This was the right thing to do because the team from Liege was one of the best-placed to observe its awakening. “With Cyrielle Opitom, who is doing her doctoral thesis on the composition of comets based on observations made with TRAPPIST, we were the first to detect a sudden burst of activity”, recalls Emmanuël Jehin. A first “explosion” on the surface (known as an outburst in scientific jargon) was noticed on the 4th of November thanks to TRAPPIST. “As I recall, an American colleague didn’t believe us”. Shortly afterwards, it happened again, there was a new outburst. It was huge, of a factor of 10 this time. “This means that there was suddenly ten times more matter and gas ejected into space, greatly increasing the brightness of the comet”! These observations were later confirmed by Keck, the enormous American telescope based in Hawaii.

A heavenly fireworks

It remains to be determined why this happened. Studies are ongoing, but some theories are already emerging. By getting closer to the Sun, Ison began to experience its great heat. This was enough to cause some parts of the comet nucleus rich in ice to “explode”. The nucleus of a comet is 50% rocks and the other 50% is made up of ice (mainly H₂O).  The inside of the nucleus is a pile of “blocks” of different sizes forming empty spaces or cavities that can fill with gas, resulting from the sublimation of the ices. When these pockets can no longer support the pressure of the ever-increasing gas under the heat of the Sun, they create a giant “fireworks display” in space.   

These two consecutive “explosions” led to the hope of an exceptional spectacle even though they could have been seen as a premature sign of the disappearance of the comet… Ison then began to become visible to the naked eye with the tail and the coma getting bigger and brighter every day. Unfortunately hopes were dashed when, a day before its perihelion passage, that is to say, very close to the Sun at a distance of only 700,000 km from its surface, the comet disappeared from screens. Disintegrated, destroyed and broken up into many pieces. There was no outgassing, no vaporization anymore. No ice was left in its nucleus. All that remained of it was probably a load of rocks, dust and perhaps an inert nucleus, which nobody up to now has been able to trace. Not even the powerful NASA’s Hubble Space Telescope (HST) which tried to detect some of its fragments by inspecting its orbit. Ison seems to have simply evaporated.

How can such an event be explained? The question will undoubtedly occupy specialists during the coming months. The following scenario could be possible, continues Emmanuël Jehin: “Ison’s nucleus must have been smaller than calculated and much more fragile. As it was detected far from the Sun, we believe it was several kilometers in diameter. In fact, we should have smelt a rat because when it arrived close to Jupiter, its activity should have been stronger due to the increasing sublimation of the water ice. But in fact it remained very quiet for a long time before getting really active only 20 days before its perihelion passage”.

Its diameter was probably smaller than 500 meters, maybe only 250 m, while “normal” comets have a nucleus of a few kilometers and the biggest are around 30 to 50 kilometers. As Ison had never visited the inner solar system before, it still contained a great quantity of hyper volatile ice on its surface, like carbon monoxide and carbon dioxide producing this atypical activity far from the Sun that gave the impression of a bigger nucleus. During its perihelion passage, under the intense heat of the sun, the temperature of its surface reached about 3,000 degrees. By penetrating the small nucleus, the heat vaporized the ice that held all the blocks together, causing its disintegration and leaving only a mass of rocky material which was unfortunately a lot less interesting to study from a scientific point of view and putting an end to all hopes of observing a giant comet.