WHILE YOU were sleeping last night, a radio dish 140ft (42m) in diameter, set in the wilds of West Virginia, was busy scanning the sky for signs of alien life. Panning from selected star to selected star, the telescope lingered on each one for just a few minutes - long enough to listen for any tell-tale artificial radio signals emanating from the star's circling planets.

This privately funded search, called Project Phoenix, has been going on for nearly five years. It arose out of the ashes of a NASA-funded project, axed by the US Congress after only one year in 1993 because, as one senator put it, "not one Martian has said `take me to your leader'". An editorial in the Boston Globe wryly commented: "This only goes to prove that there is no intelligent life in Washington."

Many astronomers are certain there is intelligent life out there. The Sun is an ordinary star with planets and other stars have planets, too. There are 200bn stars in our galaxy alone, and the chemical elements that make up life are among the most common in the Universe.?

For 40 years, a dedicated band of astronomers has been on the trail of ET. Working from the premise that all technologically driven civilisations will go through a phase of communicating by radio waves - they are fast, cheap and penetrating - the researchers have been building receivers of increasing complexity to pin down civilisations. Soon, the SETI researchers hope to pick up a signal. The effects will send shockwaves around our planet. Knowing we are not alone in space will put a different perspective on everything.

We must also decide whether to reply - an act some believe would be extreme stupidity. Anthropologist Jared Diamond begs radio astronomers tempted to send a reply to switch off their transmitters. He fears the aliens will track us down and do, in his words "what every superior civilisation has done to lesser civilisations throughout history", subjugate and eventually exterminate them.

But the SETI researchers believe that a dialogue with a more advanced race could be enormously beneficial to us. But who should reply on behalf of all humankind? And what should the message say?

Time is running out for SETI. Increasing radio interference from our electronic society is drowning out the faint murmurings crossing light years of space. Ultimately, the SETI astronomers are asking to be allocated a crater on the far side of the Moon.

What if we never pick up a signal? Then we might have to face the fact that we are alone. Then we would be responsible not just for one small planet - but for an entire Universe.

The night sky in April

It's a pretty poor month for planets. None are visible in the evening sky; in the early morning you'll catch brilliant Venus low in the twilight glow, rising little more than an hour before the Sun. Lower still is the second brightest planet, Jupiter. The two planets are steadily converging, passing closest to one another on the morning of 23 April, when they form a spectacular trio with the thin crescent moon.

April diary (all times 24-hour, BST)

3rd: 2119 Moon at First Quarter

11th: 2324 Full Moon

19th: 2053 Moon at Last Quarter

22nd: 1500 (approx) Maximum of Lyrids meteor shower - 10 meteors/hr

26th: 1241 New Moon