Ulysses Catches Another Comet by the Tail
9 February 2004Ulysses is not normally associated with the study of comets. Nonetheless, the European-built space probe demonstrated its ability as a "comet catcher" when it crossed the distant tail of comet Hyakutake (C/1996 B2) in 1996.
In an article soon to appear in Astrophysical Journal Letters, the same Ulysses teams that identified that particular comet tail present evidence for one (and possibly two) new tail crossings. This time, however, Ulysses needed a little help from the Sun to make the right connection. The comets involved were McNaught-Hartley (C/1991 T1) and SOHO (C/2000 S5). Unlike Hyakutake, these comets seemed to be at the wrong location for Ulysses to intercept their tails. By chance, a Coronal Mass Ejection moving away from the Sun enveloped both the comet and the spacecraft, carrying the cometary material to Ulysses.
Comets such as McNaught-Hartley are often likened to "dirty snowballs". They come from the deep freeze of the outer solar system, travelling on orbits that carry them close to the Sun. As they approach the inner solar system, the comets warm up, releasing gas and dust that forms a cloud around the cometary nucleus. Under the influence of sunlight and the solar wind, the gas atoms become electrically charged, and form what is known as the comet's ion tail. "The solar wind causes the ion tail to point away from the Sun, acting just like an interplanetary windshock", said Richard Marsden, ESA's Mission Manager for Ulysses. In fact, back in the 1950s, a scientist called Ludwig Biermann inferred the existence of the solar wind from observations of comet tails.
Violent eruptions on the Sun can throw huge clouds of ionised gas into space. Known as Coronal Mass Ejections (or CMEs), these clouds travel away from the Sun at speeds of up to 2000 kms-1, and are responsible for many of the effects of space weather. As they plough through the solar wind, CMEs often produce a significant distortion of the interplanetary magnetic field that is carried away from the Sun by the solar wind. It was just such a distortion that is believed to have connected the comets McNaught-Hartley and C/2000 S5 to Ulysses.
"The ability of CMEs to carry cometary ions far from their radial paths significantly increases the chance of detecting these ions", said Prof. George Gloeckler, Principal Investigator of the Solar Wind Ion Composition Spectrometer (SWICS) experiment on Ulysses that made the discovery. Serendipitous encounters with comet tails cannot replace dedicated missions like ESA's Rosetta. Even so, they are helping scientists to unravel the mysteries of these icy visitors to the inner solar system.