Ulysses Status Report - May 2007
Operations and Archive
Nutation operations commenced on 14 February as planned, with the first signs of the disturbance (caused by asymmetric solar heating of the axial boom) appearing after an open-loop slew on 10 February. As in the past, closed-loop Conscan manoeuvres have been carried out as required to control the nutation, which is expected to be present until mid-February 2008. The ESTRACK ground stations at Kourou and New Norcia are being employed to fill gaps in coverage arising from incomplete spacecraft visibility from the DSN Canberra complex. Science operations have, in general, been conducted according to the agreed payload power-sharing plan. Some modifications were necessary in March and April owing to slightly lower-than-expected power margins. Ground segment performance for the period has been excellent. The ESA Ulysses archive is accessible via the World Wide Web at URL: http://helio.esa.int/ulysses.
On 3 February 2007 Comet C/2006 P1 McNaught, the brightest comet observed from Earth in the last 40 years, was almost radially aligned with the Ulysses spacecraft, then at a heliocentric distance of ~2.4 AU and 79° south heliographic latitude. The comet was ~0.7 AU from the Sun. During a ~4.5-day interval (5-9 February), Ulysses encountered the tail region of this spectacular comet. The region of disturbance in the solar wind produced by the comet was nearly 10 Mkm wide at 2.4 AU. During the encounter, the speed of the solar wind dropped from ~750 kms-1 to a minimum of 360 kms-1 and the proton density dropped by more than 2 orders of magnitude. Simultaneously, very large fluxes of molecular and singly and doubly charged atomic ions of cometary origin were detected. The slowing, depletion and heating of the solar wind was a result of charge exchange with neutral atoms and molecules from the comet, and with the pickup up by the solar wind of the newly-born cometary ions. Although no shocks were observed during the encounter, the magnetic field strength was slightly enhanced in broad regions at the leading and trailing edges of the comet's tail. The field was generally weaker inside the tail than in the unobstructed solar wind and pointed nearly radially inward during much of the encounter, opposite to its normal outward direction in the southern polar hemisphere at this phase of the present solar cycle. There were, however, shorter periods when the field pointed nearly radially outward, indicating the filamentary structure of the comet tail. This is the third confirmed crossing of a comet tail during the mission (the others were comet Hyakutake in 1996 and comet McNaught-Hartley in 2000).