The interplanetary space probe Ulysses officially ceased operations on Tuesday after an 18-year voyage of roughly 5.5 billion miles (8.85 billion km) and nearly three complete orbits around the sun, NASA said.

Radio contact with the Volkswagen-sized spacecraft was halted by ground controllers shortly after 1 p.m. PDT/2000 GMT, but NASA project manager Ed Massey said Ulysses will continue its wide, elliptical orbit around Earth's local star indefinitely.

He said there was a chance the probe might eventually swing close enough to one of Jupiter's moons to alter its course and place it on a path that will take it out of solar system and into interstellar space. The spacecraft was about 437 million miles (705 million km) from the sun at the time that its transmitter was switched off, Massey said.

Ulysses, a joint project of NASA and the European Space Agency, was launched from the U.S. space shuttle Discovery in October 1990 and became the first probe to fly around the sun's poles. As of mid-June, it had logged 5.4 billion miles and nearly three complete solar orbits.

Named for the hero of "The Odyssey," Ulysses was designed to help scientists study solar radiation and was originally expected to last for just five years.

By staying active for the better part of two 11-year solar cycles, Ulysses collected a wealth of information that formed the basis of over 1,000 scientific articles and two books.

Among its discoveries was a finding that the solar wind, a steady stream of charged sub-atomic particles blown out from the sun at about 1 million mph (1.6 million kph), has dwindled to its lowest level in at least 50 years.

The solar wind inflates a massive protective bubble, called the heliosphere, around the solar system. As the solar wind weakens, the heliosphere is expected to contract in size and strength as well, allowing more cosmic radiation -- super high-energy electrons and protons zipping through interstellar space -- to reach the inner solar system.

The Earth remains shielded from these potentially harmful cosmic rays by virtue of a magnetic field that surrounds our planet. But the diminished solar wind and corresponding rise in cosmic rays are a concern for astronauts and spacecraft that venture beyond Earth's orbit.

The biggest implications of those fluctuations, and other observations made by Ulysses, are scientific ones.

"The data acquired during the long lifetime of this mission have provided an unprecedented view of the solar activity cycle and its consequences and will ... keep scientists busy for many years to come," Ed Smith, a scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, said in a statement.