Speeding along out there in the direction of the constellation Sagittarius is a spacecraft a bit smaller than my Toyota. New Horizons, as it’s called, is now about 2.7 billion miles from planet Earth. If all goes as planned, it will whip past Pluto and its moons in 2015 and then continue on to outer locales.

This business of where things are is more complicated, and more crowded, than you think. The Earth is on average about 93 million miles from the sun, a distance called an astronomical unit (AU). Mars is on average half again that far from the sun, and Jupiter, which is the next big thing after Mars, is on average 5.2 AU from the sun, about 484 million miles. Saturn is about 9.5 AU, Uranus about 19.2 AU, and Neptune about 30.2 AU, or around 2.8 billion miles, from the sun, on average.

All these places have been visited, so to speak, for at least a few fleeting hours by spacecraft from Earth, but Pluto hasn’t. Part of the reason is that Pluto was in the wrong place in its orbit for the Voyager or Pioneer spacecrafts to get near; but also, it’s a lot farther out, most of the time, than Neptune. Pluto’s average distance from the sun is 39.6 AU, or roughly 3.7 billion miles.

Right now, after its launch Jan. 19, 2006 (which was the 197th birthday of Edgar Allan Poe; not sure what to make of that), New Horizons is almost 29 AU (around 3 billion miles) from the Earth, and about 4.68 AU from Pluto. Its radio signals, traveling at the speed of light (about 186,000 miles per second), take almost four hours to reach Earth.

Pluto is a very small, very distant place. When it was discovered in 1930, a tiny point of light changing position on photographic plates, it was designated the ninth planet in the solar system. But within about 60 years, this had become a problematic description. Pluto is smaller than the moon. And our moon itself is smaller than several moons of Jupiter, Neptune and Saturn. Can something smaller than some moons be a planet? And as if that wasn’t weird enough, it turned out more things are orbiting the sun out beyond Pluto than were imagined back in 1930. At least one of them, Eris, is larger than Pluto. Is Eris the 10th planet, then? And are there more and more planets out there?

The astronomers describe, in a way, three different regions of our solar system.

First, there’s the inner solar system. This includes the eight major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune) and their moons. It also includes the asteroids, which are sort of miniature planets orbiting the sun mostly between Mars and Jupiter, but also other areas, among which are the centaurs in the vicinity of the outer planets.

Second, there is the region just beyond the orbit of Neptune (trans-Neptunian space). It’s called the Kuiper Belt, or Edgeworth-Kuiper Belt, depending on whom you give credit for predicting its existence. This is made up of asteroidlike chunks of rock and ice, collectively called Kuiper Belt objects, of varying sizes orbiting between about 30 AU and 50 AU from the sun. More than a thousand have been spotted so far, and it is generally (though not universally) agreed that Pluto and Eris are KBOs — not planets, but dwarf planets. The most distant large KBO detected so far is mysterious Sedna, which at its closest approach to the sun is about 76 AU, but at its farthest is around 937 AU away. It apparently takes Sedna more than 11,000 years to make one orbit of the sun. Pluto takes about 249 years.

Third, out beyond the Kuiper Belt and Sedna is the Oort Cloud, where an unknown abundance of comets slow-orbit the sun. No one has ever seen anything in the Oort Cloud because what’s out there is too small and distant for telescopes — 1,000 to 100,000 AU out. But the Oort Cloud is thought to be the origin of long-period comets, meaning the ones that come streaking into our view just once, to return near Earth only after thousands of years, or possibly never.

New Horizons is on its way out, scheduled to fly by Pluto in July 2015 and then proceed into the Kuiper Belt. It will take pictures of whatever comes into range and send back detections of molecular and atomic activity in space for hopefully another five years or so. When New Horizons reaches about 55 AU out, its signals will not be strong enough to make it back to Earth intact.

These spectacularly lonely seeming distances are actually, in a way, neither far out, nor in deep. The most distant spacecraft we still hear from is Voyager 1, about 127 AU away (radio signal: 17.6 light-hours). The nearest bright star is the Alpha Centauri system, whose light takes 4.3 light-years to get here. The nearest large galaxy to us is 2.5 million light-years away. Far out. But when did the farawayness of the horizon ever stop anyone on the beach from keeping watch?

Dana Wilde lives in Troy. His writings on the stars and planets are collected in “Nebulae: A Backyard Cosmography,” available from Booklocker.com and online book sellers. Backyard Naturalist appears the second and fourth Thursdays each month. You can contact him at [email protected]

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