The International Space Station receives roughly 14 visiting vehicles per year, a mix of crewed flights and uncrewed cargo deliveries that keep the station supplied, staffed, and scientifically productive. In the 2023–2024 period, those vehicles delivered more than 40,000 pounds of cargo and supported over 400 scientific investigations conducted by 25 crew members.
How Often Crew Missions Launch
The station maintains a permanent crew of six to seven people, organized into overlapping “expeditions” that each last about six months. Crew rotations happen roughly every six months from both the American and Russian sides, which means a crewed spacecraft launches to the station about four times per year. Two of those are typically SpaceX Crew Dragon capsules carrying NASA and partner astronauts, and two are Russian Soyuz capsules carrying Russian cosmonauts and occasionally international crew members. Boeing’s Starliner has also carried crew to the station, though on a less regular schedule.
These crew flights overlap deliberately. When a new crew arrives, the outgoing crew stays for a handover period of about a week, transferring knowledge about ongoing experiments and station systems before returning to Earth. This staggered approach means the station is never left unstaffed, and fresh crew members always have experienced hands to show them around.
Cargo Missions Keep the Station Running
Cargo resupply flights make up the majority of visiting traffic. Between SpaceX Dragon cargo missions, Northrop Grumman Cygnus freighters, and Russian Progress supply ships, the station typically receives eight to ten uncrewed cargo deliveries each year. These flights carry food, water, spare parts, new experiments, crew clothing, and personal items. A snapshot from March 2026 shows five spacecraft parked at the station simultaneously: a SpaceX crew capsule, a Cygnus cargo craft, a Soyuz crew ship, and two Progress resupply vehicles.
Not all cargo ships serve the same purpose after they arrive. SpaceX Dragon capsules can return to Earth intact, which makes them essential for bringing back scientific samples, including materials that need to stay cold during the trip home. Cygnus and Progress vehicles, by contrast, burn up on reentry. The crew fills them with trash and waste before they undock, turning them into disposable garbage trucks. This distinction matters for science: if an experiment produces results that need to come back to a lab on Earth, it has to wait for a Dragon flight.
Private Astronaut Missions Are a Newer Addition
Starting in 2022, a new category of visitor began arriving at the station. Axiom Space has now flown four private astronaut missions to the ISS, with a fifth targeted for no earlier than January 2027 and a sixth in planning. These missions typically last up to 14 days, far shorter than the six-month government crew rotations. They add one or two extra visits per year to the station’s schedule.
Private missions aren’t space tourism in the sightseeing sense. Axiom crews conduct their own scientific investigations in microgravity, test new technologies, and perform outreach activities. NASA views these flights as a way to expand access to low Earth orbit and build commercial capabilities the agency will rely on as it shifts focus toward the Moon and Mars. The agency even purchases specific services from Axiom during these visits, such as the ability to return cold scientific samples.
Why So Many Missions Are Necessary
The simplest reason is survival. The station orbits about 250 miles above Earth with no way to resupply itself. Everything the crew eats, drinks, and breathes has to come from the ground or be recycled from onboard systems. The station does generate its own oxygen by splitting water molecules and recycles a significant portion of its water supply, but those systems need replacement parts, filters, and chemical supplies that only arrive by cargo ship. Food cannot be produced on board at all.
Beyond basic life support, the station’s hardware is aging. The ISS has been continuously occupied since November 2000, and components wear out. Crew members and ground teams perform ongoing preventive maintenance, logging how systems slowly degrade over time. Some equipment that was originally considered unrepairable has been successfully fixed in orbit, but repairs require tools and spare parts delivered from Earth. Spacewalks to maintain the station’s exterior, including solar panels, radiators, and docking ports, depend on supplies and sometimes new hardware arriving on cargo flights.
The Station as a Research Laboratory
Science is the core justification for the station’s existence, and it drives much of the mission traffic. The ISS functions as a microgravity laboratory where researchers study phenomena impossible to observe on Earth. Over 400 investigations ran during the most recent reporting year alone, spanning fields like molecular crystal growth, combustion physics, fluid dynamics, cellular biology, and low-temperature physics.
A large portion of that research directly supports future deep space exploration. NASA uses the station to study how the human body responds to long-duration spaceflight, developing countermeasures for bone loss, muscle wasting, vision changes, and radiation exposure. The crew also tests life support technologies, autonomous docking systems, robotics, and spacewalk equipment that will eventually be needed for missions to the Moon and Mars. The station serves as a closed-loop life support testbed, where engineers can evaluate how well air and water recycling systems hold up over months and years of continuous use.
Other research has no connection to exploration at all. Basic science experiments take advantage of the near-weightless environment to study how fluids behave without gravity’s influence, how flames burn differently in microgravity, and how proteins form crystals more perfectly when they aren’t pulled downward. Some of this work has direct medical applications: recent expeditions have researched heart health and cancer treatments using the unique conditions aboard the station.
Who Sends Spacecraft to the Station
Five space agencies and two major private companies currently contribute visiting vehicles. On the American side, SpaceX launches both crew and cargo Dragons, while Northrop Grumman flies Cygnus cargo freighters. Russia’s space agency Roscosmos sends Soyuz crew capsules and Progress cargo ships. Boeing has contributed crew flights with its Starliner capsule.
The international roster was once even broader. The European Space Agency flew its Automated Transfer Vehicle five times before retiring the program, and Japan’s space agency launched its Kounotori (“White Stork”) transfer vehicle nine times. Those programs have ended, but both agencies still send experiments and crew members aboard other partners’ spacecraft.
All told, the station sees a new arrival roughly once a month, with busier stretches when crew handovers and cargo deliveries cluster together. That pace has held remarkably steady for years and will continue through the station’s planned operational life, currently extended into 2030.