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Neptunian Lagrangian Asteroid

August 16, 2010

In a previous article (The Witch of Agnesi, June 22, 2010), I mentioned a simple test that Isaac Asimov had to determine whether a person was a scientist or a non-scientist. There's a similar test to determine whether an astrophysicist sits more on the astronomy or the physics side of the fence. You ask her to explain a Lagrangian. Joseph Louis Lagrange was a mathematician who contributed to many fields, including number theory, classical mechanics and celestial mechanics. Physicists know him for a function called the Lagrangian, which expresses the difference between the kinetic and potential energy of a system. As such, it's a close cousin to the Hamiltonian, which expresses the sum of the kinetic and potential energies and is a way to analyze energy-conserving systems.

Astronomers know Lagrange instead for the Lagrangian Point, a place in space where a smaller object can be held relatively stationary with respect to two larger objects; for example, the Earth and the Moon. These points are analogous to geostationary orbits. For the Earth-Moon system, the Lagrangian points have been proposed as convenient parking lots for such diverse things as space telescopes and human colonies.

The Lagrangian points in the solar system are places where cosmic debris, such as asteroids, can be trapped. The length of time such trapping is effective depends a lot on whether there are transient gravitational fields that disturb those of the two main bodies. A team of astronomers has recently reported an asteroid, designated 2008 LC18, that sits in the trailing L5 Lagrangian region of Neptune.[1-3] These L5 Lagrangian points arise from the gravitational effects of Neptune and the Sun, and they appear sixty degrees ahead and behind the orbiting planet. Similar asteroids have been found at the Neptunian L4 Lagrangian points, but the detection of Neptunian L5 asteroids had been hindered by an obscuring background of stars in our galaxy. As the discovery images below indicate, separating this object from the background was no simple feat. To facilitate the search, the astronomers looked purposely at regions in which galactic dust blocked most of the background stars.

Discovery photos of the Neptunian L5 trojan

Discovery photos of the Neptunian L5 trojan.[3]

It took some considerable hardware to make this discovery - An 8.2 meter diameter telescope. Asteroid 2008 LC18 is estimated to be about 100 km in diameter; and although only this one was found, it's estimated to have about 150 companions of about the same size. These haven't been detected because of the observational difficulties. It's conjectured that these L5 asteroids were captured when Neptune was in a different orbit than it is in now. The clockwork universe has quite a bit of chaos built into it.


  1. "Asteroid Found In Gravitational 'Dead Zone'," Carnegie Institution Press Release, August 12, 2010.
  2. Scott S. Sheppard, and Chadwick A. Trujillo, "Detection of a Trailing (L5) Neptune Trojan," Science Online, August 12, 2010.
  3. "Trojan Asteroid Found in Neptune's Trailing Gravitational Stability Zone," Carnegie Institution Press Release, August 12, 2010.

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Linked Keywords: Isaac Asimov; astrophysicist; astronomy; physics; Joseph Louis Lagrange; Lagrangian; Hamiltonian; Lagrangian Point; James Webb Space Telescope; L5 Society; Neptune; clockwork universe; chaos.