Frequently asked questions
Everything you want to know about the ISS and beyond
From how fast the Space Station travels to when you can see it from your backyard — answers to the most common questions about the ISS, Hubble, Starlink, and Voyager.
The International Space Station
How fast does the ISS travel?
The ISS orbits Earth at approximately 28,000 km/h (17,500 mph). At this speed it completes one full orbit every 90 minutes and travels roughly the distance from Earth to the Moon every 14 hours.
How high does the ISS orbit?
The ISS orbits at an average altitude of around 408 km (253 miles) above Earth's surface — well within low Earth orbit (LEO). The exact altitude varies slightly due to atmospheric drag and periodic re-boost manoeuvres.
How many times does the ISS orbit Earth per day?
The ISS completes approximately 15.5 orbits per day. Because Earth rotates beneath it, each orbit passes over a different strip of ground, allowing the station to cover much of Earth's surface over the course of several days.
How many astronauts are on the ISS right now?
The ISS is typically crewed by 7 astronauts and cosmonauts, though the number varies during crew rotation periods when a Soyuz or Crew Dragon vehicle is docked. The station has been continuously crewed since November 2, 2000.
How long has the ISS been in space?
The first module of the ISS — Zarya — was launched on November 20, 1998. The station has been permanently crewed since November 2, 2000, making it home to humans in space for over 24 years continuously.
How big is the ISS?
The ISS spans approximately 109 metres (357 feet) from end to end — roughly the size of a football field. It has a pressurised volume of about 916 cubic metres, equivalent to a six-bedroom house, and a mass of around 420,000 kg.
What is the ISS used for?
The ISS serves as a microgravity laboratory where astronauts conduct thousands of scientific experiments covering biology, physics, astronomy, and materials science. It also serves as a technology testbed and a platform for Earth observation.
When will the ISS be decommissioned?
NASA plans to deorbit the ISS in January 2031 by using a series of deorbit burns to guide it into the Pacific Ocean. Commercial space stations are expected to replace it in the late 2020s and 2030s.
Seeing the ISS
Can you see the ISS with the naked eye?
Yes. The ISS is one of the brightest objects in the night sky and is easily visible to the naked eye as a fast-moving, steady white light — it does not blink like a plane. It appears brightest when sunlight reflects off its solar panels at the right angle.
How long is the ISS visible for during a pass?
A typical ISS pass lasts between 1 and 6 minutes depending on how high it arcs above the horizon. Passes that go nearly overhead last the longest. The station travels quickly across the sky — from horizon to horizon in about 4–5 minutes for a high pass.
When is the best time to see the ISS?
The ISS is best seen during twilight — roughly 30 to 90 minutes after sunset or before sunrise. At these times the sky is dark enough to see the station, but the ISS is still in sunlight. It is invisible in full darkness because it receives no sunlight to reflect.
How bright is the ISS?
At its brightest, the ISS reaches a magnitude of about −5.9, making it the third brightest natural object in the sky after the Sun and Moon. Typical passes reach magnitude −3 to −4, easily outshining Venus and Jupiter.
How can I find out when the ISS will pass over me?
Use the ISS Pass Calculator on this site at whereisiss.com/passes — click 'Use my location' to instantly see upcoming passes for the next 7 days, including time, duration, and maximum elevation. Pass times shift daily as the ISS orbit precesses, so check on the day you want to observe.
Tracking & Data
What is a TLE?
A Two-Line Element set (TLE) is a standardised format for describing a satellite's orbit using two lines of 69 characters each. It encodes the orbital inclination, eccentricity, altitude, and other parameters at a specific point in time. TLEs are published by CelesTrak and Space-Track.org and are used by this tracker to compute the ISS position.
How accurate is the ISS tracker?
For a fresh TLE (less than a day old) and using the SGP4 propagation model, the position error is typically less than 1–2 km. Older TLEs produce larger errors as orbital drag accumulates. WhereIsISS also uses live GPS-corrected data from the WhereTheISS API when available, reducing the error further.
What is SGP4?
SGP4 (Simplified General Perturbations 4) is the standard mathematical model used to predict the position of satellites from TLE data. It accounts for atmospheric drag, Earth's oblateness, and other perturbations. It is the same model used in most civilian satellite tracking software.
Why does the ISS position on this site differ slightly from another tracker?
Small differences between trackers are normal. They may be using TLEs from different epochs (snapshots in time), different propagation implementations, or different update frequencies. For casual observation and education, the differences are insignificant.
Hubble Space Telescope
How old is the Hubble Space Telescope?
Hubble was launched on April 24, 1990, aboard Space Shuttle Discovery. It has been in continuous operation for over 34 years, far exceeding its original 15-year design life.
How high does Hubble orbit?
Hubble orbits at approximately 538 km (334 miles) altitude — slightly higher than the ISS. Its orbital period is about 95 minutes.
What are Hubble's biggest discoveries?
Hubble's key contributions include determining the age of the universe (~13.8 billion years), confirming that the universe's expansion is accelerating due to dark energy, capturing the Hubble Deep Field (showing ~3,000 previously unknown galaxies), and pioneering the study of exoplanet atmospheres.
Starlink
What is Starlink?
Starlink is SpaceX's satellite internet constellation, consisting of thousands of small satellites in low Earth orbit that work together to provide broadband connectivity worldwide — including in remote areas where ground-based internet is unavailable.
Can I see Starlink satellites?
Yes. Starlink satellites are particularly visible shortly after launch when they travel in a bright train formation. After reaching operational altitude and deploying their sun visors, individual satellites become dimmer but are still detectable with binoculars during twilight passes.
How many Starlink satellites are there?
SpaceX has launched over 6,000 Starlink satellites as of 2024, making it the largest satellite constellation in history by a wide margin. The planned first-generation constellation calls for up to 12,000 satellites, with applications filed for up to 42,000.
Voyager
Where is Voyager 1 right now?
Voyager 1 is currently in interstellar space, more than 160 AU from the Sun (over 24 billion kilometres). It is the most distant human-made object ever built. You can see its live position on our Voyager tracker.
How long does a signal take to reach Voyager 1?
At the speed of light, a radio signal takes over 22 hours to travel one-way from Earth to Voyager 1. A round-trip communication therefore takes more than 44 hours.
Is Voyager 1 still working?
Yes, as of 2024 Voyager 1 is still transmitting science data back to Earth, though NASA has had to carefully manage its dwindling power supply. Its Radioisotope Thermoelectric Generators (RTGs) are expected to power at least some instruments until the early 2030s.