THE ORBIT OF SG35

The standard deviation of a "least squares" parabolic fit to the mean orbital longitude of SG35 from 1700 to 2020 is abnormally high (27 degrees, see the "Mean Elements" page), and puts in evidence that there are important orbital changes during this short period.

The following are plots of the osculating elements of the orbit of 1998SG35 during the period 1700-2020, covering a total of 293 consecutive dates separated by 400 days each. Below each graph there is a link to another plot of 384 such points from 1600 to 2020. All the angular quantities are with respect to the ecliptic and equinox of J2000.

The effects of the close approaches to Saturn on March 17, 1759, and on October 11, 1879, can be clearly seen, producing abrupt "jumps" in the otherwise normal periodic perturbations and accumulated secular drifts. The minimum distance between Saturn and SG35 was 1.71 AU in 1759 and 1.35 AU in 1879 (The previous encounter with Saturn in the longer downloadable graphs happens in 1673/74).

From the plot of the semi-major axis, it is easy to observe how Saturn pushes the orbit of SG35 away as the two bodies get closer. It is also interesting to note that the encounters are relatively mild in terms of distance, while the effects are strong, putting in evidence how unstable this orbit is.

Perturbations are of two types: secular (cumulative) and periodic, or usually a combination of both, the amplitude of a particular periodic perturbation increasing with time. It usually happens that a secular drift is really a periodic perturbation of very long period, so before reaching conclusions about the instability of SG35's orbit, plots for longer time-spans and a better orbit determination are needed.

Generally speaking, centaur orbits are by definition "chaotic", i.e., subject to radical changes and presenting discontinuities in orbital evolution in a scale of a few thousand years, sometimes only a few centuries. But no other centaur (with the possible exception of 1995SN55, for which we'll have to wait for a better orbit estimation) shows variations of such magnitude in such a short time-span, or so close to the present.

The elements used for the numerical integration were taken from Bowell's "Astorb" database and are fitted to observations covering 2 consecutive oppositions in 1998 and 1999, or one complete synodic cycle.

A similar study of the orbital changes suffered by Asbolus by an encounter with Saturn in 1702 can be found here.