An Elmhurst College professor is adding to the nation`s knowledge of whether man can survive long periods in space by growing tomatoes from seeds that orbited Earth for six years.

Biology professor Frank Mittermeyer has been growing the space tomatoes for two years, and he said that he`s getting fewer fruits this year on plants that are not as tall or as bushy as a year ago.

Though the comparison of this year`s tomatoes with the 1990 crop is hardly enough to reach any conclusions, Mittermeyer said that if the pattern continues, scientists might have to wonder whether man could sustain a space flight to Mars and beyond.

Many who took part in the much-heralded tomato project of the National Aeronautics and Space Administration have now lost interest, but Mittermeyer`s backyard garden in Elmhurst is still bearing the fruit of it.

In 1984, NASA bought millions of Rutgers-variety tomato seeds, keeping some on Earth and sending the rest aboard the Challenger space shuttle. Astronauts launched the seeds in cannisters into orbit from the shuttle. They were supposed to be in space for only a few months and be retrieved by another Challenger mission in 1986, but the shuttle exploded shortly after takeoff.

The seeds finally were plucked from space in 1990 by the Columbia space shuttle. In a project with extensive national publicity, tomato seeds from space and those kept on Earth were sent to thousands of professors, teachers and schoolchildren around the country to grow and as a way to arouse interest in science. Mittermeyer received more than 1,000 seeds.

Mittermeyer believes he is one of a few educators to persevere with the project. Last year, he said, some teachers and students abandoned their growing after an unfounded report out of California that the space seeds could produce radiation. He used a Geiger counter to measure the seeds, then the plants, and found no evidence of radiation.

Meanwhile, efforts by other students and their teachers last year to grow space tomato plants fizzled out because school ended in June and there was no one to tend the gardens, Mittermeyer said.

Mittermeyer has continued with the help of several students.

At first, he didn`t expect any of the seeds to grow because they were in space so long.

But he planted space seeds and those left on Earth as a control group and 80 percent of both seeds germinated. Last year, performance of both groups was the same in terms of size and quality of plants and fruits.

His 1990 botany students hypothesized that color and acidity of space tomatoes would be different from those grown from the seeds left on Earth, but they were the same.

Mittermeyer saved seeds from the 1990 space tomatoes and the control group to try again this year.

Looking over the 70 space tomato plants and 20 control-group plants in his garden, he asked a visitor, ''Can you tell which is which?''

One group appeared healthier than the other, with taller stems, more leaves, bushier plants and more tomatoes. Mittermeyer said this was the control group.

All plants were treated the same, with identical watering patterns and fertilizing.

Mittermeyer said he was concerned that the second crop of space tomatoes might be toxic. But groundhogs, squirrels and crows invaded the tomato patch and plucked away fruits with no signs of ill effects.

The space tomatoes are in big demand by college faculty members and other Elmhurst residents. Mittermeyer said they taste better than tomatoes from the control group, though he grants that this is a subjective judgment.