Space is Big, Empty, and Very Very Lonely.

A 2-kilometer-wide asteroid named 1998 OR2 just flew past the Earth, driving home one of the most common popular and enduring space myths: that the solar system is a crowded place, packed with flying rocks that constantly menace the Earth. This past week, many scientists and science journalists (including myself) on Twitter were bombarded with questions from people who were convinced that 1998 OR2 was a doomsday rock about to hit us.

In reality, space is overwhelmingly empty. It is empty to an extent that is far beyond human experience, far beyond most human imagination, even. We are surrounded by very little risk, because we are surrounded by extraordinary loneliness.

I understand why people worry, though. For decades we’ve been immersed in movies, TV shows, and comic books that depict a crowded version of space. The “asteroid field” in The Empire Strikes Back (shown up above) is a prime example. This kind of falsehood is almost essential for science fiction storytelling. Who wants to watch characters sailing uneventfully through empty space?

The need for recognizable, tangible drama is obvious in the stories pushed out by the tabloids (The Express in the UK is a regular offender), but it influences the language of more reputable outlets as well. “This giant asteroid will fly by Earth,” explained CNN in its headline. Veteran BBC journalist John Simpson described the asteroid’s trajectory as “a near miss.”

NASA is not immune, either. Its headline (“Asteroid 1998 OR2 to Safely Fly Past Earth This Week”) emphasized the lack of danger, but also gave a false sense of proximity. If you were reading carefully, you probably noticed that I did the exact same thing in the first sentence of this story. I wrote that the asteroid “flew past the Earth,” but the asteroid never came closer than 6.3 million kilometers from Earth. When something is 6.3 million kilometers away, does that really deserve to be called a flyby?

Part of the problem is that we have no familiar language or imagery with which to describe the emptiness of space. Kilometers and miles give us numbers to work with, but they lack context. When you say 6.3 million kilometers—that’s just 4 percent of the distance to the Sun, so it’s pretty close, right? Or it’s 15 times the distance to the Moon, so it’s pretty far, right?

Here’s an idea that might help: Try thinking about distances in space not in terms of absolute measures like kilometers, but in 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 kilometers? It’s about 500 Earth diameters.

The distance between Earth and asteroid 1998 OR2 to scale. Earth is the dot at left. The asteroid is represented by the smaller dot at right.

The distance between Earth and 1998 OR2, shown to scale. Earth is the blue dot at left. The asteroid is represented by the smaller dot at right. I had to circle the two to make them visible.

Now you have some visual grounding to work with. Here is Earth. There, 500 Earth diameters away, is 1998 OR2. I made a very simple graphical representation of what that geometry, above. I drew an image box 1,000 pixels wide. I drew Earth (at left) 2 pixels wide. I drew 1998 OR2 at right, 500 Earth diameters away. Unless you blow up the image, you may not be able to see the dots; I had to circle them to make them noticeable. That is how empty space is.

(Strictly speaking, it’s still an exaggeration of how crowded space is. If I drew 1998 OR2 to scale, you definitely wouldn’t even be able to see it. I made it 1 pixel wide, but it should actually be about 1/7,000th of a pixel wide.)

With this kind of context, you can also translate the “flyby” of 1998 OR2 into human terms. A person is roughly 2 meters tall. If there’s another person 500 human heights from you, that puts her about 1 kilometer away. I don’t know about you, but if the the nearest person on Earth were 1 kilometer away, I wouldn’t consider that close at all. I certainly wouldn’t worry that she might be about to bump into me.

Several concerned folks on Twitter asked me if 1998 OR2 could change course slightly and hit us. The answer is simple and absolute. No, no more than that woman who is 1 kilometer away could suddenly teleport and knock you off your feet.

What does it mean when people say that comet C/2019 Y4 (ATLAS) was “half the size of the Sun”? That number refers to the total size of the comet’s tail and coma—which is to say, that is the size of its entire environmental footprint in the solar system.

If you measure other things in the solar system using the same standard, you get some really unfamiliar answers. For instance, how big is the Earth if you measure its environmental footprint in the solar system? Earth’s magnetotail (the elongated bubble created by the interaction of our planet’s magnetic field with the solar wind) can stretch 5 million miles long, perhaps even 10 million miles. See: Earth’s Magnetosphere

If you go by tabloid headlines (or science fiction movies and TV shows, for that matter), Publications like

Star Trek asteroid belt (like Star Wars asteroid field).

People paranoid about 1998 OR2. Things are far apart, and things are small.

And objects are small: that comet “half the size of the Sun”

But things do collide: Kreutz Family comets.

So the Earth is 10 times the size of the Sun.

Based on measurements from the Voyager spacecraft, it seems that Jupiter’s magnetotail may extend about 300 million miles beyond the planet. See: http://www.igpp.ucla.edu/people/mkivelson/Publications/279-Ch24.pdf

So Jupiter is 300 times the size of the Sun.

But wait! The Sun itself creates a huge bubble of magnetic plasma around it; that bubble eventually bumps against the material of the interstellar medium. The outer edge of this bubble, called the heliopause, extends about 10 billion miles (give or take) in all directions.

So the Sun is 20,000 times the size of the Sun.

You see how confusing things get when you look at the solar system this way, so let’s go back to normal talk. These enormous structures are real and meaningful, but they are not at all what non-scientists mean when we ask how large something is in space. Normally we’re talking about the discrete structure (solid, liquid, gas or plasma) of the object itself. In the case of Comet ATLAS, its solid nucleus was probably less than 1 mile wide before it began fragmenting. For rounding purposes, let’s say that

C/2019 Y4 (ATLAS) was about 1/1,000,000th the size of the Sun.

Happens all the time! Hundreds of small comets plunge into the Sun every year

—in other words, it’s more or less a daily occurrence—and the Sun is none the worse for it. Keep in mind that the Sun is huge, about a quintillion (10^18) times as massive as a midsize comet. The vast majority of the comets that fall into the Sun are thousands of times smaller than that, even.

Comets also vaporize rapidly on the way in, so I’m not sure if you would really call that a “crash” into the Sun, but they can look quite dramatic before they die. The US-European SOHO spacecraft has a camera (LASCO) that has captured images of many of these dive-bombing comets. The Sun itself is blotted out to avoid blinding the camera, which is why there’s a weird circles in the center & an image representing the location of the Sun.

Here’s a comet crash from last August:

And here’s a really cool, rare shot from 1998 (also taken by SOHO) of two comets plunging into the Sun

at the same time. You can see them coming up from bottom left. In this video it sure looks like there’s a huge splash following the collision, but astronomers think that the splash is actually a random solar eruption, unrelated to the comets.

We know much less about the impacts of asteroids with the Sun, but calculations show

that gravitational interactions and radiations effects should cause small asteroids to fall into the Sun on a fairly regular basis. Nobody has ever actually seen this happening, which is not surprising considering how difficult it would be to observe. The effect on the Sun would probably be hard to distinguish from a normal solar eruption.

The solar system is also full of fine dust (visible as the Zodiacal light, an oval glow seen after sunset, under clear, dark skies), which continually spirals

into the Sun. These are tiny particles, though—more like a soft rain onto the Sun, rather than a crash.

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