Stars and Planets: February

The sky is looking magnificent this month. The winter constellations – Orion et al – are now putting on their best show, their stars glittering like diamonds on hard frosty nights. To add to the spectacle, they are flanked on either side by two brilliant planets – Jupiter and Saturn.

On 2 February, Jupiter is "at opposition" – meaning that Jupiter, Earth and the Sun are in a straight line, and that Jupiter and Earth are at their closest to each other. "Close" is, of course, a relative concept – the two planets are separated by almost 600 million kilometres – but Jupiter is by far the most brilliant object in the sky, outshining Saturn 10 times over.

Jupiter has no light of its own. It reflects sunlight like a high-flying aircraft at dusk, and shines so brightly simply because it's so vast (you could fit more than 1,000 Earths inside it). It's the nearest thing we have in the Solar System to another star. Had Jupiter been 50 times more massive, its core would have been hot enough to trigger nuclear reactions, and the giant planet would have shone like the Sun.

Like the Sun, Jupiter has its own family of smaller objects circling it. We call these worlds "moons", but two of them are actually larger than the smallest planets. And Jupiter's family is growing; on Hallowe'en night last year, Scott Sheppard of the University of Hawaii discovered Jupiter's 40th moon (in addition to the 22 others he had already found). This tiny object – only 3.5km across – takes 748 days to orbit Jupiter in a backwards direction. It also confers on Jupiter the title of the planet with the most moons – the runners-up are Saturn with 30 and Uranus with 21.

Jupiter's moon system has been under intense scrutiny since 1995, when Nasa's Galileo space probe went into orbit around the planet. Galileo's main targets were the four biggest moons, discovered by the astronomer Galileo in 1610. The probe has made more than 30 fly-bys of the Galilean satellites, recording erupting volcanoes on Io, a magnetic field on Ganymede, a vast sub-surface ocean on Europa, and another possible one on Callisto. When it arrived, it dispatched a sub-probe into Jupiter's atmosphere, which discovered that Jupiter is wracked by lightning strikes and thunderstorms many times more powerful than those on Earth.

As well as investigating Jupiter and its largest moons, Galileo also nuzzled up to its wraith-like ring – which is not a patch on the rings of Saturn – and found that it was made of dust grains blasted off the four biggest moons by the impact of meteoroids. It also surveyed Jupiter's immense and complex magnetic field. This region of lethal electrical currents, generated by Jupiter's rapidly spinning core, is so vast that were it visible from Earth, it would cover an area of sky five times bigger than the full Moon.

Galileo had a direct brush with this dangerous zone last November, when it flew just 160km above the surface of Jupiter's small, potato-shaped moon Amalthea. In doing so, the craft also came within 70,000km of Jupiter itself, and hit the magnetic field head-on. Galileo switched its instruments into "safe" mode to protect itself, and it took weeks for Nasa to coax the probe back into life.

Amalthea was Galileo's last port of call. The reddish-coloured moon, 270km long and about 130km wide, has an incredibly low density. Scientists are coming to the conclusion that it isn't solid at all, but is actually a floating raft of rubble – the result of countless collisions – loosely held together by gravity.

Now Galileo's mission is at an end. It has circled Jupiter for five years longer than scientists had intended, and it has survived more than four times the cumulative dose of harmful radiation it was designed to withstand. But its propellant – which it needs in order to manoeuvre – is running out.

If it were left to itself, Galileo might eventually collide with one of the moons. And scientists are particularly concerned that it shouldn't end up on Europa. The discovery of a warm ocean under its ice crust suggests that this moon could harbour primitive life-forms. The last thing scientists want is to contaminate Europa with a crashed space probe from planet Earth.

So Galileo will end in a blaze of glory – literally. On 22 September, the Galileo team will send their probe plummeting into Jupiter's atmosphere, where it will burn up. There won't be a dry eye in Mission Control...

What's up

The winter constellations are beautifully positioned at the moment, with Sirius – the brightest star – due south. Orion and his pals, while slipping slightly to the west, are still very prominent. But the spring constellations are starting to make their appearance. Lion-shaped Leo is rising in the east, as is the red giant Arcturus (follow the handle of the Plough downwards to locate it).

The planets Jupiter and Saturn are absolutely stunning in February's skies. Jupiter is close to the centre of the faint constellation Cancer, where there lies a cluster of stars called Praesepe, easily visible through binoculars. For some unknown reason, ancient Chinese astronomers dubbed this cluster "the exhalation of piled-up corpses".

If you're an early riser, Mars puts in an appearance at about 4am, Venus pops up just after 5am (it's a dazzling object in the dawn skies), and Mercury rises about an hour and a half before the Sun at the beginning of the month.