THE ODYSSEY CONTINUES: ULYSSES BEGINS ITS SECOND SOLAR
Six years after it first swooped over the Sun's poles, the European Space Agency's intrepid Ulysses spacecraft is about to start its second reconnaissance of these mysterious regions - just as our nearest star is reaching a peak in its 11-year cycle of activity.On 8 September, Ulysses will cross the solar latitude of 70 degrees south.
After spending four months flying over the Sun's south polar region, the spacecraft will swing towards the equator before turning its attention to the solar northern hemisphere. The robotic explorer's passage over the Sun's northern polar expanses will commence on 3 September 2001.
Although Ulysses will be returning along the same orbit that it followed six years ago, solar conditions will be quite different. and new discoveries are eagerly awaited. Solar storms are already numerous and the high latitude solar wind (the stream of electrically charged particles that blows at supersonic speeds away from the Sun) is chaotic and blustery. In contrast, during its first south polar pass in 1994, when solar activity was very low, the solar wind at high latitudes was fast (blowing at a steady 750 km/s) and steady.
Among the scientific instruments on board Ulysses is a magnetometer provided by Imperial College, London, which is measuring changing magnetic fields in interplanetary space.
"Magnetic fields from the Sun's outer atmosphere - the corona - are carried into space by the solar wind," explained Professor Andre Balogh, Principal Scientific Investigator for the magnetometer. "Our observations on Ulysses confirm that the interplanetary medium is filled with a complex tangle of magnetic field lines which play a key role in all the important phenomena.
"Ulysses also showed that we must not trust even our best theories without observational evidence. Perhaps the greatest surprise was the very high level of fluctuations in the magnetic field which is responsible for restricting the influx of cosmic rays - highly energetic particles travelling at nearly the speed of light - into our Solar System. Before Ulysses, we all thought that cosmic rays just stream in over the poles of the Sun. Now we know better." For Professor Balogh's team, the remarkable longevity of Ulysses and the dramatic scientific results from this unique mission are the payback for many years of hard work.
"We designed and built our instruments more than twenty years ago," explained Professor Balogh. "The long delay in the launch of Ulysses (caused by delays in the space shuttle programme and the Challenger disaster) in the end proved fortunate as it allowed us to complete the first full orbit around the Sun at a time when solar activity was low.
"Solar activity has been increasing for the past three-four years, and now Ulysses has started observing much more disturbed conditions at increasing solar latitudes. The passes over the poles of the Sun will show a different kind of interplanetary environment from the one we observed in the mid-1990s. Instead of coronal holes covering the polar regions, hot and active regions are generating a surprising amount of disturbance over the whole Sun now.
"The magnetic field observations already show that, at its present high activity level, the Sun throws large amounts of exploding coronal material into space at all latitudes. Our observations show that the previously observed high levels of fluctuations have given way to compressed and tangled magnetic structures, remnants of the exploding coronal loops. Cosmic rays have now an even harder time to penetrate into the Solar System." And what of the future?
"We can still look forward to another four years of operations on Ulysses, until it completes its second orbit around the Sun in 2004," he said "By the time the mission comes to an end, Ulysses will have gathered the only set of observations above the solar poles covering more than a complete 11-year solar cycle. In the absence of follow-up missions, Ulysses will continue to be the benchmark for our understanding of the three-dimensional heliosphere (the bubble-shaped region of space dominated by the Sun and its solar wind) for another generation."
Notes For Editors.
A joint ESA and NASA mission, Ulysses was launched on 6 October 1990. After using Jupiter's gravity to change course, it became the first spacecraft to be launched into an orbit outside the ecliptic, the plane in which the planets orbit the Sun. From this unique vantage point, it has revolutionised scientists' view of the heliosphere.
At solar minimum, its instruments found that the fast solar wind, emanating from small coronal holes near the Sun's poles, expands to fill a large fraction of the heliosphere. The data also revealed that the boundary between the fast wind and the slower, more variable wind from the equatorial regions, is surprisingly sharp. Another surprise was that the effects of collisions, occurring at low latitudes between fast and slow wind streams, continue to be felt all the way up to the poles.
Ulysses' discoveries, however, have not been confined to the Sun and heliosphere. It also made the first measurements of dust particles and neutral helium atoms that originated outside the Solar System. These findings have contributed to a major increase in our knowledge about the gas and dust clouds surrounding the heliosphere in interstellar space. Other measurements have led to a better understanding of processes occurring even further away, in distant supernova explosions. Further information and images on the Ulysses mission are available at the ESA Ulysses website http://sci.esa.int/ulysses
Issued by: Peter Bond, RAS Press Officer (Space Science)
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Professor Andre Balogh, Space Physics Dept., Blackett Laboratory, Imperial College of Science, Technology and Medicine,
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