New Horizons just flew by Pluto — the first spacecraft ever to do so

NASA's New Horizons probe just became the first spacecraft to ever visit Pluto. It passed within 7,750 miles of the dwarf planet at 7:49 am ET Tuesday — and for the first time in a generation, it's going to let us set eyes upon an entirely new world.

Scientists won't have absolute confirmation that the flyby went as planned until around 9 ET tonight (when they'll receive a signal from the spacecraft) and we won't have photos of the flyby until Wednesday afternoon. The delay is because New Horizons is busy collecting data during the flyby — not transmitting it — and it takes 4.5 hours for any transmission to reach Earth.

But in the coming days and weeks, we should begin to see gorgeous, high-resolution photos of Pluto. They'll be 100 times sharper than anything taken so far, showing us the beloved dwarf planet in exquisite detail.

NASA-JHUAPL-SWRI

Pluto, as seen by New Horizons the day before the flyby. (NASA-JHUAPL-SWRI)

New Horizons traveled 3 billion miles to get to Pluto

The small craft that passed by Pluto today was launched back in January 2006 and has since traveled more than 3 billion miles. In 2007 it flew by Jupiter, using the giant planet's immense gravity to slingshot itself outward.

But even moving at speeds as high as 50,000 miles per hour, it took nearly a decade to reach Pluto because of a simple fact: It's incredibly far away. By analogy, if you imagine the Earth to be a basketball, Pluto would be a little larger than a golf ball — and at the same scale, that golf ball would be 50 to 80 miles away.

This distance also meant that the craft had to be fairly light (about 1,000 pounds) to get there in a reasonable amount of time, and precluded it from carrying enough fuel to slow down to enter Pluto's orbit. Consequently, it was moving at about 31,000 miles per hour during the flyby — and passed by Pluto in just a few minutes.

New Horizons is about to reveal Pluto for the first time

Until very recently, the best photos we had of Pluto — taken by the Hubble Space Telescope — showed it as a blurry blob:

(NASA/ESA/M. Buie)

Pluto, as seen by the Hubble Space Telescope in 2010.

New Horizons quickly surpassed the quality of those images during the past few weeks of its approach to Pluto, capturing much sharper images of the dwarf planet and its moons:

(NASA/JHUAPL/SWRI)

Pluto, as seen by New Horizons on July 12, from 1.6 million miles away.

We'll see the first of these Wednesday around 3 pm ET, and the rest will trickle in over the coming weeks (because New Horizons is only capable of transmitting data at a very slow rate). They'll reveal a fascinating new landscape and perhaps even features like ice volcanoes or liquid oceans trapped under ice.

Scientists will use all this data to better understand Pluto and its place in the solar system

The spacecraft will also collect a huge amount of data on Pluto's temperature, atmosphere, and interactions with the solar wind (the charged plasma released by the sun that emanates throughout the solar system), as well as the dwarf planet's five moons. Combined with the images, they'll paint a complex portrait of a long-mysterious planet.

Already, data collected by New Horizons has allowed scientists to more precisely determine the size of the dwarf planet (its diameter is 1,473 miles, slightly larger than estimated) and revealed mysterious dark spots in its southern hemisphere.

One of the reasons scientists want to learn more about Pluto is that it likely formed at the same time as the rest of our solar system, from the same materials. What's more, it likely formed much closer in to the sun — going through the same early stages of growth as Earth and the other rocky planets — before being flung outward billions of years ago.

All the data collected on its geology, atmosphere, and moons will help scientists refine their ideas about this early era in our planet's history. "We know that the Earth went through the stage of growth that Pluto stopped at," Alan Stern, New Horizons' principal investigator, told me in April. "This will help us connect the dots."

This is the first time in a generation we'll see a new planet — and the last

(JHUAPL/SwRI)

Since the dawn of the space age, we've been striving to explore our solar system, sending spacecraft to each of the planets in turn: Venus and Mars in the 1960s, Mercury, Jupiter, and Saturn in the '70s, and Uranus and Neptune in the '80s. These probes showed us entirely new worlds, revealing beautiful moons, rings, atmospheres, and landscapes.

There's since been a generation-long gap. Many people (myself included) aren't old enough to remember a moment of such pure exploration, of seeing a planet that no one had seen before. Tomorrow, though, we're once again going to see an entirely new world.

But in a sense, this is a bittersweet achievement because it'll also be the last time this happens. Whether Pluto is officially deemed a planet or not, this mission completes humanity's initial tour of the traditional set of nine planets in our solar system. Savor this moment, because, as Dennis Overbye puts in an excellent New York Times column, "None of us alive today will see a new planet up close for the first time again."

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