How far can a comet tail reach?
An entire branch of science has been built around so-called ‘pick-up ions’ because of Ulysses observations. Pick-up ions are charged atoms which are created in the heliosphere either when a neutral atom is subject to a ‘charge exchange’ with other charged particles carried by the solar wind, or by the direct action of solar radiation (called ‘photo-ionisation’). With one of its pioneering instruments, Ulysses has detected pick-up ions, and even discovered new sources of them. For example, Ulysses found that solar wind particles appear embedded in dust grains when they are near the Sun, and then they are released to form a pick-up ion population known as the ‘inner source’. Furthermore, Ulysses could measure the isotope Helium 3 (He3) in the population of pick-up ions created from the interstellar neutral gas present within the heliosphere. Measuring isotopes of helium, one of the very first atoms to be created, is very important as it provides important clues about the evolution of mat
