The asteroid belt marks the boundary in our Solar system between the inner rocky planets and the outer gas giants.
It's made up of large, irregular-shaped, rocky and metallic bodies called asteroids, or minor planets.
It is the widest region of the Solar System between Mercury and Neptune that doesn't contain a major planet.
One of many rings of the Solar System including the rings of Saturn and the Kuiper Belt, scientists are slowly revealing the secrets of the asteroid belt.
Mass, properties and orbit
The total mass of the asteroid belt is a mere one-thousandth that of Earth’s, and a single asteroid, Ceres, comprises almost a third of this total mass.
And while science fiction has largely depicted the asteroid belt as a ring of dangerously-close space rocks, in fact the individual bodies are so far apart that a spacecraft would have no problem flying through asteroid belt.
The inner asteroids are mostly dry, S-class (siliceous) objects thought to be most similar to ordinary chondrite meteorites.
C-class (carbonaceous) objects dominate the outer belt and are loaded with carbon and water.
The orbits of the asteroids are what is known as ‘dynamically excited’.
Their circuits around the Sun aren’t neat circles within the plane of the Solar System, but have orbital eccentricities of up to 0.3 and can be inclined at angles of over 20°.
Despite how well we can characterise the asteroid belt today, we don’t know for certain how it formed.
How the asteroid belt formed
The classical view is that the asteroid belt formed within the gassy, dusty disc around the young Sun.
It's a swarm of leftover planetesimals with an initial mass of perhaps several Earth masses in total.
Over time, so this hypothesis goes, 99.9% of this material was ejected out of this orbital region by gravitational interactions from the planets (including the possible early migration of Jupiter and Saturn)
So the asteroid belt we observe today represents the remnants of a heavily depleted reservoir of planetesimals.
But the opposite could explain the characteristics of the asteroid belt, too.
Perhaps this orbital region actually started with no rocky material in it at all, and has been filled up over Solar System history with planetesimals born elsewhere in the Solar System.
In this way, the sculpting of the asteroid belt may be a story of accumulation rather than depletion.
One clue to what happened may be provided by the two largest asteroids, Ceres and Vesta.
Ceres and Vesta
While Ceres and Vesta currently occupy relatively close orbits (indeed, there’s a chance they may collide in the next billion years or so) it is likely that they actually formed in vastly different environments in the primordial Solar System.
NASA’s Dawn space probe confirmed that Ceres is rich in water ice and has a composition most similar to carbonaceous chondrite meteorites.
This giant asteroid is thought to have formed in the cold outer Solar System, but was dislodged and implanted into the asteroid belt by the formation of the gas giants.
Vesta’s composition, on the other hand, suggests it was scattered into the asteroid belt from its birthplace closer to the Sun.
A mark for accumulation perhaps?
This article originally appeared in the March 2021 issue of BBC Sky at Night Magazine.
