摘要：“新视野号”探测器将于美国东部时间周二早晨7点50分飞掠冥王星，让人类一睹这颗“矮行星”的真容。“新视野号”2006年发射，已经飞越了48亿公里。

Almost there.

After nine and a half years and three billion miles, NASA’s New Horizons spacecraft was on course to take the first close-up look at Pluto on Tuesday morning.

“Fasten your seatbelts,” S. Alan Stern, the mission’s principal investigator, said Monday at a news conference here at the Johns Hopkins Applied Physics Laboratory, which is operating the mission. “New Horizons is arriving at the Pluto system.”

Among the science findings so far: a precise measurement of Pluto’s diameter; greater than expected amounts of nitrogen leaking from the atmosphere into space; confirmation of nitrogen and methane ices at the polar region; and images that show strange, and different, landscapes on Pluto and Charon, its largest moon.

“Pluto has not disappointed,” Dr. Stern said. He described the data so far as “mouthwatering,” but said it was still too early to answer some of the mysteries, like the strange features of Pluto and Charon.

Paul Schenk, a co-investigator on the science team, said, “It looks like somebody painted it for a ‘Star Trek’ episode.”

Pluto and Charon coalesced out of the same material after two large objects in the Kuiper belt — the ring of icy debris beyond the orbit of Neptune — collided early in the history of the solar system. But the two look very different, according to fuzzy images that New Horizons took from millions of miles away. The north pole of Charon is unexpectedly dark, while Pluto’s is bright with ice. By contrast, Pluto has a belt of dark regions around its equator.

With much better data arriving soon, the scientists were reluctant to speculate. “We don’t know,” Dr. Schenk said several times.

New Horizons, launched in 2006 aboard the biggest Atlas 5 rocket available, left Earth’s vicinity at the highest speeds ever. The compact spacecraft, about the size of a grand piano, runs on just 200 watts of power, generated from the heat of 24 pounds of radioactive plutonium dioxide.

On Monday morning, New Horizons was still 650,000 miles from Pluto. Its closest approach to the former ninth planet, about 7,800 miles above the surface, was scheduled to occur Tuesday around 7:50 a.m. Eastern time. Dr. Stern said the resolution of the images would jump by more than a factor of 100, from 15 kilometers per pixel to less than 100 meters per pixel.

The photos so far have been sharp enough to give a better determination of Pluto’s girth — 1,472 miles, give or take six miles — which restores Pluto as the undisputed giant of the Kuiper belt.

The discovery in 2005 of Eris, a more distant Kuiper belt object, set off the events that led to Pluto’s demotion to dwarf planet, a new category. Eris was so bright that it seemed certain to be bigger than Pluto, and initial measurements appeared to confirm that. But later, astronomers were able to make a more precise measurement when a star passed behind Eris. Eris turned out to be 1,453 miles in diameter.

Similar measurements had been made for Pluto, but the uncertainties were greater — earlier estimates of the diameter ranged from 1,428 miles to 1,490 miles — because Pluto has an atmosphere that bends starlight. Pluto and Eris appeared almost identical in size.

With New Horizons weighing in, “that settles the debate about the largest object in the Kuiper belt,” Dr. Stern said.

The larger diameter means that Pluto is less dense than had been thought, and that in turn means a greater proportion of ice and less rock in its composition.

While Pluto is now the biggest in the Kuiper belt, Eris remains the heavyweight — 27 percent more massive than Pluto.

Scientists had expected to find that the atmosphere on Pluto was escaping into space, but New Horizons detected the charged nitrogen atoms last week, “much farther from Pluto than we anticipated,” Dr. Stern said.

He said that could mean that the flow of nitrogen off Pluto is greater than had been thought or that the flow of charged nitrogen atoms happened to be concentrated in the region that New Horizons is traveling.

Mission managers said New Horizons was operating perfectly, and they passed on their last opportunities to refine the trajectory or computer commands. “We are good to go this last bit of this trip,” said Glen Fountain, the project manager.

At 11:17 p.m. Monday, the spacecraft, by design, was to stop talking to Earth and start almost 22 hours of programmed choreography, repeatedly firing its thrusters to pivot among Pluto, Charon and four smaller moons, taking a multitude of measurements.

After the closest approach to Pluto, the trajectory is to pass through the shadows of both Pluto and Charon, which will enable additional measurements. A 70-meter radio dish in California will send a powerful signal toward Pluto. The signal will bend around Pluto, and then Charon, to the receiver on the spacecraft, providing information about Pluto’s atmosphere.

For those 22 hours, no one will know the fate of New Horizons. There is a chance — tiny, but greater than zero — that New Horizons could run into something as small as a pebble and come to a catastrophic end.

If it does not, the spacecraft will turn its antenna back toward Earth around 4:20 p.m. to send a message that it survived and a brief summary of how the day went. Four and a half hours later, the time it takes light to travel three billion miles, the message will arrive at mission control, around 8:53 p.m. “That’s going to be a very highly anticipated event,” Dr. Stern said. “I think we’re all going to breathe a final sigh of relief at 9 p.m.”

Many on the mission team have long dreamed of this Pluto visit. New Horizons rose out of the ashes of an earlier mission, Pluto-Kuiper Express, which was canceled in 2000.

“It always seemed the encounter was so far away,” Dr. Stern said. “People talk about how surreal it is that we’re actually here.”