Hubble has spotted the beginning of a new “spoke” season on Saturn.
NASA discovered these spoke features occurring across the gas giant’s rings during the Voyager mission in the 1980s. Voyager 2 captured an image of the spokes across Saturn’s rings on August 22, 1981.
Spokes – so-called because they are reminiscent of the spokes on a bicycle wheel – are seasonal, according to EarthSky.
These spokes have been observed in the years before and after the planet’s equinoxes, with them becoming more and more prominent as the equinox date draws nearer.
An equinox occurs at the point in a planet’s orbit when the Sun is directly overhead at the equator, according to the European Space Agency. Saturnian equinoxes occur about every 15 years, with the first being observed by the Cassini mission in August 2009.
“Despite years of excellent observations by the Cassini mission, the precise beginning and duration of the spoke season is still unpredictable, rather like predicting the first storm during hurricane season,” said NASA senior planetary scientist Amy Simon, according to a press release.
Saturn’s next autumnal equinox of its northern hemisphere is predicted to occur on May 6, 2025, meaning the spokes will continue to become more prominent and observable as that date approaches.
These spokes disappear when Saturn is approaching its summer or winter solstice.
Scientists have not yet defined the exact reason why the phenomenon occurs, but the prevailing theory is that the spokes have something to do with the interaction between the planet’s magnetic field and incoming solar wind.
NASA officials plan to use the Hubble Telescope to observe and study the spokes as part of the ongoing Outer Planet Atmospheres Legacy program (OPAL).
“Thanks to Hubble’s OPAL program, which is building an archive of data on the outer solar system planets, we will have longer dedicated time to study Saturn’s spokes this season than ever before,” said Simon in the release.