Monday, June 21, 2021

Why is space so empty?

A 2-km-wide asteroid named 1998 OR2 just flew past Earth, driving home one of the most enduring space myths: that the solar system is a crowded place, packed with flying rocks that constantly menace our planet. In reality, space is overwhelmingly empty. It is empty to an extent that is beyond human imagination. We are surrounded by very little risk, because we are surrounded by shocking loneliness.

Everyone, including NASA, said that the asteroid ‘fly by Earth’, but in reality, the asteroid never came closer than 6.3 million km from Earth. We have a twisted image of space, the reasons being   — we’ve all been immersed in movies, TV shows and comic books that depict a crowded version of space and that we have no familiar language or imagery with which to describe the unbounded emptiness of space. Kilometers and miles give us numbers to work with, but they lack context.

Here’s an idea that might help: Try thinking about distances in space not in terms of absolute measures like kilometers, but in terms of relevant units of size. If you are trying to picture the distance to something that is approaching Earth, think of it in Earth diameters. How far is 6.3 million km? It’s about 500 Earth diameters.

And at that distance, there is never a chance to change direction and collide with Earth. 

This unit-measure way of thinking is also a useful way to comprehend the way the universe is organized on different scales. The moon is about 60 Earth diameters from Earth. The Earth is about 110 sun diameters from the sun. The distance between the sun and the next star system. Alpha Centauri is about 30 million sun diameters away.

There’s a lot of empty space within the solar system, but it is nothing compared with the emptiness between the stars. We will never, ever witness two stars collide in our part of the galaxy. They are simply too small and too far apart from each other.

On the other hand, galaxies are quite large relative to their separations. The Milky Way has two satellite galaxies, the Magellanic Clouds, that are only a single Milky Way diameter distant. Our large neighboring galaxy, Andromeda, is less than 20 Milky Way diameters away. No surprise, then, that astronomers routinely observe galaxies in the process of colliding with each other. The Milky Way and Andromeda are on such a collision course, though it will take another 4 billion years for it to unfold.

Despite the staggering emptiness of space, collisions really do happen. An asteroid wiped out the ancient dinosaurs 65 million years ago. A much smaller asteroid injured more than 1,000 people in Chelyabinsk, Russia, in 2011. Comet fragments crashed into Jupiter in 1994, and comet chunks rain down on the sun pretty much every day. Every meteor that you see is a little collision.

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