The Astronomer (1668)
Johannes Vermeer

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How big is Comet 67P/Churyumov-Gerasimenko?
A size comparison with the city of Los Angeles can be found through here.

The Size of the Solar System

One Astronomical Unit (AU) is the distance from Earth to Sun, now defined specifically, for the sake of calculation and no longer as an approximate average only, as 93 million miles. Pluto is said to vary from 2.7 to 4.6 billion miles from Earth. The Kuiper Belt lies between 30 and 50 AUs. Thirty AUs equals two billion seven hundred ninety million miles: 2,790,000,000 miles. Sounds really far. But, nevertheless, the Kuiper Belt is practically a neighbor compared to the Oort Cloud.

Beyond the Kuiper Belt at 50 AUs, we enter an area where the sun's gravitational and magnetic influence falls precipitously as it confronts the interstellar medium--gas, in all its forms, dust, and cosmic rays--producing a turbulent bow wake as we cruise through space, circumnavigating the galaxy. If we consider the extent of the solar system to end on the outskirts of the Belt, then the diameter of the sun's reach is about one trillion miles. However, we need to travel another 950 AUs through this vast emptiness in order to enter the realm of the inner disk of the theoretical Oort Cloud.

Estimated to begin at about 1,000 AUs, it appears as a disk or torus at first, then gradually curves to a sphere of comets, tiny asteroids, and other space flotsam and jetsam. Now, 1,000 AUs is 93 billion miles: 93,000,000,000. But this is only the inner boundary of the Oort. Where it begins to curve, wraping the solar system in a cocoon, is 5,000 AUs or 465 billion miles. Purportedly, it extends to about 100,000 AUs. [These numbers vary in the astronomy community. I've also read that the average distance to the Oort is 50,000 AUs or close to one light year, without a lower or upper bound estimate, but, that amounts to the same thing.] Rounding off the AU for convenience sake to 100 million, the outer surface or boundary of the Oort is 10 trillion miles from where I'm sitting, give or take an AU. That's trillion.

[It's traditional to go with 'the average distance from Earth to sun' for the Astronomical Unit. Very romantic. A sentimental affectation, like holding on to inches, feet, and yards in light of the metric system. However, if a constant like '100 million' was chosen instead, everything would still make sense--it's just a gauge--and would make the world simpler for calculations.]

(1x105) X (1x108) = 1x1013

Diagram of theoretical distance of Oort Cloud from sun

I find the size of our solar system impossible to grasp. The very idea of a trillion miles is incomprehensible to me. To try to wrap my head around not only its meaning but also its significance for space travel leaves me numb. If we were a space-going people, used to voyaging from star system to star system, a trillion might simply be a basic unit of measure. Although, we'd probably no longer use the word trillion. Jargon might just call it a click. How many clicks to the next star? And to imagine ten of them?

To get outside the solar system from planet Earth, all the way past the K. Belt and the outer boundary of the Great Oort, you have to travel one trillion miles in a direction away from the sun, and after that, nine trillion more. That's crazy.

[Incidentally, that makes the diameter of the solar system 20 trillion miles or 3.3 light years. The Oort Cloud per se, as a physical reality, a whole of many trillions of parts, has never been seen. Purely theoretical, it's used to explain where comets come from.]

Did you know that the core of the sun is 27 million degrees fahrenheit? Didya?


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