Hal Clement is an award-winning science fiction writer, a bona fide Grand Master of the genre, but he looks more like a high school teacher as he tells a crowd of fans about subjects like "What You Can't Get Away with in Hard SF and How to Do it Anyway."

That's not too surprising -- Clement really is a retired science teacher, and he's spent decades learning how to hold an audience's attention.

He ambles back and forth in front of slides of classic SF cover paintings, pointing out the features that were facts then and are fantasy today. The 78-year-old writer read most of these books and magazines when they were first published, and several of the images are from his own stories.

Clement reminisces about an painting of Mercury from the days when it was believed to face only one side to the sun.

Editor Frederik Pohl -- also famous for his SF writing and work as an agent -- liked to commission cover stories based on paintings, and asked Clement for one based on this image.

Unfortunately, the cover depicts a fanciful volcano exploding in the background, so Clement had to put careful thought into how he would explain the presence of vulcanism on Mercury, which is actually almost completely inactive.

Like many writers, Clement has a sharp memory for editorial changes. Pohl frequently retitled stories, and Clement now remembers the alteration from "Small Change" to "Hot Planet" better than he can recall the story's plot.

Slide rules and misshapen planets

The talk on hard SF is one of several he holds regularly. He first gave it during the Voyager missions, he says, but the exact scientific content has evolved over time.

"During the Neptune flyby," he says, "I promised to give a talk on where we were wrong about Neptune the next year."

He did, and he remembers that the biggest difference was that Triton was smaller and less massive than expected.

"I'm not calling anything impossible. When you run into a problem, treatit as a challenge."
     

Clement is famous for working out all the details of his imaginary worlds, but science still has a way of catching up with science fiction. Occasionally even the grand master gets a detail or two wrong.

In his classic 1954 novel Mission of Gravity, for example, "the planet was the wrong shape."

Clement thought his high-gravity world should be an elliptical mass after doing the math on his slide rule, but a group of fans at MIT plugged the data into a computer and came up with a more spherical shape.

Getting caught in a mistake doesn't bother Clement much. His fans had fun finding it, he says, "and I did write the story to be entertaining."

Science changes, good storytelling doesn't

Despite all the advances in science over the last half century, he doesn't think SF is much different today.

As he sees it, the constant expansion of scientific knowledge "limits some stories, but it expands others."

No matter how much the science behind good SF changes, Clement points out that you need the same fundamental elements of plot and character.

The biggest change in SF, he says, is the way it keeps expanding into new territories.

"The boundary between genres has grown foggier and foggier, and as soon as you try to define it some character immediately writes a story that goes outside it," he says with a smile.

To this day, Clement doesn't know whether Campbell was serious or was merely engaging in an advanced form of author motivation.

"I've never been sure in which particular cases he was serious or challenging," he says.

He does know Campbell never let his beliefs interfere with buying a good story. Clement never went along with Campbell's late-life obsession with dianetics, but the two always maintained a strong professional relationship.

An even tougher audience than SF fans

Clement credits much of his success as a writer to his years as a teacher. He says teaching astronomy to high school students encouraged him to constantly keep up with the field.

It also taught him the importance of communicating information clearly. "I'm still trying to avoid ambiguity," he said.

High schoolers aren't the toughest audience Clement has faced, though. That would probably be the U.S. Air Force Reserve shortly after Sputnik went up in 1957.

Clement had just been promoted to Major in the Reserve and assigned to a new base. His new commander knew he was a teacher, and introduced him to the troops by asking him to explain why Sputnik wasn't falling out of the sky.

Clement lectured for an hour on Sputnik and the basic principles of rocketry behind it, but was floored when an airman stuck up his hand and asked, "But when is it going to run out of gas?"

Balanced between faith in learning and skepticism

Occasional misunderstandings aside, Clement thinks better education is the key to the future of science and space.

"I was lucky to learn in my first year as a teacher," he says, "that it was best to admit when I didn't know something."

He worries that's a lesson many teachers never learn.

Clement says that students have to learn to think for themselves. That skepticism is important, but students must also be taught they can really learn things from study and experimentation.

"You have to strike a happy medium," he says.