The New Horizons Probe Has Made Its Closest Approach to Pluto

Mission scientists have received the confirmation signal that the pre-programmed event went as planned and the craft is healthy

Pluto as seen by New Horizons on July 13, when the spacecraft was about 476,000 miles from the surface. NASA/APL/SwRI

Fists hit the air and tiny American flags fluttered overhead this morning as a huge crowd in a Laurel, Maryland, conference room cheered for the New Horizons spacecraft's closest approach to Pluto. At 7:50 a.m. ET, the spacecraft swept past Pluto's surface at a distance of about 7,706 miles, closer to the tiny world than most GPS satellites get to Earth.

After an afternoon of nervous anticipation, the jubilation hit a crescendo at 8:52 p.m. ET, as Alice Bowman, the mission operations manager at the Johns Hopkins University Applied Physics Laboratory, announced to the world that the spacecraft had made its anticipated "phone home" signal—confirmation that the flyby was truly successful and that the spacecraft is healthy and ready to transmit some of its first data from the encounter.

"There's a little bit of drama, because this is true exploration. New Horizons is flying into the unknown," mission manager Alan Stern of the Southwest Research Institute said during a morning briefing just after the flyby. 

"If you think it was big today, wait until tomorrow and the next day. This is just the beginning," NASA's associate administrator for science missions, John Grunsfeld, later told the crowd during a post-signal briefing. "As a team, we all have made history. This can never be repeated. This is in the history books."

As part of its last safety check before the flyby, New Horizons beamed back one high-resolution color picture of Pluto, seen above, showing the surprisingly varied terrain on this alien world. With a resolution of about 2.5 miles per pixel, the stunning image hints at a dynamic planet with possible tectonic activity and strong atmospheric cycles, Stern says.

"This image is oriented with north at the top. The dark regions are near Pluto's equator," he says. "We can see a history of impacts, a history of surface activity. But by tomorrow we will show you images with ten times this resolution. Pluto has a lot more to teach us with the data coming down."

The reason for the long delay between the flyby and the signal home is tied to the long journey New Horizons was designed to endure, says Bowman. To ensure its health during the voyage, the team wanted the spacecraft to have as few moving parts as possible, and so the antenna that transmits data is a fixed instrument. The spacecraft must take aim to Earth whenever it wants to communicate, and that's not always the best position for collected data.

"This is the closest approach, and this is when it gets the best science," Bowman told reporters during the pre-encounter briefing. "We don't want it to turn to Earth and talk to us—we want it to do science." Even though the spacecraft is out of touch during this scientifically critical time, the mission team remains confident that everything in its choreographed dance is happening as planned.

"We always talk about the spacecraft being like a child, like a teenager," Bowman said at the time. "Right now there's nothing the operations team can do. We just have to trust that we have prepared it well and send it off on its journey."

Stern was also resolute, noting that the team had done hours of modeling and had collected reams of data on any safety hazards, like debris around Pluto that could damage the fast-moving spacecraft. The odds of something going wrong, he said, were extremely low. "The probability of loss has an upper limit at around 2 parts in 10,000—you could fly hundreds of New Horizons through the Pluto system and expect them to survive," he said.

On the chance that something did break, New Horizons had been taking failsafe data, collecting and transmitting key snippets for the main mission objectives each time it sent back a health status report. That includes the newly arrived image, as well as shots of the large moon Charon, mapping, spectroscopy, thermal data and information about the dust and plasma environments around Pluto and its moons. Tonight's signal, while a sign of success, did not include any additional teasers.

"The signal we got tonight was entirely engineering data … no science data came home tonight," Stern said during the evening briefing. "We wanted that report to be as brief as possible because as soon as it was over, New Horizons went back to work to collect that data."

Now that mission managers know the craft is safe and loaded with data, the team is eagerly awaiting a fresh round of scientific wonders due tomorrow morning.

"It is truly amazing ... the recovery was flawless. We were up to the challenge—we met it," Bowman added. "And on a personal note, I can't express how I'm feeling to have achieved a childhood dream of space exploration. I'm pretty overwhelmed at this momet. Please tell your children and anybody out there: Do what you're passionate about. Give yourself that challenge and you will not be sorry for it."

Note: This story has been updated with news of the successful signal from the New Horizons probe.

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