The Hungaria asteroids are a group of asteroids in the asteroid belt that orbit the Sun between 1.78 and 2.00[1] AU. The asteroids typically have a low eccentricity (below 0.18)[1] and an inclination of 16 to 34[1] degrees. [2] They have an orbital period of approximately 2.5 years.[1] They have a resonance with Jupiter of 9:2 and with Mars of 3:2.[1] They are named after the largest member, 434 Hungaria, and are the innermost dense concentration of asteroids, lying somewhat inwards of the "core" of the asteroid belt that lies beyond the 4:1 Kirkwood gap.
Most Hungarias are E-type asteroids, which means they have extremely bright enstatite surfaces and albedos typically above 0.30. Despite their high albedos, none can be seen with binoculars because they are far too small: the largest (434 Hungaria) is only about 20 km in size. They are, however, the smallest asteroids that can regularly be glimpsed with amateur telescopes.[3]
The origin of the Hungaria group of asteroids is well known. At the 4:1 orbital resonance with Jupiter that lies at semi-major axes of 2.06 AU, any orbiting body is sufficiently strongly perturbed to be forced into an extremely eccentric and unstable orbit, creating the innermost Kirkwood gap. Interior to this 4:1 resonance, asteroids in low inclination orbits are, unlike those outside the 4:1 Kirkwood gap, strongly influenced by the gravitational field of Mars. Here, instead of Jupiter's influence, perturbations by Mars have, over the lifetime of the Solar System, thrown out all asteroids interior to the 4:1 Kirkwood gap except for those far enough from Mars' orbital plane where that planet exerts much smaller forces.[1]
This has left a situation where the only remaining concentration of asteroids inward of the 4:1 resonance lies at high inclination orbits, although they have fairly low eccentricities. However, even at the present time in Solar System history some Hungaria asteroids cross the orbit of Mars and in the process of still being ejected from the solar system due to Mars' influence (unlike asteroids in the "core" of the asteroid belt, where Jupiter's influence predominates).
Long-term changes in the orbit of Mars are believed to be a critical factor in the removal of Hungaria asteroids. At the highest eccentricities, similar to the extreme values observed today or even slightly greater, Mars will perturb Hungaria asteroids and force them into ever more eccentric and unstable orbits when their ascending node is close in longitude to Mars' aphelion.[4] This ultimately leads over millions of years to the formation of the short-lived Amor asteroids and Earth-crossers.
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