Understanding the ISS Orbit: Why the Station’s Altitude and Speed Change

Watching the International Space Station (ISS) circle the Earth in real time is fascinating – especially when you notice its altitude and velocity constantly changing on live trackers like issinfo.net. If you’ve ever wondered why the ISS’s height and speed aren’t fixed, you’re in the right place. In this blog post, we’ll break down the orbital path of the ISS, explain the wavy ground track you see on the map, and dive into those altitude and velocity graphs. We’ll keep things relaxed and clear, with just enough science (and a bit of maths) to satisfy your curiosity without overwhelming you.

Tracking the ISS: Live Map and Curved Ground Track

If you open the ISS live map on issinfo.net, you’ll see the station’s path traced as a curved line across a world map. This line – the ground track – shows where the ISS is passing over Earth. Why is it curved rather than a straight line? There are two main reasons:

• Orbital Inclination: The ISS’s orbit is tilted about 51.6° relative to Earth’s equator. This inclination was chosen so that launches from Kazakhstan (Russia’s Baikonur Cosmodrome) could reach it, but it also means the ISS travels in a plane that swings it as far north as 51.6° N latitude and as far south as 51.6° S each orbit. On the map, this creates a sine-wave-like path. The station isn’t literally zig-zagging; it’s following a circular orbit path, but because the map is flat and the orbit is tilted, we see a wave.
• Earth’s Rotation: The ISS orbits from west to east, but as it circles the Earth, our planet is rotating underneath it from west to east as well. In the roughly 90 minutes it takes the ISS to complete one orbit, Earth has rotated about a quarter turn beneath it. The result? By the time the ISS comes back around to cross the equator again, that crossing point has shifted westward on the map. Consequently, each orbit’s ground track is offset to the west of the previous one. This is why the line on the map curves and moves westward – it’s essentially the ISS’s path combined with Earth’s spin.

*Example from our live tracking page at issinfo.net*The ISS Live Map on issinfo.net, showing the station’s ground track (green curve) and the real-time altitude (yellow line) and velocity (blue line) graphs beneath it.*

In short, the ISS’s ground track looks like a sweeping curve looping between northern and southern hemispheres because of the orbital tilt and the rotating Earth. Over the course of a day (about 16 orbits), the track will cover different parts of the globe. In fact, with a 51.6° inclination, the ISS passes over or near about 90% of Earth’s populated areas at some point – one reason it’s so popular for spotting opportunities when it flies overhead at dawn or dusk!

How High and How Fast: ISS Orbit Basics

The ISS is really moving. On average it orbits at an altitude of around 400 km above Earth (that’s about the distance from London to Edinburgh straight up!). At that height, it whips around the planet once every 92–93 minutes, completing about 15.5 orbits per day. To sustain this orbit, the ISS must travel at an orbital velocity of roughly 27,600 km/h, which is about 7.7 km/s. Yes, you read that right – 7.7 kilometres every second.

Such dizzying speed is necessary to achieve a delicate balance: gravity vs. inertia. Here’s the basic idea of orbital mechanics in a nutshell:

• Gravity is constantly pulling the ISS downward toward Earth.
• Inertia (from the ISS’s high speed) is trying to carry it forward in a straight line out into space.

In orbit, these two effects balance out. The ISS is essentially falling around Earth. It’s moving so fast horizontally that as it falls, the ground curves away beneath it. This balance of forces is often expressed by a simple formula for circular orbits:

v = √(GM/R)

Here v is the orbital speed, M is Earth’s mass, G is the gravitational constant, and R is the distance from Earth’s centre. For the ISS, R is roughly Earth’s radius (6,371 km) plus 400 km – plug that in, and you get about 7.7 km/s. In plain language, there’s essentially one “right” speed for a given orbital height. Too slow, and gravity will win (the ISS would spiral downwards); too fast, and the station would drift into a higher orbit or even escape. This is why the ISS generally cruises at that sweet-spot velocity to stay about 400 km up.

To recap the basics: the ISS is approximately 400 km high, moves at ~27,600 km/h, and goes around the world ~16 times a day. Its orbit is tilted 51.6°, giving it that angled path. Now, if it maintained a perfectly circular orbit and there were no other forces at play, its altitude and speed would stay constant. But if you watch those issinfo.net graphs, you’ll notice they wiggle…

Altitude vs. Velocity: Why They Fluctuate

Take a look at the two line graphs on the live tracker – one shows altitude (in kilometres) and the other velocity (in km/h) – and you’ll see neither is a flat line. The altitude graph typically goes up and down by a few kilometres, and the velocity graph does the inverse, see-sawing by a few dozen km/h. What’s going on here?

These short-term fluctuations are mainly due to the ISS’s orbit not being a perfect circle but a slight ellipse. Even though the ISS is maintained as nearly circular, tiny variations exist – it might be a bit lower on one side of Earth (the “perigee”) and higher on the opposite side (the “apogee”). In the sample graph above, for instance, the yellow altitude line rises and falls over time, indicating the ISS was a few kilometres higher at one point and lower at another.

In an elliptical orbit, a satellite’s speed isn’t constant – it speeds up when it’s lower (closer to Earth) and slows down when it’s higher. This is a consequence of orbital energy and Kepler’s laws: when the ISS dips a little closer to Earth, gravity pulls it stronger and it accelerates; when it climbs slightly higher, it decelerates. The physics is similar to how a skateboarder in a half-pipe goes faster at the bottom (low point) and slower at the top (high point).

On the graphs, this shows up clearly: whenever the altitude curve reaches a low point, the velocity curve hits a peak, and vice versa. In our example, the ISS’s altitude might range roughly from about 415 km up to 435 km over part of an orbit. When it was at the low end (~415 km), the blue velocity graph spiked (around 27,600+ km/h). Later, at the high point (~435 km), the velocity dipped (perhaps ~27,500 km/h). The changes are relatively small (a few percent), but measurable. Essentially, high altitude = slightly lower speed, low altitude = slightly higher speed during each orbit.

Importantly, these ups and downs are normal and expected. The ISS’s orbit naturally oscillates a bit due to that slight eccentricity (often on the order of a few kilometres). The station’s operators don’t try to micro-adjust these small variations – it would be inefficient to constantly tweak for a perfectly circular path. Instead, they let the ISS follow the laws of orbital mechanics, and the altitude/velocity graphs reflect that graceful ebb and flow.

Atmospheric Drag and Orbital Decay

So far we’ve considered the ideal picture (just gravity and inertia). But the ISS orbits in low Earth orbit, and “low” is the key word – at ~400 km, it’s still inside Earth’s upper atmosphere. There’s not much air up there, but it isn’t a complete vacuum. The ISS continuously ploughs through sparse gases in the thermosphere, and this creates atmospheric drag (a bit like air resistance, but extremely thin).

Diagram of earths atmosphere

Drag is a small but relentless force: it steals energy from the ISS’s orbit. As the station encounters those air molecules, it slows down ever so slightly over time. When a satellite loses speed to drag, it can’t stay at the same altitude – gravity starts pulling it to a lower orbit. In other words, the ISS’s orbit is gradually decaying. On average, the ISS loses on the order of 1 to 2 kilometres of altitude each month due to atmospheric drag. (The exact rate varies with solar activity and the station’s orientation, but it’s roughly tens of metres per day.)

Left alone, the ISS would keep dropping and eventually re-enter Earth’s atmosphere after a few years of drag slowing it down. In fact, past space stations like Skylab eventually fell back to Earth because their orbits decayed. But don’t worry – the ISS has a solution to this problem: routine reboosts.

Reboosts: How the ISS Raises Its Orbit

To counteract the constant drag, mission controllers regularly boost the ISS’s orbit back up. Every now and then (typically after several weeks or a couple of months of gradual decay), the ISS gets a push. How do you push a 450-tonne space station? Usually with rockets! The ISS is fitted with thrusters (on the Russian Zvezda module) that can fire to nudge the orbit higher. Often, visiting spacecraft do the job: for example, a Russian Progress cargo ship docked to the station can fire its engines to give the ISS a gentle shove. In the past, Europe’s ATV cargo craft and even the Space Shuttle provided reboosts too.

During a reboost manoeuvre, the rockets fire in the direction of travel, which increases the ISS’s speed. That extra speed lifts the ISS to a higher orbit – just as faster = higher is the rule in orbital motion. The boost might add on the order of 1–2 m/s to the ISS’s velocity, which can raise the orbit by a few kilometres. These burns are usually done carefully and gradually – one reboost might be split over a couple of orbits (taking a few hours) to gently raise the altitude without stressing the structure.

If you were watching the altitude graph during a reboost, you’d see a sudden jump as the station’s height increases. Interestingly, after the boost, once in the new higher orbit, the orbital velocity at that altitude is actually a bit lower than before (since we learned higher orbits need slightly less speed). However, the act of boosting required a temporary speed increase. It’s a bit counter-intuitive: speed up to slow down! But what it really means is we gave the ISS more energy to climb, putting it in a higher orbit where it can afford to coast a little slower around Earth.

Ground controllers perform these boosts whenever needed to keep the ISS in its planned operational altitude range (generally around 400–420 km nowadays). The station’s orbit has even been allowed to drop lower at times (e.g. down to ~350 km) for specific reasons – for instance, during the Space Shuttle era, they sometimes let the altitude fall to make it easier for shuttles to reach with heavy payloads, then raised it back up after. Since the shuttles retired, the ISS has been kept higher on average (around 400 km) to reduce drag. Each year, several reboosts are done; keeping the ISS aloft actually uses a substantial amount of propellant – on the order of 7.5 tonnes of fuel per year dedicated just to maintaining altitude (that’s hundreds of millions of dollars worth of rocket fuel!). This is simply the cost of doing business for a large space station skimming the edge of the atmosphere.

Besides routine scheduled boosts, the ISS can also manoeuvre to avoid collisions with orbital debris if necessary. If radar tracking warns of a piece of space junk coming too close, the station can be given a small boost or even a slight dodge to change its path. These Debris Avoidance Manoeuvres are essentially small orbit adjustments – the principles are the same, though they’re usually only a kilometre or two change in altitude. Safety comes first: it’s better to burn a little fuel than risk a high-speed impact with a stray object.

Through all of this, a joint team on the ground (primarily in Moscow and Houston) monitors the ISS’s orbit and plans these manoeuvres. The astronauts on board strap in when a reboost is happening (as the station will accelerate and everyone literally feels a slight push). Otherwise, the process is automated. After each boost, the cycle of drag and decay begins anew – very slowly – until the next correction.

Quick Facts About the ISS Orbit

To round up our exploration of the ISS’s orbit, here are some quick-hit facts and figures:

• Typical Altitude: ~400 km above Earth on average. The ISS’s orbit is usually maintained between about 370 km and 420 km altitude. If it dips too low, atmospheric drag grows stronger; too high, and it’s harder to reach by spacecraft.
• Orbital Speed: ~27,600 km/h (about 17,100 mph). In more relatable terms, that’s 7.7 km/s – fast enough to cross the Atlantic from New York to London in about 10 minutes if it were travelling in a straight line! This speed is what’s needed to “fall around” Earth at 400 km up, balancing gravity’s pull.
• Orbit Duration: ~92–93 minutes per orbit. The ISS sees a sunrise or sunset roughly every 45 minutes! Over a 24-hour day it circles Earth about 15 to 16 times.
• Inclination: 51.6°. This is the tilt of the orbit relative to Earth’s equator. It means the ISS’s ground track ranges between 51.6° N and 51.6° S latitude. Thanks to this inclination, the ISS flies over the majority of Earth’s inhabited regions at some point (about 90% of humanity lives below 51.6° latitude).
• Ground Track: Appears as a wave-curving line on maps. The curvature is due to the inclined orbit and Earth’s rotation, causing the track to shift westward each orbit. If you watch for a full day, the pattern repeats roughly every 3 days (with slight shifts), as the orbit ground tracks eventually realign over the same areas.
• Atmospheric Drag: Even at 400 km up, a thin trace of atmosphere causes the ISS to lose altitude (≈1–2 km per month) if no action is taken. Drag also gradually slows the ISS’s orbital speed, which is why the station needs help to stay fast enough for orbit.
• Reboosts: Performed every so often using onboard thrusters or visiting spacecraft (like Russia’s Progress freighter). Reboosts increase the ISS’s speed by a tiny amount, raising it to a higher orbit. These manoeuvres happen multiple times a year and in total require several tonnes of propellant annually.
• Staying in Orbit: The ISS remains aloft because its centripetal force (due to moving along a curved path) is equal to the pull of Earth’s gravity at that altitude. In essence, it’s perpetually falling towards Earth but moving forward so fast that it keeps missing it! This balance is described by the equation (v = \sqrt{GM/R}) which dictates the required speed for a stable orbit at radius R.

In conclusion: the International Space Station’s orbit is a dynamic balancing act. The next time you check out the live ISS tracker on issinfo.net, you’ll understand why the map shows a looping path and why those altitude and speed graphs dance up and down. It’s all driven by orbital mechanics – from the ISS’s tilted trajectory around our rotating Earth, to the trade-off between height and velocity in an ellipse, and the never-ending battle with the thin atmosphere. The ISS literally rides the line between Earth and space, and staying up means adjusting to nature’s forces. It’s a brilliant feat of engineering and physics that this orbital home stays put, circling our planet every 1.5 hours.

So keep an eye on those little fluctuations – they tell the story of a space station in motion, constantly falling, constantly being nudged, and endlessly soaring around Earth. Happy ISS tracking!

Sources: NASA, ESA, Wikipedia, Universe Today, and live data from issinfo.net.