of night, in winter, high in the sky → unlikely to have been the ISS.
Red-green blinking, humming → an airplane, not a satellite.

The sober take

A steadily moving point of light without blinking, seen in the evening or at dawn, is in the overwhelming majority of cases a satellite: the International Space Station or another bright satellite reflecting the light of the Sun that has already set. The sudden disappearance is no mystery either, but the Earth's shadow. On a prediction site you can check in a few minutes what passed overhead – and if nothing fits what you saw, that is where the truly interesting question begins.