Astronomers from Canada, Japan and the United States have discovered the first retrograde asteroid orbiting the planet. The orbit and orbital period of 2015 BZ509 approximately corresponds to the orbit of Jupiter, but at the same time the celestial body is moving towards the planet. Scientists estimate that the object has been following such an exotic trajectory for the past million years. It was not possible to establish its origin. The research is published in the journal Nature.
The vast majority of the celestial bodies of the solar system revolve around the sun counterclockwise - as viewed from the north pole for the ecliptic. It is believed that this is due to the rotation of the protoplanetary disk. However, among the over 700,000 known asteroids, there are 82 retrograde objects rotating clockwise. Probably the most famous retrograde celestial body is Halley's comet. The mechanism of evolution of such orbits is not known for certain.
Orbit of a celestial body Paul Wiegert et al. / Nature, 2017
The orbits of a large group of asteroids (such as Trojans or centaurs) are associated with the gravity of massive planets such as Jupiter. The joint attraction of the planet and the Sun creates several stable groups of orbits - Lagrange points. They are located in orbit behind and in front of the planet, three more points lie on the line connecting the planet and the Sun. Asteroids can be at Lagrange points almost forever - the only disturbing factor is the attraction of other planets. Obviously, however, objects at these points are moving in the same direction as the planets.
Stable orbits for retrograde asteroids also exist. For example, asteroids 2006 BZ8 and 2008 SO218 are in resonance with Jupiter, while moving in retrograde orbits. However, their period of revolution differs from that of Jupiter (2: 5 and 1: 2). Astronomers have assumed that there may be objects moving co-orbit (1: 1) with Jupiter, but such a celestial body was discovered only recently.
The authors of the new work studied the asteroid 2015 BZ509, discovered in January 2015 by the Pan-STARRS group. At the time of the discovery, the orbit of the celestial body was not entirely clear, but scientists have already assumed that it may be retrograde. A series of 52 observations with the Large Binocular Telescope (LBT, Arizona), covering 300 days, made it possible to refine these data and prove that the body not only moves in a retrograde orbit, but also coorbital with Jupiter (revolves around the Sun with the same period).
From the point of view of a stationary observer on Jupiter, the orbit of 2015 BZ509 is like a trisector - a closed curve with a loop that encloses the Sun. Although a celestial body flies near the gas giant twice per revolution, the short-term nature of these encounters does not allow disrupting its behavior. According to the authors' estimates, the orbit is unusually stable - the asteroid has been on it for about a million years.
Astronomers could not determine the nature of the retrograde object - according to one of the assumptions, it could be a comet, but at the time of approaching the Sun in 2015, BZ509 showed no signs of a cometary coma. The authors classify it as Damocloid.
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