24 May 2004
The Jupiter Distant Encounter (JDE) campaign that commenced on 25 January was completed successfully on 8 March. The on-board tape recorder has been switched on again, marking the end of more than 40 days of 24-hour per day real-time coverage by the Deep Space Network.
With the exception of the gamma-ray burst (GRB) instrument, the scientific payload was operated continuously during the JDE campaign without the need for power sharing. Attempts to operate GRB without its heater, a power-conservation measure, resulted in unstable instrument behaviour related to the low temperature. An investigation into the exact cause is on going, but no permanent damage is expected. The spacecraft and scientific instruments are otherwise in good health.
On 30 June, Ulysses will be at its maximum distance from the Sun (5.41 AU) heading south, having crossed the heliographic equator on 20 February.
Operations and Archive
With the exception of the anomalous GRB behaviour reported above, all science operations during the reporting period have been nominal. The ESA Ulysses archive is accessible via the World Wide Web at URL: http://helio.estec.esa.nl/ulysses/archive/.
Analysis of the data acquired during the Jupiter campaign is still in progress. Nevertheless, a number of interesting results have already emerged. The DUST instrument detected streams of dust particles flowing from Jupiter. First observed by the same instrument in 1992, the dust streams comprise grains no larger than smoke particles, and are believed to originate in the volcanoes of Jupiter's moon Io. The dust particles, which carry an electric charge, are strongly influenced by Jupiter's magnetic field. Electromagnetic forces propel the dust out of the Jovian system, into interplanetary space. The recent observations include the most distant dust stream ever recorded, at 3.3 AU from Jupiter. Another unusual feature is that the streams occurred with a period of about 28 days. This suggests that solar wind streams that co-rotate with the Sun play an important role. The most intense peaks showed fine structure not seen in 1992.
During its second encounter, Ulysses approached Jupiter from high northern latitudes, opening a window on previously unexplored parts of the Jovian magnetosphere. This was of particular interest to scientists studying Jupiter's natural radio emission, since a distinctive type of radio signal is believed to originate in the high-latitude auroral zones of Jupiter. These signals, which have a repetitive, burst-like character, have indeed been detected throughout the campaign period. The radio and plasma wave experiment on board Ulysses first detected bursts of radio waves occurring approximately every 40 minutes during the Jupiter fly-by in 1992. These so-called "Quasi-periodic", or QP-40 bursts were present for several hours, then faded away and reappeared a number of hours later. More recently, NASA's Chandra X-ray observatory detected similar QP-40 pulsations in X-rays emitted in hot spots in Jupiter's northern polar regions. Although not fully understood, these phenomena also seem to be triggered by streams of high-speed solar wind hitting Jupiter's magnetosphere.