The Standard Six-Month Rotation
If you ask NASA or Roscosmos how long a crew member stays on the International Space Station, the answer is almost always the same: about six months. A typical ISS rotation, formally called an "increment," lasts roughly 180 days. The crew launches aboard a SpaceX Crew Dragon or a Russian Soyuz capsule, spends their half-year aboard the station, and comes home when the next crew arrives to take over.
This six-month standard has been the norm since around 2010, when the station transitioned from three-person to six-person crews. Before that, early ISS expeditions varied more widely, with some lasting four months and others stretching past seven. But the roughly 180-day increment settled into place as the default, and it has held steady for over 15 years now.
Why six months specifically? It comes down to a balance of three factors. First, crew health: six months is long enough to get meaningful science done but short enough that the physical effects of microgravity (muscle loss, bone density decline, fluid shifts) remain manageable with the exercise countermeasures available on the station. Second, logistics: the ISS needs regular resupply of food, water, and experiments, and the rotation schedule syncs with cargo mission planning. Third, vehicle certification: both Crew Dragon and Soyuz are rated for roughly 200 days docked in orbit, which puts a hard ceiling on how long a crew can stay without a fresh vehicle.
Extended Missions: Going Beyond Six Months
Six months is the standard, but some astronauts have stayed considerably longer. These extended missions are rare and they generate valuable medical data that no six-month stay can provide.
The most famous extended ISS mission is NASA's Year in Space study. In 2015 and 2016, astronaut Scott Kelly and cosmonaut Mikhail Kornienko spent 340 consecutive days aboard the station. The mission was designed specifically to study the effects of long-duration spaceflight on the human body. Kelly's identical twin brother Mark, a retired astronaut himself, served as a ground-based control subject. The resulting Twins Study revealed changes in gene expression, immune function, and cognitive performance that took months to normalize after landing.
Then there is the unplanned record. In 2023, NASA astronaut Frank Rubio arrived at the ISS expecting a standard six-month rotation. Shortly after, his Soyuz MS-22 capsule was struck by micrometeorite debris, which damaged its cooling system and rendered it unsafe for reentry. A replacement Soyuz had to be launched uncrewed, and Rubio ended up spending 371 days in space, the longest single spaceflight by an American. His experience was a stark reminder that spaceflight timelines do not always go according to plan.
These extended stays provide data points that are directly relevant to future deep-space missions. A six-month dataset only tells you so much. The 340-day and 371-day marks start to get closer to what a crew might face on a transit to Mars.
The All-Time Records
The record books for long-duration spaceflight are dominated by Russian cosmonauts, largely because the Soviet and Russian space programs have prioritized endurance missions since the Salyut and Mir station eras.
The single-mission record belongs to Valeri Polyakov, who spent 437 days aboard the Mir space station from January 1994 to March 1995. That is nearly 14 and a half months in continuous microgravity. Polyakov, a physician by training, volunteered for the mission specifically to prove that the human body could tolerate a Mars-length spaceflight. He reportedly walked unassisted from his Soyuz capsule after landing, a point of pride for the Russian space program.
For cumulative time in space across multiple flights, Gennady Padalka holds the record at 878 days, accumulated over five separate missions between 1998 and 2015. On the NASA side, Peggy Whitson holds the American cumulative record at 665 days across three flights.
Here are the five longest single spaceflights in history:
| Rank | Astronaut | Days | Station | Year(s) |
|---|---|---|---|---|
| 1 | Valeri Polyakov | 437 | Mir | 1994 to 1995 |
| 2 | Sergei Avdeyev | 380 | Mir | 1998 to 1999 |
| 3 | Frank Rubio | 371 | ISS | 2022 to 2023 |
| 4 | Vladimir Titov | 365 | Mir | 1987 to 1988 |
| 5 | Musa Manarov | 365 | Mir | 1987 to 1988 |
Four of the five longest flights took place on Mir. That station, which operated from 1986 to 2001, was the testing ground for nearly everything we know about keeping humans alive and functional in space for a year or more. The ISS inherited that knowledge, and Frank Rubio's unplanned long stay added a modern data point to the list.
Shenzhou Mission Lengths
China's crewed spaceflight program has followed a similar trajectory to the ISS partnership, gradually extending mission durations as its station infrastructure matured.
Early Shenzhou missions to China's prototype stations (Tiangong-1 and Tiangong-2) were short. Shenzhou-9 in 2012 lasted about 13 days. Shenzhou-10 in 2013 was 15 days. Shenzhou-11 in 2016 pushed to 33 days, which was a significant step up and gave China its first month-long crewed mission.
Once the full Tiangong space station became operational in late 2022, mission lengths jumped to match the ISS standard. Shenzhou-15, Shenzhou-16, and subsequent crews have each spent roughly six months aboard, with three taikonauts per mission. The handover pattern mirrors the ISS approach: the incoming Shenzhou docks while the outgoing crew is still aboard, the two teams overlap for about a week, and then the departing crew returns to Earth.
China has not yet attempted anything close to a year-long mission on Tiangong, but the station's life support systems are designed to support continuous habitation. As the program matures, extended-duration stays are a possibility, especially if China begins planning for its own deep-space crewed missions.
Track active missions: Want to follow current crew missions as they happen? Skytrail shows every active crew member, which station they are on, and when you can see that station pass overhead from your location.
Private and Short-Duration Missions
Not every spaceflight lasts six months. The rise of commercial crew missions has created a new category of short-duration flights that would have seemed unlikely a decade ago.
Axiom Space has sent several private crews to the ISS, with each mission lasting roughly 10 to 14 days. These Axiom missions (Ax-1, Ax-2, Ax-3, and beyond) dock a SpaceX Crew Dragon at the station and carry a mix of private astronauts, researchers, and international partner crew members. The stays are brief, but the crews participate in real science experiments and follow the same safety protocols as long-duration residents.
Even shorter are the free-flying missions that never dock with a station at all. SpaceX's Inspiration4 mission in September 2021 spent three days orbiting Earth in a Crew Dragon capsule, making it the first all-civilian orbital flight. The Polaris Dawn mission in 2024 lasted five days and included the first commercial spacewalk. These flights are self-contained: the crew lives inside the capsule itself, with no station to visit.
Space tourism on Soyuz has a longer history than most people realize. Beginning with Dennis Tito in 2001, several private citizens flew to the ISS on Soyuz spacecraft, with stays typically lasting about 10 days. The tourist would launch with a professional crew, spend a week or so on the station, and return with the outgoing crew. These flights paused for several years but set the precedent for the commercial missions that followed.
What Happens to the Body
The reason mission length matters so much comes down to biology. The human body did not evolve for weightlessness, and the longer you stay in microgravity, the more it adapts in ways that are not helpful back on Earth.
Muscle atrophy begins almost immediately. Without the constant load of gravity, muscles that support posture and movement start to waste away. ISS crew members exercise about two hours per day using a resistive exercise device (called ARED) and a treadmill to slow this process, but they still lose muscle mass over a six-month stay.
Bone density loss runs at roughly 1% to 1.5% per month in weight-bearing bones like the hip and spine. That rate is comparable to what osteoporosis patients experience over a full year on Earth, compressed into just 30 days. Some of this bone loss is recoverable after landing, but studies show that not all of it comes back, even years later.
Fluid shifts cause some of the most visible changes. Without gravity pulling blood and other fluids toward the feet, they redistribute toward the head. Astronauts develop puffy faces and thinner legs within the first few days, a phenomenon sometimes called "bird legs." More concerning, the increased intracranial pressure can cause vision changes (a condition called Spaceflight-Associated Neuro-ocular Syndrome, or SANS) that persist long after the mission ends. Some astronauts have returned needing new eyeglass prescriptions, and in a few cases the changes were permanent.
Radiation exposure is the long-term wildcard. The ISS orbits within the partial protection of Earth's magnetic field, but crew members still receive radiation doses roughly 10 times higher than what people experience on the ground. A six-month ISS mission delivers about 80 to 160 millisieverts, depending on solar activity. On a Mars transit, outside the magnetosphere, the dose would be significantly higher.
Recovery from a six-month mission typically takes several months. Astronauts often describe the first few days back on Earth as disorienting: their balance is off, their muscles are weaker than expected, and simple tasks like walking feel strange. Most crew members return to baseline fitness within three to six months, though bone density recovery can take longer. For a year-long stay, the recovery timeline stretches further. All of this data is informing plans for Mars missions, which at two to three years round-trip would push the human body well beyond anything we have tested on the ISS.
You can follow along with current missions using Skytrail, which shows you each crew member currently aboard the ISS and Tiangong along with how long they have been in space. I check it regularly to see mission day counts tick upward.