Orbital laboratory

The International Space Station

The ISS is the largest object humans have ever put in orbit and, on a good pass, the brightest thing in the night sky after the Moon. It circles the planet roughly sixteen times a day.

The first module, Zarya, launched on 20 November 1998. Since 2 November 2000 the station has never been empty — a continuous human presence in low Earth orbit for a quarter of a century. It is a joint project of NASA, Roscosmos, ESA, JAXA and the Canadian Space Agency, and it has been continuously crewed by rotating expeditions of typically seven people.

Why it moves so fast

Orbital speed is set by altitude. At around 410 km the station needs 7.66 kilometres per second — about 27,600 km/h — to stay in free fall around Earth rather than dropping back into the atmosphere. That works out to one full orbit in roughly 93 minutes, so the crew sees about 15.5 sunrises and sunsets every day.

The 51.64° inclination is the reason the station never flies directly over the poles, and why it never appears high in the sky from latitudes far above 51.6° or far below it. From northern Europe or Canada it is always a relatively low pass; from the equator it can pass almost overhead.

Seeing it with the naked eye

You do not need a telescope. The ISS looks like an unusually bright, steady star that moves smoothly across the sky in a few minutes — no blinking strobe, no engine noise, no contrail. It is sunlight reflecting off the solar arrays and the pressurised modules.

That reflection is also why the station is only visible during a narrow window. The sky has to be dark enough for a bright object to stand out, but the station itself still has to be in sunlight. In practice that means the first one to two hours after sunset, or the last one to two hours before sunrise. In the middle of the night the station is in Earth's shadow and simply disappears mid-pass.

How big is it?

The integrated truss structure spans about 109 metres, the pressurised modules run roughly 73 metres end to end, and the solar arrays cover around 2,500 square metres. Mass is about 420 tonnes. Despite that size, at 400 km altitude it subtends only about 40–60 arcseconds — the size of a small crater on the Moon — so it stays a point of light to the unaided eye.

Track it live

The tracker on the front page renders the station's current sub-point onto a globe using the latest CelesTrak two-line elements. Position, altitude, velocity and orbit period update continuously. For a guide to reading a predicted pass, see how to read a pass.