Back in the days when there were no women in the astronaut corps, one of the National Aeronautics and Space Administration's arguments for keeping it that way was that women lacked the upper-body strength required to perform spacewalks.

As one former chief of the astronaut office explained it seven years ago:

"Spacewalking is an exhausting job. It involves using the hands, arms and shoulders in the most arduous ways imaginable. I frankly think women could never be spacewalkers."

Well, as somebody else once said: Never say never. Not only have two American women already flown in space, a third is about to become the first American woman astronaut to undertake a spacewalk.

Kathryn D. Sullivan, 34, a New Jersey native who has a PhD in geology, is scheduled Oct. 10 to step out into the cargo bay of the space shuttle Challenger for a three-hour spacewalk with astronaut David Leetsma.

Hidebound chauvinists at the space agency are quick to point out that Leetsma will do most of the major tasks that require upper-body strength, such as using tools to open and close valves to transfer 550 pounds of fuel between two tanks. Sullivan will hand him the tools and photograph him as he performs the fuel-transfer experiment, the first of its kind ever attempted under weightless conditions. The same officials point out that at 5 feet 6 inches tall and 150 pounds, Sullivan is the most robust of the women astronauts and is "possibly the only woman astronaut" who could successfully walk in space. Theirs may be a self-fulfilling prophecy. No other woman astronaut has been picked to walk in space, and NASA doesn't have a spacesuit that would fit the others. MAKE A DETOUR? . . .

America's space scientists have formally asked NASA to consider re-routing the Galileo spacecraft so that it passes by the asteroid Amphitrite in December 1986. A space shuttle mission is scheduled about five months before that to launch the satellite on a three-year flight to Jupiter.

"It's an opportunity to undo the damage of the American decision not to rendezvous with Halley's Comet in 1986," one scientist said. "A rendezvous with Amphitrite would give us the first encounter with an asteroid and not leave it to the Russians and Europeans the way we left Halley's Comet to them."

NASA has not responded to the scientists' request, largely because it wants to "keep its hands on the stick" of the Galileo spacecraft as it speeds through the asteroid belt between Mars and Jupiter. There is a chance this first-of-a-kind spacecraft will need some delicate handling when it passes through the belt.

Nevertheless, a trajectory that would take the spacecraft within 10,000 kilometers of the asteroid is being plotted and will be loaded into Galileo's navigating computer. An agency official explained: "This will keep our options wide open so that if we have no trouble with the spacecraft on its outward journey, we can bring it right alongside the asteroid on our way to Jupiter."

NASA and the Los Alamos National Laboratory in New Mexico have agreed to a plan in which a space shuttle would deploy a satellite in late 1986 or early 1987 that will eject 38 cannisters of barium and lithium. The gases will be released into the atmosphere over sites such as New Mexico, Puerto Rico and the Marshall Islands where powerful radar stations and cameras will monitor the results.

What's going to happen? The lithium and barium will glow with such intense light when they're struck by sunlight that they will be easy to photograph as they follow the Earth's magnetic field around the globe.