The Oort Cloud

Yesterday, NASA published a logarithmically scaled map, below, of the solar system and surroundings, to accompany the news that Voyager 1 has left the heliosphere and entered interstellar space.

Solar System, in Perspective by NASA/JPL-Caltech

Over at geek blog io9, where I first saw the image, George Dvorsky noted the amazing size of the Oort Cloud, the scattered collection of small objects which surrounds the solar system, held by the Sun’s gravity and providing the source of many of the comets which periodically make closer visits:

“It should take the spacecraft around 300 years to reach the inner edge of the Oort cloud — that massive expanse of celestial debris that sits at the outer realm of the solar system. And massive it is. The one thing that strikes me most about this scaled map is how unbelievably large the Oort Cloud really is. And in fact, it’ll take Voyager about 30,000 years to fly through it.”

Even this seems like an understatement. If Voyager takes 300 years to reach it, and 30,000 years to pass through, presumably it’s 100 times wider than the little pocket in the middle in which we live. Of course, the transit time could be due to Voyager slowing down, but checking the figures on Wikipedia confirms the order of magnitude difference in scale.

The inner edge of the Oort Cloud is approximately 2000-5000 AU from the Sun: in other words, that’s the size of the hollow in its middle. The outer edge has been estimated at between 50,000 and 200,000 AU from the Sun. That means that the size of the hollow, which contains the Sun, all the planets, the Kuiper Belt, and far out objects like Sedna, is only about 1-3% of the size of the massive spherical cloud in which it sits.

NASA’s map is a little misleading in artistically representing the Oort Cloud as much denser than it really is – justifiably, otherwise it simply wouldn’t be visible at all. The Cloud contains objects of sizes up to 20km, though the vast majority are smaller, and these objects are typically tens of millions of km apart. You could – and presumably Voyager will – fly right through it without seeing a thing. The total mass of Oort Cloud objects is estimated to be 3×1025 kg, or about five Earths, so in bulk terms, the Cloud is barely noticeable compared to the far more significant planets and moons of the solar system, and of course the Sun.

However, in terms of structural features, it’s interesting to think of our stellar environment as primarily an enormous sphere of icy debris, with a few tiny objects rattling around a little empty bubble in the middle.

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