The European Space Agency (ESA) yesterday chose as its next major scientific project a joint U.S.-European exploration of Saturn's moon Titan, a body that may resemble Earth before life evolved here.

The decision is expected to put pressure on the National Aeronautics and Space Administration to come up with an estimated $1.5 billion for its share of the project at a time when the U.S. space agency is struggling to pay for its own costly programs.

Space shuttle costs have soared well beyond the levels that existed before the Challenger explosion. And NASA wants to move ahead with developing a permanent space station. But the incoming Bush administration is expected to push hard to cut spending to hold down the federal budget deficit.

Most scientists believe that knowledge about the solar system and the universe can be gained most efficiently through unmanned projects. But in recent years, budgetary pressures have forced space science to take a back seat to the more glamorous but less scientifically rewarding manned space program. Many U.S. unmanned space science projects have been delayed, some for a decade.

The European decision is seen as pressure to redress the imbalance that has developed in recent years. NASA is seeking approval from the Office of Management and Budget to include the first stage of funding for the project in its 1990 budget.

ESA's scientific director, Roger Bonnet, said ESA's decision means it is "introducing planetary exploration as a major theme in Europe's long-term space science program, Horizon 2000."

The joint mission, called Cassini after the 17th century French- Italian astronomer who discovered some of Saturn's moons, has been envisioned by scientists on both continents as of unusual scientific value.

The ringed planet's moon Titan may have environmental conditions resembling those of the Earth before life arose billions of years ago. Titan is the only moon known to have a thick atmosphere rich in nitrogen, as is Earth's atmosphere, and also rich in various organic compounds. Chemical reactions now taking place there may be like those that gave rise to life on Earth. Titan's cold surface, 290 degrees Fahrenheit below zero, may be covered by lakes or even oceans of the organic compounds methane and ethane.

Plans for the joint mission call for a NASA rocket, launched in 1996, to propel a U.S.-built Cassini spacecraft into a course that would reach Saturn in 2002. Then, just before going into orbit around Saturn, Cassini would release a European-built probe that would veer toward Titan and attempt to parachute softly to its surface. An automated chemical laboratory, it will analyze Titan's atmosphere on the way down and, if it survives the landing, analyze the soil and radio data back to Earth.

The Titan probe, called Huygens for the Dutch astronomer who discovered Titan and Saturn's rings in 1656, is the only part of the project to be built by the Europeans. ESA officials say the probe is expected to cost about $260 million.

The main Cassini spacecraft is intended to stay in orbit around Saturn for four years, studying the planet and, when orbital conditions are right, Titan. It is planned that Cassini's orbit and that of Titan would bring the two within 600 miles of one another more than 30 times over four years.