Cady Coleman goes to work in a rocket. As a Nasa astronaut, she'll soon be off to space for six months. Her son, Jamey, and husband, Josh, have had to get used to her extreme job

When Catherine Coleman, known as Cady, goes to work, waving goodbye to her son Jamey, 10, she doesn't drive or take a train – she blasts off in a spaceship. Many more women now work in space, but Cady's next Nasa mission is a big one. In December, she leaves Earth on a Soyuz rocket for the International Space Station, where she will live and work for six months. This will be the longest mission undertaken by a Nasa astronaut who is the mother of a young child.

The Cady Coleman who greets me at the door of her 200-year-old farmhouse deep in the woods of New England is smaller than I had expected. This is a woman who walked into an American Air Force centrifuge programme as a volunteer and walked out with a world record for endurance. She once spent six weeks camping in Antarctica, learning that "my mother was correct when she told me to dress in layers." She has also lived 18m underwater in a giant tank, as part of Nasa's extreme environment training, an experience she took in her stride as the daughter of a navy diver. And she has clocked up 500 hours in space on two previous short-duration shuttle missions, in 1995 and 1999.

Cady is 49, 5ft 4in, slim and narrow-waisted. She confesses to having had trouble getting space suits small enough. There is excitement in her voice when she talks about the possibility of a space walk during the coming mission – but the reality is that committing to a mission at all has required soul-searching. She needed the full support of her husband, Josh Simpson, and she needed Jamey to understand, too. "We talked a lot about whether I would even sign up to go – but it is part of who I am – and it's part of who Jamey's mother is and Josh's wife is. If I'm not doing those things, I'm not sure I'm being as much of a mum as I could be. It is part of me."

How does an astronaut with a child come to terms with the risks inherent in space travel? Cady answers by putting those risks into a broader context. Soldiers are deployed all the time to places "not nearly as nice to be as the International Space Station" she says.

However, she and her husband do their best to shield Jamey from the starker realities of his mother's profession. They recall sitting down as a family to watch a television series about space flight and then, when the topic turned to the Columbia disaster in 2003, deciding calmly that it was Jamey's bedtime. It's harder to protect him from the tactlessness of the public. "People come up to me all the time, with Jamey there, and say: 'Aren't you afraid that rocket's going to blow up?'" says Josh. "I'm always taken aback."

These days, more thought is put into pre-mission preparation, especially when children are involved. "Everyone in the family has to be committed," says Al Holland, a Nasa psychologist. "It's not just one person on one mission."

Holland recognises that older children have more sophisticated coping mechanisms and bigger support networks – and it's possible to appeal to their sense of adventure. Families are invited to Star City in Russia to see the training facilities and to attend shuttle launches. Nasa astronaut Mike Barratt, whose five children were aged between nine and 21 when he carried out a long mission last year, says that his offspring are adventurous by nature and took it in their stride. "We've sailed across the Gulf of Mexico in small sailing boats and hiked and camped and climbed – and this was just another thing," he says.

It's tougher on younger children as they can have trouble understanding the concept of space travel. British-born astronaut Nicholas Patrick's three-year-old son was under the impression that his father went to space every day to work. Consequently, when Patrick's shuttle launched in February, the little boy couldn't understand why his daddy didn't come home that evening.

Holland advises parents of younger children to give them constant reassurance, and to tell them "I am proud of you – these five words go a long way to resolving anxiety in young kids who may think [a long absence] is their fault," he says.

Once the mission is under way, communications between the space station and Earth are sophisticated – so much so that psychologists recognise that the families of astronauts can feel far less isolated than those of soldiers on deployment. A daily telephone call between an astronaut and their family is pretty standard. There's also email and a weekly audio-video conference.

When Nicole Stott, an astronaut and mother of Roman, seven, spent time on the space station last year, she made a point of telephoning her husband and son after school each day. She would ask, "So what did you do today?" and with typical seven-year-old panache, Roman would reply: "Oh, nothing."

Mike Barratt used email to read his teenage daughter's English essays from the space station and to help his youngest child come up with ideas for school projects. During his recent mission, Nicholas Patrick was surprised to receive an email from his six-year-old daughter urging him to hold on tight. She had seen a much publicised photograph of him holding on to the space station with just one hand during a space walk.

But family relations between Earth and space aren't always plain sailing. Barratt remembers a particularly difficult phone call with one of his sons. He told him, "If you do that, you'll be grounded," and there was a small silence as his son processed the fact that his father was in space and therefore unable to enforce the punishment. "It becomes up to the kid to make the right decision," says Barratt.

Astronauts say that combining parenthood with space travel is like being subjected to opposing forces. Looking at Earth from space and knowing that your family is there is a pull stronger than gravity. But experiencing the beauty and magic of space can be addictive. "At the end of 199 days, I was happy to be going home – but it was hard to leave the station," says Barratt.

Meanwhile, on Earth, while Cady's family come to terms with her impending six-month absence, she is under pressure of a different kind. Experts acknowledge that the time before flying is often tougher on the astronaut than the mission itself. In fact, it was while waiting for a delayed second mission in early 2007 that the then Nasa astronaut Lisa Nowak, who has three children, famously unravelled, driving 950 miles from Houston to Orlando to accost a romantic rival.

Cady, though, is cheerfully upbeat in the face of her giant responsibilities. Her intensive schedule means that she's been abroad in Germany, Russia or Japan, for 40% of the time for two and a half years. During the mission she'll be in charge of the robotic arm that grabs the three supply vessels that will visit the station. These are the size of buses. If they run into the station, a lot of damage could be done. "I have to practise a lot," she says. She's also brushing up her Russian so she can converse with three of her fellow crew members.

Josh has taken on the role of Jamey's primary carer, but it's a role he knows well, thanks to the family's unconventional lifestyle. He and Cady have never lived together. His home is the New England farmhouse, and his business is focused on the glass studio he built 40 years ago across the drive. As an acclaimed glass artist, with a dozen employees, moving to Houston, where his wife works, has never been an option. The couple spend as much time together as they can, and Jamey divides his time between them. That means two schools and two understanding head teachers. It's tough on Jamey, and he has asked why his mum can't work in New England.

Cady says that they have had to be strong in their convictions to make this lifestyle work and that she has relied on family and friends for support, notably her sister. "I can call her up and be in tears that I am the only mum who doesn't go to the zoo or on field trips," she says. But having a child has taught her how to juggle a lot of things – and this ability to multi-task is a skill she says is essential in space.

Today, while the adults talk, Jamey tidies his bedroom and potters around the garden and the kitchen. There are two robotic dinosaurs sitting on the kitchen floor, and he asks me if I want to borrow one so we can have a fight. Cady smiles and rolls her eyes and says it's all fighting with boys at this age.

Then she tells me she has just had some good news from Nasa: her shirt allocation for the mission has been doubled. She will get two clean shirts a month instead of one. She's also being asked what children's books she wants to take with her, so she can read to Jamey while she's away. And someone wants to know if she'll need ordinary bras or if sports bras will do. Everyone wants her attention. Everyone wants to teach her something. "In general, the women [Nasa] has had have been super achievers," she says. "I hope to live up to this."