What a week it has been for comet-lovers. First came the news that the European Space Agency’s Solar Orbiter had sailed serendipitously through the tail of doomed Comet ATLAS, then new analysis suggested that the first interstellar comet, Comet 2I/Borisov, will likely make it out of the solar system relatively unscathed. Turning this duo into a trio is the discovery of the longest cometary tail yet measured.
At least 7.5 times as long as the distance between the Earth and the Sun (over a billion kilometers, or 621 million miles), Comet 153P/Ikeya-Zhang’s ion tail is nearly double the length of the previous record-holder, Comet Hyakutake. In order to make this immense discovery, however, researchers had to revisit data from nearly 20 years ago.
When NASA’s Cassini spacecraft was traveling between the orbits of Jupiter and Saturn back in 2002, it detected an increased number of protons. This event has gone unexplained for all these years, but a team of UK and US researchers now believe that these protons in fact came from the ionized tail of Comet 153P/Ikeya-Zhang.
Interactions between the Sun and an orbiting comet can cause it to shed two types of tail. Perhaps the more familiar one is the dust tail, produced as the solar radiation melts the comet’s core, spewing out a trail of dust and gas that had been trapped in the so-called “dirty snowball.” The other is the ion tail, formed when neutral gas in the comet’s core becomes ionized by the radiation. In Comet 153P/Ikeya-Zhang’s case, protons stripped from hydrogen gas during this ionization could be transported by the solar wind in the direction of the spacecraft.
Therefore, as the team concluded in their paper, available on the pre-print server arXiv, the comet’s location coupled with the proton numbers registered by Cassini comprehensively suggests that the spacecraft did in fact fly through Comet 153P/Ikeya-Zhang’s ion tail. A closer look at the detected protons told the researchers that they had traveled a distance of 6.5 astronomical units (the distance between the Earth and the Sun) from the comet coma, which, when the comet’s orbital change during the measurements is accounted for, implies a minimum tail length of more than 1 billion kilometers.
As probes have only crossed comet’s tails a handful of times, the possibility of this record being broken again is highly likely, researchers told New Scientist. Astronomers will be greatly anticipating the results from the Solar Orbiter’s interaction with Comet ATLAS.