Findings from NASA鈥檚 Cassini spacecraft have revealed a record-breaking comet, with an ion tail that stretched more than seven times the distance between Earth and the sun.
Geraint Jones at University College London and his colleagues looked through past data from the mission in 2002, when Cassini was on its way from Jupiter to Saturn. The spacecraft used its plasma instrument from 2001 to 2003 to study charged particles from the sun called the solar wind.
Jones and his team found a notable increase in protons detected by the spacecraft in March 2002 that had gone unexplained until now. The culprit appears to have been hydrogen ions pushed off a comet called 153P/Ikeya-Zhang by the solar wind. 鈥淚t matches up really well,鈥� says Jones.
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Comets are known to have a dust tail, which comprises dust and gas left in its wake, and an ion tail, caused by the solar wind pushing ions behind the comet. Both grow longer as the comet approaches the sun. Based on the number of protons detected and the position of the comet, Jones and his team believe Cassini detected comet 153P鈥檚 ion tail.
At the time of the measurement, comet 153P was 6.5 astronomical units (AU) away from Cassini 鈥� 1 AU is the distance between Earth and the sun 鈥� but as the comet follows a curved orbit around the sun, the team estimates the tail to be at least 7.5 AU, or more than a billion kilometres long. The previous record tail length was comet Hyakutake鈥檚 ion tail, measured at 3.8 AU by NASA and the European Space Agency (ESA)鈥檚 Ulysses spacecraft in 1996.
Such measurements are only possible when a spacecraft just happens to cross a comet鈥檚 path, which has only occurred a handful of times. For example, ESA鈥檚 Solar Orbiter spacecraft looks likely to pass through the tail of comet C/2019 Y4 (ATLAS) this month.
These chance measurements mean it is likely that comet tails can stretch much longer than 7.5 AU. 鈥淭here鈥檚 no reason to expect [comet tails] can鈥檛 survive all the way to the edge of the heliosphere, where the solar wind reaches the interstellar medium,鈥� says Jones.
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