What New Horizons reveals about Pluto

Unless you have been, well, on another planet, you know that July 14 was the date of closest approach to Pluto by the New Horizons spacecraft, which has now sent back unprecedented images from this historic encounter.

Passing Pluto at barely 7,750 miles (or 12,400 kilometers for savvy metric readers), the seven experiments on the New Horizons spacecraft are busily gathering extensive data on Pluto and its five known moons, Charon, Styx, Nix, Kerberos and Hydra.

It will be months before these data are fully transmitted to Earth, in part because, like many of us, New Horizons needs to focus all its effort on one essential task at a time. But even the bare-bones data that we’ve gotten so far have offered up several surprises.

For one thing, Pluto is a little bit larger than previously thought, with a diameter of roughly 2,360 kilometers. This means it is slightly less dense than we thought, so it has more ice and less rock. Irregular icy regions (one in the shape of a heart) are visible on the surface, adjacent to reddish regions the color of Mars, both clearly cratered.

Lest Pluto’s promotion in size reignite the is-it-or-isn’t-it-a-planet debate, let’s recap: Pluto was the only solar system planet discovered in the last century, by American astronomer Clyde Tombaugh. For the next 80 years, millions of schoolchildren proudly memorized nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

Then in 2006, to the distress of many, the International Astronomical Union coined a new term, «dwarf planet,» for Pluto and other similar objects in the Kuiper Belt, a vast array of rocky bodies that are the building blocks of future planets. The inner planets of the solar system probably formed out of similar material, albeit more rapidly because the density of blobs was higher closer to the sun. So studying the Kuiper Belt, which extends some 5 billion kilometers past Neptune (itself about 4.5 billion kilometers from the sun), may tell us a great deal about the origins of Earth.

You see, Pluto is one of many similar objects. If it’s a planet, so are thousands of other Kuiper Belt objects. Absent a minimum diameter for a planet, look out: At least 70,000 Kuiper Belt objects are bigger than 100 kilometers across. Hard to think of names for 70,000 would-be planets, much less schoolchildren memorizing them all!

Pluto’s demotion to dwarf planet status was controversial but scientifically required. The IAU requires bona fide solar system planets to meet three criteria: (1) they orbit the sun; (2) they are big enough that gravity makes them round; and (3) their neighborhood is not filled with rival would-be planets. Pluto failed this last criterion; it is one of the biggest Kuiper Belt objects known so far, but it is not unique.

Also, Pluto is unlike the eight solar system planets in several ways: It’s smaller; its moon Charon is nearly half its mass (the Earth’s moon is 1.2% of the Earth’s mass); its orbit is less circular and actually crosses the orbit of Neptune, plus it’s in a different plane. And, in violation of the third criterion, it shares space with thousands of other similar dwarf planets.

New Horizons spacecraft took nearly a decade to reach Pluto. This sounds like a long time, but it’s moving much faster than earlier missions to the outer planets such as Pioneer and Voyager. Wouldn’t it be nice if, having arrived at Pluto, NASA could hit the brakes and let New Horizons linger for a while?

Unfortunately, the physics doesn’t allow it. You see, there is nothing to brake against — unlike a car’s tires against the road. If the spacecraft had enough fuel, it could generate the power to turn left or right and insert itself into orbit around Pluto, but that much fuel would have weighed too much to launch with the requisite speed. The choice was between a brief but timely flyby or a trip too slow and expensive to fund.

So New Horizons is snapping pictures as frantically as any tourist on a five-day trip to 10 European capitals, repeatedly imaging the surfaces of Pluto and its five known moons. And it will continue taking data as it moves away from Pluto.

By the way, Australia, where I am a visitor to the University of Melbourne, is part of the Deep Space Network that collects signals from NASA spacecraft. As Australian television news noted with pride, the data from Tuesday night’s closest approach of New Horizons was seen here first. People of Oz, this American thanks you!

The next phase of the mission, as long as spacecraft consumables and NASA’s budget permits, will be to explore further in the Kuiper Belt. There are more Plutos and Plutinos out there, some of them may be potentially short-period comets, all of them have material from which planets are made, and all never before observed up close and personal.

Fast-forward a few billion years and who knows, maybe some of those lumps will have turned into a bona fide solar system planet. IAU of the future, here’s a suggestion: Why don’t you name it Pluto Veritas?

Meg Urry is the Israel Munson professor of physics and astronomy at Yale University and director of the Yale Center for Astronomy and Astrophysics. The opinions expressed in this commentary are solely those of the author.

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