a Luxembourg team observes a geyser on a comet

A Luxembourg team was the first to observe the eruption of a geyser on comet 29P/Schwassmann-Wachmann, one of the strangest objects in the solar system.

On April 23, 2023, every astronomer’s dream came true for Matt Dawson, scientific collaborator in the geo/astrophysics section of the National Museum of Natural History in Luxembourg, and Eric Buttini, the head of this section: according to the MNHN, they were indeed the first to observe an exceptional phenomenon, in this case the eruption of a geyser on comet 29P/Schwassmann-Wachmann, one of the strangest objects in the solar system.

“I didn’t see a geyser per se,” says Matt Dawson. “We are talking about an object 60 km in diameter located a billion kilometers beyond the orbit of Jupiter, it is really very far! But I noticed a change in brightness as well as color. And we could see the tail of the comet.” An observation that was later confirmed by telescopes in Australia and the United States.

Comet 29P, discovered in 1927 by German astronomers Schwassmann and Wachmann, is a “centaur”. In Greek mythology, the centaur is a half-man, half-horse creature. In astronomy, centaurs are celestial objects whose nature is not clear due to lack of sufficient data: are they real comets (objects composed of rocks and ice, leaving behind a trail of gas and dust )? Or asteroids (small bodies composed of rocks, metals and ice, geologically dead)? “29P is an asteroid that goes around the Sun and suddenly has a big tail like a comet, then suddenly becomes very dark and very small,” summarizes Matt Dawson.

These icy bodies orbit the Sun between Jupiter and Neptune. They are generally small in size, with the largest centaur known to date (Chariclo) measuring only 250 km in diameter. “We know about 500 centaurs. Some erupt every 20 years, but 29P is the only one that constantly erupts,” says Matt Dawson. Comet 29P Schwassmann-Wachmann seems to explode several times a year. In fact, steam and dust that shroud the nucleus are regularly released from the comet’s frozen surface. “You have to imagine an ice cream that would be frozen on the outside, but wet on the inside. The pressure inside builds, and the ice outside cracks. Gas, stone and frozen carbon monoxide are escaping.”

Enlightened amateurs

The Luxembourg team is taking part in an international project to monitor this comet and search for signs of activity thanks to a semi-professional observatory set up two years ago in Calabria, in southern Italy, where light pollution is well lower than in Luxembourg. The observations of Matt Dawson and Eric Buttini, as well as their colleagues around the world, are invaluable to astrophysicists. “We watched the comet almost every night for two years. When we have our eyes so riveted on it, we can detect the slightest change. The data collected then makes it possible to deduce many things. For example, the smallest of the variations in luminosity makes it possible to know the period of rotation of the object. You can also establish patterns, such as the number of explosions per year.”

This observation work is carried out by amateur astronomers, but who are in no way amateurish, strongly insists Matt Dawson, rocker by profession and avid astronomer since he was six years old: “It just means that we make these observations by passion and not for the money. But the work is professional quality. And then, our contribution is very interesting for the professional community, because we look very specifically at an area of ​​the sky and at a particular object. The largest telescopes in the world, like those in Hawaii for example, are very expensive to operate. They sometimes discover things, but at 10,000 dollars an hour, they cannot go back to an area to observe it longer. So it’s people like us who continue to do this vital work.”