to 14 January. Perigee (closest approach to the Earth) was 15 January 2007, at a distance of 0.82 AU. After passing the Sun, McNaught became visible in the Southern Hemisphere. In Australia, according to Siding Spring Observatory at Coonabarabran, where the comet was discovered, it was to have reached its theoretical peak in brightness on Sunday 14 January just after sunset, when it would have been visible for 23 minutes. On 15 January the comet was observed at Perth Observatory with an estimated apparent magnitude of −4.0. Ulysses probe The Ulysses spacecraft made an unexpected pass through the tail of the comet on 3 February 2007. Evidence of the encounter was published in the 1 October 2007 issue of The Astrophysical Journal. Ulysses flew through McNaught's ion tail 260 million kilometres (160 million miles) from the comet's core and instrument readings showed that there was "complex chemistry" in the region. The Solar Wind Ion Composition Spectrometer (SWICS) aboard Ulysses measured Comet McNaught's tail composition and detected unexpected ions. It was the first time that O3+ oxygen ions were detected near a comet. This suggested that the solar wind ions, which did not originally have most of their electrons, gained some electrons while passing through the comet's atmosphere. SWICS also measured the speed of the solar wind, and found that even at 260 million kilometres (160 million miles) from the comet's nucleus, the tail had slowed the solar wind to half its normal speed. The solar wind should usually be about per second at that distance from the Sun, but inside the comet's ion tail, it was less than per second. Prof. George Gloeckler, the principal investigator on the Solar Wind Ion Composition Spectrometer (SWICS), said the discovery was important as the composition of comets told them about conditions approximately 4.5 billion years ago when the Solar System was formed. Period Comet C/2006 P1 took millions of years coming directly from the Oort cloud. It follows a hyperbolic trajectory (with an osculating eccentricity larger than 1) during its passage through the inner Solar System, but the eccentricity will drop below 1 after it leaves the influence of the planets and it will remain bound to the Solar System as an Oort cloud comet. Given the orbital eccentricity of this object, different epochs can generate quite different heliocentric unperturbed two-body best-fit solutions to the aphelion distance (maximum distance) of this object. For