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Trojan Asteroids Found In Neptunian Orbit
Agent Provocateur writes to mention a release at Science Daily about three rogue asteroids discovered by the Carnegie Institute. The objects are in about the same orbit as Neptune, lending evidence that the planet has a cloud of these 'Trojan' celestial bodies. From the article: "Trojan asteroids cluster around one of two points that lead or trail the planet by about 60 degrees in its orbit, known as Lagrangian points. In these areas, the gravitational pull of the planet and the Sun combine to lock the asteroids into stable orbits synchronized with the planet. German Astronomer Max Wolf identified the first Jupiter Trojan in 1906, and since then, more than 1800 such asteroids have been identified marching along that planet's orbit. "
I believe it was saying that 1800 Jovian Trojans have been found. These are the first Neptunian Trojans to be discovered. Being that much farther from the sun, they are far more difficult to detect. Also, since Neptune's mass is less than Jupiter's and it is further from the main asteroid belt, it might not have as many to begin with.
Constitutionally Correct
At this distance they're more likely to be captured Edgeworth-Kuiper Belt Objects and therefore more likely to resemble comet nuclei. Neptune already has a large number of EKBO's in a 3:2 resonance, including the "planet" Pluto - we sometimes call objects in the 3:2 resonance with Neptune 'Plutinos'. So, the fact that some objects get caught in this stable 1:1 resonance hardly surprises me, but it's nice to have someone actually identify such objects.
The first Neptune trojan was discovered in 2001. These three were discovered since then over the course of the last 2-3 years. Not particularly new, but the paper finally got published.
The burning question is - how can they call it a "trojan asteroid" if it doesn't occupy the same orbit as Neptune? A significant orbital inclination vis a vis Neptune makes it a passing stranger at best, not something captured in Neptune's Lagrange points.
Not quite. The L4 and L5 Lagrange points are kind of like gravitational collection points. There is a fairly large area surrounding these points where objects can play around based on whatever other forces are affecting them, but still remain trapped by the Lagrange point. So, if you look at a "top-down" view of the solar system, the asteroid would be moving in lock-step with Neptune's orbit at the Lagrange point. But if you look at a "side-on" view, the orbit would follow kind of a wave patern, with one period equal to one orbit.
It's been proven that you can't have stability using only attractive forces in a static system.
The classical proof is, take any number of magnetic, electrical, and gravitational forces that are fixed relative to each other. There is no arrangement of these forces that produces a point of both equilibrium and stability.
Good thing planets move. Trojan points are just two of the five lagrange points. When any two gravitational bodies orbit each other, there will exist points of equilibrium where all gravitational forces cancel each other out - this is called a lagrange point. That's great but you also have to consider the concept of stability and sensitive dependence on initial conditions. For instance, L1, the simplest to calculate of the lagranges, is a point on a line connecting the centers of mass of the two bodies such that the pull from each body is the same. Slip a little in either direction, and thanks to the inverse power law, you see a greater pull from the body you slipped closer to, pulling you further out of equilibrium. The L1 point is not stable - objects there don't tend to stay there.
Think of it like a hill with a flat top. You can put a marble perfectly balanced at the top of the hill, but eventually something will push it a little to the side and it'll roll down.
The other points are L2 and L3, both on the same line as L1, but beyond either the smaller or the larger of the bodies, depending on if you're talking about L2 or L3. L4 and L5 are the trojans, 60 degrees ahead or behind of the smaller body in its orbit. L1 through L3 are unstable, though certain very non-circular orbits about those points are stable over periods of time. L4 and L5 are stable; a minor perturbation will pull the object slightly out of the point, and then coriolis effects pull it into orbit about the lagrange point. If you've ever read The Smoke Ring or related works you're familiar with this concept: go east to go up, go up to go west, go west to go down, go down to go east. The more scientific way of putting would be that faster orbits rise, slower orbits fall. If you move vertically in your orbit you don't change the speed of the orbit, and the average distance from your orbital center will always be the same for a specific speed for the same mass that you're orbitting. Move vertically simply makes your orbit more or less elliptical.
Now why do these coriolis forces affect only the L4 and L5 points in this manner? Because L4 and L5 are valid orbital points even if you take out the second body and convert those to basic orbits - orbital distance of an object is not dependant on that object's mass (until it gets massive enough to noticeably perturb the object around which it orbits - such as in the earth/moon system), only on it's speed. At the L1, L2, and L3 points the object is in equilibrium but it's natural orbit without the second mass would be highly elliptical, thus minor perturbations there tend to make elliptical orbits around one of the masses in the system (and that's why highly perturbed orbits about these points are quasi-stable), while at the L4 and L5 points a minor perturbation results in minor orbit changes, but the point of equilibrium stays the same.
I am disrespectful to dirt! Can you see that I am serious?!
To add to the detailed explanation by merlin_jim, here's Wikipedia's entry on Lagrange points, which includes a couple of diagrams showing where L1-L5 are located in an orbital system.