America's final Gemini space walker has plunged with scarce notice into a Baltimore swimming pool to see if being an underwater dog paddler is anything like being a human satellite.

Apparently, it is. And so astronaut Edwin E. Aldrin Jr., who spent two hours submerged in the McDonogh School pool earlier this month, is coming back again soon for another dip, reports the space agency.

The fieldhouse pool of the preparatory school for 800 boys off Reisterstown Road, Baltimore County, is the unlikely scene of an experiment the space agency hopes will solve one of its most vexing problems.

Hard to train for

The puzzle is that spacemen have found walking around in orbital weightlessness is more complex, more tiring and harder to train for than expected. Gemini 11's Richard Gordon was the latest to find this out.

Aldrin will become the last American out of the hatch for at least a year in mid-November when he steps into the void on the four-day Gemini 12 flight. Project Gemini ends then and no walks are planned for early Apollo missions.

The 75-foot-long McDonogh pool has been used for almost two years for simulated weightlessness research by a tiny, ten-man Randallstown firm called Environmental Research Associates.

Tries E.R.A. system

After Gemini 9's Eugene Cernan experienced real troubles doing even the simplest of tasks this year, the space agency decided to try the novel system worked out by E.R.A.

Astronaut Cernan, with air hose and all, dunked himself in early July to see if a swim approached the difficulties of zero gravity. It did, and astronaut Aldrin came here in early September for the same.

Below surface was a full-scale mockup of Gemini capsule's back end, where most spacewalkers' chores are done. Fully suited in special gear, Aldrin was kept suspended in the 10-foot-deep end by a system of weights and floats.

For more than two hours, he ambled around in free movement doing the type of chores he would do in November.

Like weightlessness

"The pool system," said a National Aeronautics and Space Agency spokesman at Houston's Mission Spacecraft Center, "has proven of value. It can be compared with weightlessness and it allows some good training."

With air bubbles rising from the strange scene, astronaut Aldrin worked around the Gemini mockup, his tether connected to it as in the real thing. He removed small items, replaced them and did other work.

"Some of our boys watch them on a limited basis," said one McDonogh teacher. "They seem very interested. But much of this work is done in the early morning and other times when no one is around."

Back-pack dropped

Aldrin's originally scheduled walk around the globe with a power back-pack has been dropped because of the difficulty Gordon had with lesser assignments outside the capsule.

But the swimming pool idea is catching. NASA is building a tank of its own at Houston "so we can do more of this simulation ourselves."

William Bruchey and Samuel Mattingly have directed the E.R.A. project at McDonogh. E.R.A.'s own personnel have plunged in place of other spacewalkers, Michael Collins and Gordon. Mr. Mattingly and Harry Loats are co-partners of the five-year-old firm.