Pluto largest known object beyond Neptune orbit: NASA

06-21

0 2015-07-15 08:52Xinhua Editor: Gu Liping

The latest and most detailed image of Pluto sent back to Earth. This was captured by New Horizons at about 4 p.m. EDT on July 13, about 16 hours before the moment of closest approach. The spacecraft was 476,000 miles from the surface.(Photo provided to China News Service)

An unmanned spacecraft, New Horizons, making its closest flight by Pluto after a decade-long journey, has proved that the planet is larger than all other known objects beyond the orbit of Neptune, the U.S. space agency said.

At 7:49 a.m. EDT (1149 GMT) on Tuesday, the spacecraft passed a point some 12,500 km from Pluto, which is about 4.77 billion km away from Earth as of this month, and will keep traveling in the surrounding space area, called the Kuiper Belt, according to the National Aeronautics and Space Administration (NASA).

Scientists have found Pluto to be 2,370 km in diameter, somewhat larger than many prior estimates. Images acquired with the Long Range Reconnaissance Imager aboard New Horizons were used to make this determination, NASA said.

"The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest," said mission scientist Bill McKinnon with Washington University in St. Louis, Missouri.

Measuring Pluto"s size has been a decades-long challenge due to complicating factors from its atmosphere. A bigger size means that its density is slightly lower than previously thought, and the fraction of ice in its interior is slightly higher. Also, the lowest layer of Pluto"s atmosphere, called the troposphere, is shallower than previously believed.

So Pluto can now be regarded as the king of the Kuiper Belt, a space region of thousands of icy worlds that orbit Sun beyond Neptune. The new discovery also indicates that Pluto is larger than Eris, another distant world with a diameter of 2,326 km across.

Though it may be smaller in size, Eris remains about 30 percent more massive than Pluto, said astronomer Mike Brown, who has been called a "Pluto killer" as the discovery of Eris played a part in Pluto"s demotion to a dwarf planet in 2006 by the International Astronomical Union.

The closest approach to Pluto is scheduled later Tuesday, when New Horizons will pass within 10,000 km of the planet"s surface, traveling at a speed of 43,000 km per hour.

After a long voyage, New Horizons is almost 4.8 billion km away from Earth, where radio signals, even traveling at light speed, need 4.5 hours to reach Earth. Round-trip communication between the spacecraft and its operators requires about nine hours.

The piano-sized probe will perform the first close-range study of the remote, icy world and its five attendant moons before continuing its journey deeper into the Kuiper Belt to examine one or two of the ancient, icy small worlds in that vast region, which is at least 1.6 billion km beyond Pluto.