It took 10 years to get there, but NASA’s New Horizons spacecraft flew past the planet Pluto early Tuesday morning. The outermost planet in our solar system is now officially the most distant object ever visited by humankind.

Human Race Reaches Pluto

NASA reported that the closest approach of the New Horizons Pluto probe was anticipated to occur at 4:49 a.m. PDT when the craft will pass a mere 7,800 miles away from the dwarf planet. Astronomers point out that this is close enough to take high-resolution images of Pluto’s landscape, another historic first.

According to NASA officials, telemetry data shows that New Horizons actually began its pass by of Pluto 72 seconds earlier than originally calculated.

This also means the planetary probe passed by the rocky planetoid 43 miles closer than originally planned, but the few mile difference does not impact the mission, New Horizons principal investigator Alan Stern explained in a media press conference.

The space agency is reporting that an initial batch of images is expected to be available by Thursday or Friday.

More on the New Horizons Pluto mission

Keep in mind that the budget for the New Horizons Pluto mission was a small-by-today’s-standards $700 million. The mission was overseen by the Johns Hopkins Applied Physics Laboratory, which also designed and built the spacecraft.

In an interesting historical footnote, Pluto was considered a full planet when the mission launched, but It was demoted just a few months later because the International Astronomical Union said the planetoid is not massive enough to prevent similar-sized objects from forming nearby.

NASA had to send the New Horizons on a fast course to the Kuiper belt given the distances involved, and since it was traveling at the speed of at 30,800 mph, the craft could not orbit or even slow down much as it approached Pluto and its five moons..

In order to apply its full resources to scientific data collection, the spacecraft will not communicate with mission controllers until Tuesday night.

“We lost the signal as planned at 11:17 p.m. Eastern and there is nothing we could do but trust we had prepared it well to set off on its journey on its own,” noted Alice Bowman, New Horizons’ mission operations manager, at the presser Tuesday.

Bowman also explained why several members of the New Horizons team remained in the operations room after the probe went quiet. “Even though we knew it wasn’t going to be talking to us, we wanted to be there so that we would be with it while it went through that journey,” he said.

New Horizons scientific instruments have already made a number of new discoveries about the dwarf planet.

From the initial data sent in by the New Horizons Pluto probe, researchers found out the planetoid is slightly larger than earlier calculations. Previous estimates suggested the radius of Pluto was between 715 and 746 miles, but the new data shows the actual radius is 736 miles, in the upper end of the estimate.

Stern pointed out the new measurements suggest that Pluto’s mantle and core is less dense with more ice and less rock than anticipated. He also said that it appears there may be snow on the surface of Pluto.

Lack of fuel for braking prevented slowdown of probe

The fact that New Horizons was limited to a brief flyby was simply a matter of physics, which meant it was not possible to slow the craft down as it approached the planet.

For starters, Pluto is only 65% the size of the moon, which means its gravity is weak (1/15 of the gravity on Earth).

Astronomers point out that it’s relatively easy to use the gravitational pull of a large planet like Jupiter, which is about 300 times as big as Earth, to “catch” a spacecraft and slow it down. However, it is a completely different story with Pluto, as a craft can only get caught by gravity in an orbit around Pluto if it is moving relatively slowly.

A spacecraft can theoretically be slowed down ahead of its rendezvous by using propulsion. But, based on calculations by Ben Montet, an astronomy grad student at Caltech, that meant the probe would have had to bring along 70,000 pounds of fuel. Given the entire spacecraft weighed barely 1,000 pounds when it was launched, it’s clear the idea of slowing down the probe as it reached its destination is a nonstarter.