Houston — Hundreds of miles above Earth, possibly the greatest spaceflight drama since the struggles to save Apollo 13 and the crippled Skylab space station has been unfolding in recent weeks.
Soviet Cosmonauts Vladimir Lyakhov and Alexander Alexandrov remain in orbit long past their scheduled return date: Their replacements narrowly escaped death when their launch rocket blew up underneath them. Widespread reports have it that the cosmonauts in orbit are marooned. Their Salyut 7 space station has also been the subject of dramatic rumors. The most critical one concerns a possible rocket fuel leak that may have crippled most of the station's maneuvering capability.
To make sense out of these events it is necessary to understand what the Soviets have been trying to do in space recently.
What the Soviets attempted to do Sept. 26 was nothing short of making space history with a new ''giant leap for mankind'' - a continuous human presence in space. Cosmonauts Vladimir Titov and Gennady Strekalov were to have undertaken the world's first space station relief mission, allowing the two current inhabitants of Salyut 7 to return home while the space station continued to function.
In the past, long-term crews have flown to an empty station, activated it, received visiting crews, then after many months deactivated the station and left it empty. That was not supposed to have happened this time - and this goal may still be reached.
Formerly, visiting crews spent about a week aboard the Salyut. This time, the overlap may have been scheduled to last two or three weeks: The aborted launch attempt (to have been called Soyuz T-10) occurred at least 10 days earlier than would have been the case for a routine week-long visit. This would have given the new crew sufficient time to get used to space conditions and to be fully briefed by the departing crew on the station's routines and idiosyncrasies.
The purpose of routine week-long visits is tied in with another problem that the launch failure led to. The seven-ton Soyuz craft, unlike the multiyear-mission, 20-ton Salyut module, has a limited lifetime in orbit. As a general rule, such spaceships only spend 60 to 80 days in orbit before returning. Three times before, Soyuz ships had remained in space for slightly more than 100 days, but all three were contingency cases.
During missions of six or seven months' duration, the Soyuz craft attached to the Salyut must be periodically swapped for a fresh one. About every 58 days a new visiting crew is launched.
Soviet space officials have never directly explained the reason for the duration limitation rule, but reasonable analogies with American space vehicle characteristics can provide some insight. First among them is the presence of highly corrosive ''hypergolic'' propellants - fuels that, when mixed, ignite spontaneously - in the Soyuz service module tanks. As weeks pass, the exposed valves and seals in the engine could be expected to deteriorate. They may suddenly leak all the fuel, or they may not function at all when called upon during the critical return to Earth. Also, batteries aboard the Soyuz may slowly lose their charges. Finally, exposure to 16 cycles of daily sunrise-sunset thermal stress may have a cumulative impact on the mechanical hardware of the ship.
It was this gradual aging of the Soyuz T-9, which brought cosmonauts Lyakhov and Alexandrov to Salyut 7 late in June, that set off alarms around the world on Oct. 19, the day that the Soyuz exceeded in age the previously longest Soyuz mission.
The alarm was sounded in a BBC report from London, which said that the cosmonauts could not return home safely, nor could they remain aboard the failing Salyut space station.
The report was based on authentic anxieties. With the danger of a return to Earth aboard the aging Soyuz T-9 growing daily, the cosmonauts may also have been faced with serious doubts about the security of the Salyut. A report by Aviation Week & Space Technology magazine described a serious rocket fuel leak that was said to have occurred Sept. 7. Two of three sets of fuel tanks on the Salyut were reportedly involved. The magazine, evidently relying on United States intelligence sources that routinely monitor conversations between the cosmonauts and mission control in Moscow, claimed that the tanks drained overboard due to a ruptured pipe. Aviation Week also reported that the cosmonauts were about to evacuate the station in emergency mode before mission control advised them to stay. At that time, the station had been pumping a new supply of rocket fuel from the unmanned tanker spacecraft Progress 17, so a failure in the fuel transfer apparatus may also be plausible.
Since the date of the alleged fuel leak, the Soviet cosmonauts have been following a flight plan consistent with such a situation. They have greatly restricted the station's pointing maneuvers over ground targets for photography, while concentrating instead on passively drifting through space. This flight plan is very useful for materials-processing experiments, which must avoid disturbances induced by firing rockets. According to Soviet news reports, the cosmonauts are conducting those kinds of experiments.
But even if the fuel-leak story were true, it does not spell the end of Salyut 7 and the latest big Soviet push in space. Perhaps the pipes can be repaired during a spacewalk, using special tools possibly sent up on the Progress 18 robot freighter that linked up Oct. 22. (Indeed, such space walks took place Tuesday and Thursday, according to the official Soviet news agency Tass - although the agency said that the two cosmonauts were installing a new solar battery.) Perhaps the leak actually involved only equipment aboard the previous robot tanker, Progress 17.
And if the propulsion system failure is real and cannot be fixed, the Soviets still have one ''ace in orbit'' left. This is another unmanned space vehicle, designed to act as an add-on module to the Salyut space station. Once attached, the module - which is vaguely similar in size, weight, and capabilities to the Salyut - can serve as a specialized laboratory, a storeroom (with at least three times the cargo capacity of a Progress craft), or even a greenhouse. The module has its own power supply (solar panels) with which to augment the Salyut, and - most important - it has its own propulsion system.
When such a vehicle was tested earlier this year - under the code name ''Kosmos 1443'' - it spent several months linked up with the Salyut 7. Future launches of such modules are expected shortly. Once a new craft of this type can be attached to the Salyut 7, all propulsion difficulties will be moot.
While the West watched anxiously for some sort of Soviet rescue operation in orbit, the path through space of the Salyut 7 space station made it clear that indeed the Soviets were preparing a new launching.
At 12:59 p.m. Moscow time on Oct. 20, the Soviets launched an unmanned supply ship, Progress 18. It linked up successfully at 2:34 p.m. Moscow time, two days later. A few hours later, as the station crossed North America, radio listener Richard Flagg in Melbourne, Fla., heard the two Russian spacemen laughing and joking as they opened letters and gifts from home. ''A happy crew'' is how Mr. Flagg, an experienced Soviet space-watcher, put it.
Such supply flights must occur every two months or so to restock the life-support consumables aboard the station, and to bring up new equipment as needed. According to Moscow, the ship was also carrying rocket fuel to replenish the Salyut's drained tanks.
Meanwhile, the Soyuz T-9 continued to grow older. But under automatic interrogation from the ground (Flagg and his colleagues were picking up intense bursts of radio telemetry data from the Soyuz, informing Soviet space engineers of the status and health of all its systems) the Soyuz seemed to be receiving a clean bill of health. It would take up to three weeks to unload the Progress and clear the docking port it occupied, so by the time any new ship could be sent up the Soyuz T-9 would be at least 135 days old. But perhaps even that unprecedented age can be tolerated.
It would certainly benefit the Soviets to qualify their Soyuz ships for missions of that duration; thus, they may receive a major bonus by being forced to do so this time. Such an extended lifetime would allow a reduction in the required number of ''Soyuz-swap'' visiting missions, at great savings in money and crew training time.
A new attempt to send up a relief crew could occur within a few weeks, after the Progress 18 freighter has been unloaded and jettisoned. If by then the Soyuz T-9 has lost enough reliability for a safe return trip, an unmanned Soyuz could be sent up first.
When this long-anticipated crew hand-over occurs, Western observers will realize that space may never again be empty of human travelers. There will henceforth always be people in space, living, working, observing, . . . and speaking Russian.