At a meeting in Paris on February 11 and 12, ESA's Science Programme Committee unanimously approved a proposal to continue operating the highly successful Ulysses spacecraft until March 2008.

An artist's concept of Ulysses. Credit: NASA/ESA

This latest extension, the third in the history of the joint ESA-NASA mission, will enable Ulysses to add an important chapter to its survey of the high-latitude heliosphere. In 2007 and 2008, the European-built space probe will fly over the poles of the Sun for a third time.

Unlike the recent high-latitude passes in 2000 and 2001 that brought Ulysses over the solar poles near the maximum of the Sun's activity cycle, conditions for the third set of polar passes are expected to be much quieter. In fact, they are likely to be similar to those in 1994/1995 when Ulysses first visited the Sun's poles. There will be an important difference, however.

While the Sun's activity follows the well-known 11-year sunspot cycle, conditions in the heliosphere are driven by the 22-year solar magnetic (or Hale) cycle.

"The Sun's magnetic field is rather like that of a bar magnet, with a north and a south pole," says Richard Marsden, ESA's Mission Manager for Ulysses.

"The north and south poles changed places at the time of the recent solar maximum, so the Sun's magnetic polarity in 2007/2008 will be opposite to that of the previous solar minimum."

The magnetic field of the Sun influences the way in which charged particles (cosmic rays, solar energetic particles, and even interstellar dust grains) move through the heliosphere.

"We'll be looking for differences in behaviour related to the magnetic reversal," says Marsden.

There is another difference between the solar minimum polar passes in 2007/2008 and those in 1994/1995. During its first high-latitude exploration, Ulysses did not have the benefit of being part of a fleet of solar and heliospheric spacecraft that now includes SOHO, Cluster, and NASA's ACE and Cassini. These missions did not come on line until later. The launch of new missions like NASA's dual-spacecraft STEREO, and Solar Dynamics Observatory, will add a further dimension over the next few years.

The study of our solar system's local neighbourhood using data from Ulysses has proven to be remarkably fruitful. Several of the instruments on board Ulysses are able to measure the properties of the gas and dust clouds that surround the heliosphere by detecting individual atoms and grains that are able to penetrate into the heliosphere. Many of the key measurements, however, can only be made when Ulysses is close to its maximum distance from the Sun. The period 2004-2006 represents a unique opportunity to extend these studies.

"The best way to improve the precision of our measurements is to detect as many atoms and dust grains as we can," says Marsden.

Because Ulysses is a joint project with NASA, programmatic decisions have to be agreed on by both agencies. A proposal was presented to NASA's 2003 Sun-Earth Connection Senior Review Panel who recommended continuing the mission until 2008. Based on this recommendation, NASA has already indicated its intention to go ahead. So things are all set for a new round of solar polar adventures.

"We're all excited by the wonderful scientific opportunities that Ulysses will give us in the next four years," says Richard Marsden.