Much excitement this morning. A small group of enthusiasts based in Knoxville, Tennessee has been building "Spirit of Knoxville" unmanned balloons with the target of crossing the Atlantic. The balloons are equipped with a computer, GPS and an amateur radio transmitter; they're designed to ride the jetstream all the way to Europe.

Their fourth attempt is currently aloft – just. Launched just over a day ago, it hit 12,000 metres at its highest. It came back on the air this morning at 7:47am following an overnight silence – the shortwave frequency it uses to report closes down during darkness – and since the last message received before the blackout indicated that it was falling dramatically, nobody was at all sure it would survive the night.

It's very cold up there, which affects the electronics and the frequency of transmission, and shortwave is notoriously prone to interference, fading and the vagaries of the ionosphere. Receiving the data is quite an art: if you fancy it, the balloon's on 10.1463 MHz transmitting a mix of CW and RTTY every ten minutes or so.

As well as the team in Knoxville, there are amateur radio operators co-operating all around the world. They've got shortwave receivers plugged into decoding software, with automatic reporting fed back to HQ over the Internet. Others, like me, just keep an ear on the frequency and an eye on the chatroom where people swap reports – and there's little to compare with the feeling of being there when the first weak signals come in and you know the balloon is alive.

Things are a bit bleak right now. It was at 4000 metres at 8am, but now it's down to around 2600 metres and falling at around 6 metres a minute – the hope is that now it's in daylight, the temperature will rise and warm the helium in the balloon, but the weather isn't good and it's not picking up. On the bright side, this effort has gone twice as far as the previous effort.

You can see live reports from the balloon on the mission dashboard. But be quick. Or sign up to their email alert, and be ready for the next one. They're not giving up until they make it.