At the end of 2013, many telescopes around the world were looking to comet Ison, a comet that had been spotted a year earlier during its slow progression toward the sun. The comet raised the hopes of many professional and amateur astronomers: it promised to be one of the brightest comets of the last decades  and to provide new and important insights into the knowledge of these enigmatic solar system bodies. However, in the end, the comet failed at its closest approach to the Sun. TRAPPIST, the robotic telescope of the University of Liege, was at the forefront of an international observing campaing to follow this event. Astronomer Emmanuël Jehin recounts the fate of Ison and the information which it was able to supply despite the turn of events.

Ison had a rendezvous with the sun, a rendezvous that promised to be exceptional: from Brussels to New York and as far away as Tokyo, the comet was expected to be observed in December by the naked eye from the entire Northern hemisphere, displaying a long tail above the horizon for many nights. But comets are sometimes capricious and unpredictable: despite the hopes of astronomers, the expected show did not take place.

Ison had everything to be the comet of the decade or even of the Century like other famous comets such as Halley’s, Hale-Bopp and Hyakutake. Its story began in September 2012 when it was spotted for the first time by amateur Russian astronomers. The astronomers then realized that Ison had the stuff that big headlines are made of. Discovered very far from the sun, it originated from the Oort cloud, that immense reservoir that contains billions of comet nuclei at around one light-year from the sun, at the boundary of interstellar space.

While calculating its orbit, astronomers noticed that Ison belonged to the category of sungrazers, comets that pass very close to the surface of the sun. Ison was detected very early in its journey, almost a year before its passage close to the Sun. This was a first: specimens belonging to the sungrazers category are generally only detected a few days before their fateful encounter with the Sun. The SoHo satellite, which has been observing the Sun for some 20 years, detected hundreds of them before they plunged into the Sun. However, these are usually very small comets with a nucleus of just a few tens of meters in diameter. Even at large distance from the Sun, Ison was very active, emitting gas and dust that fed the coma surrounding its nucleus. This could be explained if Ison had a large nucleus of several kilometers which promised to provide a brilliant spectacle like some of the great comets that were observed in the past.

TRAPPIST and the comet

From the La Silla observatory in Chile, in the middle of the dry and rocky Atacama desert, the robotic TRAPPIST telescope of the University of Liege had been monitoring the progressive descent of Ison towards the Sun. Since October 2013, the telescope was aimed at the comet every night, examining its every move. This was a long-term study of the comet that few other instruments throughout the world were able to carry out. “In general you have to book months in advance for a telescope and you are usually granted only one or two nights of observation”, explains Emmanuël Jehin, a Research Associate at the FRS-FNRS and astronomer at the University of Liege.  

The situation is very different when you are the owner of your own equipment. Since 2010 and the beginning of the Chilean adventures of TRAPPIST (abbreviation for Transiting Planets and Planetesimals Small Telescope), the specialists at the university are the sole users and can use their time as they see fit. Even better, this telescope was designed specifically for the observation of comets. It is equipped with special filters which enable it to take pictures in the light emitted by an entire series of molecules and therefore to determine the composition of the atmosphere (coma) of comets.  Also, when Ison began to show itself as potentially “the comet of the century”, the team of observers from Liege did not hesitate to study it in all its aspects.