The interloper this time is 2002AJ129, a body with a highly eccentric, comet-like orbit. Its perihelion is a near-suicidal 0.116397 AU, far inside Mercury’s orbit, and its orbital eccentricity is 0.915097, typical of short-period “periodic” comets. That means that the orbital period is short enough so that it actually returns to the inner Solar System, where we humans presently reside, on human time scales. Its orbital period is only 1.61 years, so we have had several opportunities already to see it since its discovery (in 2002, as its name tells us).
On each trip around the Sun it coasts out to aphelion at 2.625502 AU, comfortably far away from any planet. The orbit is inclined 15. 47941 degrees relative to the plane of the ecliptic.
This asteroid, because of its size and Earth-crossing orbit, qualifies it as a PHA, a Potentially Hazardous Asteroid. Its albedo has not been measured, so converting the observed brightness into a size is uncertain. This uncertainty leads cautious astronomers to estimate a diameter of 600 meters to 1.2 kilometers. The aphelion distance suggests a possible origin in the heart of the asteroid belt, in a region in which dark C-type carbonaceous asteroids outnumber the brighter S-type stony asteroids. An educated guess, that C-type is moderately more probable than S-type, would place the albedo (reflectivity for visible light) down around 0.04, which favors a size well above 1 kilometer.
By the way, it’s not going to hit us in the foreseeable future. In the long term, however, all bets are off: since AJ129 crosses the orbits of Mercury, Venus, Earth and Mars on every trip around the Sun there are lots of opportunities for tweaking its orbit. Over millions of years the odds are about even that it will end its career by hitting Earth.
The threat is real, but not imminent enough for me to change my investment strategy.