Comet 29P/Schwassmann-Wachmann behaves like no other object we know, and even by its own high standards, the last few months have been deeply odd. Although classified as a comet, 29P undergoes eruptions for reasons that are still unknown but are not the same as those seen on other comets. After the biggest set of eruptions in 40 years in September, it's just had another.
When discovered 29P seemed like an ordinary comet, distinguished only by having a nearly-circular orbit and being large enough to see at a great distance. Almost a century later, however, we know that's definitely not the case, but we still haven't explained why it is unlike anything else we know.
Outbursts are common on comets, but the reason is too much exposure to the Sun. Comet 73P/Schwassmann-Wachmann, as the name suggests, was discovered by the same astronomers, and is a typical short-period comet that approaches the Sun every five years and brightens in the process as ice escapes into space, carrying dust with it. On recent visits, it has broken up into many pieces as a result.
29P, on the other hand, never gets closer than the orbit of Jupiter, and so shouldn't get warm enough to melt its ices. Nevertheless, it experiences numerous eruptions – usually reported as seven a year, but far more according to some recent estimates – the biggest of which cause it to brighten 100-fold. Jupiter's moon Io is more volcanic, but at least there we know the reason why.
This year, 29P experienced a major eruption on August 25, a super-eruption a month later, and another four weeks after that. The most recent one saw it jump almost three degrees of magnitude in a day. The timing was good for observations from Earth, as 29P is currently in Auriga, as seen from Earth, making it visible through most of the night from the Northern Hemisphere.
There had been plans for Hubble to take a look at 29P, which might have provided some important explanations of its behavior, but that was scuttled by another shutdown. Few professional Earth-based telescopes have made it a priority, so most of what we know comes from amateur observations.
The general explanation for 29P's strange behavior is that it is a ball of ice approximately 60 kilometers (40 miles) wide that somehow supports many ice volcanoes, powered by nitrogen and carbon monoxide. The brightenings are a consequence of large comas appearing when material carried off-world by the gasses has a much larger surface area to reflect sunlight.
Although most comets have less than one-tenth the diameter, and therefore a thousandth of the volume, it is still not clear why 29P shows such complex behavior when other objects don't. The Megacomet C/2014 UN271 (Bernardinelli-Bernstein) is thought to be more than twice the diameter, yet so far we have not observed any similar outbursts.
The rotation period for 29P is thought to be a relatively stately 58 days. Dr Richard Miles of the British Astronomical Association told SpaceWeather; "The latest eruption has taken place some 59 days after a similar event on August 25th, and may be an example of an outburst from the same cryovolcano erupting a second time on the next rotation of the nucleus." Whether the fact the even larger late September outburst, thought to have involved four volcanoes going off in 56 hours, was almost exactly halfway in between was a coincidence or not, no one knows.