It's official: NASA's New Horizons became the first spacecraft ever to fly by Pluto today, passing within 7,750 miles of the dwarf planet at 7:49 am ET.
This fact was widely celebrated this morning (including on this site), but in reality no one knew whether the probe successfully made it until scientists received a signal this evening. That's because New Horizons was busy collecting data during the flyby — not transmitting it — and once it did send a signal, the transmission took 4.5 hours to reach Earth.
Now, after receiving the signal this evening, mission scientists have confirmed that the probe made it through as planned (there was roughly a 1-in-10,000 chance it could have been hit by a piece of errant space debris). The first photos of the encounter should arrive sometime tomorrow, and over the next weeks and months, we'll see gorgeous, high-resolution photos of Pluto — 10 times sharper than anything taken so far.
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 means that the craft had to be fairly light (about 1,000 pounds) to get there in a reasonable amount of time, which 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 traversed the diameter of 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:
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. But even this will be outdone by the photos taken today during the actual flyby, which are projected to be 10 times sharper.
We should see the first of these Wednesday afternoon, and the rest should trickle in over the coming weeks because New Horizons is only capable of transmitting data at a very slow rate.
They should reveal a fascinating new landscape, shedding light on the ice caps and plains scientists have spotted in recent images, as well as features we simply haven't seen yet.
Scientists will use all this data to better understand Pluto and its place in the solar system
The spacecraft also collected lots of data on Pluto's temperature, atmosphere, and interactions with the solar wind (the charged plasma released by the sun), 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 previously 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 world
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 (including me) aren't old enough to remember a moment of such pure exploration, of seeing a planet that no one had seen before. When we get the photos tomorrow, though, we're once again going to see an entirely new world for the first time.