How do spacecraft avoid collisions in the asteroid belt?

The main belt located between the orbits of Mars and Jupiter is so sparsely populated that spacecraft pass through as though it were empty space.

How do spacecraft avoid collisions when passing through the asteroid belt?

Val-David Smithson
Pleasant Grove, Utah

Let’s begin by reviewing some astronomical “history”: Han Solo and his rebel cohorts Leia Organa, Chewbacca, and C-3PO are nestled uncomfortably together in the Millennium Falcon’s cockpit while their ship is pursued by a battalion of Imperial fighters. Unable to accelerate to light-speed due to a damaged hyperdrive motivator, the supremely confident Solo recklessly directs his craft into an asteroid field. Despite C-3PO’s dire warning that “the possibility of successfully navigating an asteroid field is approximately 3,720 to 1” (he must have meant 1 in 3,720), Solo flies through the field, deftly maneuvering his ship above, below, and around the tumbling asteroids, evading his pursuers and, as stipulated in his contract, survives through the end of the movie. It’s a perfect popcorn moment.

Now, please forget all that as we address the question: How can spacecraft in this solar system pass through the asteroid belt unscathed?

The simple answer is, it’s easy. There’s hardly anything there. The asteroid field we encountered in that galaxy far, far away a long, long time ago was purely fictional and preposterously overcrowded. The main belt located between the orbits of Mars and Jupiter is so sparsely populated by asteroids that spacecraft can pass through it as though it were empty space. 

That notion may seem absurd because asteroids in the main belt number in the millions. Moreover, astronomers estimate there are some 1.1 million to 1.9 million asteroids there with diameters larger than about half a mile (1 kilometer). That’s a lot of large, tumbling rocks darting about. However, if you gathered all those asteroids together into a single world, the resultant body would be a dwarf planet with a diameter of only 930 miles (1,500 km) — for comparison, Pluto’s diameter is 1,477 miles (2,377 km) — with a mass about 3 percent that of the Moon. And again, remember that these bodies are strewn about an immensely large volume of space. The region defined as the main belt encompasses roughly 4.7 x 10²⁵ cubic miles (2 x 10²⁶ cubic km); see the May 2024 Ask Astro column for more details on how this is calculated. And the average distance between asteroids is about 600,000 miles (965,600 km), though this separation distance is a simple average and does not take into account asteroid families, which can cluster more closely together. Thus, even the preternaturally anxious C-3PO should be able to calmly tell any space smuggler that “the probability of colliding with an asteroid in the main belt is 1 in a billion.”

Related: How closely packed are Jupiter’s Trojan asteroids?

To describe the situation more simply: Reduce the Sun to the size of a softball. On this scale, the main belt would be a disk with an inner boundary 76 feet (23 meters) from the softball-sized Sun, while the outer boundary would be 113 feet (34 m) away. All the asteroids put together would be half the diameter of the wire in a paperclip. Now, divide that minuscule bit of wire into millions of much smaller pieces and scatter them around the 37-foot-wide (11 m) disk, between the inner and outer boundaries. The result? A highly rarefied asteroid belt. 

We should note that many spacecraft, including the Voyagers, the Pioneers, Galileo, Cassini-Huygens, and New Horizons, have all traversed the main belt unscathed, and future spacecraft will be able to pass through it without so much as seeing an asteroid, too — unless, of course, their mission is to study one of them.

Edward Herrick-Gleason
Staff Member, Southworth Planetarium, University of Southern Maine, Portland, Maine