| Jupiter trojans Orbits ofplanets Sun | Hilda group Asteroid belt Near-Earth objects(selection) |
TheHilda asteroids (adj.Hildian) are adynamical group of more than 6,000asteroids located beyond theasteroid belt but withinJupiter's orbit, in a 3:2orbital resonance with Jupiter;[1][2] that is, while Jupiter orbits twice, the Hildas orbit three times. The namesake is the asteroid153 Hilda.
Hildas move in their elliptical orbits in such a fashion that they arrive closest to Jupiter's orbit (i.e. at theiraphelion) just when either one of Jupiter'sL5,L4 orL3Lagrange points arrives there.[3] On their next orbit their aphelion will synchronize with the next Lagrange point in theL5–L4–L3 sequence. SinceL5,L4 andL3 are 120° apart, by the time a Hilda completes an orbit, Jupiter will have completed 360° − 120° or two-thirds of its own orbit. A Hilda's orbit has asemi-major axis between 3.7 and 4.2AU (the average over a long time span is 3.97), aneccentricity less than 0.3, and aninclination less than 20°.[4] Twocollisional families exist within the Hilda group: theHilda family and theSchubart family. The namesake for the latter family is1911 Schubart.[5]
The surface colors of Hildas often correspond to the low-albedoD-type andP-type; however, a small portion areC-type. D-type and P-type asteroids have surface colors, and thus also surface mineralogies, similar to those ofcometary nuclei. This implies that they share a common origin.[4][6]
The asteroids of the Hilda group (Hildas) are in 3:2mean-motion resonance with Jupiter.[4] That is, theirorbital 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 theJupiter 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 triangularlibration points of Jupiter—the "Hildas Triangle".[3] The "asteroidal stream" within the sides of the triangle is about 1AU 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 ownelliptic 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 aretrograde perihelion motion. On average, the velocity of perihelion motion is greater when the orbital eccentricity is lesser, while the nodes move more slowly. All typical objects in aphelion would seemingly approach closely to Jupiter, which should be destabilising for them—but the variation of the orbital elements over time prevents this, andconjunctions with Jupiter occur only near the perihelion of Hilda asteroids. Moreover, theapsidal line oscillates near the line of conjunction with different amplitude and a period of 2.5 to 3.0 centuries.
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. Theorbital periods of these asteroids are approximately 7.9 years, or two thirds that of Jupiter.
Although the triangle is nearlyequilateral, some asymmetry exists. Due to the eccentricity of Jupiter's orbit, the sideL4–L5 slightly differs from the two other sides. When Jupiter is inaphelion, the mean velocity of the objects moving along this side is somewhat smaller than that of the objects moving along the other two sides. When Jupiter is inperihelion, the reverse is true.
At the apexes of the triangle corresponding to the pointsL4 andL5 of Jupiter's orbit, the Hildas approach theTrojans. At the mid-sides of the triangle, they are close to the asteroids of the external part of theasteroid belt. The velocity dispersion of Hildas is more evident than that of Trojans in the regions where they intersect. The dispersion of Trojans ininclination is twice that of the Hildas. Due to this, as much as one quarter of the Trojans cannot intersect with the Hildas, and at all times many Trojans are located outside Jupiter's orbit. Therefore, the regions of intersection are limited. This is illustrated by the adjacent figure that shows the Hildas (black) and the Trojans (gray) along theecliptic plane. One can see the spherical form of the Trojan swarms.
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.
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 nearconjunction with the mid-sides of the Hildas Triangle, because that is when the asteroids are closest to Earth, and in opposition with the Sun. They are therefore at their brightest during these moments which occur every 4 and 1/3 months. In these circumstances thebrilliance of objects of similar size could run up to 2.5 magnitudes as compared to the apices.[citation needed]
The Hildas traverse regions of theSolar 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]