Why the Asteroid Belt Doesn’t Threaten Spacecraft

When you think of the asteroid belt, you probably imagine a region of rock and dust, with asteroids as far as the eye can see. Such a visual was popularized in movies, where spaceships must swerve left and right to avoid collisions. But a similar view is often depicted in more scientific images, such as the artistic rendering above. Even the first episode of the new Cosmos series described the belt as a dense collection of asteroids. But the reality is very different. In reality, the asteroid belt is less crowded than often depicted. How much less might surprise you.

The Sloan Digital Sky Survey (SDSS) has identified over 100,000 asteroids in the solar system. Not all of them are in the asteroid belt, but there are about 80,000 asteroids in the belt over a kilometer. Sure, there are asteroids smaller than that, but they’re harder to detect, so we don’t know exactly how many there are.

The pyramid-shaped zodiacal cone of light is centered on the same path the sun and planets take in the sky, called the ecliptic. This map shows the sky 90 minutes after sunset in early March facing west. Created with Stellarium

We have a pretty good idea, though, because the observations we have indicate that the size distribution of asteroids follows what’s called a power-law distribution. For example, with a power law of 1, for every asteroid 100 meters wide, there would be 10 with a diameter of 10 meters and 100 with a diameter of 1 meter. Based on SDSS observations, asteroids appear to follow a power law of about 2, which means there are probably about 800 trillion asteroids larger than one meter in the belt. That’s a lot of rock. So much so that sunlight scattered from the asteroid belt and other dust in the solar system is the source of zodiacal light.

But there is also a lot of volume in the asteroid belt. The belt can be said to occupy a region around the Sun about 2.2 to 3.2 times the Earth’s distance to the Sun from the Sun (AU), with a thickness of about 1 AU. A little math puts that at around 50 billion billion cubic kilometres. So even though there are billions of asteroids, each asteroid on average has billions of cubic kilometers of space. The asteroid belt is hardly something you would consider crowded. It should be pointed out that the belt asteroids are not evenly distributed. They are grouped into families and groups. But even such a grouping is not significant compared to the vast space it occupies.

A real image of the interior of the asteroid belt, taken by the NEAR probe as it headed towards Eros (center).  Credit: NASA
A real image of the interior of the asteroid belt, taken by the NEAR probe as it headed towards Eros (center).
Credit: NASA

You can even do a very rough calculation to get an idea of ​​the vacuum level of the asteroid belt. If we assume that all asteroids lie in a single plane, then on average there is 1 asteroid in an area roughly the size of Rhode Island. In the United States, there are said to be around 2,000 asteroids, most of them only one meter in diameter. The odds of seeing an asteroid along a cross-country road trip, let alone hitting one, would be surprisingly low. So you can understand why we don’t worry about space probes hitting an asteroid en route to the outer solar system. In fact, getting even close to an asteroid takes a lot of effort.

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