Airships may not be green flight of fancy

Because environmentalists tend to have a feeling for material constraints, we recognize that solutions cannot be conjured out of thin air. In some cases they just don't appear to exist.

There are two reasons why we make such a fuss about flying. The first is that, even as governments promise to cut emissions, everywhere airports are expanding. In Britain, the government expects the number of airline passengers to rise from 228 million in 2005 to 480 million in 2030. Before long, there will scarcely be a patch of sky without a jet in it.

The other is that there are no alternative means of propelling people through the air which are not more destructive than burning ordinary aviation fuel. Or so we think.