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Human exploration of Mars, explained

Mars is the planet most like Earth — and we've been dreaming of going there for a very long time.

Why are people so obsessed with Mars?

Mars is the second-closest planet to Earth and, in many ways, the most similar. As long as people have been thinking about space exploration, they've been dreaming of visiting Mars. And it now seems possible that a human mission there might actually happen sometime in the next few decades, though a lot hinges on levels of funding to NASA.

Since 1897's War of the Worlds, sci-fi writers have fantasized that Mars was home to intelligent life, partly because of the planet’s striking color, proximity, and visible polar ice caps. Our initial probes, sent in the 1970s, showed it to be a dry, sterile place, but more recently we've found water locked in its ice caps and soils, as well as elements — such as nitrogen, oxygen, and carbon — that are essential to life as we know it. We've also found evidence of ancient lakes and flowing water on Mars's surface — suggesting the planet may have once been home to extraterrestrial life.

It's unlikely that any life survives on the surface of Mars today: the planet is relatively frigid and has no atmosphere or magnetic field to protect life from the radiation of space. But compared with other options nearby, like Venus and the moon, it remains the most intriguing target for space exploration.

All this is why NASA has been fixated on Mars for decades, putting seven probes in orbit around it and landing another seven on its surface. The technical and medical barriers to sending people to Mars are considerable, and many experts argue that it’d be easier and more fruitful simply to continue sending robotic probes. But NASA hopes to launch a human mission to Mars within the next few decades — and if the agency succeeds, it'd serve as a singular moment in the history of the human species.

How hard would it be to get humans to Mars?

Really, really hard. Think of it this way: the farthest place we've ever sent astronauts, the moon, is about 240,000 miles away. Earth is about 34 million miles from Mars at its closest point. A journey there would take at least six months, one way.

Other big challenges: we don't yet have powerful enough rockets to send a craft heavy enough to carry all the supplies astronauts would need, nor do we have the technology to safely land a heavy craft on the planet once it arrives.

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A rendering of NASA's SLS rocket, currently in development, that could theoretically take people to Mars. (NASA/MSFC)

All this is possible to engineer, but it'd cost a lot of money: estimates run as high as $500 billion. NASA has a rocket in development that could theoretically send people to Mars, but right now it isn't getting nearly enough money to afford actual missions using it.

There are also some issues that can't readily be solved with money. Traveling to Mars and back would expose astronauts to dangerous radiation from deep space, increasing their lifetime risk of developing cancer. It's also believed that an extended period of weightlessness could cause a number of other health problems, including degraded muscle, bone, and eyesight. Some experts believe the psychological difficulties of spending years cooped up with just a few other people would be the most daunting health problem.

Still, the allure of Mars is such that when the organization Mars Oneannounced plans to send four people on a one-way trip there in 2024 — funded, in theory, by a reality TV show about the journey — they got more than 2,000 applications. If we can develop the technology, it seems, some people will sign up despite the risks.

NASA has expressed skepticism about Mars One and other private-sector Mars projects, but has announced its own plans to send more rovers to Mars over the next few decades, with the ultimate goal of sending a manned mission in the 2030s, perhaps in partnership with other countries' space agencies. If NASA can secure enough funding and develop the technology, it's a possibility.

What would it be like to live on Mars?

Very, very difficult. Imagine the harsh, self-sufficient life of a settler on the Oregon trail — except there's no bison to hunt, no water to drink, no oxygen to breathe, and no other humans living within tens of millions of miles.

For a relatively short-term, round-trip mission — like NASA is theoretically planning for the 2030s — astronauts would probably bring all their food and water with them. After the eight-month journey to Mars, they'd likely spend at least 80 percent of their time indoors, in small modules that provide the space of about two bedrooms per person. Some of this time would need to be spent on treadmills or stationary bikes to repair muscles and bones that are eroding in Mars's reduced gravity.

Sure, there'd be some exploration and outdoor experiments, but Curiosity and other probes scheduled to visit Mars in the intervening years make better explorers than humans, and limiting astronauts' exposure to the dangerous radiation that bathes the planet's surface would be a priority.

For a longer-term colonization mission, though, things would be very different, because the astronauts would have a lot of work to do.

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(NASA)

Although we could send an advance spacecraft stocked with some food, water, and oxygen ahead of the astronauts, they'd still need to provide these supplies for themselves after a short period of time. They might establish a base near the planet's water ice caps, melting the ice to provide water and converting some of it into oxygen through chemical reactions.

The oxygen could also be supplemented by some given off by plants grown for food inside greenhouses (though this might be difficult due to the reduced gravity and limited amount of sunlight). NASA has identified a few hardy, dense crops that might be worth growing on Mars: lettuce, spinach, carrots, tomatoes, green onions, radishes, bell peppers, strawberries, fresh herbs, and cabbages.

Is there life on Mars right now?

It's pretty unlikely, for a few different reasons.

One is that Mars is very cold: because it's about 50 million miles farther from the sun than we are, its average temperature is around minus 81°F. This is too cold for water to exist as a liquid (except in tiny quantities), which is necessary for all the life we've found so far on Earth.

Mars also has a very thin atmosphere and weak magnetic field, so its surface is constantly bombarded with harmful radiation from the sun and outer space that would wipe out living organisms.

Some scientists have speculated, though, that microbial life could evolve and survive beneath Mars's surface. The planet has geologic activity that could generate enough heat underground to melt ice into water. Additionally, there's evidence that at the surface, high levels of salt act as an antifreeze, keeping water liquid at frigid temperatures. (This could explain why scientists have occasionally spied dark streaks on the planet's surface that look a lot like water seeping out of the soil.)

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A 3D image shows streaks that look like water seeping out of Mars's surface. (NASA)

Other ingredients necessary for life as we know it — such as elements like carbon and nitrogen — are also likely to exist in cracks deep under the planet's surface. On Earth, scientists have found microbes living miles underground, powered by chemicals naturally released by rocks, and it's possible — though still unproven — that the same scenario could unfold on Mars.

There also may be some indirect evidence for it. Curiosity has detected plumes of methane gas coming from the planet's surface, and on Earth, methane is generally produced by living microorganisms, including those that live underground. Methane can also be produced by geologic processes that have nothing to do with life, so this isn't proof of underground microbes — but it's probably the most promising finding we've made thus far.

Was there ever life on Mars in the past?

It's uncertain, but some scientists are hopeful.

Data collected by NASA's Curiosity rover indicate that 3 or 4 billion years ago, Mars may have had a thicker atmosphere, along with a robust magnetic field like Earth's.

Together, these could have solved both of the main problems that would plague any life on Mars today. A thicker, carbon dioxide–filled atmosphere could have warmed Mars by trapping warmth emitted by volcanic activity, as well as from the sun. Both the atmosphere and the magnetic field would have also acted as protective shields, limiting the amount of harmful radiation from the sun and outer space that would reach Mars's surface.

Pieces of evidence for this scenario are the rock and soil formations indicating that things like flowing water and lakes were once plentiful on Mars. And chemical analysis of the water vapor currently in Mars's atmosphere also suggests the planet once had much more water, which presumably evaporated out to space since.

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A rendering of the ancient lake that may have existed in Mars's Gale Crater billions of years ago. (NASA/JPL-Caltech/ESA/DLR/FU Berlin/MSSS)

When Curiosity has drilled into Martian rock, it has discovered organic molecules — compounds of carbon bound to other elements, which serve as the building blocks for all life on Earth — providing even more hope that conditions may have been just right for life billions of years ago on Mars.

However, not all scientists buy this theory. Study of the craters on Mars's surface has indicated that Mars's atmosphere was thin for most of the planet's history, allowing asteroids to make it through without burning up significantly. Meanwhile, modeling of Mars's early climate has suggested that it may have merely been warmed by volcanic activity for very brief periods of time — perhaps as short as a few years. This could both explain the historical presence of liquid water and make the evolution of life seem rather unlikely.

Additionally, we still haven't found evidence of life itself on Mars. Despite lots of searching with rovers on Mars (as well as in pieces of Mars that have arrived on Earth as meteorites), no fossils or other traces of ancient life have turned up yet.

How have humans explored Mars so far?

Over the past few decades, humans have put 13 probes in orbit around Mars and successfully landed nine intact on its surface. These machines have taken thousands of photographs, sampled rocks and soil, and analyzed the extremely thin atmosphere.

Exploration began in the 1960s and '70s, with programs by both the Soviet Union and the United States that tried — and often failed — to put probes in orbit. The first big success came in 1971, when NASA's Mariner 9 successfully entered orbit, closely followed by two Soviet probes that arrived in the next month. In 1976, NASA landed a spacecraft on Mars for the first time.

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Mariner 9, the first spacecraft to enter Mars's orbit. (NASA)

Exploration took a big step forward in August 2012, with the landing of NASA's Curiosity rover — the largest and most sophisticated probe to land on Mars. Curiosity has since rolled several miles across the planet's rocky terrain, drilling holes in rocks and even shooting them with lasers to analyze their composition. The rover has made quite a few interesting discoveries so far, like the fact that Mars may have been warmer and wetter billions of years ago.

In addition to Curiosity, there is one other operational rover on Mars —NASA's Opportunity, launched in 2003 — and five working probes in orbit: NASA's 2001 Mars Odyssey, Mars Reconnaissance Orbiter, and MAVEN, the European Space Agency's Mars Express, and India's Mangalyaan. There are also dozens of now-defunct crafts.

What is the Curiosity rover?

NASA's Curiosity — the largest and most sophisticated rover to be sent to Mars so far — landed there in August 2012, and has made a number of big discoveries since.

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The Curiosity rover. (NASA/JPL-Caltech/Malin Space Science Systems)

Simply landing the car-size rover after its eight-month journey was a huge challenge, because of the need to slow Curiosity down enough that it wouldn't be burned up by Mars's thin atmosphere or crash on impact. NASA engineers ended up developing a rocket-powered crane that suspended the craft and slowly lowered it to the surface.

Since then, Curiosity has traveled several miles across Mars's rocky terrain, powered by a small electric generator that harvests the energy given off by plutonium packed aboard. After landing and making a brief detour to a place called Yellowknife Bay (where the craft found evidence of an ancient freshwater lake), Curiosity is being remotely guided to a huge mountain called Mount Sharp. On the way there, it's traversed a number of different rock layers that formed over the course of 2 billion years, so by sampling them, it's told us a whole lot about Mars's history.

Curiosity is packed with a ton of scientific instruments, including three cameras, devices called spectrometers — which beam X-rays and lasers at pieces of rock, using the energy that's reflected back to determine the rock's composition — a drill and scoop that collect pieces of Martian rock, and even a little Easy-Bake Oven that cooks them to analyze what's inside.

Engineers are remotely controlling Curiosity and using these tools to fulfill a few different objectives: to look for indirect evidence of life (such as organic compounds), to learn about the geologic processes that produced Martian rocks, to collect data on the large amounts of radiation that constantly hit the planet, and to provide information on the planet's ancient atmosphere and how it may have changed over time.

Curiosity has already done a pretty solid job of hitting many of these goals. So far, it's told us that Mars may have had a thicker, protective atmosphere and flowing water long ago, and contains elements that serve as the building blocks for life. It's also found traces of liquid water in the planet's soil, and may have found organic compounds — combinations of carbon atoms bonded with other elements that are ubiquitous across all forms of life, but scarce outside them — but it's unclear whether these were inadvertently carried there by Curiosity itself, or occurred on Mars naturally.

What's the point of sending people to Mars in the first place?

Given the success humans have had exploring Mars with robots, you might ask why NASA and others are so bent on sending people there — which will be way more difficult, more expensive, and more dangerous.

The cynical answer: Egotism and foolishness. There's no good reason to send a person to Mars. Unmanned probes like Curiosity can explore it way more effectively than humans for a fraction of the cost, with no health risk to astronauts.

Long-term plans to try making Mars habitable for humans are escapist, and absolve us of the responsibility of taking care of the perfectly habitable planet we already have. And politicians' grand announcements of plans to send people there are delusional, unrealistic fantasies.

The inspirational answer: A spirit of exploration has defined our species from the start. It's what's led us to cross the oceans and climb the world's mountains — and now it compels us towards the next frontier, that of interplanetary travel.

A human Mars mission would mark a new era in the history of our species. Like the moonshot, it would demonstrate the remarkable heights we can achieve when we devote huge amounts of energy and resources toward a truly ambitious goal, rather than war or incremental technological progress. It would give us firsthand knowledge of the planet most like Earth — and open up the possibility that humans might someday be able to colonize it, in case Earth's environment is irrevocably damaged.

Who would NASA send to Mars?

A trip to Mars would be long and physically grueling — it would probably take a minimum of about 30 months, round-trip. The most obvious requirement for any astronaut would be that he or she is in good, stable health, without any conditions that require ongoing medical care. Some have pointed out that it'd make sense to exclusively send women to Mars, since they consume much less food.

Mars One, an organization that seeks to send four people on a one-way trip to Mars in 2024, emphasizes five psychological characteristics in their application criteria: resiliency, adaptability, curiosity, ability to trust, and creativity. They also require physical fitness.

These sorts of qualities are so important because of what many experts anticipate will be the biggest health problem for anyone going to Mars — the challenge of not going crazy being locked up in an RV-size craft for many tedious months at a time with just a few crewmates. Unlike the astronauts currently in the International Space Station, Mars voyagers would lack real-time communications with Earth, because any radio signals we send to Mars take at least 10 minutes or so to travel there. Establishing a permanent colony would further increase the psychological toll, because it'd mean that any astronauts would be forever abandoning all loved ones on Earth.

A few different experiments have been carried to probe the human mind's tolerance for these sorts of conditions and see who might be best to pick. On a barren volcano in Hawaii, NASA is conducting the latest in a series of simulations in which a small group of volunteers are isolated in a Mars base mock-up for months at a time with limited outside contact, donning "space suits" in order to go outside.

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NASA's simulated colony, designed to test the psychological rigors of isolation on Mars. (HI-SEAS/NASA)

Of course, if a permanent colony were the goal of a Mars mission, rather than a round trip, the criteria would be somewhat different. In order to create a sustainable, self-replicating population, things like youth and genetic diversity would be more important.

What are the dangers of living on Mars?

Permanently living on Mars would wreak all sorts of damage on the human body.

Radiation: The Earth's magnetic field and atmosphere naturally protect us from most of the harmful radiation that emanates through space. Without this protection, any astronauts living on Mars would be exposed to dangerous levels of it, increasing their long-term risk of cancer and potentially eroding their decision-making skills.

NASA has calculated that a round-trip voyage to Mars, combined with a six-month stay there, would increase an astronaut's lifetime risk of developing fatal cancer by about 5 percent. The longer someone stayed there, the higher this number would climb. This radiation would also increase a person's chance of having a child with a harmful genetic mutation — a big problem for any plans to establish a self-sustaining colony there.

There have been various mechanisms suggested to shield astronauts from some of this radiation, like Martian soil piled up against their living quarters, and a special heavily lined protection room they could retreat to in case of a solar flare.

Reduced gravity: Normally, your body's muscles and bones have to work to stand up against the force of Earth's gravity. Due to Mars's reduced gravity, astronauts' muscles would quickly atrophy: in experiments, rats flown in space lost a third of the muscle bulk in their legs within a few days.

For some muscles, this could be minimized by exercise on stationary bikes or treadmills, but the reduced gravity would still be a problem for the skeletal system (which would lose calcium, becoming more brittle) and for the mechanisms inside the inner ear that normally allow you to balance (potentially leading to vertigo and disorientation).

There's also some evidence that an extended period of time in reduced gravity can be harmful to astronauts' eyesight, perhaps because it causes the fluid pooled near the optic nerve to expand, changing the shape of the eyeball.

Boredom: Some experts think that psychological tensions of spending years cooped up with a couple of crewmates and few distractions would be the most difficult part of a Mars journey.

In Mars mission simulation experiments, for instance, researchers have noticed that participants often experience something called "third-quarter syndrome." They do all right for the first half of a nine-month mission, but as it wears on, they have a noticeable dip in morale, with the lowest moments coming before the fourth quarter, when they're close to the finish. For a 30-month round-trip mission to Mars (or a permanent colony there), you'd have to imagine this problem would be a lot more severe.

Why should NASA go to Mars instead of, say, Venus or the moon?

Well, it's not exactly an either/or proposition — NASA can and has done both, sending probes to Venus and both probes and people to the moon.

But as a destination for a manned mission, Mars is a bit more compelling than either Venus or the moon for a few reasons.

For one, we've already landed 12 people and 10 unmanned probes on the moon, and learned a lot about it. Unlike Mars, it doesn't appear to have frozen sheets of ice (just trace amounts of water molecules), and there's no indication it was more habitable long ago, like Mars may have been, so we probably won't find evidence of ancient organisms. Given that it's cheaper and easier to learn about space by sending an unmanned probe, the main motivation for manned space travel is the desire to send people somewhere new — something the moon can no longer provide.

Venus, on the other hand, would be a new destination for humans — but would also present a very good chance of killing them immediately.

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An image of Venus's surface, created by radar data. (NASA)

That's because Venus's extremely thick atmosphere — with surface air pressures about 90 times as strong as those on Earth — would probably crush any spacesuits or landing modules we sent there. During the 1980s, the Soviet Union landed 10 probes on Venus, but none survived more than an hour or two because of this problem. For humans, the extreme heat (about 850°F) would also be rather uncomfortable. Some, however, have theorized that it might be possible to send floating blimps to explore Venus's atmosphere, instead of trying to reach the surface.

Compared with Venus and the moon, Mars is a happy medium — exotic and new enough to (perhaps) justify the huge expense of a manned mission, but mild enough that it actually might allow astronauts to survive more than a few minutes there.

Still, many argue that devoting all our resources to getting to Mars is shortsighted — especially when there are other destinations in our solar system, like Europa (Jupiter's icy moon that may harbor liquid oceans), that are more likely to have life.

What are the plans for exploring Mars in the next few years?

Funding for space projects can dry up at any time, but there are currently a few projects in the works that seem fairly certain to happen.

2016: In 2016, NASA will send a stationary lander called Insight to Mars with the goal of studying the planet's interior. Scientists hope it'll help us learn more about the geologic processes that created the planet, and perhaps the other rocky planets that orbit the sun.

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The Insight lander. (NASA/JPL-Caltech)

As part of the multi-phase ExoMars project, the European Space Agency and Roscosmos (the Russian Space Agency) plan to send a stationary lander (which will serve as a test for landing a rover on Mars) along with an orbiting probe (which will study the planet's atmosphere) in 2016.

2018: In 2018, they hope to send a rover with a suite of scientific instruments. Its landing location will be based on where the previous orbiting probe detects methane — a gas believed to potentially reflect the presence of microbial life forms. Funding for the 2018 phase, though, is still uncertain.

2020: In 2020, NASA will send a new rover to Mars. The vehicle will use the same chassis and landing technology as Curiosity, but will carry scientific instruments aimed at a different goal: locating and extracting the rocks most likely to contain evidence of past life, and caching them for eventual retrieval and return to Earth.

There are a few other proposed near-term government missions — such as a Finnish-Russian plan to set up weather-recording stations on Mars and several different proposals to send a craft that will scoop up and return Martian soil to Earth — but at the moment, none of these plans are concrete.

Are there any plans to send humans to Mars?

There are a few different projects — both from government agencies and the private sector — that could hypothetically lead to astronauts landing on Mars sometime in the next few decades.

NASA's plan, laid out by President Obama in 2010, would use the Orion space capsule and SLS rocket to put humans on an asteroid by 2025 and on Mars sometime in the 2030s.

Given present levels of NASA funding and other political difficulties, though, this ambitious plan might not be realistic. A recent National Research Council report found that NASA's current plan isn't detailed enough and isn't getting enough funding to be feasible.

The private sector plans to bring people to Mars range from overwhelmingly unlikely to somewhat plausible.

The Netherlands-based Mars One organization plans to establish a permanent colony on Mars in 2024, with new crews of four joining the founders every four years. The organization claims the mission will cost
$6 billion, and be funded largely by a reality show made about the journey. Given the lack of progress and how difficult it'd be to transport humans to Mars even once, this price tag and timeline seem extremely far-fetched.

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(Inspiration Mars)

The Inspiration Mars Foundation's proposal might be a bit more plausible. The plan is to use a 2018 launch window during which Earth will be relatively close to Mars to send two astronauts there, have them fly by the planet, then return, all in a total of 501 days. Although we don't yet have rockets that are powerful enough to put a sufficiently big vehicle into space, this sort of simplified mission is technologically feasible — but the aggressive timeline and limited funding available make it unlikely.

The most realistic private Mars mission comes from SpaceX, a company that's already launched rockets that have reached Earth's orbit and the International Space Station. As part of its Red Dragon mission, the company would outfit its most powerful rocket (still in development) with an unmanned capsule that could land on Mars, collect a soil sample, and return to Earth. The latest plans call for the Red Dragon to launch in 2022.

What is Mars One?

Mars One is a Dutch organization that in theory plans to send people on a one-way trip to Mars in 2024, in order to seed a permanent colony that will grow over time.

The organization says it will begin with an uncrewed probe mission in 2018, followed by cargo missions in 2022 and 2023, which would set up life support modules that grow food, as well as extract water and oxygen from the Martian soil. Starting in 2024, new groups of four astronauts would depart every two years to live there permanently.

The $6 billion needed to pay for the mission would come from unorthodox sources, including a reality TV show made about it. Mars One has attracted a lot of publicity throughout the whole process, especially as it recently narrowed down the contestant pool to 100 people.

However, pretty much anyone with any expertise in space exploration says Mars One is more of a publicity stunt than a realistic mission.

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A rendering of Mars One's life support modules. (Mars One)

There are a number of reasons Mars One's plan is so far-fetched. On the most basic level, Mars One is a small nonprofit, rather than an experienced aerospace company, and experts say it's underestimating the cost of its plan by several orders of magnitude.

In the short term, Mars One needs to get moving on its 2018 probe — but though it ordered a concept design from Lockheed Martin, it hasn't actually asked the company to build the probe. This is probably because it doesn't have the money on hand, as it hasn't sold the broadcast rights to the mission, and failed to even meet its $400,000 goal on Indiegogo (a probe would cost at least tens of millions).

In the longer term, the habitat Mars One has designed is very heavy — and at this point, no one has the rocket technology needed to lift it all into space or land it safely on Mars. Scientists still haven't worked out how to keep humans healthy in the weightlessness and high-radiation environment of deep space for the multiyear journey there. A recent MIT study, meanwhile, found that Mars One's planned habitat would likely lead the astronauts to suffocate within months of landing.

Mars One has disputed this study, along with other criticisms, but hasn't offered any real answers. We might someday establish a colony on Mars, but don't bet on Mars One doing it next decade.

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