By Stephen Mulvey
BBC News Online
The European Union has taken off and is flying east without a clear idea of where or when it is going to land.
On 1 May it will reach the borderlands of the old USSR.
It will then glide into the Balkans in 2007, and in theory it could land in Turkey 10 years from now, giving "Europe" a border with Iraq.
But that might not be the limit.
Belarus, Moldova and Ukraine are widely regarded as potential EU states and the Caucasian states of Armenia, Azerbaijan and Georgia have not been ruled out.
Even Israel and Russia are seen as future members by a few.
It is not beyond the realms of possibility that today's EU of 15 could one day become a club of more than 40 states stretching from Iceland to the Caspian - no line has yet been drawn on the map.
What kind of organisation such a big union would become is anybody's guess.
Even the EU of 25, which will come into existence this year, will be very different, it is generally agreed, from today's club of 15.
France and Germany will no longer be able to dominate - weakening the EU's "motor" - and the hard-up new members are expected to slam the brakes on the pace of EU policymaking.
Further ahead, it is hard now to imagine the Common Agricultural Policy subsidising the semi-nomadic shepherds of Azerbaijan or cohesion funds extending to Moldova, where GDP per capita is about 1% of the current EU average.
The EU's draft constitution echoes earlier treaties by saying that any country may join as long as it shares the union's values - and is in Europe.
But where does the continent of Europe end?
Morocco's bid for membership in 1987 was rejected outright on geographical grounds, but some other countries occupy a grey area.
Europe's traditional boundary markers - the Bosphorus and the Ural mountains - leave Turkey and Russia partly in and partly out and consign the three Caucasian states, which all want to join, to no-man's-land.
Cyprus too, which lies off the Turkish coast and is closer to Syria than Greece, is not obviously European by geography, though it is one of 10 states joining the EU in 2004.
"Europe's political elite needs to have a discussion about what Europe really is, what the concept of Europe is," says German member of parliament Klaus-Juergen Hedrich, over tea in his office, just to the east of the old Berlin wall.
"We have to do that to explain to our electorates where we are heading. If we don't we will sooner or later run into difficulties."
Some EU officials agree that the limits of possible future enlargement should be spelled out, in the interests of transparency.
But others think the issue is best left unresolved, because the possibility of future membership is a carrot which could encourage countries to make democratic reforms and become better, more stable neighbours.
Member states are divided on how much more expansion the EU can take after the anticipated admission of Bulgaria and Romania - and possibly Croatia - in 2007.
Least controversial would be Norway, Switzerland and Iceland (the first two have both made and withdrawn applications before) followed by the western Balkans.
Albania, Macedonia, Bosnia-Hercegovina and Serbia-Montenegro have all been promised membership if and when they fulfil the conditions.
But Turkey, whose bid stirs great controversy, is currently ahead of them in the queue.
Turkey won candidate status in 1999 and the promise, in 2002, that membership talks could start as early as January 2005. However, there remains a significant body of opinion - including Klaus-Juergen Hedrich's Christian Democratic party, for example - which says the country simply is not European.
Even Turkey's supporters acknowledge that it would be very difficult to "digest", being bigger than all existing members except Germany and poorer even than the poorest new member from eastern Europe.
Simon Taylor, the author of a new study on enlargement based on off-the-record interviews with European Commission officials, says many were "unpleasantly surprised" when Turkey moved closer in 2003 towards meeting the political criteria for membership.
"The reaction was one of 'Blow me, we never expected they would be able to do this'," he says.
"They thought they had set Turkey an impossible task. It casts some doubt on their sincerity, and calls into question whether Turkey will ever be able to join."
Currently, European leaders are far more enthusiastic about enlargement than the public they represent. Mostly they regard it as a hugely successful operation to consolidate democracy and stability in central Europe.
But Simon Taylor expects the difficulties of absorbing the 10 new members joining in 2004 to result in a change of mood.
"There will be a severe reality check because of the ambitious scale of the enlargement already committed to," he says.
"It will be a very difficult digestion process. These countries are going to say No to new environmental, social and employment legislation that increases labour costs and reduces competitiveness. The pace of initiatives will inevitably slow down."
He expects the EU to delay further expansion - but warns of a possible crisis if Ankara's reformers help to end the division of Cyprus, and are not then rewarded with the opening of membership talks.
Ultimately, Turkey, Bulgaria and Romania have a commitment from the EU, which it will be hard for anyone to tear up and throw away.
But the hopefuls who have not yet received a public commitment could be waiting for a long time.