Cassini helped to find the comet with the longest ion tail

Astronomers have discovered in the data collected by the apparatus Cassini, evidence that the length of the ion tail of the comet 153P/Ikeya—Zhang, flying past the Earth in 2002 could amount to more than a billion kilometers. Preprint available at

The tails of comets are formed when it approaches the Sun, and her solid core gradually begins to evaporate. The ion tail consists of ionized molecules of various gases — among them usually stand out of carbon monoxide molecules, which join with the electrons give a characteristic blue color. The dust tail consists of small dust particles that glow due to the fact that reflect sunlight, and usually the closer the comet comes to the sun, the more noticeable it becomes.

Geraint Jones (Geraint Jones) from University College London and his colleagues studied archived data probe “Cassini” in 2002, when he flew past Jupiter and went to Saturn. Astronomers noticed that the plasma spectrometer spacecraft, from 2001 to 2003, registered the charged particles, found a surge of hydrogen ions, which could not be explained by the solar wind (because the tool was rarely aimed in his direction) or the output of interstellar hydrogen shadow — a region of space where the hydrogen is ionized, and then is pushed out by the solar wind.

Astronomers have discovered that a statistically significant surge of ions was observed from March to April 2002, and at about the same time long-period comet 153P/Ikeya—Zhang passed through the area where then Cassini has detected an anomaly. Analysis of the particle flux registered by the apparatus, showed that with high probability, a plasma spectrometer, noticed the ion tail of a celestial body.

Calculations show that at the time the comet Ikeya—Zhang was 6.5 astronomical units from Cassini (one astronomical unit equals the average distance from the Earth to the Sun). However, given the fact that a comet is not a straight line but on a highly elliptical orbit, the study authors estimate the tail length at least 7.5 astronomical units. If the calculations are correct, then the ion tail of the comet of Ikeya—Zhang — longest measured today. The previous record holder was the ion tail of the comet Hyakutake measured in 1996 by the spacecraft, NASA and ESA “Ulysses”. Its length was 3.8 astronomical units.

Such measurements become possible only when the probes directly cross the tails of comets, which happens rarely. But the last such event happened recently: spacecraft Solar Orbiter a few days ago, flew through the tail of C/2019 Y4. At this time, the probe was supposed to collect scientific data that will help you better understand the processes of comets.

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