The venerable Ulysses spacecraft has gone south.
That’s just what solar scientists had been counting on. Earlier this month, the craft passed directly below the sun, looking at the south pole from a distance of 329 million kilometers. In its 16 years of orbiting the sun, Ulysses has flown past the south pole only twice before, a feat no other solar probe has attempted.
Initially, Ulysses attained this rare perspective thanks to a gravitational assist from Jupiter that bent the craft’s orbit out of the plane in which the planets orbit the sun. In its 6.3-year-long orbit, the craft passes from the south pole to the equator and then over the north pole. The spacecraft has a good view of the sun’s turbulent atmosphere and brewing solar storms.
As in 1994, the first time that Ulysses flew over the solar south pole, the sun is at the nadir of its 11-year activity cycle. However, the sun’s magnetic field has reversed direction since the earlier passages, and the polar fields are now half the strength that they were in 1994, notes Ed Smith, Ulysses project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Studying the poles under these changed conditions, he says, will enable Ulysses to provide information about how the sun’s magnetic field controls the solar wind and shields Earth and other planets from cosmic rays coming from elsewhere in the Milky Way.