A comet’s last hours

WHEN comet 2012/S1 (ISON) was discovered in the autumn of 2012, astronomers hoped it would eventually light up the night sky and become the “comet of the century” when it whizzed past the sun in late 2013. Orbital analysis had shown that the comet’s closest approach to the sun, the perihelion, would be 1.8 million kilometres above the sun’s visible surface on November 28, 2013. On the basis of its early brightness, scientists had expected to witness a unique research object and, should it survive that flyby event, a stunning celestial phenomenon in the weeks preceding Christmas. However, these hopes were belied as during the final phase of the approach to perihelion the comet’s tail became increasingly faint. ISON’s activity had either ceased or its nucleus had completely disintegrated. What exactly happened remained unclear.

A new reconstruction of the comet’s activity during its final hours by researchers at the Max Planck Institute for Solar System Research (MPS) has revealed that ISON stopped producing dust and gas shortly before it raced past the sun and disintegrated. The analysis is based on data from the European Space Agency/National Aeronautics and Space Administration’s space-based Solar and Heliospheric Observatory (SOHO) spacecraft, which has been sending back a stream of data about the sun and thousands of sun-grazing comets for more than 18 years.

Hours before ISON reached perihelion, stunning images taken by SOHO’s Large Angle and Spectrometric Coronagraph (LASCO) showed the bright elongated tail of the comet rushing in its sun-grazing trajectory. Unfortunately, the trajectory was so close to the sun that the final phase of the encounter was obscured by LASCO’s occulting disc, which blocks light from the sun in order to make structures in the sun’s vicinity visible.

The MPS researchers turned to data from SOHO’s spectrograph SUMER (Solar Ultraviolet Measurements of Emitted Radiation), which was the only instrument capable of obtaining any data of the comet during the minutes of its perhelion though detection of a faint comet is not what it is designed for. “Our measurements and calculations indicate that ISON ran out of steam before perihelion,” said Werner Curdt, from the MPS, first author of a paper published in the latest issue of Astronomy & Astrophysics. Apparently, 8.5 hours before the comet should pass by the sun, a short and violent outburst occurred that set free a great amount of dust. The dust production completely stopped within a few hours.

By operating SUMER in camera mode, the researchers were able to record images of the comet’s tail in ultraviolet light, which was emitted from the solar disc and reflected by the dust particles into space. Images of the dust tail with SUMER, obtained during the perihelion were key to the new insight and show a slightly curved, pointed tail with a length of at least 240,000 km. To understand the tail shape, the images were compared with computer simulations with different sizes of the dust particles, their speed and the time of their emission. The model was most consistent with a scenario where ISON had stopped producing dust and gas hours before. Whether the nucleus had been completely disintegrated cannot be settled without doubt, according to co-author Hermann Böhnhardt, MPS. However, some evidence indicates that this was the case. Calculations showed that the comet must have emitted around 11,500 tonnes of dust at this time. It is most likely the final break-up of the nucleus triggered this dusty firework. Gas and dust trapped inside the nucleus would have been abruptly released in such an event.