# Why is the asteroid belt shaped like a triangle?

So, in this question, the user JollyJoker posted this image depicting the orbits of the asteroids in the asteroid field in the comments:

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In this image, you can see that while the individual asteroids follow elliptical orbits, the asteroid belt as a whole is a giant triangle with the Jovian Trojans at two corners, and a third corner directly opposite Jupiter, and the whole triangle orbits in sync with Jupiter.

Is this image accurate? Is the Asteroid Belt actually a giant triangle, rather than the loose circle it is often depicted as? If so, why is this the case?

• Scheirich has more visualizations of the inner solar system here. The main belt is shown in three shades of red. Feb 12, 2020 at 14:26
• FYI that image host is considered "adult content" by some corporate blockers. Feb 13, 2020 at 9:52
• You point out in the question that the individual red objects each have an elliptical orbit but the collection looks like a triangle that points away from Jupiter. A similar animation that shows more clearly that the individual objects have elliptical, not triangular orbits is here: sajri.astronomy.cz/asteroidgroups/hildaorb.gif Feb 13, 2020 at 21:14

## 1 Answer

It's not. The image doesn't show the main asteroid belt. It shows the Jovian Trojans (in green) , and the Hilda Asteroids (in red).

The Hildas are a dynamical group of a few thousand known asteroids in elliptical orbits that are locked in a 3:2 orbital resonance with Jupiter, and reach aphelion coinciding with the regions near Sun-Jupiter Lagrange points L4, L3, and L5 in succession.

"Animation showing Hildas with 3 indicated orbits" from "Asteroid (and Comet) Groups", Petr Scheirich, Retrieved 2020/02/14.

In accordance with Kepler's Second Law, objects in elliptical orbits move slower near aphelion than perihelion, and as a result, when you just look at the Hildas, they bunch up near the Lagrange points, and the pattern they appear to show at any one time resembles a triangle.

A more complete view of all the Asteroids inside of Jupiter looks like this.

• youtube.com/watch?v=yt1qPCiOq-8
– uhoh
Feb 12, 2020 at 14:15
• Not really coinciding with the Lagrange points, but approaching them – not because they are Lagrange points, but because those are the phases where an orbit with that period, with periapsis under Jupiter, must have its apoapsis. Feb 13, 2020 at 7:10
• This video is both enlightning in showing where asteroids are, and also very mesmerizing to watch (it shows atseroids both orbiting and being discovered through the years) Feb 13, 2020 at 23:10
• @Bilkokuya I've updated the answer with an image illustrating the elliptical orbits, and added some language about how they bunch up into the triangle. Feb 14, 2020 at 12:39