The 85-ton Skylab, whose last astronaut visitors left it more than three years ago, will be revived again in April in an attempt to slow down a descent that could bring it crashing to earth by the fall of 1979.

From the Mission Control Center in Houston, flight directors will attempt to recharge Skylab's dormant batteries by using the space station's solar panels, which in springtime will be almost in line with the sun as Skylab follows the southernmost part of its orbit. The recharged batteries then would be commanded to fire shall jet engines to force Skylab into a slow end-over-end tumble to reduce the drag forcing the space station toward earth.

"The tumble would be so slow it could be likened to the minute hand moving around a clock," said an engineer at the Marshall Space Flight Center in Huntsville, Ala., where Skylba was built. "We could put Skylab nose on end into the wind, so to speak, just like a football where there's almost no dragon the ball after it's been perfectly place-kicked."

Skylab is in a slowly descending orbit that brings it as close as 230 miles to earth. If left alone, its orbit could drop 80 miles more in the next 18 months, and bring it perilously close to falling into the earth's atmosphere.

Tne National Aeronautics and Space Administration is concerned that Skylab might burn and break up in the atmosphere into pieces of jagged metal big enough to damage or harm whatever it might strike on earth. The size of a four-bedroom house, Skylab is by far the biggest satellite circling the earth, dwarfing the five-ton Soviet surveillance satellite that burned up over western Canada on Jan. 24.

It has been the space agency's plan to have astronauts rendervous the space shuttle with Skylab in October 1979 to install a small rocket engine in the space station's docking port. The rocket engine could then be fired remotely to lift Skylab into a higher and safer orbit where it could stay unattended for 10 years.

Recent calculations show that Skylab might fall out of orbit before the space shuttle could reach it. These calculations suggest Skylab could re-enter the earth's atmosphere as late as April or May of 1980 and as soon as might be too late.

The space shuttle will not make its maiden flight until June 1979 and could not be flown to rendervous with Skylab until October, by which time it might be too late.

Countless things could go wrong with the effort to revive Skylab in April, foiling any attempt to tumble the space station and extend its orbital lifetime. The sun might, have degraded its solar cells. It's batteries might not recharge. It's jets might not fire to begin the tumble.

"We might not even be able to communicate with it." worries Flight Director Eugene Kranz at Houston's Johnson Space Center. "And if we can't communicate, the ball game's over."

Even if Skylab's lifetime cannot be extended by remote maneuvers, the shuttle astronauts have a chance of doing it by hand if they can reach the space station before it enters the atmosphere.

If Skylab comes down before the astronauts get to it, the 85-ton machine could break up anywhere in the world along its orbital path. An estimated 200 pieces of space debris have fallen to earth in places as far as apart as Michigan and Rhodesia. None fell where it was predicted to fall.