New Horizons made its closest approach of Pluto, a dwarf planet smaller than Earth's moon, at 1149 UTC Tuesday and spent another eight hours studying the planet's atmosphere and nighttime side. Four and a half hours later, scientists on the ground 5 billion kilometers (3.1 billion miles) away received the good news: New Horizons had made a successful pass of the highly symbolic orb and its five moons.

"This is truly a hallmark in human history," said NASA associate administrator John Grunsfeld.

The ship had not yet transmitted 99 percent of its data before emerging from the Pluto system on its 50,000-mile-per-hour (30,000 kph) pass. A debris strike (odds: 10,000 to 1) could have destroyed all the data. However, the $700-million (635-million-euro) spacecraft stayed its course just 7,750 miles from Pluto.

"Now the solar system will be further opened up to us, revealing the secrets of distant Pluto," British cosmologist Stephen Hawking said in a message broadcast on NASA TV. "We explore because we are human and we want to know," he added. "I hope that Pluto will help us on that journey."

Downgraded but significant

New Horizons launched from Cape Canaveral in 2006, back when scientists still considered Pluto the solar system's ninth planet. However, six months later, the International Astronomical Union's annual general assembly, held that year in Prague, voted on the meeting's final day to downgrade the plutoid to a dwarf planet, a decision still disputed.

Pluto orbits the sun just once every 248 Earth years, on an elliptical path that crisscrosses the more regular path of its larger neighbor - and now the solar system's last officially full-fledged planet - Neptune. Other plutoid spheres also orbit in the Kuiper Belt, the region beyond Neptune. Scientists consider the objects remnants from the formation of the solar system 4.6 billion years ago.

Images and measurements relayed from New Horizons have changed scientists' understanding of the dwarf planet, which has yielded evidence of past and possibly present tectonics, or crust movement.

Even at light speed, it will take about 16 months for New Horizons to transmit back all the thousands of 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.

mkg/cmk (Reuters, AFP, AP)