When Jane first met John, she knew that they would spend the rest of their lives together — literally. The pair spent more than eight years in space flight training before leaving Earth without the possibility of return.

As members of the first Mars colony, Jane and John naturally gravitated towards each other because they share the same future of an isolated life on a new planet. And as their mental bond grew, so did a fervent, passionate physical urge for each other. Now they face an obstacle for which they never trained: sex on the Red Planet.

Jane and John are fictional characters. But if a handful of Mars colonization projects have their way, their lives could be a reality in just 10 years.

SEE ALSO: Are We Meant to Live on Mars?

On the surface, this story sounds like the beginning of a wonderful extraterrestrial romance. However, there's a dark side — having sex on Mars is unexplored and could potentially be life-threatening.

Space Sex: Too Taboo for Study

When it comes to sex in space, federal agencies' lips are sealed. NASA claims that no sexual experiments have ever been carried out, and former astronauts play coy when asked about the topic. But this is partly because sex hasn't been an issue on short-term missions.

However, putting a human on Mars is in our future. If NASA does it first, it will require an extended period of space flight that could stretch two years. If a private colonization venture sends humans to the Red Planet, it will be on a one-way flight without the possibility of return to Earth.

In either case, sex will have to be addressed, especially for a mission with a mixed-gender crew. Yet even in 2013, both federal agencies and private ventures shy away from the topic like a 1950s sitcom couple that sleeps in separate beds.

This is a huge problem for our future in space.

"There are basic human drives to have shelter, eat and procreate, so to negate not thinking about one of them, you're doing a disservice to space exploration," says Dr. Saralyn Mark, a women's health specialist who consults with agencies including NASA.

The biggest question mark is how our bodies will react to prolonged exposure to 40% gravity on Mars.

The Mars Society, an organization that grew out of the Mars to Stay concept from the early 1990s, advocates for a one-way manned mission to colonize Mars. While members of the group differ on when that will happen, Mars Society regularly carries out mock missions in its Mars Desert Research Station, a structure in Utah that was built to simulate a Martian environment.

Every two weeks, five or six people run through routine tasks similar to the job functions that future members of a Mars colony would conduct every day in order to be self-sufficient, such as growing their own food. Dr. Kris Lehnhardt, chief flight surgeon for MDRS, believes that procreation and women's health should also take top priority when it it comes to preparing for a mission like this.

"If you think about colonies historically, they are at some point supposed to become self-sufficient," he says. "If you didn't allow mating, you are not setting up a colony that would populate over time. And, in theory, they would die on Mars because there wouldn't be a propagation of species."

How Do You 'Do It' on Mars?

MDRS hasn't conducted any sex experiments to account for the gravity on Mars. However, Lehnhardt, who also teaches a class on human health and space flight at George Washington University, has an idea about what sex on Mars would be like, based on what we've learned about our bodies in microgravity from missions on the International Space Station.

We've all seen videos of astronauts spinning in microgravity on the ISS, but Mars will be a little different, where there is some gravity. If you're speaking purely in terms of physical pleasure, sex on Mars wouldn't be that different than what we're used to. It would just be a little bumpy at first.

"You could probably just do it like you do it on Earth," says Lehnhardt. "The reality is that you would have to deal with physics."

Just like astronauts on the ISS, colonists wouldn't need to wear space suits inside their stations because the atmosphere would be as it is on Earth. The trickiest part of having sex in low gravity would be finding the most optimal way to remain in contact because the two bodies would repel.

"It's kind of a misnomer in space because there is no down, Lehnhardt says, noting that astronauts on the ISS sleep in bags attached to the wall. "In terms of positions it would be more about staying in contact with each other. It isn't impossible, but it would be awkward."

Keeping that close connection would be easier on Mars than in orbit because of the 40% gravity. "It's not like the moon where you are bouncing up and down, but it will be lower than Earth," Lehnhardt says.

Birth Control on Another Planet

When it comes to sex on Mars, the bigger issue here is procreation. If the goal is to truly find another planet on which humans can live, sex and pregnancy have to be heavily reviewed.

"Most people will say that it is something we will have to figure out once we permanently live off the planet," says Lehnhardt. "But it is obviously essential. If you're talking about sending a group to Mars and setting up a colony, you have to have procreation."

However, some Mars colonization advocates don't feel that pregnancy will be an issue for the first group of settlers on the Red Planet.

In January, Mashable interviewed Norbert Kraft, medical director for Mars One, a private venture that wants to send a group of colonists — who will be chosen by the public via national television — to Mars in 2023. When asked about how Mars One will prepare its astronauts for sex or possible pregnancy on Mars, Kraft didn't see it as a problem, noting that the organization would try to "make colonists aware of the risks associated with having sex."

In Mars One's ideal situation, the first few waves of colonists — a new group of about 30 people will be sent to the planet every two years — won't procreate at all. Kraft says once the team on Earth has more information about the environment on Mars, "perhaps they will then try sending animals to the planet to breed."

Kraft says the team would pack contraceptive supplies. However, the birth control methods used on Earth may not work on Mars. We have very little knowledge about how a woman's body will adapt to a lower gravity environment, particularly how her hormone levels will be affected. Therefore, it would be hard to develop a contraceptive prior to going to Mars that would actually prevent pregnancy while there.

Some medical experts have proposed inserting an intrauterine device (IUD) before launch. But the problem with even a non-hormonal IUD is the increased risk of movement once in lower gravity, which could puncture the uterus.

"With an IUD, there's a foreign object in your body, and now you're going into a foreign environment. It would be hard to extract in an emergency," says Dr. Mark. "Keeping things as simple as possible is the best way to go."

There is, however, a highly controversial proposed solution to an unexpected pregnancy on Mars: female sterilization.

Mars colony advocates discuss removing inessential organs — such as appendices and tonsils — before flight in order to avoid a very dangerous emergency surgery once on Mars. But when it comes to removing the uterus, the ethical issues divide health experts.

"When you're going to a new world, the last thing you want to think about is bringing an offspring until you're settled," says Mark. "But what's considered an organ that you need? To what extreme do we go? When you remove a uterus, some women have problems because of blood flow. So unless you really have to, I wouldn't recommend it."

The First Human Martian

Perhaps talking about pregnancy prevention on Mars is putting the cart before the horse. Is it even possible for a woman to get pregnant in low gravity?

Before you can discuss a pregnancy on Mars, you have to start with menstruation. "On Earth we have a 28-day cycle, how will you shift to Martian moon cycle with a longer orbit? One day is 24.65 hours," Mark points out. You would have to find some way to account for the extra 40 minutes per day.

Another issue is the gravity's effects on bones. In space, astronaut bone loss varies from 1% to 5% per month. Hormonal milieu impacts bone health and lower sex hormone levels, which would affect menstruation and, therefore, ovulation. Mark cites eating disorders as an example — women who suffer from extreme cases of bulimia or anorexia experience bone loss and lower hormone levels, eventually halting their periods.

However, on previous short-term missions to the ISS, women were able to have their periods while in orbit.

"In theory, if a woman menstruates and ovulates, there's the possibility of getting pregnant in space," Lehnhardt says. "If you're able to [physically have sex], then the issue is sperm motility. Will it function in order for fertilization to occur? If it does, can an egg implant properly in the uterus, and can the embryo develop normally?"

The best example we have to draw from is a study that sent rodent embryos into space. When they came back down to Earth's gravity, they were born as normal. A 2009 study that examined mice's embryo fertilization found that implantation at normal gravity yielded healthy mice, but fertilization rates were lower for the embryos fertilized in microgravity.

However, no mice or rats have fully developed while in microgravity throughout the entire developmental cycle. Lehnhardt says there hasn't been a stronger push for rodent birth in space because of a lack of support and rules allowing that research to happen.

"It's triaged as a lower priority research topic compared to other risks astronauts face right now," he says. "But if we truly want to become more than a one-planet species, we are going to have start doing more research into every step along the way."

So will Jane and John's love story ever find its happy ending? Marital bliss isn't a guarantee, but one thing is for sure — sex on Mars will eventually happen.

"We are biological creatures. We are animals," says Lehnhardt. "We try to fight [the urge to have sex] all the time. But most of the time, biology wins."

Illustration by Bob Al-Greene/Mashable.