“We’re flying into the unknown”, Stern said. “This is the risk we take with all kinds of exploration”.

“This is a tremendous moment in human history”, John Grunsfeld, NASA’s science mission chief, said at a news conference.

By NASA’s calculations, we’ve made it to Pluto.

For decades, the Plutonian system was the only one of the original 9 planetary systems in the solar neighbourhood unexplored by the space agency.

The pictures already gathered by New Horizons are “mind-boggling to put it mildly”, Bolden said.

“All of these things that we know tonnes about about all the other planets, we just haven’t a clue with Pluto”.

Three new discoveries were revealed Monday, a tantalizing sneak preview as the countdown to closest approach reached the 21-hour mark. Earthbound comparisons for space are hard, at best, but one project scientist characterized Earth as a basketball and Pluto as a golf ball, with the basketball in Washington, D.C., and the golf ball north of Baltimore. Data confirms that it has a polar ice cap and its composition is more of methane than the dark band material splotches that can be seen near its equator.

New Horizons’ most recent reports show the dwarf planet to be 2,370 kilometres across, which is slightly larger than was estimated when the planet was downgraded from planet status to a dwarf planet. During the flyby, New Horizons will go radio silent so as to ensure that all the computing power is available to the scientific instruments.

As for pictures, the resolution is going to increase dramatically. While the orbiting Hubble telescope has been able to provide small, grainy images of Pluto, New Horizons will be able to photograph Pluto at such a fine level that features the size of a football pitch will be visible. New Horizons, which weighs about 1,000 pounds and is about the size of a baby grand piano. The closest the spacecraft will get to the previously unexplored planet is 7,767 miles, and that’s expected to happen around 7:49 a.m EDT.

The New Horizons probe over the weekend took the finest image of four dark spots on Pluto that continue to captivate millions on the Earth.

Darren Bly of the Kern Astronomical Society said, “this is pure science, it’s pushing the envelop a little bit”. As it has gotten closer to Pluto, its Long Range Reconnaissance Imager has transmitted back dozens of black-and-white images, providing progressively sharper photos.