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It has taken more than nine years for an American spacecraft to reach the far edge of our solar system. The spacecraft, named New Horizons, sailed past the "dwarf" planet Pluto on Tuesday. New Horizons flew 4.88 billion kilometers to reach Pluto.

Guest and New Horizons team members countdown to the spacecraft's closest approach to Pluto, Tuesday, July 14, 2015 at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. Photo Credit: (NASA/Bill Ingalls)

The spacecraft completed the fly-by Tuesday morning, after reaching an area called the Kuiper Belt, which lies outside the planet Neptune. This success tops off a 50-year effort by scientists to explore our solar system.

"It's truly a mark in human history," said John Grunsfeld. He is the associate administrator for science at NASA, the American space agency. He is with the mission control center at Johns Hopkins University's Applied Physics Laboratory.

The spacecraft is so far from Earth, that radio signals, traveling at the speed of light, take about four-and-a-half hours to reach scientists on our planet. More information is expected to arrive Tuesday night. That is when New Horizons is expected to send final word that it has passed some 12,550 kilometers from Pluto.

"We will get information from the spacecraft and know if it's healthy," said Alice Bowman, the mission operations manager.

Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA’s New Horizons spacecraft, taken on July 13, 2015, when the spacecraft was 476,000 miles (768,000 kilometers) from the surface.

By that time, New Horizons will have spent almost a day in radio silence. It has been quiet while it works on a series of observations as it flies past Pluto and its five moons. New Horizons is traveling at speeds of about 14 kilometers per second.

The spacecraft is about the size of a baby grand piano. It is fueled by nuclear power. For several hours after the closest pass by Pluto, New Horizons will look back at the planet. It will study its atmosphere using radio signals sent from Earth as a probe. New Horizons does not have the ability to fire rockets to slow down and go into orbit around Pluto. So its cameras and scientific instruments must work on the fly, without slowing down.

Scientists want to know more about Pluto. When New Horizons was launched in 2006, Pluto was considered our solar system's ninth planet. But since then scientists decided it was a "dwarf planet" after other Pluto-like ice and rock worlds were discovered in the Kuiper Belt.

Those objects are believed to be left over pieces from the creation of the solar system some 4.6 billion years ago. Scientists will have to be patient to wait to receive all of New Horizons' information. It will take about 16 months for the spacecraft to send back all the images and other data taken during the Pluto flyby. By then, it will have traveled even deeper into the Kuiper Belt. There it may be heading for a possible visit with one of Pluto's neighbors.

I'm Anne Ball.

Anne Ball adapted this story for Learning English with information from Reuters. George Grow was the editor

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Words in This Story

solar system–n. our sun and the planets that move around it

dwarf– adj. something of relatively small size

probe–n. an instrument that is used for examining something