The 'oddball,' called Valetudo, has a prograde orbit that crosses the retrograde orbits.
It doesn't belong to any known group of moons, and it follows a strange prograde orbit that occasionally crosses the path of the retrograde moons.
Most objects in our solar system circle the sun in what's known as a prograde orbit, wherein their motion is counter-clockwise when seen from "above." All but a handful of the million or so known asteroids in our solar system exhibit prograde motion - a direction inherited from the spinning disk of debris that birthed them roughly 4.6 billion years ago.
Since these asteroids are present in stable points, and since they are locked in prograde orbits, they do not (and will not) collide with the gas giant.
For the collision a prograde orbit
for the satellite is chosen (see Fig.
Close encounters with another planet or a companion star can yank a planet out of its circular, prograde orbit
and into a highly eccentric and inclined orbit.
Table 1: Calculated period P (in hours) and predicted speed-change for prograde orbits
[delta][v.sub.yr] (in mm/s per year), and the predicted speed-change for retrograde orbits [delta][v.sub.yr] (in mm/s per year), for a spacecraft in a near-Earth orbit with [epsilon] = 0.5, [[alpha].sub.eq] = 45[degrees], [[lambda].sub.p] = 45[degrees], = 14 [v.sub.k], and for [r.sub.p] ranging from 2 [r.sub.E] to 8 [r.sub.E] .
Conventionally, bodies orbiting in a direct sense, with orbital angular momentum vectors within 90deg of the direction of the Earth's orbital angular momentum (or the rotational angular momentum of the primary), are defined to have inclinations from 0deg to 90deg and are said to be on prograde orbits
. Bodies traveling in the opposite direction are defined to have inclinations from 90deg to 180deg and are said to be on retrograde orbits.
Matese and Whitman also note that meteoroids hitting comets in prograde orbits
would produce only one-third the punch, on average.
Grun was puzzled and surprised by the finding, since most dust particles in the inner solar system have slower speeds and prograde orbits