Spacecraft's solar polar pass

The Ulysses space probe has gone where no probe has gone before

By BBC News Online science editor Dr David Whitehouse

The pioneering European Space Agency's (Esa) Ulysses spacecraft flew over the Sun's north pole on Saturday, reaching a high northern solar latitude of 80 degrees.

Ulysses, launched in 1990, has been this way before, circling the Sun in a wide orbit. It measures the stream of particles thrown out by our star called the solar wind.

Ulysses' track allows the probe to sample the different properties the wind displays in different directions.

It is the first spacecraft to explore interplanetary space at such high solar latitudes. Ulysses orbits the Sun at a distance that varies from just more than one to five times the Sun-Earth distance (149 million kilometres or 93 million miles).

Polar passage

Although Ulysses was built to study the Sun, the closest the probe has ever been to the star was when it was being constructed on Earth.

After launch, Ulysses headed towards Jupiter - away from the Sun - and used Jupiter's gravity to alter the craft's trajectory to swing above the orbits of the planets and over the poles of the Sun.

Over the next few years, the Sun's activity will decline

Ulysses has already passed over the poles twice, discovering that the solar wind comes in two forms: a slow wind at the Sun's equator, and a fast wind at its poles.

Ulysses will complete its north polar passage on 10 December, when it passes 70 degrees north solar latitude on the other side of the Sun.

Then, for the second time during its 11-year lifetime, the probe will head out towards Jupiter's orbit on the long leg of its six-year circuit around the Sun.

The spacecraft will continue to monitor the Sun's outpourings as solar activity calms down from the maximum level activity it has experienced this year through to its quiet period - the next solar minimum - due in a few years.