Monday, August 3, 2015

The Alinda Family of Asteroids

The asteroid 887 Alinda has long been known to follow an orbit that is nearly resonant with the orbital periods of both Jupiter and Earth: its orbital period of 3.915 years is close to the 1:4 Earth resonance and close to the 3:1 resonance with Jupiter.  In recent years the rate of discovery of previously unknown asteroids has been enormous, with thousands of new asteroid discoveries each year, so it is not surprising that a number of other Alindas have been found.  Membership in this family requires an orbital period very close to 4 Earth years, which in turn requires that the mean distance from the Sun (the orbital semi-major axis) must be close to 2.54 AU.  That places these bodies in the inner asteroid belt—except for the excursions brought about by the eccentricities of their orbits. 
Orbits close to a Jupiter resonance are not only subjected to the gravitational perturbations exerted by Jupiter on all asteroids, but experience repeated perturbations with the same approximate geometry.  This allows, like the resonant pumping of a child on a swing, a constant buildup of self-reinforcing disturbances, which cause a constant growth in the eccentricity of the asteroid’s orbit, making an ever more elongated ellipse. Eventually, this growth in eccentricity imperils the asteroid by extending its orbit inward to perihelion distances ever closer to the Sun, crossing the orbits of one or more of the terrestrial planets, while also stretching the orbit outward so that its aphelion distance can approach Jupiter.  Close encounters with any planet can seriously disturb an asteroid’s orbit; the closest encounters, resulting in collisions, are fatal to the asteroid and may be seriously disruptive to the target planet.
The 23 Alindas now known include eleven in low-eccentricity orbits (e ranging from about 0.30 to 0.34).  These bodies roam the reaches of the Solar System from about 1.7 to 3.4 AU from the Sun, spending most of their time in the asteroid belt and never approaching any planet closely. They are the "young" Alindas, recently nudged into resonant orbits.  In such orbits their resonant relationship to Jupiter causes their orbits over time to gradually become more eccentric.  They are not in immediate danger except for the small probability of colliding with other asteroids, but they are in for serious trouble in the long run.
Three of the known Alindas (6318 Cronkite, 8709 Kadlu, and 6322 1991 CQ) have orbital eccentricities between 0.465 and 0.475, sufficient to have them cross the orbit of Mars.  These three Alinda Mars-crossers do not cross the orbit of any other planet; Mars has a small mass and cross-section area, and cannot remove these bodies as rapidly as Jupiter can replenish them and move them on to even more eccentric orbits.
Then there is the namesake of the family, 887 Alinda itself, with an eccentricity of 0.564.  Its perihelion distance (q) of 1.084 AU qualifies it as a near-Earth asteroid (NEAs by definition have q < 1.300 AU).  It grazes but does not cross Earth’s orbit, making it an Amor asteroid as well as an Alinda family member.
Even more pumped-up Alinda clan members include eight (with eccentricities between 0.57 and 0.75) that cross Earth’s orbit: at perihelion they are closer to the Sun than Earth is at aphelion, 1.017 AU.  They are therefore Apollo-family NEAs as well as Alindas.  Since all Alindas are Earth-resonant, they may fly by Earth repeatedly at close range at 4-year intervals for decades at a time, affording radar observation and spacecraft launch opportunities—and collision opportunities—over that time period.  One such asteroid is 4179 Toutatis, which was the target of a close flyby by the Chinese Chang-e 2 spacecraft in 2013.  Two members of this group, 7092 Cadmus and 8201 1994 AH2, could be termed Venus-grazers, having perihelia inside 0.76 AU.  The most eccentric of the Alindas is 3360 Syrinx, a Venus-crosser with e = 0.743.  Its orbit makes six crossings of planetary orbits every four years (twice each for Mars, Earth, and Venus), a highly unstable situation that suggests a short life expectancy.  Interestingly, all three of these most-eccentric Alindas have aphelia close to 4.3 AU.  None of the Alindas approach Jupiter closely, a wise precaution.  A close encounter with Jupiter could swallow the asteroid whole, kick it out of the Solar System permanently, or wreak other orbital havoc.
The Alindas serve as a reminder of the role Jupiter plays in sending hazardous bodies toward us; a fringe benefit is the opportunity to have many repeated launch opportunities to a given asteroid.  The Alindas are loose cannons, subject to disturbance by Jupiter, Mars, Earth, and Venus.  These asteroids are both carrot and stick, guaranteeing that we will hear a lot more about them in the future--such as when Toutatis comes by again in 2016! 

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