MIAMI (AFP) - Nasa briefly lost touch this weekend with an unmanned spacecraft on its way to a historic encounter with Pluto, but the United States space agency said on Monday that the glitch had been fixed.

The communications blackout on Saturday lasted nearly an hour and a half, and perplexed scientists as they tried to figure out what had gone wrong aboard the New Horizons spacecraft nearly 4.9 billion km from Earth.

The mission is being closely watched by millions of space enthusiasts because the spacecraft's flyby of Pluto on July 14 will offer the world's first close look at the distant dwarf planet.

"The New Horizons spacecraft experienced an anomaly the afternoon of July 4 that led to a loss of communication with Earth," Nasa said on its website.

"Communication has since been reestablished and the spacecraft is healthy."

The spacecraft was pre-programmed to switch from its main to a backup computer in the event of such a problem.

"The autopilot placed the spacecraft in 'safe mode,' and commanded the backup computer to reinitiate communication with Earth. New Horizons then began to transmit telemetry to help engineers diagnose the problem," Nasa said.

No hardware or software problems were found.

Instead, the diagnosis was "a hard-to-detect timing flaw in the spacecraft command sequence that occurred during an operation to prepare for the close flyby", the US space agency said on Monday.

The glitch required the spacecraft to stop collecting scientific data, but those operations are on track to resume on Tuesday.

"I'm pleased that our mission team quickly identified the problem and assured the health of the spacecraft," said Mr Jim Green, Nasa's director of planetary science.

"Now - with Pluto in our sights - we're on the verge of returning to normal operations and going for the gold."

Pluto was long considered the ninth planet in the solar system, and the farthest from the sun. It was reclassified as a dwarf planet in 2006.