INEVITABLY, the explosion of the space shuttle has stirred a debate about the worth of the manned space program. Yet why? There is obvious reason for NASA to find out everything it can about what went wrong aboard Challenger. That will take time, and it will no doubt crimp the space agency's plans to fly a record 15 shuttle missions this year. It may also quicken the Pentagon's interest in finding an alternative launcher for its special payloads. Beyond the underlining of the necessary cautions, however, nothing significant has changed. The loss of a $2.2 billion vehicle and the death of its seven-member crew only made real what too many people had tended to overlook: space flight, for all the appearance of safety and routine that the previous 24 shuttle missions had imparted, entails audacious risks.

The loss is bound to make people think again of why there is a manned space program. The great costs of the manned space station that NASA is now designing tend to force the same question. It is observed, and it is perfectly true, that unmanned machines in space do fantastic things. Just the other day the Voyager 2 spacecraft, completing a journey of 3 billion miles and 81/2 years, provided man's first close look at the planet Uranus. The information it sent back is invaluable and could not possibly have been retrieved by man.

The uses of manned flight, however, are not really competitive with those of unmanned flight, and they remain what they have been through the last quarter-century. There is scientific, military and conceivably some commercial work to be done in space, notwithstanding the marvels of the instruments and robots in unmanned spacecraft. There is the great and unending adventure of exploration and discovery. There is an enduring current of national prestige.

The costs and uncertainties of manned space were already imposing a certain scrutiny of the program before this week. The tragedy off Cape Canaveral adds further impetus to this review. All this is fine, but it is necessary to keep in mind that no part of the rationale for manned flight ever was that it was easy and safe.