Soviet astronomers are proposing an orbiting, 100-ton "superobservatory," more massive than anything planned in the West, to be launched late in this century.

Nikolai S. Kardashev, a member of the Soviet Academy of Sciences, said this week that preliminary engineering studies of the superobservatory envision three powerful instruments to gain unprecedented views of the heavens.In an interview at the 20th assembly of the International Astronomical Union, Kardashev said plans call for parts of the observatory to be sent into Earth orbit aboard the Soviet's powerful Energia rocket.

The observatory would be assembled in orbit "either by robots or by astronauts and cosmonauts," he said.

A second rocket then would send the observatory to the so-called Lagrangian point L-2 about 900,000 miles from Earth where the gravitational attraction of the Earth, moon and sun balance each other. An object left in L-2 would remain at that point.

"This would give a very high resolution of the whole universe," said Kardashev. He said the sun, Earth and moon would be visible at all times from L-2 in one direction, while the rest of the universe could be seen by pointing instruments outward, away from the sun.

An observatory at L-2 would orbit the sun in about the same period as the Earth.

Kardashev said instruments proposed for the observatory are an X-ray and gamma-ray detector, a 400-meter radio telescope and a 10-meter optical telescope.

The optical telescope would be about four times more powerful than the $1 billion space telescope planned for launch next year by the National Aeronautics and Space Administration.

Kardashev said observation data collected by the instruments would be sent by radio signals to ground stations or satellites, and the observatory would be able to operate around the clock.

"This would be a very big project and we are very interested in international cooperation," he said.

Kardashev said the cost would be "one billion rubles or dollars," but American astronomers say a project of the scale decribed by the Soviets would cost much more than that.

The Soviet scientist said it is believed that if the project is started soon, it would be ready for launch by the year 2000.

The Soviets said in a statement the superobservatory "can only be realized in the context of broad, international collaboration."

"We herewith invite all interested parties to contact us at their earliest convenience," the announcement said.