Incredible Pluto Flyby the End to an Era of Exploration

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Today is exactly half a century after Mariner IV flew past Mars and humans first closely viewed another planet, starting our initial reconnaissance of the solar system.

Until this week, Pluto has been nothing but a point of light in the night sky, visible only with a big telescope. Today, as the New Horizons probe zoomed past and shot some pictures of it, many have said that it will be the last time in history that we will have viewed a planet in the solar system up close for the first time.

This, of course, is using the traditional definition of the nine planets, of which Pluto was one until it was unceremoniously but officially demoted to a “dwarf planet” a few years ago, to the dismay of many scientists, and schoolchildren of all ages. Technically, it’s a Kuiper Belt Object (KBO), of which astronomers now believe there are hundreds of thousands. But the existence of the Kuiper Belt, far out in the solar system beyond Neptune, the most distant (current) official planet, hadn’t even been postulated, let alone verified at the time a young Clyde Tombaugh discovered Pluto, its first known member, in the 1930s.

Because it has a slight atmosphere, the body’s size hasn’t been known precisely until now, but one of the first things we’ve learned this week from the spacecraft is that it’s the largest known one, edging out its KBO “twin” Eris, which had previously been thought to be the ninth-largest body in the solar system. Both are less than one fifth the diameter of our own planet, and smaller than our own moon.

In May, it was determined that Pluto has a total of five moons: Charon, its major moon, and the much smaller Styx, Nix, Kerberos and Hydra. The other thing we’ve learned as the probe has approached is that Charon is much larger than expected, making Pluto almost a dual planet system, both engaged in a wobbly dance around each other, and making the orbits of the other moons chaotic and unpredictable.

There are indications of outgassing of the atmosphere as well, with nitrogen escaping from the equator. Charon seems to have a dark red pole, and the initial speculation is that it is due to trapping of the leaking gas. The early data also indicate that Pluto itself has a polar  ice cap of frozen methane and nitrogen. The fact that it’s larger than expected for its mass means that it is of lower density than previously speculated, indicating perhaps that it has a higher content of ice than originally thought. Eris is in fact quite a bit more dense, indicating that it is probably rockier than Pluto.

The planet is currently over thirty times as far from the sun as earth, and getting farther away (one of the reasons that Dr. Alan Stern, the acknowledged father of the mission, pushed to make it happen over the past couple of decades). It’s been a long nine-year journey, and it required a “sling shot” maneuver around Jupiter, stealing an unmeasurable amount of that gas giant’s momentum, to get there at all.

But as it approached the distant bodies, things started to happen very rapidly. In a press conference yesterday, Stern characterized it as walking on an interminably long escalator, then stepping aboard a supersonic transport. It will be passing through the Plutonium system at a relative velocity of 14 km/s, which (as the invaluable XKCD points out) would make a real mess if it hit your car at that speed. This morning, it made its closest approach to the system, taking pictures  as it did so, and more in the “rear-view mirror” as it sped away through the belt.