ORBIT
A FIELD GUIDE TO ISS & MIR
Ten chapters on why spacecraft don't fall, how two stations were built four hundred kilometres overhead, and what it costs to bring one home. Scroll to begin the mission.
BEGIN — CHAPTER 01 →Falling and missing the ground
Every satellite, station, and spacecraft in orbit is falling. Gravity never releases its grip. What keeps them from hitting the ground is not an absence of gravity — it is sideways speed.
Isaac Newton described it with a thought experiment: fire a cannonball fast enough, horizontally, from a tall enough mountain, and the curve of its fall will match the curve of the Earth falling away beneath it. It never lands. It orbits.
Adjust the launch speed and watch the same cannonball find three different fates. The curves are true orbits around a point-mass Earth, fired from the top of an imaginary mountain 100 km high; as in Newton's drawing, air is ignored. Up there a circular orbit takes 7.84 km/s and escape 11.09 km/s, a little less than the surface values in the next chapter.
Three thresholds, three destinies
Soviet and Russian spaceflight literature names three critical speeds. Each is a threshold: to orbit Earth, to escape it, and to escape the Sun.
The speed of a circular orbit skimming the surface, if there were no air. Below it, an object falls back to Earth. Higher up it is less: 7.66 km/s at the ISS's 415 km.
Escape velocity — the speed needed to break free of Earth's gravity entirely and never return, coasting outward with no more fuel spent.
Measured relative to Earth, for a launch near the surface aimed along Earth's orbital motion, this is what's needed to leave the Solar System entirely. At Earth's distance, escaping the Sun takes 42.1 km/s; Earth's own 29.8 km/s orbital speed supplies most of it, so a craft must leave Earth's gravity with 12.3 km/s to spare — and that takes √(12.3² + 11.2²) ≈ 16.6 km/s at launch. Pioneer and Voyager reached solar escape only with a gravity assist from Jupiter (see SLINGSHOT).
Humanity's largest orbital laboratory
Assembly began in 1998, when the Russian module Zarya and the American node Unity were joined in orbit. More modules, trusses and solar arrays followed for more than a decade, built and run by a partnership of five space agencies from 15 countries [1]. It has been crewed without a break since November 2000 [1], and it circles about 415 km up, in an orbit tilted 51.6° to the equator [2].
FIG. 3.1 — SCHEMATIC MODULE MAP (NOT TO SCALE). SELECT A MODULE.
A day with no up or down
The station runs on Greenwich Mean Time, regardless of the roughly sixteen sunrises its crew sees each day. Select a phase of the schedule.
Sixteen sunrises a day
At about 415 km [2] the ISS laps the planet once every 93 minutes while Earth turns underneath it, so its ground track shifts west with every pass. The track never goes beyond 51.6° north or south, but the station can be seen from farther than that: from 415 km up it clears 10° above the horizon for anyone within about 1,400 km of the point below it. Pick a city to count the passes.
Model: circular orbit at 415 km and 51.6° [2], spherical Earth; passes counted when the station climbs above 10° elevation, day or night, averaged over every starting position of the orbit. Only passes with the station sunlit against a dark sky can be seen by eye.
Why the crew floats
A common answer is that there is no gravity up there. There is: at the station's height Earth still pulls with about 90 percent of its surface strength [4]. The crew floats because the station and everything in it are falling together around the Earth — the cannonball of Chapter 1. Slide the altitude and watch what changes, and what doesn't.
Model: circular orbit around a spherical Earth (R = 6,378 km, GM = 398,600 km³/s²), no air. Laps per day counts orbits in 24 hours; the number of sunrises is almost the same. A person standing on a tower this tall would weigh the share shown; aboard the station a scale reads zero because it falls with them.
Why a laboratory in freefall
Microgravity removes one variable — weight — that every experiment on Earth has to work around. That makes the ISS a testbed for medicine, materials, and the technology that will carry humans further.
The legendary predecessor
Mir — Russian for both 'peace' and 'world' — orbited for fifteen years, three times the five it was designed for [9]. It hosted 28 long-duration expeditions and, in all, 125 cosmonauts and astronauts from 12 countries [10]. Scrub through the timeline.
A station built one module at a time
Mir pioneered the principle every later station would copy: a small core, expanded module by module — five research modules added between 1987 and 1996 [11], most of them docked around a node at the core's front end. Select one.
A controlled ending
By the autumn of 2000 every effort to find money to keep Mir flying had failed, and RKK Energia and the Russian government decided to bring it down [11][12]. Rather than let 135 tonnes of hardware [11] re-enter wherever drag happened to take it, controllers planned a deliberate descent into the South Pacific. Step through the sequence.
The braking burns, by the numbers
A braking burn at the high point of an orbit lowers the opposite, low point. The calculator finds the smallest burn that does it — fired exactly at the high point, all at once — and compares it with what Mir's Progress actually flew [11]. The real burns cost more because they also had to place the low point, and the debris, over the target zone.
Model: two-body Kepler orbits around a spherical Earth, altitudes above 6,378 km, one instantaneous burn along the direction of motion at the high point. Mir's orbits and burn sizes are ESA's reconstruction of 23 March 2001 [11]; the 110–100 km breakup height is from the pre-entry plan [12].
The ISS will end the same way
Status as of 10 October 2026: the United States, Japan, Canada and ESA's member states have committed to operating the station through 2030 [13]. Russia, committed only through 2028 until then, agreed in July 2026 talks with NASA to support operations through 2030 as well; NASA confirmed the account [14]. Congress has directed NASA to keep the station running through at least 30 September 2030 [2]. In June 2024 NASA chose SpaceX to build the US Deorbit Vehicle, a contract worth up to $843 million, to push the station down into a remote stretch of ocean [13].
Why not park it higher instead? At about 415 km the station would fall within one to two years without reboosts [2]. NASA estimated that raising it to an orbit lasting 100 years would take 120–140 m/s, and to one lasting more than 10,000 years 760 m/s, against 57 m/s for a controlled deorbit [15]. Higher orbits are also more crowded: the expected time between penetrating debris impacts drops from about 51 years at 415 km to under 4 years at 800 km [2] (see DEBRIS).
Model: two-burn Hohmann transfer between circular orbits starting at 415 km, no drag. NASA does not state the altitudes behind its 100-year and 10,000-year figures; matching them to 633–670 km and 2,000 km is this model's inference.
From Salyut to whatever comes next
Every station is an argument for the next one. Fifty-five years of continuous, overlapping design lineage sit between the first crewed station and the ones now being planned for low orbit, the Moon and beyond. For the stations in between, see SALUTE and PALACE.
Sources cited throughout this guide
Bracketed numbers in the text above correspond to the sources below.