NASA and its industry partners are aiming to send humans to Mars as early as the 2030s. After the iconic Apollo missions to the Moon, landing humans on our neighboring world is the next giant leap, and we’ve been preparing for that moment right here, in Earth orbit.

This year marks the 20th anniversary of the ISS. For two decades, there’s been a giant space laboratory speeding around Earth. Bigger than a Boeing 747 airplane, the International Space Station (ISS) is the largest human satellite ever built.

Since the launch of its first two modules in 1998, the ISS has been significantly expanded in size and function. It now boasts more than a dozen unique modules built by a total of five collaborating space agencies. All this time the ISS has sustained human presence, hosting over 200 astronauts in total. Rockets have been launched every few months to send supplies in the form of food, equipment and tools, and experiments. With the ISS, we have successfully built and operated a long-term human habitat in space. 

Most notably, the ISS provides a stable environment for human researchers to carry out long-term space experiments. This ability has been critical for researchers looking to solve the problems of zero gravity habitation. The ISS is teaching us how to work together, do science, and simply survive on timescales comparable to future missions to Mars.

With current technology, a trip to Mars could take at least 6 months. Future explorers are in for a lengthy interplanetary cruise in a zero gravity environment. By having astronauts stay on the ISS for months and even a year, researchers are able to study the effects of prolonged space exposure and reduced gravity environments on the human body, and devise methods to keep astronauts healthy.

A 2006 study found that long term exposure to microgravity environments caused considerable bone and muscle loss in ISS astronauts. This means future astronauts would have trouble moving after landing on Mars and would be prone to fractures.

And humans sweat more during exercise when in orbit — this may seem like a minor thing, but it’s not. The lack of convection in space means that heat envelops astronauts’ bodies like an aura, and sweat stays where it originated due to lack of gravity. Such a warm and humid environment is an ideal entry point for bacterial infection.

To remedy this, the Europe-led SpaceTex experiment produces special fiber clothing that astronauts wear during exercise. Its materials dissipate heat effectively and reduce microbiological contamination while keeping astronauts comfy. All necessary for a long trip to Mars and beyond.

A more recent study of 11 ISS astronauts on six-month missions revealed that more than half of them experienced stagnated blood flow. One astronaut developed a blood clot and needed to be treated with anticoagulants for the rest of the spaceflight.

To prevent blood clots, the ISS has a “Chibis suit” in one of the Russian modules. These vacuum-sealed pants lower pressure below the waist to ensure adequate blood flow in the upper parts of the body. The Chibis suit is a promising solution for preventing blood clots in astronauts on long duration interplanetary flights.

On the Moon and Mars, astronauts’ lungs may become easily irritated or inflamed by dust particles. The reduced gravity on such celestial bodies makes floating dust a real threat for humans. To solve this problem, The European Space Agency (ESA) has been leading a study to monitor astronauts’ lungs for more than 10 years.

The ISS astronauts breathe into a specially developed instrument that measures nitric oxide levels, an indicator of lung inflammation. These measurements are taken at reduced pressures in an airlock to simulate conditions in future Moon and Mars habitats. This information is helping researchers devise ways to ensure the health and safety of astronauts on landing missions beyond Earth’s orbit.

The ISS is also a lab for studying long-term changes due to space exposure on plants and microorganisms. Healthy habitats for plants and microbes on other worlds are an assumed requirement, because they are crucial for food and ecosystems.

Most of us were amused when Mark Whatney, in the film The Martian, grew potatoes on Mars with his own feces as fertilizer. The ISS hasn’t gone exactly that route with their fertilizers, just yet, but learning to grow our own food on the ISS and Mars is a serious necessity. 

Enter Tomatosphere, a joint Canadian and US experiment. Since 2001, millions of tomato seeds have been flown to the ISS over multiple missions. These seeds are exposed to space radiation for several months and then brought back and planted on Earth to see if — and how — they grow differently from regular tomatoes. Space-exposed tomato seeds show clear differences in how they germinate, like how fast they grow to visual changes in the size and color of their leaves. But other than that, they are just as easily consumable.

A mission to Mars can’t carry all the food required for the trip, or count on timely supplies. The crew will need to produce their own food to survive. By teaching us how to grow plants effectively in a radiation-filled environment, Tomatosphere and other such ISS experiments are enabling an interplanetary future.

ESA leads a series of experiments on the ISS under the label EXPOSE. In each experiment, microorganisms like bacteria, lichens, fungi, and algae are placed in containers outside the ISS, where they are exposed to the harsh conditions of space.

The longest of these experiments exposed microbes for 533 days. In the end, some microbes of every species survived the prolonged space exposure. To everyone’s surprise, some lichens survived too, despite being more complex life forms.

Thanks to the EXPOSE experiments onboard ISS, we now know that many microorganisms can in fact survive under Martian conditions. The result is promising for building initial habitats on Mars. It’s also particularly interesting, because some of these microbes might be used to bioengineer Mars, terraforming it into a more habitable planet for us — much like cyanobacteria did to Earth 2.5 billion years ago.

Another Martian challenge the ISS is helping to solve is computing. You see, space computers are not as capable as ones on Earth, as they need to be hardened for reliability in the harsh space environment. Engineers work around this by offloading intense computing tasks to computers on Earth, much like the Apollo missions did. This works well for human missions to the Moon where the communication lag is just a few seconds. But a 10-20 minute signal lag for Mars is simply not feasible.

So engineers have been testing powerful supercomputers on the ISS, built to withstand harsh space radiation. In 2017, a SpaceX cargo supply mission to the ISS brought one such supercomputer by NASA dubbed Spaceborne. During its mission, Spaceborne successfully performed over a trillion calculations per second for 207 days straight without requiring a single reset.

Another such computer, SG-100, was tested for two years with zero resets. SG-100 processes data 12 times faster than a standard ISS computer, and costs a quarter the price. These tests are proving the capabilities of powerful computers for interplanetary missions, removing the need to downlink data to Earth entirely.

“In low-Earth orbit, we are able to shrink the data down to a minimal set before you send it down,” said Trent Martin, primary investigator for SG-100. “In deep space, it allows you to actually do your processing there. Now, if we are doing the processing on Mars, rather than taking the time to send it back to Earth and then back to Mars—that is 16 minutes that’s just eliminated.”

One of the most critical problems for sustained human presence in the solar system is fuel. For instance, a trip to Mars and back would require at least 10-20 times more fuel than Apollo 11, depending on mission specifics. Dragging all that fuel out of Earth’s deep gravity well wouldn’t just be inefficient — but incredibly expensive and non-sustainable.

To solve this problem, NASA has been testing advanced cryogenic fuel storage and transfer technology on the ISS. With this technology, spacecraft landing on the Moon and Mars can be refueled to either return to Earth, or venture farther out. Note that this is precisely what Elon Musk plans to do with Starship, as well.

NASA launched three experiments to the ISS under the banner of Robotic Refueling Missions. The first two of these demonstrated preparation tasks that lead up to cryogenic fuel transfer in orbit, such as removing caps and valves, installing coolant line adapters, etc.

One major issue with cryogenic fuel however is that it quickly boils off to space, making it impossible to use for Mars missions. For this, the third of the Robotic Refueling Missions was launched in December 2018 to the ISS. Unlike traditional methods that use passive cooling, it used active cooling and advanced multilayer insulation to avoid boil off. In all, it demonstrated storage of cryogenic fuel for four months in orbit, a promising progress towards usability on interplanetary trips.

These are just some highlights from the hundreds of experiments at the ISS that have directly laid a foundation for living and working on Mars and beyond. What’s most striking about the ISS is that it’s been a spectacular example of global cooperation despite political conditions. In 2011, Alexey Krasnov, head of human space flight program for Russia's space agency, Roscosmos, wrote,

“When compared with partners acting separately, partners developing complementary abilities and resources could give us much more assurance of the success and safety of space exploration. The ISS is helping further advance near-Earth space exploration and realization of prospective programs of research and exploration of the Solar system, including the Moon and Mars."