NASA restored full contact with Voyager 1 after the space probe stopped sending data to Earth this week - the first such malfunction since it was launched in 1977 to explore Jupiter and Saturn.

"We're relieved, of course. But now there's the question of why did it happen and is it going to happen again?" John Tullius, senior mission controller for Voyagers 1 and 2 at the National Aeronautics and Space Administration's Jet Propulsion Laboratory, said Thursday.But flight engineering manager Lanny Miller said the partial loss of contact "wasn't too worrisome" because engineers were confident of their ability to correct the malfunction.

The Voyagers were launched from Florida in 1977. They explored Jupiter in 1979. Voyager 1 explored Saturn in 1980, followed in 1981 by Voyager 2, which went on to study Uranus in 1986 and Neptune in August. The twin spacecraft learned so much that scientists have been forced to revise astronomy textbooks.