The true dimensions of Space

It is far, far emptier than you ever imagined

Some time ago I designed a computer game. I asked my son Tommy to program it, and he became genuinely interested. We would call it Asteroids (or something), and it involved steering a spaceship through the Asteroid Belt, from Mars to Jupiter. It would all be in vector graphics with 3D controls.

The Asteroid Belt is a disk of objects circling the sun between the orbits of the planets Mars and Jupiter — billions of objects, ranging from pebbles to piles of rubble to massive rocks, all the way to the minor planets. Four of them, Ceres (I described how it was discovered in 1801 by the Italian priest Piazzi), Vesta, Pallas and Hygiea account for half of the belt’s total mass. There are about 80,000 objects with a diameter of a kilometer or more, and the orbits of about ten thousand of them have been mapped.

This is a typical depiction of the Asteroid Belt:

Here is an artist’s impression of what it would look like when you approach the Asteroid Belt in a space ship (click to enlarge and get a full impression):

Some take it a step further and have produced images like this:

Well, our little game was supposed to paint a more accurate picture. In reality traversing the Asteroid Belt in a space ship would look like this:

Space is so vast that, despite the fact that there are millions of substantial objects in the Asteroid Belt, the distance between them is extremely large — each asteroid has, on average, billions of cubic kilometers of space to itself. So your spaceship would fly through the Belt without any chance encounters. NASA has sent a dozen probes through the Belt, and none have crashed into an object — or even passed close enough to photograph one.

Well, our game never materialized. We were planning to plot the background sky, insert a few thousand asteroids and allow the user to search for them. But that expanded the project dramatically and made it too big for a simple joke. I considered making a version I myself could program in 30 minutes: just a black screen with fake controls, where nothing ever happened. At least it would be the most boring game ever published. Compare it to this action-packed 1979 Atari game of Asteroids.

We now come to the main didactic point of today’s article. I am sure you have all seen this kind of depictions of our Solar System:

This picture is not accurate, it is really, really not what our Solar System looks like. The following image at least gets the relative sizes of the planets right:

Here you see the relative sizes of Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars and Mercury.
And this is a real picture of the Earth and our Moon, shot by the High Resolution Imaging Science Experiment camera on NASA’s Mars Reconnaissance Orbiter

NASA and JPL are quick to admit that in the above image “Earth and the moon appear closer than they actually are because the observation was planned for a time at which the moon was almost directly behind Earth.” What the duo would normally look like, in a telescope from Mars, is this:

Let us now turn to the distances of planets to the sun. You want reality? Well here it is, a truly instructive depiction of the Solar System by Josh Worth, humorously entitled A tediously accurate map of the solar system

Click on the above image or link to proceed to this extraordinary screen. I am not going to spoil it all for you, and urge you to scroll through the mile-wide page to get a true impression of space. However, if you are reading this on a mobile phone, and for those of you who get tired of scrolling, here are a few examples that will explain what it is all about.

This is the sun, with a diameter of 1,400,000 km. In the “tediously accurate” graphic depiction it is about 400 pixels across, with each pixel representing 3475 km, or the diameter of Earth’s moon.

The first planet we encounter, a dozen pages or so to the right, is Mercury, which orbits around 58 million km from the sun. It is just under 5,000 km in diameter and hardly deserves more than a single pixel (look closely, you can see it).

Venus is very slightly smaller than Earth, around 12,000 km in diameter, and 108 million km from the sun.

Next comes our own planet, 13,400 km in diameter and orbiting 150 million km from the sun (in my childhood that was 93 million miles). It has a single moon, 3,480 km in diameter, which orbits Earth at 384,000 km. It is represented by a single pixel, the unit of this graphic.

Next, at 228 million km from the sun, we have Mars, 6800 km in diameter. After the Red Planet you have to scroll a long way to come to the next one:

At 778 million km from the sun we find Jupiter, 140,000 km in diameter, and containing 2½ times the mass of all the other planets in the Solar System combined. It has at least 63 moons, four large enough to earn a pixel or more in the “tediously accurate” graphic.

Scroll on to the right — it is excellent exercise for your index finger. In the darkness of space you will see occasional factoids, like:

The New Horizons space craft that launched in 2006 only took 13 months to get to Jupiter. Don’t worry. It’ll take a lot less than 13 months to scroll there.

You would need 886 of these screens lined up side-by-side to show this whole map at once. If this map was printed from a quality printer (300 pixels per inch) the earth would be invisible, and the width of the paper would need to be 475 feet.

After scrolling 122 pages to the right we come to the Solar System’s gem, Saturn. This ringed planet is 118,000 km in diameter, the second largest in the Solar System. It orbits at 1.4 billion km from the sun. If you look at the scroll bar at the bottom you see that we are just over a quarter into our graphic.

At 2.9 billion km we encounter Uranus, which is 51,000 km in size, and then at 4.5 billion km the other gas giant, Neptune. And that is basically it, for the planets. The last one was recently demoted because it is too small and too eccentric to qualify for full status.

Pluto is close to six billion km from the sun, but its orbit is so elliptical that at its perihelion it comes to 4.4 billion km, which is closer than Neptune. It is just 2,400 km in diameter and is now considered a “dwarf planet” in the Kuiper belt, only the second most massive one we know.

We might as well stop now. As the author of this wonderful page, Josh Worth, says: “We’ll need to scroll through 6,771 more maps like this before we see anything else.” That would be Proxima Centauri, I’ll assume. And that makes it vividly clear to us: the universe is basically empty. When you see beautiful pictures of giant galaxies, containing hundreds of billions of stars, you hardly realize that it is 99.9999999999999999999958% empty space you are looking at, with stars much less closely packed than the planets in our example above. And don’t get me started on atoms, particles, quarks and strings. They are even less dense than the planets above. The universe is far emptier than you can imagine. But that is the subject of a later article.

Written by

Frederic Alois Friedel, born in 1945, science journalist, co-founder of ChessBase, studied Philosophy and Linguistics at the University of Hamburg and Oxford.

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