The Density of the Asteroid Belt
Jul 30, 2025
By Ivy Knudsen
In films, we often see heart-racing, high-speed chases through asteroid belts, with our favorite characters narrowly escaping head-on collisions with huge asteroids. Have you ever watched and wondered if you could accomplish such a feat if you were piloting a ship in our asteroid belt?
The good news is, even with no spaceship piloting experience, you’d probably be alright.
The real asteroid belt is far less densly packed than those often shown in the in movies. It isn’t completely empty! But, as the Lucy spacecraft travels through the asteroid belt on its way to the Trojan asteroids it will never have to fire its thrusters to avoid colliding with an asteroid.
While most of the objects visible in the optical navigation images Lucy took as it approached (52246) Donaldjohanson are background stars (or galaxies), Lucy also spotted points of light that moved relative to the background stars between images. We’ve been able to identify at least three of them as known and numbered asteroids—33196 Kaienyang, 181310 (2006 QE35), and 165680 (2001 OY58)—with many others appearing for just a moment as they pass through the field of view. Was Lucy just lucky to have seen these all these asteroids? How populated is the asteroid belt? Let’s break it down.
First, how much mass is in the asteroid belt? An estimate from the Dawn spacecraft gives a value of 2.1×1021 kg, which is equivalent to ~1.6 million Mount Everests. Not a very convincing unit of measurement if you’re told the asteroid belt won’t be as full as it looks in films. Let’s try another unit. The total mass in the asteroid belt is also equivalent to just 3-4% of the mass of the Moon. Furthermore, over half of the total mass in the asteroid belt is in just four large asteroids, with ~1/3 of that mass entirely within the dwarf planet Ceres.
Second, how many asteroids are there? To think about this, consider that with each collision in the asteroid belt, more smaller objects are made than larger ones. Additionally, asteroids can have different compositions, with differing amounts of ices and metals. Keeping that in mind, there is an estimated 0.7 - 1.9 million asteroids with a diameter of 1km or greater, with around 10,000 larger than 10 km. If we keep going down to dust sizes, there are estimated to be hundreds of millions of asteroids.
And third, and most importantly, how big is the asteroid belt? If there are millions of asteroids, how densely packed are they? The asteroid belt takes up the region of space between Mars and Jupiter, from a distance of 2.1AU to 3.2AU (1AU = 1 Astronomical Unit, the distance between the Sun and the Earth). This distance is equivalent to ~160 million kilometers, or running between the Earth and the Moon 800 times. And don’t forget, this distance represents just the 2D distance through the asteroid belt in the plane of the solar system. The asteroid belt encompasses an entire orbit around the Sun and has thickness in the plane of our solar system, of about 1AU. Meaning, the volume of the asteroid belt is ~a few × 1025 km3, which would fit ~5 × 1015 Earth’s inside of it, or more than 30 million suns. If you were able to take the volume of all of the oceans on Earth and dump them out every second into the asteroid belt, it would take you ~1.6 billion years to fill it.
So, with 3-4% of the mass of one Moon distributed throughout a volume of ~2 quadrillion Moons, you can gain a better intuition for why the asteroid belt really isn’t all that dense. The average distance between asteroids larger that 1 km is about in the main belt is a few×105 km, or several hundred thousand miles. Meaning you’d almost certainly be alright flying through the asteroid belt in your spacecraft, a fact that scientists are happy to take advantage of.
But what would that mean for the number of asteroid we could expect to see in any random L’LORRI image? The instrument’s field-of-view is 0.3 degrees, a bit smaller than the full moon as seen from Earth. For an extremely rough estimate, lets assume Lucy is looking through nearly the entire width of the asteroid belt (lets say 1 AU) so that each L’LORRI snapshot images about 0.01 AU3 of the asteroid belt. If this entire area has the rough averaged, that implies that there should be several hundred asteroids larger than 1 km in every L’LORRI image, and even a few over 10 km. Of course, these are short images designed to see an asteroid that is less than 100,000 km away, so the vast majority of those asteroids would be too far and too dim to be seen. Still, this shows that we shouldn’t be suprised that we see these interlopers.
There have been ~15 missions that flew spacecraft into and/or past the asteroid belt, something we can simultaneously only do because there are so many interesting asteroids, but also because it isn’t packed enough to be a hazard. So, while the asteroid belt isn’t dense enough to make for a very exciting chase scene, it is certainly full enough to encourage great science and exciting missions!
Banner Image: NASA Eyes on Asteroids. Credit: NASA/JPL