If all goes as planned, astronaut Sunita Williams will join a small and select group Saturday afternoon.
When she opens a hatch on the international space station and steps outside into the vacuum of space, she will become one of the few women ever to do a spacewalk.
"Everybody would love to do a spacewalk. It's probably the most beautiful view of the Earth," Williams said a few months ago, as the reality of her upcoming walk started to sink in. "But as it's dawned on me pretty recently, it's real serious business."
Spacewalking is dangerous, and at NASA, it gets serious respect. The first American woman to walk in space was Kathryn Sullivan in 1984. Sullivan says that being an astronaut is an amazing experience in general, but that spacewalking is something special.
"Slipping outside a spacecraft in your own little body-shaped spacesuit, it is a neat thing to get to do," Sullivan says. "It is one of the kind of cool, coveted things to do."
But even though more than 150 male astronauts have spacewalked, only seven women have gone outside. Partly it's because NASA didn't send women into space until 1983.
But veteran spacewalker Mike Fincke says there's another reason.
"Our spacesuits only come in medium, large and extra-large," Fincke explains. "Anybody who is on the smaller side … they will not be able to have a chance to go outside."
The limitation imposed by small spacesuits was confirmed by Steve Doering, manager of NASA's spacewalking office at Johnson Space Center in Houston. He says the agency last looked into the issue in 2003, and at that point, none of the male astronauts were limited by suit size. But about one-third of the women -- eight out of 25 female astronauts -- couldn't fit into existing suits.
Sunita Williams says the astronaut corps realizes that it's "stuck" with the sizes NASA has.
But why is NASA, a $16 billion-a-year agency, stuck with only medium, large and extra-large?
First of all, a spacesuit is not like a sweatshirt for sale at the mall. A spacesuit costs millions of dollars to design and build. Lara Kearney, who develops suits for NASA, says a suit is like a tiny spaceship.
"It's not as big and as glorious and as wonderful as rockets that shoot fire and things like that," Kearney says. "But it literally is a small vehicle."
And these tiny spaceships have to fit right. For example, the arms and legs are inflated with air. So it's hard to bend your elbows and knees unless built-in joints line up properly . And if you're too small for the suit, in zero gravity, you'll float around inside.
The Apollo astronauts had custom-fitted suits. But in the 1970s, the vision for the space shuttle was that hundreds of people would go into space. Building custom suits for every one would take too long and be too expensive. So NASA built a flexible spacesuit system of mix-and-match parts: arms, legs, torso. It came in four sizes: small, medium, large and extra large.
In the early 1990s, a technical glitch forced NASA to develop a new suit. And spacewalking official Steve Doering says that just then, the agency was hit with budget cuts.
"One of the things that was eliminated as a result of this cost-cutting exercise was the small and the extra large size," Doering says.
The remaining medium and large suits fit most people. But not astronauts like Nancy Currie, who is 5 feet tall.
"People my size are in fourth grade. Literally," Currie says. "I mean, some fourth graders are bigger than me."
Eventually, NASA did build an extra-large suit. And it started on a small suit, too; Currie worked on a prototype. But more budget cuts came, and NASA killed the small suit.
Women's groups cried foul, and NASA's equal-opportunity officer looked into the matter. Lara Kearney worked on the small suit and says that its cancellation is a sensitive subject.
"It's very easy to turn it into a gender issue because the small people tend to be women," she says. But she doesn't see it that way.
"It is truly, in my opinion, not about gender," she says.
To her, it's logistics and cost-effectiveness.
For example, unlike larger sizes, the small suit had certain unique parts that didn't mix and match. So if part of a small suit broke in space, the astronauts couldn't easily replace it.
And astronaut Nancy Currie says there's another issue. Even if a small astronaut had a suit that fit, it could still be hard to work outside the space station, which was designed when only larger people were suited up. So, for instance, hand-holds spaced far apart would present a challenge for a small astronaut.
The question NASA essentially faced, says Currie, is: "Do we spend around $15 million to accommodate, relatively speaking, a few more people than we could today? Or, do we take that money, and turn it towards the suit development for the next generation?"
The space shuttle and its suits are expected to be retired in 2010. NASA will need all new equipment to return to the moon and go on to Mars. Glenn Lutz heads a NASA office that is making plans for moonwalks. His team is making a long wish list for the ideal new suit. For example, the suit should be lightweight, rugged and fit basically everyone.
"But eventually, we will come to the point in the program when we will need to make a cost vs. capability trade," notes Lutz. Down the road, technical or money constraints may squeeze the size range.
One former astronaut who hopes that history won't repeat itself is Bonnie Dunbar.
"I do not want to turn to a young girl who has all the talent in the world, becomes an extraordinary engineer, but isn't the right size, to tell her, 'I'm sorry but our nation can't build a suit for you,'" Dunbar says. "It's not the biggest expenditure. And it's not an engineering challenge that can't be overcome."
NASA officials point out that not every astronaut has to spacewalk and there are many other important jobs. During a shuttle mission this summer, for example, two men went spacewalking while two female astronauts stayed inside and operated a robotic arm. When asked if the two women could fit in a spacewalking suit, a NASA spokesperson replied that they are "both on the very edge of possibility" in the suits -- so they focused on robotics.