Neptune Trojans. There may be thousands.

3488 · Jan 30, 2007

The first Neptune Trojan was discovered in 2001 as part of the NASA funded Deep Ecliptic Survey at the Lagrange region 60 degrees and 3.1 billion miles (5 billion kilometers) ahead of Neptune. A further three Neptune Trojans between 37 and 87 miles (60 and 140 kilometers) in diameter and shaded a pale red color have since been identified by Scott Sheppard of the Carnegie Institution of Washington and Chadwick Trujillo of the Gemini Observatory in Hawaiiusing the 6.5-meter Magellan telescope in Chile. Despite their diminutive size and brightness, the Neptune Trojans quickly betrayed their existence by their distinct motion against background stars. The most recent Trojan discovered by Sheppard and Trujillo is moving at an unusual inclination of 25 degrees relative to the plane of the solar system (the ecliptic). ”The sky we covered searching for Neptune Trojans was all within 1.5 degrees of the ecliptic,” Sheppard said. “High inclination objects will spend the majority of their time off the ecliptic. Thus detecting a high inclination Trojan in our survey suggests there is a large population of such objects. In fact, the high inclination objects appear to outnumber the low inclination objects by a ratio of four to one." If so, there would be swarms of Trojans accompanying Neptune, perhaps up to twenty times more than at Jupiter. The sheer number of Trojans Neptune is thought to harbor reveal that these objects are an established part of Neptune’s entourage, dating back to shortly after the planet’s formation. “Neptune cannot currently efficiently capture Trojans for long periods of time,” Sheppard said. “Just after the planet formation epoch Neptune's orbit was likely much more eccentric due to its interactions with the other planets. Neptune's interactions with the myriad small bodies around its orbit which included comets, Kuiper Belt objects and other debris which formed nearby would have slowly circularized Neptune's or <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

MeteorWayne · Jan 30, 2007

Andrew, Where did you get this article from? I'd like to keep track of followup observations. Thanx Wayne <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

mikeemmert · Jan 30, 2007

Hi, MeteorWayne. The link: http://www.space.com/scienceastronomy/070130_st_neptune_trojans.html. Our very own Space.com. I read the description of how Lagrange points work. It wasn't very detailed. So I will try to fill in some of the points that the graphic on the article missed. The Lagrange points were discovered mathematically by Jean Joseph Louis Lagrange in 1755, thus earning him a huge prize from the French Academy of Sciences for showing up England's Isaac Newton (France and England were at the time engaged in a Cold War). Lagrange points happen because three forces are balanced: the gravitational force of the largest object, the gravitational force of the smaller object, and centrifugal force around the center of mass of all three objects. Unless two of the objects are massless test objects, the center of mass of the system does not coincide with the center of mass of any of the objects. Notice that in various places in the article the L4 point is described as "60 degrees ahead of Neptune" in it's orbit about the Sun, and L5 is described as "60 degrees behind". It's actually somewhat greater than 60 degrees, because all these things orbit the center of mass of the system. Their locations were also described as being at the apex of an equilateral triangle. This is the exact mathematical solution. This system can only work if one of the masses is close to the center of gravity of the system. The center of gravity is closest to the largest mass. This mass has to be 25 times larger than the second mass; masses at the Lagrange points need to be "negligible". This condition is satisfied for all eight planets; they all have Lagrange points. It is also satisfied for all the moons of the eight planets, but not for dwarf Planet Pluto. Charon is about 1/10 the mass of Pluto; as a result, this system h

brellis · Jan 30, 2007

hellos wayne and mike, thanks mike for your illuminating comments on LaGrange points. My favorite moments on SDC occur when a post like yours gets the ol' light bulb flickering in my head! I had been wondering if at the end of its extended mission Cassini could be sent to a Trojan location following Titan or Enceladus around Saturn. I assume Cassini would have to slow down a great deal to do this, but if it has enough fuel, wouldn't it be ideal to leave it in a longterm position observing the geysers of Enceladus, or the changing season on Titan? <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

MeteorWayne · Jan 31, 2007

Even though I knew what they were, that's a great explanation, and I hadn't thought about it that one of the points is really the barycenter, not the largest mass. Thanx!! And thinks for the link. I've been sick for the last few days, so my browing around has been limited. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

robnissen · Jan 31, 2007

GREAT POST!! <img src="/images/icons/smile.gif" /> Most of the explanations I have seen describing Lagrange points leave out centrifugal force. They merely state that the gravitational force of each object is equal. But that only describes L1. I never understood how L2-L5 existed. But thanks to your simple, eloquent post, I now understand that centrifugal force is the explanation for the other LaGrange points. Thanks again. One question though. You state: This condition is satisfied for all eight planets; they all have Lagrange points. It is also satisfied for all the moons of the eight planets, but not for dwarf Planet Pluto. I don't understand how Mercury can have Lagrange points, when it doesn't have a satellite. Doesn't it take two masses to create Lagrange points. Perhaps you are referring to the system of the Sun and Mercury. But if that system has Lagrange points, then doesn't the Sun Pluto/Charon system have Lagrange points?

mikeemmert · Jan 31, 2007

Ouch! <img src="/images/icons/blush.gif" /> Well, you got me. What I should have said is that "all known moons of the planets have Lagrange points" or something to that effect. I put in the "eight planets" because the Pluto/Charon system does not have Lagrange points. In case you haven't noticed, Pluto was "demoted" recently...I think the truth about Pluto is more interesting than if it has some fancy label. I was indeed referring to the Sun/Mercury system. And yes, the Sun/Pluto Charon system has Lagrange points (<img src="/images/icons/blush.gif" />} but they're so weak I doubt if they'll hold anything. Neptune's pretty certain to rip anything out of the Sun/Pluto Lagrange points. As I stated, the Pluto/Charon system has no Lagrange points. Thanks for the constructive criticism. If I ever write a book about all this stuff, I will try to remember how careful one must be when writing about these things. However, it's really easy to sound "too" careful, I've noticed that, too. The only way I know of to avoid both problems is to not be descriptive enough and I want to avoid that problem as assiduously as possible <img src="/images/icons/wink.gif" /> .

brellis · Jan 31, 2007

hi mike Neptune's pretty certain to rip anything out of the Sun/Pluto Lagrange points. Does the "attraction" to an L-point itself strengthen as one approaches it? I assume not. How does the "halo orbit" around an L-Point work? I take it this term simply describes how objects drift between gravitational influences. <div class="Discussion_UserSignature"> I'm a recovering optimist - things could be better. </div>

mikeemmert · Jan 31, 2007

For my next act, I am going to place an object at only one Lagrange point (at a time). This gives you what is called a "three body problem". Lagrange's limited solution to the three body problem is what got him a huge prize from the French Academy of Science. (He had to invent new mathematics to do that, and his solution has lots of practical uses in a wide variety of fields.) The problem as stated was (excuse my bad French and fourth-hand translations), "given three bodies alone in the Universe and their initial postions and velocities, can their relative positions be determined for all of time?". If you place one object at any of the Lagrange points, the three will stay in those positions relative to each other foreever. Unfortuneately, if your positioning at L1, L2, or L3 is even the slightest bit off, then the gravities of the two objects and centrifugal force will not add up to zero. There will be an unbalanced force, which will have a direction. At first the force is very small, but as the object moves away from the Lagrange point, the unbalanced force will increase. They're unstable. Now, the little graphic that came with the article that Andrew (3488) was so kind as to post shows what happens if you're a little off on placing an object at Lagrange points L4 and L5. Let's go over what happens to an object that's a little off. For simplicity, I am going to be refering to a central mass for the system more than 25 times the second mass, but less than infinitely more massive; imagine an alien system who's star is the mass of the Sun and whose only planet is the mass of Neptune (the article's about Neptune). Furthermore, the planet (we'll call it Neptune anyway) is in a perfectly circular orbit around the "Sun". OK, a perfect little system, stays like that foreever. See, but we messed up <img src="/images/icons/blush.gif" /> . We put the spacecraft (the third object of the system)(we'll call it Lost Horizons, or LH

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