The Destination That Changed

Seventy-eight days ago, we strapped a piano-sized spacecraft to an Atlas V rocket and flung it off the planet at 58,000 kilometers per hour. New Horizons left Earth faster than any human-made object ever has — passing the Moon's orbit in nine hours, a trip that took Apollo astronauts three days. Today it crossed the orbit of Mars, 243 million kilometers from the Sun, traveling at 21 kilometers per second.

Its destination: Pluto. Three billion miles away. Arrival: sometime around 2015.

Here's the thing nobody at NASA will say out loud at the press conference, though everyone in planetary science is thinking it: by the time New Horizons gets there, Pluto might not be a planet anymore.

The Fastest Thing We've Ever Built

The numbers on this mission are genuinely absurd. New Horizons is moving so fast that if you pointed it at the nearest star instead of Pluto, it would still take roughly 78,000 years to arrive. Space is incomprehensibly large even at incomprehensible speeds. The fastest spacecraft in human history, and it's still a bottle tossed into an ocean so vast that "ocean" is an insult to the scale.

Right now, at this moment, New Horizons is closer to Earth than to Mars. Mars is on the far side of its elliptical orbit, trailing the probe by some 299 million kilometers. The spacecraft isn't visiting Mars. It's just passing through the neighborhood on its way to the outer solar system, moving too fast to stop, which is exactly how you have to travel if you want to reach Pluto before your funding expires or your instruments die of old age.

Next stop: Jupiter, in February 2007. The probe will use Jupiter's gravity like a cosmic slingshot, picking up speed for the long cruise to the Kuiper Belt. This is elegant engineering — using the gravity of one giant planet to throw a machine at a tiny one, across a void so dark that sunlight at Pluto is about as bright as twilight on Earth.

The Classification Problem

Last year, astronomer Mike Brown and his team confirmed the discovery of a body in the Kuiper Belt that appears to be roughly Pluto's size — possibly larger, definitely more massive. They're calling it 2003 UB313 for now, though Brown's team has submitted a proper name to the International Astronomical Union. The IAU, for their part, is expected to take up the question of planetary definition at their General Assembly in Prague this August.

The problem is simple and entirely human: if Pluto is a planet, then 2003 UB313 is a planet too. And probably several other Kuiper Belt objects. Either the solar system has twelve planets and counting, or it has eight.

New Horizons was launched to study the ninth planet. The probe doesn't have an opinion on the taxonomy. It will arrive at Pluto — whatever we're calling it by then — and do exactly what its instruments were designed to do: map the surface, analyze the atmosphere, characterize the moons. The physics doesn't change because we moved a name from one column to another in a database.

What the Probe Knows That We Don't

Here's what's cosmically funny about this situation: the fastest object humanity has ever launched is heading toward a destination whose identity is under review. We spent $700 million and fourteen years of development to visit something that a committee might reclassify before the spacecraft arrives. The gravity hasn't changed. The nitrogen ice hasn't changed. The orbital mechanics haven't changed. Only the label is up for debate.

Pluto has been orbiting out there for four billion years without caring what we called it. It was a god before it was a planet. It'll be whatever it is after we finish arguing.

The universe doesn't consult our taxonomies. Classification is a human obsession — the compulsive need to sort reality into categories that make us feel like we understand it. Pluto doesn't fit neatly. That's not Pluto's problem. That's ours.

New Horizons is 243 million kilometers from home and accelerating. It'll get where it's going regardless of what we decide to call the destination. The probe, at least, has its priorities straight.

Sources:

• Pluto-Bound Probe Passes Mars' Orbit — Space.com, 2006-04-07
• Outbound for the Frontier, New Horizons Crosses the Orbit of Mars — Johns Hopkins APL, 2006-04-07
• New Horizons Mission Page — NASA Science, 2006-04-07