The International Space Station completes one full orbit of Earth in roughly 90 minutes, traveling at about 28,000 kilometers per hour (around 17,500 miles per hour) at an altitude of approximately 400 kilometers. That speed means the crew sees 16 sunrises and 16 sunsets every 24 hours, and the station covers a distance equivalent to a round trip to the Moon and back each day. But the 90-minute figure is an average that shifts slightly depending on altitude, atmospheric drag, and how recently the station received a boost from visiting spacecraft.
The Speed and Altitude Behind the Number
To stay in orbit without falling back to Earth or drifting off into deep space, any satellite has to travel at a very specific speed for its altitude. At around 400 kilometers above the surface, that speed works out to roughly 7.66 kilometers per second. Slower than that, and the station would gradually spiral downward. Faster, and it would climb to a higher orbit. The balance between gravity pulling the station toward Earth and its forward momentum carrying it along a curved path is what keeps the ISS perpetually falling around the planet rather than into it.
At that velocity, the ISS covers about 675,000 kilometers in a single day. One orbit traces a path of roughly 42,600 kilometers, accounting for the slight extra circumference added by being 400 kilometers above the surface rather than at sea level. Divide one by the other and you get just over 15.5 orbits per day, which translates to an orbital period of about 92 minutes. The number fluctuates by a minute or two in either direction depending on the station’s exact altitude at any given time.
Why the ISS Flies at That Altitude
The choice of roughly 400 kilometers is a compromise. Go too low and atmospheric drag becomes fierce, requiring constant fuel-burning maneuvers to keep the station from re-entering the atmosphere within weeks. Go too high and you run into the Van Allen radiation belts, which would expose the crew to dangerous levels of charged particles. The 350-to-420-kilometer band that the ISS occupies sits in a sweet spot where drag is manageable, radiation shielding is adequate with the station’s existing hull, and resupply vehicles can reach it without extraordinary amounts of propellant.
Altitude also matters for the launch vehicles that service the station. The lower the orbit, the less energy rockets need to deliver cargo and crew. Soyuz capsules, SpaceX Crew Dragon, and cargo vehicles like Northrop Grumman’s Cygnus all benefit from the ISS being parked at a relatively modest height. A station at 800 kilometers, for instance, would demand significantly more fuel per mission, driving up costs and reducing the mass of supplies each flight could carry.
Atmospheric Drag and Why the Orbit Slowly Decays
Even at 400 kilometers, the atmosphere has not completely vanished. A thin veil of gas molecules still exists at that altitude, and the ISS, with its massive solar arrays spanning roughly the area of a football field, presents a large cross-section for those molecules to collide with. Each collision is tiny, but collectively they produce a drag force that steadily robs the station of orbital energy, causing it to sink lower over time.
The intensity of this drag is not constant. Solar activity plays a major role. When the Sun is more active, it emits higher levels of extreme ultraviolet radiation and energetic particles. These heat the upper atmosphere, causing it to expand and push denser air into altitudes where the ISS flies. During solar maximum periods, drag on the station can increase substantially compared to solar minimum years, accelerating orbital decay and requiring more frequent altitude corrections.1Transactions of the Japan Society for Aeronautical and Space Sciences, Aerospace Technology Japan. Theoretical Model of Drag Force Impact on a Model International Space Station Satellite due to Solar Activity The station can lose several kilometers of altitude per month during high solar activity if left unboosted, compared to a gentler decline during quiet solar periods.
This means the 90-minute orbit is not perfectly stable. As the station drifts lower, it speeds up slightly (counterintuitively, a lower orbit is a faster orbit), and the orbital period shortens by a small amount. After a reboost pushes it higher, the period lengthens again. The variations are on the order of seconds to a minute, not enough for anyone aboard to notice, but important for mission planners who coordinate rendezvous with incoming spacecraft.
Reboost Maneuvers That Keep the Station Aloft
Left entirely to its own devices, the ISS would re-enter Earth’s atmosphere within a year or two, depending on solar conditions. To prevent that, the station’s orbit is periodically raised through reboost maneuvers. These have traditionally been performed by firing the engines of docked Russian Progress cargo vehicles or, occasionally, the station’s own thrusters. The engines burn for anywhere from a few seconds to several minutes, nudging the station into a slightly higher orbit.
The typical reboost strategy follows a pattern: let the station drift downward under the influence of atmospheric drag, then fire engines to push it back up before it drops below a safe threshold. Researchers have explored alternatives to chemical propulsion for this purpose, including concepts that would use electromagnetic forces generated by a conductive tether interacting with Earth’s magnetic field to provide thrust without burning fuel.2Acta Astronautica. Bare Photovoltaic Tether characteristics for ISS reboost Whether or not such technologies are adopted before the station’s planned retirement, the reboost cycle is a defining feature of ISS operations and directly affects the orbital period from week to week.
Reboosts are also timed around visiting vehicle arrivals and departures. When a crewed capsule or cargo ship is approaching the station, controllers need the orbit to be precisely where they expect it. A reboost scheduled too close to a docking window could create complications, so mission planners build altitude adjustments into a long-range calendar that accounts for expected drag losses, upcoming launches, and debris avoidance maneuvers.
Sixteen Sunrises Every Day
One of the more surreal consequences of a 90-minute orbit is the rapid day-night cycle. Roughly 45 minutes of each orbit are spent in sunlight, and 45 minutes in Earth’s shadow. The transition is abrupt: the crew can watch the terminator line sweep across the planet below, and within seconds they go from brilliant daylight to complete darkness outside the windows. Then, less than an hour later, the Sun rises again.
This wreaks havoc on the body’s internal clock. On Earth, your circadian rhythm is anchored to a single sunrise and sunset. Aboard the station, there is no natural cue for when to sleep and when to be awake. To manage this, the crew follows a schedule set to Coordinated Universal Time (UTC), with a designated eight-hour sleep period. The station’s interior lighting has been upgraded over the years to include tunable LED panels that can shift color temperature, mimicking the bluer light of morning and the warmer tones of evening to help the crew’s biology stay on track.
Despite these measures, sleep disturbances remain one of the most persistent complaints among astronauts. The combination of microgravity, a confined living space, and the relentless 90-minute light cycle makes it difficult for many crew members to get consistent, restful sleep. Some astronauts report waking up during orbital “night” simply because the experience of floating in a sleeping bag strapped loosely to a wall is inherently different from lying in a bed.
The Ground Track and Why the ISS Does Not Fly Over the Same Spot Twice
If you could paint a line on the globe tracing the ISS’s path, you would not see a single circle. Instead, you would see a sinusoidal wave stretching between about 51.6 degrees north latitude and 51.6 degrees south latitude. That 51.6-degree orbital inclination was chosen largely for practical reasons: it is the latitude of the Baikonur Cosmodrome in Kazakhstan, the launch site for Russian Soyuz rockets, and launching directly into the station’s orbital plane from that location is the most fuel-efficient option.
Because Earth rotates beneath the station while the station orbits above it, each successive pass shifts westward by about 22.5 degrees of longitude. After 16 orbits, the pattern does not quite repeat, so the ground track drifts over days. This means the ISS eventually passes over the vast majority of Earth’s populated surface between those latitude limits. Major cities like New York, London, Tokyo, Sydney, Buenos Aires, and Cape Town all fall within the station’s ground track at various times, which is why almost anyone living between roughly 52 degrees north and 52 degrees south has a chance to see it overhead.
The inclination also determines what the crew can observe and photograph. Astronauts aboard the ISS have captured stunning images of everything from hurricanes forming over the Atlantic to city lights across Europe at night. The 51.6-degree tilt gives them a view of about 90 percent of the inhabited world, though they never pass directly over the polar regions.
Spotting the ISS From Your Backyard
The station is the third-brightest object in the night sky after the Moon and Venus, and under the right conditions it outshines everything else overhead. It appears as a steady, bright point of light moving smoothly from one horizon to the other over the course of about four to six minutes. Unlike airplanes, it does not blink or flash, and it moves faster than any aircraft you are likely to see.
The best viewing opportunities come during the hour or two after sunset or before sunrise. At those times, you are standing in darkness while the station, high above, is still catching direct sunlight. During the middle of the night, the station passes through Earth’s shadow and becomes invisible. NASA’s Spot the Station website and several third-party apps provide exact times and directions for upcoming passes at your location, including the brightness, the direction of appearance, and the direction where it will fade from view as it enters Earth’s shadow.
Passes vary in quality. A high-elevation pass, where the station arcs nearly overhead, can last five or six minutes and reach a brightness that is hard to miss even from a light-polluted city. A low-elevation pass, where the station skirts close to the horizon, may last only a minute or two and be dimmed by the thicker atmosphere near the horizon. Checking the predicted maximum elevation before heading outside helps you decide whether a particular pass is worth watching.
How the ISS Orbit Compares to Other Spacecraft
The 90-minute orbital period places the ISS firmly in low Earth orbit, the most crowded band of space around the planet. Most Earth-observation satellites, many communications constellations, and all crewed orbital stations to date have operated in this general zone, typically between 200 and 2,000 kilometers altitude. Satellites near the lower end of that range orbit even faster than the ISS: a satellite at 200 kilometers completes a lap in about 88 minutes, though atmospheric drag at that height is so fierce that it would need constant propulsion to survive more than a few weeks.
Higher orbits take proportionally longer. The GPS constellation, at roughly 20,200 kilometers, takes about 12 hours per orbit. Geostationary communications satellites sit at about 35,786 kilometers, where their orbital period matches Earth’s rotation and they appear to hover over a single point on the equator. The Moon, at an average distance of 384,400 kilometers, takes about 27.3 days to complete one orbit. The pattern is straightforward: the farther from Earth, the longer the orbital period, and the slower the required orbital velocity.
For the crew aboard the ISS, the practical consequence of their 90-minute orbit is that the world below is always changing. A glance out the cupola window at one moment might show the Sahara Desert; 20 minutes later, they are over the Indian Ocean. The pace is fast enough that astronauts often describe the feeling of the planet rolling beneath them as one of the most profound experiences of spaceflight. Time-lapse videos shot from the station capture this beautifully, compressing hours of orbital footage into minutes that show coastlines, thunderstorms, and auroras sweeping past in a continuous stream.
Debris Avoidance and Its Effect on the Orbit
The ISS does not simply follow its planned orbit undisturbed. Space debris is an ongoing concern at the station’s altitude. Thousands of tracked objects, from defunct satellites to fragments from past collisions and rocket stages, share the low Earth orbit environment. When tracking data indicates that a piece of debris will pass uncomfortably close, controllers can command a debris avoidance maneuver, using thrusters to shift the station’s orbit just enough to increase the miss distance.
These maneuvers are small, typically changing the station’s velocity by less than a meter per second, but they do alter the orbital period briefly. A maneuver that raises the orbit adds a few seconds to the period; one that lowers it trims a few seconds off. The crew is usually informed in advance, and if a threat is identified too late for a maneuver, astronauts may be directed to shelter in their docked spacecraft as a precaution until the debris passes.
The frequency of these avoidance maneuvers has increased over the years as the tracked debris population has grown. In some years, the station has performed three or more such maneuvers. Each one ripples through the scheduling of future reboosts, visiting vehicle arrivals, and spacewalks, making orbital maintenance a dynamic, ongoing process rather than a set-and-forget operation.
What Happens When the ISS Is Finally Deorbited
The station is currently expected to operate into 2030, after which it will be intentionally deorbited. The plan involves using a dedicated deorbit vehicle to push the station into a controlled re-entry over an uninhabited stretch of the South Pacific, far from any landmass. Without that controlled push, the station would eventually re-enter on its own as atmospheric drag pulled it lower, but the timing and location would be unpredictable, posing a risk to people on the ground given the station’s massive size.
During the final descent, the station’s orbital period would shrink rapidly as it dropped into denser atmosphere. At around 150 kilometers, drag would overwhelm any remaining orbital velocity within minutes. Most of the structure would break apart and burn up from aerodynamic heating, but some denser components, like certain structural joints and engine bells, are expected to survive re-entry and splash down in the ocean. The entire process from final deorbit burn to ocean impact would take roughly an hour, a last rapid orbit that ends not with another sunrise, but with one of the most spectacular controlled demolitions ever attempted.