While traversing its orbit, Ulysses encountered the tails of three distinct comets, McNaught, McNaught-Hartley, and the largest, Hyakutake. Although Ulysses was hundreds of millions of miles from the comet in May 1996, and outside its visible tail, scientists were getting unexpected readings. Their analysis led them to the conclusion that Hyakutake’s tail extended far beyond what they originally thought - more than 480 million kilometers (300 million miles), the longest ever recorded.
Ulysses first encounter with a comet’s tail caused confusion on Earth. US project scientist Edward J. Smith of NASA's Jet Propulsion Laboratory said: "As we were not looking for comets, we did not realize the significance of the data right away. The solar wind seemed to almost disappear and was replaced by gases not normally found in the solar wind, and the magnetic field in the solar wind was distorted."
Ulysses SWICS instrument - pictures scanned from a science magazine, 1990.
Ulysses' solar wind ion composition spectrometer instrument (SWICS) developed by University of Michigan heliophysicist George Gloeckler, found that even at such a great distance, the tail had filled the solar outflow with unusual gases and molecules.
Solar wind was discovered in 1958, < http://www.jpl.nasa.gov/news/features.cfm?feature=1502 > after scientists pondered how a comet's ion tail always points away from the sun, whether the body is traveling toward or away from the sun along the comet's elliptical orbit.
When Ulysses came upon the effects of McNaught, the satellite and the nucleus of the comet were 160 million miles apart. Still, its tail filled the solar outflow with unusual gases and molecules.
Another McNaught namesake, the McNaught Hartley comet crossed paths with Ulysses in 2004. McNaught-Hartley didn’t seem to be in the right place for Ulysses to encounter its tail. However, an eruption of particles from the sun’s surface called a coronal mass ejection, carried cometary material to Ulysses.
As Ulysses continues to orbit, unmonitored, we wonder if it run into more than just the tail of some other comet.
Editor's note: This is the third and the last part of our tribute to Ulysses. Part I and Part II are available if you click on their respective links:
Part I: Ulysses space craft says goodbye to Earth
Part II: Solar orbiting spacecraft says "Mission Accomplished"
Part III: Rendezvous with the comet Hyakutake in outerspace