Just when we thought we were starting to understand the mysterious object 29P/Schwassmann-Wachmann, it has confounded us again. Four eruptions from its ice volcanoes in quick succession led to a 289-fold brightening. This is the largest set of eruptions since 2021 – and unlike a recent, smaller outburst, we did not see it coming.
Comet 29P/Schwassmann-Wachmann is a centaur, a class of objects that orbit between Jupiter and Neptune. However, it displays some very unusual characteristics seen in few other centaurs, which remain a puzzle to astronomers. At first, these can look like common cometary behavior, but with a significant difference.
Like other comets, 29P/Schwassmann-Wachmann is made up of various kinds of ice, along with some dust. Most comets travel on very elongated orbits, and as they approach the Sun, they warm up. When this happens, types of ice with the lowest boiling points escape first, taking dust with them. Sometimes internal pockets of a particular ice will explode, causing an ice volcano. The dust that is carried into space when this occurs reflects a lot of sunlight, causing a dramatic brightening.
However, 29P/Schwassmann-Wachmann has an unusually round orbit, its distance from the Sun varying by less than 10 percent. Consequently, its temperatures should not vary much – yet 29P/Schwassmann-Wachmann is possibly the most eruptive object in the Solar System after Io, experiencing frequent (although certainly not regular) outbursts, including four between November 2 and 4.
We would expect the eruptions to be larger and more frequent when 29P is closest to the Sun (perihelion), but it reached that point more than five years ago, and will not reach another perihelion for a decade. Although early November’s eruptions were not the largest we have ever seen – some have caused increases in brightness of around a thousand times – they were still well above average. The biggest eruption in 40 years occurred in September 2021, a time when 29P was closer to the Sun than average, but still well after perihelion.
Nevertheless, seemed for a while we were getting closer to understanding this enigma. A 57-day pattern to 29P/Schwassmann-Wachmann’s eruptions has been observed. This is thought to indicate this is the period of the centaur’s rotation, an unusually slow rate for an object just 60 kilometers (40 miles) across which would allow temperatures on the day side to build up.
In 2023 there was even a successful prediction of an eruption. Although that one was substantially smaller than the latest (brightening only by a factor of 10), it was the first time anyone had seen one coming beforehand.
The clue was a dimming in the area around the cometary nucleus. Astronomers took this as a sign that the usual slow release of gases from within 29P/Schwassmann-Wachmann had paused. Since they were confident gas was still being produced inside, they concluded pressure would build up, eventually leading to an explosion – as it did a day later.
Recently, the JWST saw 29P/Schwassmann-Wachmann, in a way we had never managed before. It observed a jet of carbon monoxide emerging from the object’s center, while carbon dioxide came from what might be called its poles.
The jets were interpreted as evidence that 29P is composed of two bodies with different compositions.
Nevertheless, all these advances in our understanding of this strange world were still not enough to allow anyone to predict this series of explosions, nor explain why they are so powerful.
Most observations of 29P/Schwassmann-Wachmann are made by amateurs, and coordinated by the British Astronomical Association. However, when not exploding, the centaur is almost a billion times too faint to see with the naked eye, so you need some hefty equipment to be able to participate.
