Laurel (Maryland), July 14 (Reuters): More than nine years after its launch, a US spacecraft sailed past Pluto today, capping a 4.88 billion km journey to the solar system's farthest reaches, Nasa said.

The craft flew by the distant "dwarf" planet at 7:49am after reaching a region beyond Neptune called the Kuiper Belt that was discovered in 1992. The achievement is the culmination of a 50-year effort to explore the solar system.

"It's truly a mark in human history," said John Grunsfeld, Nasa's associate administrator for science from the mission control centre at Johns Hopkins University Applied Physics Laboratory.

Final confirmation that New Horizons passed some 12,550km from Pluto was expected at 8:53pm. The spacecraft is so far from Earth that radio signals, travelling at the speed of light, take about four and a half hours to reach Earth. "We'll get information from the spacecraft and know if it's healthy," said Alice Bowman, the mission operations manager.

By then, New Horizons will have spent nearly a day in radio silence, running through a tightly choreographed series of observations as it shoots past Pluto and its entourage of five moons, travelling at about 14km per second.

For several hours following closest approach, the diminutive nuclear-powered probe, which is about the size of a baby grand piano, will look back at Pluto, now backlit by the sun, to study its atmosphere using radio signals beamed from Earth as a probe.

Scientists have many questions about Pluto, which when New Horizons launched in 2006 was still considered the solar system's ninth planet. It was demoted to the status of "dwarf planet" after the discovery of other Pluto-like, ice-and-rock worlds.