Ulysses Status Report - May 2006
Operations and Archive
All science operations during the reporting period have been nominal. The spacecraft team has performed a thorough review of the power and thermal situation on board the spacecraft, taking particular account of recent experience when platform temperatures reached their lowest values during the mission to date. Based on this, it has been decided to modify the current payload power-sharing plan. As a result, instruments that are not part of the "core payload" (and are presently turned off) will already be switched on again for short periods this year, prior to switch-off of the 25-watt Cold Case Heater in May 2007. The latter is needed to keep critical parts of the platform at a safe temperature when the spacecraft is far from the Sun.
Preparations, including the plans for ground-station coverage, are underway for the next nutation season that is predicted to start in mid-February next year. The nutation disturbance arises when the axial boom moves out of the shadow of the spacecraft body and is subject to asymmetric heating. As on previous occasions (in 1994/95 and 2000/2001), nutation will be controlled using the onboard CONSCAN system. The ESA Ulysses archive is accessible via the World Wide Web at URL: http://helio.esa.int/ulysses.
Science Highlights
After a journey of 12.5 years and more than two complete orbits the Sun, Ulysses is once again fully immersed in the fast solar wind from the coronal hole that covers the Sun's southern polar cap around solar minimum. This time around, the spacecraft had to climb higher in latitude, to almost 50°S, before leaving the slower, more variable wind behind. In mid-1993, the transition occurred nearer to 35°S. The explanation is related to the fact that the Sun's magnetic equator is still more highly inclined with respect to the rotational equator, even though the solar activity cycle is closer to its minimum, compared with the previous occasion. (At solar minimum, the Sun's rotational and magnetic axes are most closely aligned). Since the slower, more variable solar wind is mainly confined to the magnetic equator, this in turn means that, until recently, Ulysses continued to encounter slow wind once per solar rotation.
Observations from the solar wind instrument on Ulysses over the past year showed regular excursions from slow (~400 km s-1) to fast (~700 km s-1) wind every rotation. At the start of the year, however, the speed profile began to show a systematic increase in the minimum speed encountered with each subsequent solar rotation, and since early March, the speed has remained high. Given the fickle behaviour of the Sun, with large outbursts of activity occurring relatively late in the declining phase of the present cycle, it will be interesting to see whether Ulysses remains in the fast wind until its trajectory brings it to lower latitudes again after traversing the southern pole in February 2007.