Before comet ISON raced past the Sun, it stopped producing dust and gas, as data from the SUMER spectrograph on SOHO show.
On November 28th of last year, thousands of people worldwide watched comet ISON’s fiery ride past the Sun. Researchers at the Max Planck Institute for Solar System Research (MPS) have now reconstructed the comet’s activity during its final hours. Their conclusion: already hours before its perihelion passage, ISON stopped emitting dust and gas into space. The new analysis is based on data from the spectrograph SUMER on board the solar observatory SOHO, a joint space mission of ESA and NASA. SUMER was the only instrument that was able to obtain data of the comet during the minutes of its closest approach to the Sun.
Images of the comet ISON taken by the spectrograph SUMER on board the solar observatory Soho on 28 November 2014 at 6.01 pm (Central European Time) reveal the tail & shape. The red dots mark the predicted positions of the comet & nucleus in intervals of one minute, the red cross depicts the last position at the time of the picture recording. The white arrow indicates the direction to the sun. Brightness contours and centreline of the tail clarify its appearance.
When comet ISON was discovered in the autumn of 2012, scientists and amateur astronomers alike hoped for a “comet of the century”. On November 28th, 2013, only 1.8 million kilometres would separate ISON from the Sun. Due to its brightness, the comet promised to be a unique research object – and, should it survive its flyby of the Sun, a stunning celestial phenomenon in the weeks preceding Christmas. But the story took a different course: already during the final phase of the approach to perihelion, the comet’s tail grew fainter and fainter. It soon became clear, that ISON’s activity had ceased or that the nucleus most likely had completely been disintegrated.
However, what exactly happened on November 28th, 2013 1.8 million kilometres away from the Sun is still not entirely clear. Did the comet break apart before it reached the point closest to the Sun? Or did it withstand the indescribable heat a while longer? Did the dust tail that could be discerned after the flyby maybe even block the view of a remaining solid nucleus? “Our measurements and calculations indicate that ISON ran out of steam before perihelion”, says Dr. Werner Curdt from the MPS, first author of the new publication. 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. After that, the dust production completely stopped within a few hours.
