The Asteroid Belt

Where the belt is, and where its gaps are

The main belt runs from about 2.06 AU to 3.28 AU — between Mars and Jupiter — and it is not evenly filled. Every edge and every clean lane in it is a resonance with Jupiter: an asteroid whose orbital period is a simple fraction of Jupiter’s year gets the same gravitational tug at the same point over and over, and is eventually pushed somewhere else.

That means the whole structure can be calculated rather than remembered. A body in a p:q resonance orbits at Jupiter’s distance times (q/p)^2/3, and the table below is that formula applied to Jupiter’s own semi-major axis — the same number the simulator draws Jupiter with. Change one and the other moves.

The belt is already switched on in the picture above — jump to the belt or the Jupiter view to see it drawn, gaps and all. The Trojan clouds move with Jupiter, so they are wherever Jupiter is on the date you have chosen.

Why the belt doesn’t fall into the sun

The sun pulls on every asteroid in the belt, hard, without a break, and has done for four and a half billion years. Nothing holds them up. What keeps them out there is the same thing that keeps a planet out there: they are moving sideways fast enough to keep missing. An orbit is a fall that never arrives, and an asteroid obeys that rule exactly as Jupiter does — it is not too small to fall in, and its own mass appears nowhere in the calculation.

The speed is not a choice. At a given distance from the sun there is exactly one speed that turns a fall into a circle, and every rock still out there is travelling close to it — anything that was not either left long ago or is on its way somewhere else. From the sun’s gravity and the belt’s own edges:

The belt does not turn as one thing. Every asteroid keeps its own orbit and its own year, and the inner ones go round faster than the outer ones — 3.0 years against 5.9 — so the belt shears past itself rather than rotating like a wheel. That difference is exactly why the resonances above matter: it decides which asteroids keep meeting Jupiter in the same place, and those are the ones that get removed.

The orbital velocity simulator lets you set a distance and a sideways speed and watch what gravity makes of it — a circle, a long ellipse, an escape, or a fall into the sun. Set it out at the belt and try to drop a rock into the sun: it takes shedding almost all of that 20.7 km/s before the near end of the new orbit reaches the sun at all.

The four largest

Between them these hold about half the mass of the entire belt, and Ceres alone is roughly a third of it.

The simulator draws the belt as a band and never as individual dots. The orbit of an asteroid is a fact that needs no date; where it sits on that orbit today is something this site does not solve for, and a dot in an invented place would be exactly the kind of confident wrongness these pages are built to avoid.

The belt: things worth knowing

• Everything in the belt put together comes to about 3% of the mass of the Moon. Ceres alone is roughly a third of it, and the four largest objects are about half.
• It is not crowded. The typical separation between belt objects large enough to name is on the order of a million kilometres; every spacecraft sent to the outer solar system has flown straight through without a course change for debris.
• The gaps in it are Jupiter's fingerprints. An asteroid whose orbital period is a simple fraction of Jupiter's gets the same tug at the same point every few orbits, and is eventually pushed out — which is why the belt has clean lanes at the 3:1, 5:2 and 7:3 resonances and edges at 4:1 and 2:1. This page computes all of those from Jupiter's orbit rather than drawing them from memory.
• The same resonances that empty parts of the belt fill others. The Hildas sit at 3:2, and the Trojans share Jupiter's orbit exactly, in two clouds 60° ahead of and behind it — which is why this simulator draws them wherever Jupiter happens to be.
• It was never a planet. There is not enough material there to make one, and there is no evidence of a body that broke up; Jupiter's gravity stirred the region hard enough that collisions smashed rather than stuck.

Recently learned about the asteroid belt

Findings reviewed August 2026. Space science moves; a date on a finding is part of the finding.

Make a link to a particular view

The date, the zoom, the span, the speed, the layers and any flight path are all in the address bar, so copying the URL shares exactly what is on screen. Set it up above, then take the link — it is the quickest way to hand a class one specific thing to look at.

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The other simulator

This one is about the whole system. If the question is where the sun and the moon are from where you are standing — what time the sun comes up, why tonight's moon is the shape it is — that is the Sun, Earth & Moon movement simulator, which has a page for every city and a slider over a day, a week or a month. Between the two is the three bodies moving together — Earth going round the sun, the moon going round the Earth, on one screen and openly not to scale, keeping only the real ratio between the two periods.

And for why any of it stays up: the orbital velocity simulator takes one planet and lets you set its distance and its sideways speed by hand, so you can watch the balance that holds every orbit here — and break it, into an ellipse, an escape, or a fall into the sun.

Comets · Jupiter, which made the belt what it is · Also: the classroom guide · sunrise & sunset by city · moon phase & moonrise · lunar eclipses · how the positions are worked out