One of the Solar System’s Strangest Objects Just Exploded Again

Comet 29P/Schwassmann-Wachmann acts like no other object we have seen before, and the previous several months have been strange even by its own high standards. Despite being categorized as a comet, 29P has outbursts for unexplained causes that are not the same as those witnessed on other comets. It just experienced another eruption after the largest in 40 years in September. 29P seemed to be an average comet when it was found, with the exception of having a nearly circular orbit and being massive enough to seen from a significant distance. We now know that it does not happen almost a century later but we still do not understand why it is not like anything else we know.

On comets, outbursts are common, but the cause is too much exposure to the Sun. Comet 73P/Schwassmann-Wachmann, named after the scientists who found it, is a typical short-period comet that approaches the Sun every five years and brightens as ice escapes into space, dragging dust with it. As a result, it has disintegrated into several pieces over previous trips.

29P, on the other hand, never approaches Jupiter’s orbit and so should never become heated enough to melt its ices. Despite this, it has a number of eruptions each year — normally seven, but according to some recent estimates, considerably more – the largest of which causes it to brighten 100-fold. Io, Jupiter’s moon, has higher volcanic activity, but we know why. On August 25, 29P had a large eruption, followed by a super-eruption a month later, and another four weeks later. In the most recent one, it increased by over three orders of magnitude in a single day. The timing was ideal for studies from Earth since 29P is now in the constellation Auriga as seen from Earth, making it visible from the Northern Hemisphere for the majority of the night.

Hubble had planned to investigate at 29P, which may have supplied some crucial insights for its behavior, but it was thwarted by yet another shutdown. Because few professional Earth-based telescopes have made it a priority, we have to rely on amateur observations for the majority of what we know.  The most accepted explanation for 29P’s odd behavior is that it is a 60-kilometer-wide ice ball containing a swarm of nitrogen- and carbon-fueled ice volcanoes. Large comas form when material is taken off-world by the gasses has a considerably bigger surface area to reflect sunlight, resulting in brightenings.

Despite the fact that most comets have a diameter of less than a tenth of a kilometer and therefore a thousandth of a kilometer, it is still unclear why 29P exhibits such intricate behavior whereas other objects do not. Although the Megacomet C/2014 UN271 (Bernardinelli-Bernstein) is believed to be more than twice the diameter, no such outbursts have been recorded so far.

The rotation period for 29P is believed to be 58 days, which is rather long. “The newest eruption took occurred some 59 days after a similar occurrence on August 25th, and maybe an example of an outburst from the same cryovolcano erupting a second time on the following rotation of the nucleus,” Dr. Richard Miles of the British Astronomical Association told SpaceWeather.

 Nobody knows if the fact that the considerably greater late September eruption, which is considered to have comprised four volcanoes erupting in 56 hours, occurred almost exactly midway between the two was a coincidence or not.