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Astronomers have measured Comet McNaught, which is believed to be the biggest comet to date.
In January 2007, people around the world watched comet McNaught streak across the sky.
It was the brightest comet seen since 1965 and, in some places, was visible to the naked eye in broad daylight.
The visible tail was about 35 degrees long, or roughly the same apparent size as 70 full moons lined up in the night sky.
By chance, the European Space Agency's Ulysses spacecraft plowed through the width of the comet's tail in February 2007.
Designed to study the sun's atmosphere, the probe could record information on how the comet's passage affected the solar wind, which is actually charged particles constantly streaming from the sun.
When a comet nears the sun, heat vaporizes the space rock's ices, releasing dust, which forms the comet's shining tail. But comets also have invisible tails of charged gases, or plasma, that interact with the solar wind.
Like a boat stirring up a wake, a comet's plasma tail creates "bow waves" in the solar wind on either side of the comet.