Identification of comet Hyakutake's extremely long ion tail from magnetic field signatures

Observations of the varying orientations of comet tails led to the suggesti on of the existence of the solar wind-a continuous outflow of ionized mater ial from the Sun(1). It is now well established that gas from comets is ion ized by several processes and joins the solar wind(2), forming an ion (plas ma) tail that points away from the Sun. The plasma environments of three co mets have been measured in situ, but only in the upstream direction or less than 8,000 km downstream of the nucleus. Here we report a fortuitous cross ing by a spacecraft of the plasma tail of comet Hyakutake (C/1996 B2), at a distance of more than 3.8 astronomical units (550 million kilometres) from its nucleus. This surpasses the tail length of 2 AU determined for the Gre at March Comet of 1843 (C/1843 D1)(3). Our measurements reveal that, at thi s distance, the tail of comet Hyakutake was a structured entity at least 7 million kilometres in diameter.