Observation of the interstellar helium cone by the NOZOMI spacecraft

The Japanese Mars probe, NOZOMI, is staying in the interplanetary space (1- 1.5 AU) until its Mars' orbit insertion scheduled in early 2004. Every 16 m onths on this interplanetary orbit the spacecraft crosses around 1 AU the ' gravitational focusing cone' of the interstellar helium, which are penetrat ing into the inner heliosphere under the solar gravity. During the first cr ossing of the cone in the season of March-May 2000, we observed these heliu m particles after the solar wind pickup process with an E/q type ion detect or aboard NOZOMI. We have estimated the original temperature of the interst ellar helium as 11 000 K.