Exploring space with the help of a hand computer

It's only a humble-looking hand computer, not much different in appearance from many other so-called ''pocket calculators.'' But for shuttle astronauts Jack Lousma and Gordon Fullerton, it represents powerful computing power in the palm of a hand.

For some important, even critical, tasks, it frees them from dependence on the large computer complex at the mission control center, or even their on-board units.

The astronauts, at this writing, have started their second day in orbit on what, so far, has been a smoothly running mission. As they move around Earth, putting the space shuttle Columbia through various tests, they constantly lose and re-establish communication with Houston. One of their two hand-held computers keeps track of their position. When they are out of contact, it locates the next relay station. It tells the astronauts when they will be within its range and which radio frequency to use. Once contact is established, the computer tells how long it can be maintained.

Should something happen to force the astronauts to decide to make an emergency landing when out of touch with mission control, the little computer can pick a landing site and figure the timing of the retrorocket burn needed to bring Columbia in.

There are six designated landing sites. At least one of these is available during each orbit. In selecting the right site, the computer takes account of the requirements that the site be within 995 miles cross-range of Columbia's orbit path and be in daylight at the time of landing. The computer then picks the site based on the mission's elapsed time and of Columbia's velocity and position. This is a task for the little computer that the National Aeronautics and Space Administration (NASA) has designated as ''flight-critical.''

Astronaut Terry Hart, who is serving as a mission control communicator for Columbia here, has taken a special interest in the computer. He says that one of its big advantages is ease of programming. It can take as much as two years to come up with a new program for a large computer, even for Columbia's on-board navigational computers. But complex programs for the hand-held unit can be written, tested, and rewritten if necessary within a few weeks.

Moreover, programs can be quickly changed in these machines. They can be stored on (and read from) magnetic tape or small magnetic cards. An optical scanner can even load programs recorded in bar code, similar to the price codes on grocery items.

Mr. Hart explains that the seach for a hand-held computer began when astronaut Robert Crippen requested one before the first shuttle flight. NASA settled on a machine made by Hewlett-Packard called the HP-41. It evolved out of the line of programmable scientific calculators that the company developed during the 1970s. What appealed to NASA, Hart says, was the machine's versatility, large memory (for a small computer), and the fact that it has an ''alphanumeric display.'' That means it gives messages to the astronauts in English words as well as numbers.

Special modules called ROM's (read only memory) plug into the machine to enhance its capabilities. For example, these enable it to perform complex financial calculations, act as a navigational computer, or even a game machine. The astronauts are using a timer module that not only keeps track of the mission's elapsed time but also reminds them when to perform various tasks.

Also, Hart notes, unlike most ''high tech'' shuttle equipment, the HP-41 is a consumer item widely available around the world. He adds that they bought the astronauts' units off the shelf from a Houston retail outlet and the units are being used without modification.

Although the HP-41 was unique when NASA chose it, other hand-held computers are beginning to appear. The Japanese company Sharp has introduced two models over the past couple of years which can be programmed in the computer language Basic.

Although these new computers are not yet as versatile as the HP-41, the little computer that is helping the astronauts manage Columbia has started a trend in which computer power, recently available only in desk-top or larger units, will become an item of personal equipment like a wristwatch or a simple calculator.