It was like New Year's Eve in Times Square as the countdown clock ticked down to zero.
"We're going to do our 10-9-8 thing and you can get your flags out," S Alan Stern, the principal investigator for Nasa's New Horizons mission to Pluto told the people gathered here at the Johns Hopkins Applied Physics Laboratory, which is operating the mission. "We're going to go absolutely ape."
About 7:50 am Tuesday, Nasa's New Horizons spacecraft made its closest pass by Pluto, coming within 7,800 miles of the surface.
The crowd, which included the children of Clyde Tombaugh, the astronomer who discovered Pluto in 1930, cheered.
As soon as it arrived, New Horizons was leaving, speeding along its trajectory at 31,000 miles per hour.
For now, no one knows how the spacecraft is faring.
New Horizons, which is in the middle of 22 hours of automated scientific observations, will not check in with mission controllers for several more hours, with the signal scheduled to arrive on Earth at 8:53 pm. By Wednesday, the spacecraft will be mostly finished with the data-collecting phase of the mission and begin sending back the trove of information for scientists to delve into.
Nasa released the newest colour picture of Pluto, which was sent down on Monday and offers the clearest view yet.
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. On Monday, 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," 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.
The photos so far have been sharp enough to give a better determination of Pluto's diameter - 1,472 miles, give or take six miles - which restores Pluto as the undisputed giant of the Kuiper belt. (Earth, for comparison, has a diameter of 7,918 miles.)
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.
With New Horizons weighing in, "that settles the debate about the largest object in the Kuiper belt," Stern said on Monday.
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 per cent 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.
Stern 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 travelling.
At 11:17 pm Monday, the spacecraft, by design, stopped talking to Earth and started its programmed choreography, repeatedly firing its thrusters to pivot among Pluto, Charon and four smaller moons, taking a multitude of measurements.
New Horizons will turn its antenna back toward Earth around 4:20 pm Tuesday 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.
"I think the spacecraft is going to do just fine," Stern said. "See you at 9 pm tonight."
©2015 The New York Times News Service
