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NASA's New Horizons mission to Pluto, explained

NASA's New Horizons mission to Pluto is about to show us the dwarf planet up close for the first time.

Until very recently, the best photos ever taken showed it as a small dot. But at 7:49 am ET on July 14, the probe flew within 7,750 miles of Pluto, coming closer than its five moons. The first photos of the flyby should arrive back to Earth Wednesday afternoon.

This is a staggering technological accomplishment. Pluto is the farthest destination for any space mission in history. It took New Horizons nine years to cover the 3 billion mile trip there — which means it's currently using decade-old technology, traveling a route that was calculated years ago. And because New Horizons was traveling at such a high speed (about 31,000 miles per hour) and couldn't slow down, the flyby was over in a matter of minutes.

It's not inaccurate to say that this mission was a bit like putting a GoPro on a speeding bullet and shooting it at a spot in the sky, with the knowledge that Pluto would be waiting there years later for a photo shoot.

But more than its technological prowess and amazing photos, New Horizons' brief moment with Pluto marks a historic moment in space exploration. For the first time in a generation — since the Voyager 2 probe swooped by Neptune in 1989 — we're going to see an entirely new world for the first time. But it'll also be the last time. 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."

1) We're about to see Pluto for the first time

Lots of people assume we've seen Pluto before, but they're likely thinking of artists' renderings or illustrations. Until very recently, the best photos we had of the dwarf planet looked like this:

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

(NASA/ESA/M. Buie)

But that's quickly changing. Over the past few weeks, as New Horizons closed in on Pluto, it gave us better and better photos — including this one, taken on July 12:


It's a big improvement, but it's nothing compared to what New Horizons will capture at its closest approach. "We’re going to turn points of light into a planet and a system of moons before your eyes," Alan Stern, the mission's principal investigator, said in April.

We won't receive the best photos during the flyby itself — because the spacecraft will be too busy taking them to transmit data back to Earth. But starting July 15, we'll get beautiful, color, high-resolution maps of Pluto's surface that will reveal all sorts of geologic features.

The Planetary Society's Emily Lakdawalla has calculated the level of detail we can expect, based on New Horizons' camera and distance from Pluto. This isn't Pluto — it's one of Jupiter's moons — but it's shown at roughly the resolution we'll see in New Horizon's images:

(NASA/JPL/Emily Lakdawalla)

At this level of detail, these are more than photos — they're detailed topographical maps, just like those provided by the satellites that orbit Earth. They could reveal mountains, ice caps, volcanoes, or even an ocean of liquid water under the ice. "Who knows what kind of bizarre things we'll find up close?" Stern said.

2) We'll also learn about Pluto's moons

(NASA, ESA, and L. Frattare)

Pluto is very unusual in that it's a fraction of the size of Earth but has five moons. The dwarf planet slightly orbits the largest moon, Charon (leading some to suggest the pair could best be described as a double planet system), and three of the smaller moons are swept up in the pair's gravitational field, leading to bizarre, chaotic orbits of their own.

pluto charon


Pluto and Charon, photographed by New Horizons on July 8 from 3.7 million miles away. (NASA-JHUAPL-SWRI)

At the moment, though, we don't know much more than that. Until 2012, we hadn't even discovered the smallest two moons (and until recently, scientists thought New Horizons might spot a few more). But the probe collected lots of data on all these moons, which may reveal possible surface features and perhaps even a thin atmosphere on Charon.

3) This mission has been in the works for decades

(Jason Davis/The Planetary Society)

Scientists have dreamed of sending a craft to Pluto for years. NASA engineers actually considered sending the Voyager 1 probe to Pluto after it flew by Saturn in 1980, but chose to send it to Saturn’s moon Titan instead. Soon after Voyager 2's 1989 Neptune encounter (which meant eight of the then-nine planets had been visited), scientists began proposing missions to Pluto.

For years, they were unsuccessful. During the 1990s, NASA scientists proposed four different Pluto missions — some of which would have included multiple spacecraft, or probes much larger than New Horizons — but NASA and Congress ultimately failed to provide money for any of them.

Finally, in 2003, Congress approved a slimmed-down, relatively small probe to launch to Pluto in 2006: New Horizons.

4) The tiny probe had a 3 billion mile journey to Pluto


It's hard to comprehend how far away Pluto really is. If Earth were the size of a basketball, Pluto would be a golf ball. And if you put that basketball in, say, Washington, DC, the golf ball would be farther away than Baltimore — somewhere between 50 and 80 miles away, depending on its orbit.

To go that far in a reasonable amount of time, engineers chose a small probe that would be launched atop a very large rocket (an Atlas V), so it could be sent off at the fastest speed possible. It left Earth's atmosphere traveling faster than any spacecraft ever: 36,373 miles per hour.

New Horizons' 2006 launch.

(Red Huber/Orlando Sentinel/MCT via Getty Images)

It spent most of the subsequent journey in hibernation, though it did wake up when flying by Jupiter in 2007, taking photos of the giant planet and using its gravity to slingshot itself outward to Pluto. That maneuver shortened the probe's journey by about three years — and with just a few slight correctional thruster burns (to keep the probe on the right track), it's followed a path plotted out by engineers a decade ago.

5) The journey to Pluto wasn't always easy

new horizons 3

An illustration of New Horizons.


Still, there have been challenges. In 2011 and 2012, astronomers discovered two new moons of Pluto — Kerberos and Styx — leading mission scientists to worry that Pluto might be surrounded by more debris than anticipated, possibly endangering the spacecraft. Subsequent calculations showed the chance of such an impact was extremely low, but the mission scientists remained on alert.

Additionally, ten days ago, New Horizons unexpectedly went into safe mode, switching to a backup computer because of a software glitch. NASA says the problem has been solved and won't pose any further issues, but it was still a huge scare, so close to the flyby.

6) The mission scientists aren't too worried about whether Pluto is a planet

(Joss Fong)

Just eight months after the probe was launched, the International Astronomical Union (IAU) officially decided that Pluto was no longer a planet — instead, it'd be categorized as a dwarf planet. The reasoning was partially based on Pluto's tiny size, and partially on the fact that, in recent years, astronomers found several other similarly sized objects in the same region of space (called the Kuiper belt), including Eris, a dwarf planet that's actually more massive than Pluto.

As Mike Brown, the Caltech astronomer who discovered Eris (and wrote How I Killed Pluto and Why It Had It Coming), told me a few months ago, "There are eight bodies in the solar system that dominate the others. Those eight are planets, and the other thousands are not."

The Kuiper belt.

(NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker)

On the other side are some scientists who argue that the IAU's definition of a planet is arbitrary. Any object large enough that its gravity pulls it into a round shape, they say, should be considered to be a planet. This would render most of the dwarf planets — including Pluto — full-fledged planets.

When I spent time with New Horizons scientists in April, I found that they generally belonged to the latter group — but more than anything, they don't really care what Pluto is officially designated, as long as their spacecraft can get there and explore it. At this point, nomenclature debates within the IAU are the last thing on their minds.

7) The spacecraft will tell us lots of other things about Pluto, too

It wasn't just taking photos: New Horizons has seven different scientific instruments aboard.


  • The main cameras (Ralph, Alice, and LORRI) will also be able to tell us the chemical composition and temperature of Pluto's surface, as well as the density and temperature of its atmosphere.
  • PEPSSI (which stands for Pluto Energetic Particle Spectrometer Science Investigation) detects particles escaping from the dwarf planet's nitrogen-based atmosphere, telling us what other elements are present.
  • SWAP (Solar Wind Around Pluto) provides data on how these particles interact with the solar wind — the charged plasma released by the sun that emanates throughout the solar system. Together, these two data sets will help us understand how Pluto's atmosphere fluctuates over time due to its varying distance from the sun.
  • The Student Dust Counter — designed and built by students at University of Colorado Boulder — provides new information on the density of dust in the outer solar system.
  • Right after the flyby, REX (Radio Science Experiment) transmitted and received radio signals through its atmosphere. By analyzing these signals when they reach radio dishes on Earth, we'll learn about the pressure, temperature, and composition of the atmosphere.

8) New Horizons also has some surprising stuff packed on board

The ashes of Clyde Tombaugh, the discoverer of Pluto, on board New Horizons.


Apart from its scientific instruments, the probe is carrying a few interesting mementos. Among them:

  • The ashes of Clyde Tombaugh, who discovered Pluto in 1930
  • CD-ROMs with the names of 434,000 people who signed up
  • Two state quarters and two US flags
  • A 1991 stamp that says "Pluto: Not Yet Explored"

9) All the data will come back at a snail's pace

(Estelle Caswell)

Because New Horizons is so far away, it takes about 4.5 hours for any data it sends back to reach Earth. And the signal is so faint that NASA has to use 200-foot-wide radio dishes (one each in Australia, California, and Spain) to pick it up.

This means an extremely low rate of data transmission: about 1 kilobit per second, more than 50 times slower than a 56k modem from the '90s. So it takes more than 42 minutes for New Horizons to fully transmit an image that's 1024 pixels wide.

Consequently, after its flyby the craft will continue sending data for more than a year. Beautiful photos will start coming just a day after the flyby, but scientists will have to wait a while for the full cache of data from all the instruments on board.

10) New Horizons will continue on to another destination

After Pluto, plans call for the spacecraft to fly to visit another Kuiper belt object — one of thousands of icy chunks of rock orbiting the sun beyond Neptune, of which Pluto is one of the largest.

Scientists are deciding between two potential objects, both of which are about 30 to 50 miles wide. If all goes to plan, the probe will reach one of them in 2019, taking more photos and collecting more data about these mysterious objects millions of miles away.

11) This mission is pure exploration

Pluto and Charon, as seen from 15 million miles away.


In one sense, New Horizons will give us very practical information that scientists can use to answer one of the biggest questions about our solar system: how it formed. It's believed that Pluto 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. As Stern told me, "This will help us connect the dots."

But in another sense, New Horizons will gratify something deeper: our constant urge for exploration. After mapping Earth, we continued outward, 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 1980s.

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. But after decades of planning and 3 billion miles of flight, it's finally time. We're about to see Pluto for the first time.

Click here to see all of our stories about Pluto and the New Horizons mission