A New Neptunian Trojan

by Paul Gilster on August 12, 2010

It’s almost exhilarating to find that the volume of space studied in new work on the Trojan asteroids near Neptune includes an area through which New Horizons will pass on its way to Pluto/Charon. This used to seem like an all but unknowable region until Voyager 2 made its Neptune pass, and although it’s been a long time since we’ve had a spacecraft there, we’re learning much more about the outer system from Earth-based resources, as the discovery of objects like Eris and Sedna makes clear. We can surely look forward to more surprises as New Horizons moves toward its 2015 flyby and pushes on into the Kuiper Belt.

The latest find, based on data from the Subaru Telescope in Hawaii and the Magellan telescopes in Chile, is the first Trojan asteroid found at Neptune’s L5 Lagrangian point. Both the L4 and L5 Lagrangian points, 60 degrees ahead of and behind the planet, are stable, meaning that objects tend to collect there over time. Six Neptune Trojans are known in the L4 region, but until now the L5 point was hard to study because from our vantage on Earth, the line of sight is near the center of the galaxy. That called for a strategy using places where galactic dust clouds black out background light, revealing foreground asteroids. The result was the object called 2008 LC18.

Image: Discovery images of the L5 trailing Neptune Trojan 2008 LC18, taken at the Subaru telescope on June 7, 2008 Universal Time. The Neptune Trojan is seen moving from right to left near the center of the image. Each image is separated by about one hour in time. The background stars are stationary. This image only shows about 1 percent of the area of one image from the telescope. Credit: Scott Sheppard/Chad Trujillo.

Scott Sheppard (Gemini Observatory), explains the result:

“We estimate that the new Neptune Trojan has a diameter of about 100 kilometers and that there are about 150 Neptune Trojans of similar size at L5. It matches the population estimates for the L4 Neptune stability region. This makes 100-km-wide Neptune Trojans more numerous than similar-sized bodies in the main asteroid belt between Mars and Jupiter. There are fewer Neptune Trojans known simply because they are very faint since they are so far from the Earth and Sun.”

So now we’ve identified another Trojan population linked to Neptune to join the L4 asteroids there and the Trojans associated with Jupiter. The objects are useful adjuncts to planet formation theories. In this case, the fact that 2008 LC18 has an orbit that is highly tilted to the plane of the Solar System parallels the similar orbits of some L4 Trojans, and suggests the objects were captured during the early years of the Solar System, when Neptune itself was moving in a different orbit than today. We have much to learn about planetary migration as the giant planets refined their orbits and a chaotic system gradually settled into place.

The paper is Sheppard and Trujillo, “Detection of a Trailing (L5) Neptune Trojan,” published online in Science Express August 12, 2010 (abstract).


Doug M. August 12, 2010 at 19:33

So apparently Saturn and Uranus have no detected Trojans yet, and aren’t expected to have many?

I’ve read this, but I don’t understand the reasons why.

Doug M.

Tulse August 12, 2010 at 20:43

According to New Scientist, the finding suggests the Neptunian Trojans may hold more large asteroids than the Asteroid Belt.


(And would it have killed them to have explained in more detail about Lagrangian points?)

Doug M. August 13, 2010 at 4:46

If I understand correctly, Neptune’s Trojans don’t have to sit right on the Lagrange points. They can have significant eccentricity (up to 0.1) and quite a lot of inclination (up to 25 degrees or so).

This means they could be spread across a really quite large volume. So there could, in theory, be quite a few of them.

Doug M.

Doug M. August 13, 2010 at 9:32

Incidentally, New Scientist seems to be kinda sloppy there. IIUC, the article doesn’t say more rocks than the main belt — it says more large (>100 km radius) bodies.

Anyway. So I dug around a little, and apparently Jupiter and Saturn are in a 5 : 2 orbital resonance. And this is why Saturn has no Trojans.

— Okay, I’m still grappling with this whole orbital resonance thing. I understand how resonances create gaps, like in the main asteroid belt or Saturn’s rings. What I don’t understand is how resonances work between large bodies, like with Jupiter’s 1 : 2 : 4 moons, or Pluto and Neptune. Why does the effect work to keep them /in/ the resonance, instead of pushing them out? It’s particularly fraught in this case; the resonance seems to be holding on to Saturn, but kicking its Trojans away. How?

Doug M.

andy August 13, 2010 at 16:35

Doug M.: Jupiter and Saturn are close to the 5:2 resonance but not actually in it, though apparently the 5:2 resonance is a significant factor in the chaotic nature of orbits around Saturn’s Trojan points.

Another issue for the apparent lack of Saturnian and Uranian Trojans, it may be a combination of gravitational interactions with the other giant planets, and the migration history of the gas giants in the early solar system, which may have resulted in various resonance crossings that could have destabilised the Trojan points and cleared out the objects located there.

James M Essig August 15, 2010 at 10:20

Hi Folks;

Given the following excerpt from above;

“We estimate that the new Neptune Trojan has a diameter of about 100 kilometers and that there are about 150 Neptune Trojans of similar size at L5. It matches the population estimates for the L4 Neptune stability region …”

I assume that a total of about 300 (~ 100 km wide) Trojans, 150 at L5 and 150 at L4 may exist. This would be equivalent to a single body having a diameter of roughly (100)[300 EXP (1/3)] km or 670 kilometers.

This is a vast territory for which to prospect for rare and exotic metals and possibly resources for manned star ships such as nuclear fission fuel.

I read an article by Toni Feder at Physics Today about a year and a half ago regarding the concept of fission reactors that would burn up 99.8 percent of their waste and draw additional energy from the burn up process. The nuclear reactors would use a neutron source to enhance the reduction of waste to benign isotopic forms.

Since these asteroids are so far away, perhaps they could be launch points for fission reactor powered ion, electron, and/or photon rockets that would provide a greater Isp than the 0.04 for which U-235 is capable of all the while providing a safely distant launch location so as to avoid showering the Earth with dangerous ionizing radiation. However, if the waste could be largely reduced to non-radioactive materials, then it could simply be safely discarded and/or used as a reaction mass.

The cool thing about fission reactors is that we already know how to safely operate them.

Project Ikarus should definately be worked on with full vigor, since fusion fuel does offer an Isp which is considerably higher that that for fission fuels, but perhaps we should look at nuclear fission in parallel with the efforts of Project Ikarus just so that we have atleast two research pathways to the stars.

The large number of Trogan asteroids could definitely help in this regard.

ljk October 6, 2010 at 18:49

Planet Neptune Not Guilty of Harassment


“New research by a University of Victoria PhD student is challenging popular theory about how part of our solar system formed. At the meeting of the American Astronomical Society’s Division of Planetary Sciences in Pasadena, California, Alex Parker presented evidence that, contrary to popular belief, the planet Neptune can’t have knocked a collection of planetoids known as the Cold Classical Kuiper Belt to its current location at the edge of the solar system.”

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