File Photo: Pluto (R) and its moon Charon are pictured from about 6 million kilometers in this July 8, 2015 NASA handout photo. (Reuters Photo)
Scientists at Nasa's New Horizons mission control centre are celebrating the spacecraft's dramatic flyby of Pluto, the last unexplored world in the solar system.
The US probe shot past Pluto at more than 45,000 km/h (28,00mph) at 12.49pm UK time on a trajectory that brought the spacecraft within 12,500km of the body's surface.
The moment marks a historic achievement for the US, which can now claim to be the only nation in the world to have visited every planet in the classical solar system.
"It should be a day of incredible pride," said Charlie Bolden, Nasa's chief administrator.
In 2006, seven months after New Horizons blasted off on its mission, astronomers at the International Astronomical Union changed the definition of planet, a move that downgraded Pluto to a dwarf planet.
In a live interview on Nasa TV on Tuesday, Bolden said he hoped that the scientists would reconsider the name. "I call it a planet, but i'm not the rule-maker," he said.
Bristling with cameras and other instrumentation, the New Horizons probe was programmed to gather a wealth of images and data as it sped past Pluto and its five small moons, Charon, Styx, Nix, Hydra and Kerberos.
Mission scientists at Johns Hopkins University in Baltimore were out of contact with the spacecraft as it hurtled past the icy body 4.8bn km from Earth. Instead the probe captured images and took measurements automatically and stored them on board to send back later.
At such a great distance, direct control from the ground is impossible, because radio signals take more than nine hours to travel to the spacecraft and back again.
Scientists now face an agonising wait for news from the spacecraft, which is due to call home at 2am UK time on Wednesday morning. Only when that signal is received will Nasa officials know whether New Horizons survived the flyby.
The greatest hazard posed to the spacecraft is dust that may form a hazy cloud around Pluto after being knocked off its moons by meteorite strikes. Hal Weaver, a scientist on the mission, said that colliding with a dust particle the size of a grain of rice could potentially destroy the mission. But the risk of such a catasrophic failure is low, at less than one in 10,000.
Images beamed back from New Horizons have shown Pluto in shades of red, with valleys, mountains and craters. Fresh measurements from the probe found that Pluto was larger than previously thought, at 2,370km across.