Comet 29P/Schwassmann-Wachmann recently had its biggest eruption in over three years. This image, taken by NASA's Spitzer telescope, shows the comet after a significant outburst in 2003. NASA
A mysterious comet, 29P/Schwassmann-Wachmann, has recently reawakened, causing scientists to be puzzled by its unexpected behaviour. Within less than 48 hours, the comet released four major eruptions, making it shine almost 300 times brighter than usual. This was the largest outburst in more than three years, adding to the intrigue surrounding the comet’s unpredictable eruptions.
29P is a massive icy comet that spans approximately 37 miles (60 kilometres), about three times the length of Manhattan. It belongs to a unique group of comets known as "centaurs," which orbit within the inner solar system. Among these, 29P is even rarer as it is classified as a cryovolcanic or "cold volcano" comet. These comets have icy shells that contain gas, dust, and ice. When they absorb enough sunlight, their icy insides heat up, creating pressure that eventually leads to the comet's icy surface cracking open and releasing a plume of material into space.
This eruption produces a reflective cloud, known as a coma, that expands, causing the comet to appear much brighter. The same phenomenon was observed in another well-known comet, 12P/Pons-Brooks, which became a media sensation when it made its close pass to Earth. 29P’s eruptions in early November 2024 were particularly significant, marking its first major outburst in nearly two years, followed by three more in rapid succession. The eruptions expelled massive amounts of debris that reflected 289 times more light than the comet's core, according to astronomers from the British Astronomical Association (BAA), who have been closely monitoring the comet.
Experts believe the expanding coma may form an unusual shape due to the multiple eruptions, similar to the behaviour of the “devil comet” that made headlines recently. Richard Miles, an astronomer with the BAA, predicted that a complex debris cloud would continue to develop over the coming days. This type of eruption is extremely rare and brings about new questions as to why these explosions happen sporadically.
Cryovolcanic comets become brighter as their coma grows following an eruption. These images capture 29P growing brighter after an outburst in October 2021. NASA
29P's eruptions are relatively easy to predict. Unlike other cryovolcanic comets that have highly elliptical orbits, which take them far from the sun before returning to the inner solar system, 29P has a circular orbit. It remains at a consistent distance from the sun, similar to the orbit of Jupiter. As a result, it should erupt more regularly, but observations have shown that it behaves unpredictably. This strange behaviour has left scientists baffled as to what triggers the eruptions, with no clear answer so far.
For example, in April 2023, scientists successfully predicted an eruption, which was the first such forecast for 29P. However, predicting eruptions is challenging due to their irregularity. The comet’s lack of a tail, like other comets, further sets it apart from its counterparts. While comets like Tshuminchan-ATLAS, which made a rare approach to Earth recently, develop tails when they get close to the sun, 29P remains devoid of this feature.
As researchers continue to observe 29P, they are hoping to unlock the mystery behind its peculiar behaviour. The comet's eruptions remain one of the most baffling phenomena in space, offering both a challenge and a unique opportunity for astronomers to study the unpredictable nature of cryovolcanic comets.
