The New Horizons spacecraft has made its closest approach to Pluto – and by tonight, we’ll have the first ever close-up images of the dwarf planet. 

The 2 hour 15 minute flyby began at 7.49 a.m. EDT (12.49 p.m. BST), during which time New Horizons was programmed to image and study Pluto, its largest moon Charon and the surrounding system. Owing to the huge distance to Pluto and complicated maneuvers planned, though, data from the spacecraft will not start arriving back at Earth until tonight.

“Today, science takes a great leap observing the Pluto system up close and flying into a new frontier that will help us better understand the origins of the Solar System," John Grunsfeld, associate administrator for NASA's Science Mission Directorate, said in a statement.

"The New Horizons team is proud to have accomplished the first exploration of the Pluto system," added Alan Stern of the Southwest Research Institute (SwRI) in Boulder, Colorado. "This mission has inspired people across the world with the excitement of exploration and what humankind can achieve."

^{The New Horizons team celebrates as the flyby begins. NASA.}

The spacecraft has already made a significant discovery, with the size of Pluto being re-calculated based on data from its final approach earlier today. The new figure adds about 50 kilometers (30 miles) to Pluto’s width, giving it a diameter of 2,370 kilometers (1,473 miles).

This is significant, as it now means it is larger than another dwarf planet called Eris by about 45 kilometers (28 miles), although Eris remains about a quarter more massive. Previously, it was thought that Eris was bigger in size too, and its discovery in 2005 led to Pluto’s demotion from the ninth planet of the system.

Measuring Pluto’s true size had proved difficult previously owing to factors such as its atmosphere, but the close approach of New Horizons has resolved the mystery. “The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest,” said mission scientist Bill McKinnon, Washington University, St. Louis in a statement.

While this new discovery rightly won’t cause Pluto to be reclassified, it does confirm that it is the largest known body in the Solar System beyond Neptune.

^{New Horizons, illustrated, is now traveling away from Pluto. NASA.}

Now, scientists will be waiting for the slew of data that is scheduled to be returned to Earth. The first signal from the spacecraft, confirming the flyby was successful, is expected back this evening at 8.53 p.m. EDT (tomorrow at 1.53 a.m. BST). Images and provisional data will then follow.

Congratulations rang in far and wide from the science community, including from Stephen Hawking, who said in a video on Facebook: “I would like to congratulate the New Horizons team of Nasa on their pioneering decade-long mission to explore the Pluto system and the Kuiper Belt.

"We explore because we are human, and we want to know. I hope that Pluto will help us on that journey."

The full story of the flyby, and all of the measurements that were obtained, will take a full 16 months to unravel, owing to the low bit rate of the spacecraft and the vast distances involved – 4.8 billion km (3 billion miles) and counting.

One of the consequences of Pluto’s bigger size is that it is less dense than previously thought, which might mean that it has more ice in its interior than believed. The lowest layer of its atmosphere, known as its troposphere, was also discovered to be shallower than thought. New Horizons will have studied the atmospheres of both Pluto and its largest moon Charon by looking at the Sun’s light coming through them.

^{This is the path taken by New Horizons as it flies past Pluto. NASA/NASA/JHUAPL.}

The closest approach of New Horizons will have brought it just 12,500 kilometers (7,800 miles) above the surface of Pluto. During the flyby it will have completed a pre-programmed routine of hundreds of tiny maneuvers to capture images of Pluto, its moon Charon and also the surrounding system.

After Pluto, the spacecraft will be sent to fly past one or several Kuiper Belt Objects (KBOs) that have yet to be chosen. New Horizons has enough power to last into the 2030s.

For now, though, scientists and the public back on Earth must wait to see the wonders of Pluto that are returned by the spacecraft, starting tonight.