If the original plans were to be followed, the four-year mission by the Cassini probe at Saturn would be drawing to a close by the fourth anniversary of its arrival on July 1. As it is, and given its near flawless performance, Nasa has prolonged the mission by two more years to allow another 60 orbits of the planet and a further 36 flybys of Titan and Saturn's other moons. The extension will cover the interesting period next year when Saturn's rings turn edge-on to the Earth and the Sun.

One focus will be Enceladus which, despite being only 500km across (one seventh as wide as our Moon) has geysers of water and hydrocarbons erupting from its southern polar regions. Ice is a major constituent of Saturn's moons, as it is of most of the solar system's lesser bodies from Jupiter outwards. In Enceladus, at least, it seems that tidal heating has melted the ice into underground reservoirs of liquid water.

Could these provide an unlikely niche for alien life? Cassini swept within 50km of Enceladus in March and two similar grazing encounters are earmarked for later this year.

In other news, Nasa's Phoenix remains on course for its hazardous touchdown in the Martian Arctic early on May 26, while the GLAST gamma-ray observatory, mentioned here last time, is likely to be launched a few days later than its intended date of May 16. Meanwhile, the ISS is visible again before dawn, remaining on show until it disappears into the evening twilight early next month.