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| Ulysses |
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| Ulysses Mission Status March 3, 1999 With the spacecraft and its scientific payload in excellent condition, the Ulysses spacecraft is well into its second orbit of the Sun. Today, the spacecraft is about 22 degrees south of the Sun's equator, traveling at a heliocentric velocity of about 33,000 kilometers per hour (approximately 20,000 mph) with respect to the Sun. Flight controllers continue to perform Earth-pointing maneuvers every two days to keep the spacecraft's high-gain antenna pointed at Earth. Some data were lost on February 15 when onboard systems detected an error in readouts during initiation of a pointing maneuver and caused Ulysses to switch to a safing mode, in which the spacecraft operates on a minimum of power and relies only on essential systems. Normal operations were quickly restored. Ulysses is beginning to move closer to the Sun and is preparing for its next high-latitude pass over the southern solar polar region. As it nears the Sun, flight controllers are anticipating phenomena that occur when the spacecraft's 7.5-meter (25-foot) axial boom receives direct sunlight more frequently. Increased sunlight caused a slight wobbling onboard the craft during Ulysses' first passages over the poles in 1994 and 1995. The NASA-European Space Agency flight team understands the situation and has procedures in place to minimize motion in the boom if it begins to wobble again in December 2000. Minor wobbling will most likely continue through November 30, 2001. The next phase of the Ulysses extended mission will be to study the Sun's polar regions under conditions of high solar activity, culminating in a high-latitude pass over the south polar region from September 2000 to January 2001, and the spacecraft's flight over the north polar region from September 2001 to December 2001. Conditions in the polar regions during the upcoming passage are expected to be dramatically different from those encountered during the prime mission when the Sun was quiescent. In particular, the simple configuration of the corona - the Sun's outer skin - with large coronal holes over the polar caps, is expected to be much more complex. Scientists expect to encounter high latitude "streamers," which are distinctive jets or corridors of high-density coronal material radiating outward from the Sun's surface. In the most active part of its 11-year solar cycle, the Sun is likely to produce transient events such as solar flares and massive ejections of solar particles from the corona. These events will most likely dominate the solar environment as Ulysses nears high-latitude regions again, and could greatly disturb the underlying structure of the solar wind observed during the spacecraft's primary mission. Solar activity may influence other phenomena, such as the passage of cosmic rays and energetic solar particles through the heliosphere, a three-dimensional bubble of ions, protons and electrons called plasma, that shields the solar system from interstellar space. -JPL |
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