More than nine years after its launch, an American spacecraft has sailed past Pluto, capping a journey of almost 5 billion kilometres to the farthest reaches of the solar system.

Nasa’s New Horizons spacecraft passed by the planet and its five moons last night in what NASA says is truly a hallmark in human history.

The event culminated an initiative to explore the solar system that the space agency embarked upon more than 50 years ago.

The spacecraft is so far away that radio signals, traveling at the speed of light, take about four and a half hours to reach Earth.

When New Horizons was launched in 2006 Pluto was still considered the solar system’s ninth planet but was shortly after demoted to the status of “dwarf planet”.

It was expected to continue observing Pluto for several hours after it made its closest approach to the unexplored world, coming within 12,472 km of the sphere.

The diminutive spacecraft, which is about the size of a piano, is not equipped with the propellant needed to brake and slip into orbit around Pluto. Like Nasa’s Voyagers and sister predecessor exploratory spacecraft, New Horizons was designed to conduct its survey on the fly.

Managers estimate there is a 1-in-10,000 chance that a debris strike could destroy New Horizons as it traverses the Pluto system.

With 99 percent of the data gathered about Pluto still on the spaceship, its survival is critical to the mission.

Already, the trickle of images and measurements coming from New Horizons has changed scientists’ understanding of Pluto. Once considered an icy, dead world, the planetoid has yielded signs of geological activity, with evidence of past and possibly present-day tectonics, or movements of its crust.

Pluto circles the sun every 248 years in a highly tilted orbit that creates radical changes from season to season. Pluto travels closer to the sun than the orbit of Neptune before it cycles back into the solar system’s deep freeze more than 40 times farther away than Earth.

Scientists have many questions about Pluto, which was still considered the solar system’s ninth planet when New Horizons was launched in 2006. Pluto was reclassified as a “dwarf planet” after the discovery of other Pluto-like spheres orbiting in the Kuiper Belt, the region beyond the eighth planet, Neptune.

The objects are believed to be remnants from the formation of the solar system 4.6 billion years ago.

It will take about 16 months for New Horizons to transmit back all the images and measurements taken during its pass by Pluto. By then, the spacecraft will have traveled even deeper into the Kuiper Belt, heading for a possible follow-on mission to one of Pluto’s cousins.