You, too, may speak with an astronaut.

What do you do when you're orbiting 135 miles above Earth, have some spare time on your hands, and obviously have nowhere else to go? The answer, at least for STS-9 mission specialist Owen K. Garriott, is to pick up a small, hand-held radio and talk to some folks back on Earth.

It may be a footnote to this historic mission, but Dr. Garriott (his call sign is W5LFL) will be the first ham radio operator in history to transmit from space.

But if it's a minor note to space historians, it is cause for great excitement among others. Even if you discount the hams clustered around a console of radio equipment here last night, or many of the rest of the estimated 1.5 million to 2 million hams worldwide, there are the 42 first-graders at the Avery Street School in South Windsor, Conn.

They were to gather today at 10:50 a.m. Eastern Standard Time to watch ham radio operator Sally O'Dell try to contact Dr. Garriott during Columbia's 49th orbit. Her husband, Peter (also a ham), says, ''The kids are really excited about this.'' His source? Their daughter, who is one of the first-graders.

But to some, there's more to this aspect of the Spacelab flight than high-tech show-and-tell.

One longtime National Aeronautic and Space Administration (NASA) engineer here says, ''NASA has always rigidly controlled who can communicate with astronauts during a mission.''

The fact that NASA allowed Garriott to operate while in space, he says, ''is a real breakthrough,'' perhaps bringing the US space program into more intimate contact with everyday people - and some not-so-everyday people. Among the stations Garriott is expected to listen for are those of Sen. Barry Goldwater (R) of Arizona and Jordan's King Hussein.

This event is an evolutionary step for ham radio's use of space. In 1961, three hams built the first privately owned satellite, OSCAR 1. It cost about $60 and was lofted into orbit on a rocket already carrying an Air Force payload. The unit wasn't very sophisticated: It orbited the Earth sending out the characters HI in Morse code. This message could either be taken as a greeting or as the first cosmic joke: HI is a telegraphy abbreviation for laughter.

OSCAR 1 was the forerunner of a series of amateur radio satellites, the most recent of which is OSCAR 10, launched last summer on Ariane 6. The vehicle is a sophisticated voice- and data-relay satellite.

The idea for letting a ham operate in space first surfaced when Richard Fenner, a NASA engineer and avid ham, proposed the idea for the Skylab 3 mission in 1973, Garriott's first space trip.

But the idea ran aground on the shoals of money, a lack of suitably small radios, and - perhaps most of all - the fact that Mr. Fenner wanted to put another antenna on the orbiting laboratory.

That last item, he says somewhat whimsically, ''was a very unpopular idea in the NASA engineering community.''

But the idea didn't sink. And last April, NASA approved a proposal put forth by the American Radio Relay League, a national amateur radio organization, and the Amateur Radio Satellite Corporation, a nonprofit organization dedicated to lofting ham radio satellites.

The two organizations asked that they be allowed to supply an amateur radio transceiver for use by astronauts who were hams. Permission granted, said NASA.

The effort represents a lot of personal time invested: Members of the amateur radio club at the Johnson Space Center developed the specifications for Garriott's five-watt, hand-held radio. They also spearheaded the integration of the project into the shuttle mission. The ham radio club at a Motorola Corporation facility in Fort Lauderdale, Fla., designed and built the radio.

And Garriott himself did much of the planning for the ham-in-space activity. His ham radio operation in space is also in his spare time.

Garriott is operating on portions of the two-meter amateur band, which runs from 146 MHz to 148 MHz. And while hams are the only ones who will be able to call Garriott, anyone with a VHF scanner that can tune in 145.55 MHz will be able to pick up his signals when he is passing overhead. In addition, payload specialist Ulf Merbold, though not a ham, will be allowed to contact hams in West Germany during appropriate orbits.

For information on where and when to listen for Garriott's transmissions, call 1-800-SCANNER.