The Death of a Comet: ISON's Post-Mortem

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When the “Comet of the Century” died to a solar roasting last year, the world was sad. Hopes of a beautiful cometary display rapidly evaporated as space observatories relayed images of the once-proud dirty snowball become a pulverized cloud of sublimating ice and scattered dust.

So what happened to Comet ISON and its promise of a celestial spectacular?

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According to new research published in the journal Astronomy & Astrophysics, it seems the fate of the icy mass was sealed before it made its suicidal 1.8 million kilometer close approach of the sun (an event known as perihelion).

“Our measurements and calculations indicate that ISON ran out of steam before perihelion,” said lead researcher Werner Curdt, of the Max Planck Institute for Solar System Research (MPS).

Using data from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on board the joint NASA/ESA Solar and Heliospheric Observatory (SOHO), Curdt and his team were able to get a very privileged view of ISON as it barreled deep into the solar corona, the sun’s multimillion degree atmosphere.

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Images of Comet ISON taken by SUMER on Nov. 28, 2013, reveal the tail’s shape. The red dots mark the predicted positions of the comet’s 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 centerline of the tail clarify its appearance.
MPS

Only hours before ISON’s Nov. 28, 2013, close approach, the comet stopped producing a tail of vapor and dust, which is strange considering its surface would have been rapidly heating up. According to analysis of SUMER observations, around 8.5 hours before perihelion, the comet released a “short and violent outburst” that extinguished any further release of gas and dust.

Before the outburst, ISON was acting just as it should during solar approach — a beautiful, long tail created a bright arc in observations by the Large Angle and Spectrometric Coronagraph (LASCO) also on board SOHO. LASCO is used by solar astronomers to observe the region surrounding the sun to track space weather events such as coronal mass ejections (CMEs). But LASCO also has a dazzling track record of tracking ‘sungrazing comets’ that regularly drop deep into the corona.

As ISON dropped closer to the sun, the LASCO view was lost as the comet disappeared behind the instrument’s occulter — a component of LASCO that blocks the glare of the sun so faint structures in the corona can be resolved. So the only instrument that could track ISON’s final moments was SUMER.

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Although SUMER isn’t designed to analyze disintegrating comets, it was able to recover images of ISON’s tail in ultraviolet light. “The only instrument that could obtain serviceable data at this time was SUMER,” said Curdt. “For everyone involved, this was a huge challenge.”

During perihelion, SUMER revealed a curved, pointed tail at least a quarter of a million kilometers long. But there was no sign of the comet’s nucleus. The Max Planck team simulated various scenarios that could replicate the tail they were seeing, assuming the nucleus was still intact and active.

“We were not able to reconstruct anything resembling our images, assuming that ISON was still active during the SUMER observations,” said co-investigator Hermann Böhnhardt, also from the MPS.

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This has led the researchers to believe that the SUMER observations most likely fit computer simulations of a cometary nucleus that ceased producing gas and dust hours before perihelion.

Although the researchers cannot definitely say that the nucleus disintegrated before close approach, the fact that there was a rapid outburst 8.5 hours before is indicative of a disintegration event. The outburst generated a cloud of approximately 11,500 tons of dust in a sphere 280 meters across.

So these observations support the idea that ISON broke apart before its closest approach, leaving just a cloud of debris to continue ISON’s orbit like an eerie cometary apparition, and certainly nothing resembling the Comet of the Century.

Source: Max Planck Institute for Solar System Research (MPS)

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