Hilda asteroid

The Hilda asteroids (adj. Hildian) are a dynamical group of more than 5,000 asteroids located beyond the asteroid belt but within Jupiter's orbit, in a 3:2 orbital resonance with Jupiter.^[1][2] The namesake is the asteroid 153 Hilda.

Hildas move in their elliptical orbits in such a fashion that they arrive closest to Jupiter's orbit (i.e. at their

The asteroids of the Hilda group (Hildas) are in 3:2 mean-motion resonance with Jupiter.^[4] That is, their orbital periods are 2/3 that of Jupiter. They move along the orbits with a semimajor axis near 4.0 AU and moderate values of eccentricity (up to 0.3) and inclination (up to 20°). Unlike the Jupiter trojans they may have any difference in longitude with Jupiter, nevertheless avoiding dangerous approaches to the planet.

The Hildas taken together constitute a dynamic triangular figure with slightly convex sides and trimmed apices in the triangular libration points of Jupiter—the "Hildas Triangle".^[3] The "asteroidal stream" within the sides of the triangle is about 1 AU wide, and in the apices this value is 20–40% greater. Figure 1 shows the positions of the Hildas (black) against a background of all known asteroids (gray) up to Jupiter's orbit at January 1, 2005.^[7]

Each of the Hilda objects moves along its own elliptic orbit. However, at any moment the Hildas together constitute a loosely-triangular configuration, and all the orbits together form a predictable ring. Figure 2 illustrates this with the Hildas positions (black) against a background of their orbits (gray). For the majority of these asteroids, their position in orbit may be arbitrary, except for the external parts of the apexes (the objects near aphelion) and the middles of the sides (the objects near perihelion). The Hildas Triangle has proven to be dynamically stable over a long time span.^{[citation needed]}

The typical Hilda object has a

In addition to the fact that the Hildas triangle revolves in sync with Jupiter, the density of asteroids in the stream exhibits quasi-periodical waves. At any time, the density of objects in the triangle's apexes is more than twice the density within the sides. The Hildas "rest" at their aphelia in the apexes for an average of 5.0–5.5 years, whereas they move along the sides more quickly, averaging 2.5 to 3.0 years. The orbital periods of these asteroids are approximately 7.9 years, or two thirds that of Jupiter.

Although the triangle is nearly

At the apexes of the triangle corresponding to the points

When moving along each side of the triangle, the Hildas travel more slowly than the Trojans, but encounter a denser neighborhood of outer-asteroid-belt asteroids. Here, the velocity dispersion is much smaller.

Left: A schematic of the orbit of 153 Hilda (green), with Jupiter (red); Middle: Hildas (black) and Trojans viewed from the ecliptic plane near 190 degrees longitude on Jan. 1, 2005. Right: Orbits of two idealized asteroids of the Hilda group, in the rotating reference frame of Jupiter's orbit. Black: eccentricity 0.310; aphelion at Jupiter's orbit. Red: eccentricity 0.211, the critical value for existence of a cusp.

Research

The observed peculiarities in the Hildas' motion are based on data for a few hundred objects known to date and generate still more questions. Further observations are needed to expand on the list of Hildas. Such observations are most favorable when Earth is near

The Hildas traverse regions of the Solar system from approximately 2 AU up to Jupiter's orbit. This entails a variety of physical conditions and the neighborhood of various groups of asteroids. On further observation some theories on the Hildas may have to be revised.^[citation needed]

References