Vacation to Mars: don’t forget to pack your immune system

Morgan Greenewood
Jun 26
7 min read

Writer: Morgan Greenewood

Editors: Sarah Brockway, Jackie Nguyen, Kiana Schulze, and Michaela Murphy

Illustrator: Jackie Nguyen

Where are you going for summer vacation this year? Somewhere warm? Somewhere tropical? What about Mars?

As commercial spaceflight becomes more common and federal agencies like the National Aeronautics and Space Administration (NASA) push toward long-duration missions, what once felt like science fiction is edging towards possibility. If a trip to Mars ever makes it onto a booking website, there is one thing you won’t see listed on the packing list: your immune system. And yet, it may be the most important thing you bring.

In reality, a trip to Mars wouldn’t quite fit within a typical summer vacation window. On average, Mars sits 140 million miles from Earth. To put that in perspective, if you board a commercial airplane bound for Mars, you’d arrive in about 70-80 years – assuming you found a nonstop flight. Even with our current spacecrafts, astronauts would spend seven to nine months traveling just one way, and they couldn’t just come back whenever they pleased. Then they must spend roughly another 18 months on Mars waiting for the planets to realign before enduring the return journey. That is nearly three whole years away from Earth – the longest summer vacation trip ever.

To prepare for this long expedition, you will need to pack 1,095 days’ worth of food, water, oxygen, tools, spacecraft parts, clothes, medicine, and everything else needed to survive. There is no pharmacy if you forget your toothpaste, no supply ship to bring extra batteries, and no roadside assistance if your engine breaks down. Everything must be anticipated – the engineering challenges alone are enormous.

But there remains another challenge: what space does to the human body.

The spaceflight exposome

Upon leaving Earth’s protective atmosphere, our bodies enter an environment unlike anything we experience on Earth. Scientists refer to this collection of new stressors as the “spaceflight exposome”. These stressors include microgravity, radiation, psychological strain, and changes in the microbes that live on and inside us. These factors impact astronauts physically and mentally. As we work toward making space travel more accessible, it is important that we understand the specifics of what happens to the human body during spaceflight, and how we can harness space medicines to combat the harmful effects that occur. So, let’s dive into each of these exposome components and how they affect human health.

Microgravity: When cells lose their bearings

On Earth, gravity is constant. With every step you take, gravity pulls you toward the ground. Your bones, muscles, and even individual cells have adapted to that steady force. As you get farther away from Earth, such as during your nonstop flight to Mars, gravity’s pull weakens. In space, astronauts experience microgravity, which can lead to a feeling of weightlessness. Think of the brief floating sensation you have during a huge drop on a roller coaster – astronauts feel a constant version of that in orbit. But, while this floating may seem fun, our cells get a little confused in the absence of gravity’s constant force. Microgravity alters their internal scaffolding, which not only helps cells hold their shape, but also acts as the cell’s railway system, allowing for transport of cellular cargo and messages throughout the cell. So, when this complex cellular skeleton and communication network get disrupted, it wreaks havoc on cellular function.

One group of cells that are particularly altered during spaceflight is immune cells. Our immune cells have a broad range of functions, but two of the most important are protecting our bodies against foreign invaders (such as harmful bacteria, viruses, and parasites) and eliminating dead or cancerous cells. These functions are crucial to maintain human health. Researchers have determined that immune responses can become dysregulated in space, reducing their readiness to respond to threats and leaving astronauts in a vulnerable immune state.

Many studies suggest that there are significant changes to the immune system throughout the duration of spaceflight. Eventually immune cells succumb to the stressors of space and their function declines. But due to the complexities of the immune system, the specific effects are still being determined. Gravity, it turns out, is something our immune system quietly depends on.

Our immune system relies on the cardiovascular system to travel throughout the body, and this system is also affected by microgravity. Think about the lightheaded feeling you get when you stand up too quickly. This occurs because your cardiovascular system needs a moment to adjust, temporarily reducing blood flow to the brain. Astronauts experience a similar phenomenon when they return to Earth's gravity after prolonged spaceflight. During their time in microgravity, the cardiovascular system no longer has to work against gravity to circulate blood. As a result, upon returning to Earth, the body must readapt to these demands. This cardiovascular deconditioning can leave astronauts feeling lightheaded when standing and was one of the earliest symptoms reported during the first decades of human spaceflight. Because immune cells travel through the bloodstream, these changes in circulation can influence how immune cells are distributed throughout the body during and after spaceflight.

Space agencies have been researching and implementing ways to combat microgravity using treadmills with harnesses and specialized resistance equipment to mimic gravity-like forces while astronauts are in space. But these systems are large and resource-intensive. For deep space missions to Mars, engineers and scientists are still working to design more efficient solutions. And while these tools help maintain muscle mass and bone strength through exercise, whether they also lessen the effects of microgravity on our cells is still unknown.

Radiation: the invisible threat

In deep space, radiation isn’t just a background risk – it is one of the greatest biological challenges astronauts face.

The Earth is constantly bombarded by solar and galactic cosmic radiation, which harms the human body in two major ways: by directly breaking DNA strands, and by generating free radicals, highly reactive molecules that damage our cells. Fortunately, Earth’s magnetic field, the magnetosphere, deflects much of this harmful radiation. Once outside the magnetosphere, however, astronauts are far more exposed.

Even in low Earth orbit, which is just outside the Earth’s atmosphere, astronauts can experience radiation levels equivalent to receiving multiple chest X-rays within a single day. On a years-long Mars mission, radiation exposure would be substantial and dangerous to our health. This constant bombardment with radiation leads to cellular injury, which triggers immune activation. The immune system shifts into repair mode, attempting to clear damaged cells and maintain tissue integrity. But immune cells themselves are also vulnerable to radiation. Over time, their numbers and function can decline as well, leading to weaker immune defenses.

As such, long-term radiation exposure increases one’s risk for infection, cancer, cardiovascular disease, neurological effects, and organ damage. To limit these effects, precautions must be taken. For long-duration space missions, physical shielding that can block radiation, early detection of radiation injury, and potential drug treatments to combat radiation are active areas of research in the world of space medicine.

Psychological stress and the body

Imagine you are sitting in a confined spacecraft, where every window looks out onto infinite darkness. You have relatively few crew members to interact with, many of whom were strangers when you boarded. Communication delays with Earth could stretch up to 22 minutes one way, making real-time conversation impossible. To add to that, you are constantly aware that your survival depends on every system functioning perfectly. Is the oxygen recycler working? Is that noise normal? Did we ration food correctly?

Chronic psychological stress has well-documented effects on immune function. Prolonged stress-hormone production, such as cortisol, suppresses certain immune responses, making the body less effective at fighting infections and slower to heal wounds. Social isolation compounds this effect; as humans, we are wired for connection. Extended confinement alters mood, sleep, and overall well-being.

Sleep presents another psychological challenge. On Earth, our circadian rhythm – the internal clock that tells our bodies when to sleep and wake – is synchronized to the 24-hour light-dark cycle. In space, natural sunrises and sunsets disappear. Although the International Space Station operates on a structured 24-hour artificial lighting schedule, astronauts still experience circadian rhythm disruption. The body depends on environmental signals, such as light, temperature fluctuations, and sound, that it has adapted to on Earth. When our circadian rhythm breaks down, the consequences extend beyond fatigue, affecting blood pressure, hormone balance, metabolism, and even immune cell function. In space, time itself becomes a biological stressor.

Microbes come along for the ride

Where humans go, microbes follow. Our microbiota – the trillions of bacteria, viruses, and fungi, that live in and on our bodies – play a critical role in training the immune system, processing nutrients, and protecting against harmful pathogens. Normally, this microbe-immune relationship is balanced; however, space disrupts that equilibrium.

Microgravity, radiation, altered diets, and stress affect microbial communities just as they do our cells. In space, some harmful bacteria become more infectious, have increased antibiotic resistance, and can better adhere to different surfaces, while beneficial microbes may decline. We still do not fully understand how long-duration missions reshape the human microbiota, but research on the use of probiotics and prebiotics to help stabilize these communities during spaceflight is currently in progress.

In addition to alterations in their microbiota, many astronauts experience viral reactivation. Dormant viruses are viruses that live silently in the host without causing symptoms, such as varicella zoster (chickenpox/shingles), Epstein-Barr virus, and cytomegalovirus. When immune surveillance weakens during spaceflight these dormant viruses can wake up and lead to illness. While often mild, these reactivations signal that immune regulation has become disrupted. Understanding and preventing immune dysregulation in the first place remains a priority for space medicine.

Is it worth the trip?

Despite all these challenges, imagine this: stepping onto the Martian surface after months of transit. The sky is dusty and pink. The horizon stretches wide and silent. In the distance rises Olympus Mons, the tallest mountain in our solar system. You are one of the few humans ever to stand on another planet.

The logistical hurdles are immense. The engineering challenges are staggering. The biological questions are likely just as difficult. To travel safely to Mars, we must not only design better rockets but also understand how to protect the immune system so that it can properly protect us.

So, if a vacation to Mars ever makes it onto your travel itinerary, remember: gather your food, water, oxygen, tools, space parts, clothes, medicine and courage - and don’t forget to pack your immune system.