Ever since his days as a physics major, Richard Binzel ’80 has been captivated by Pluto. Today he’s one of the world’s leading experts on asteroids and Pluto. Binzel was part of the committee in 2006 that determined whether Pluto should still be considered a planet—though his opinion was in the minority. “I was Pluto’s defense attorney,” he says. “I didn’t win the case, so today we call Pluto a dwarf planet.”

In 1989, Binzel was part of a team that began working on sending a spacecraft to Pluto. Finally launched in 2006, the New Horizons mission returned data to Earth more than nine years later. The data revealed discoveries that baffled and amazed Binzel. “Pluto knocked our socks off,” he says.

Last October, Binzel spoke to a Mac audience about how the New Horizons mission changed our understanding of the edges of the solar system. His remarks are excerpted here.

—Rebecca DeJarlais Ortiz ’06

Richard Binzel ’80

At an American Geophysical Union meeting in 1989, I was part of a group of 12 scientists who presented papers on Pluto. Each of us essentially said, “This is what we know, and we’ve reached the limits of our understanding.” At dinner afterward, the group’s ringleader said, “We need a mission to Pluto. Voyager is about to get to Neptune—we can get to the outer solar system. Why not now?” We were basically graduate students—the audacity of youth.

The Pluto mission cost $850 million—for the average taxpayer, the equivalent of a cup of Starbucks coffee. NASA funded us to bring five concepts forward, and said, “Convince us, and we’ll give you the money.” Five times, they said, “This is great. But you’re canceled.”

That changed when we realized that Pluto is part of a new region of the solar system called the Kuiper Belt, an asteroid zone even more populated than the one between Mars and Jupiter. It’s full of these icy worlds, frozen time capsules from the beginning of the solar system. This drove home the point that there was a region of the solar system that we did not understand. NASA finally gave its approval, and we were underway with this mission called New Horizons.

Remember your Motorola flip phone? That’s when technology developed to fit enough computing and interpretive power into a handheld device. That breakthrough for phones was a breakthrough for our spacecraft, too. It allowed us to build instruments low-power and lightweight enough to fly across the solar system.

Our Pluto spacecraft was the size of a grand piano. You need a lot of velocity to get to Pluto in a reasonable amount of time. Your spacecraft doesn’t live forever. Nor do the scientists—and we were selfish. We wanted to still be alive when the spacecraft got there. If that took 100 years, that’s no good for us. But by keeping the spacecraft’s mass small, we maximized its velocity. We equipped the spacecraft with a complex set of instruments that weighed only about 10 kilograms.

We assembled this spacecraft, and then we tortured it to see if it could survive a violent launch. Nearly 20 years later, we got it off the launch pad. My overwhelming reaction was, “My gosh, it worked!”

Sending a signal to the spacecraft and getting a response took nine hours to traverse the round-trip distance. We had to get there fast, and we couldn’t orbit Pluto because that would have required carrying fuel exceeding the capacity of the largest rocket available. Because this was a fly-by mission, it had to be programmed, rehearsed, and practiced. That’s what you do for the 3,463 days it takes to get to Pluto, and then you say “What did we forget?” Any tiny mistake could have jeopardized the whole mission.

For the last million miles, all we could do was wait. And everything worked perfectly—we did it. We could hardly believe how spectacularly our little spacecraft worked.

New Horizons Discoveries

Pluto has glaciers flowing on liquid nitrogen: In an ice sheet that’s tens of meters thick, the pressure at the bottom increases enough to intersect the liquid phase of nitrogen. That means Pluto has glaciers that are floating on layers of liquid nitrogen.

Pluto has a volcano: Pluto has mountains as high as the Rockies, probably made of water-ice because frozen methane or nitrogen don’t have enough strength. One looks like a volcano, and we’re baffled by that.

Pluto has tropical regions: Earth tilts about 23 degrees, but Pluto tilts roughly 120 degrees. One pole faces the sun and the other pole has a century-long winter, then it switches. This creates complex seasons. Because its Arctic Circle reaches its mid-latitudes, Pluto has regions that are both tropical and arctic. It’s a complicated distribution that we’re working to decode.

Pluto has a blue sky: Like Earth, Pluto has nitrogen gas in its atmosphere. On Earth, scattering nitrogen molecules makes the sky blue. That happens on Pluto, too.

February 2 2018

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