Seth Shostak

A factory that can turn carbon nanotubes into a sheet a yard wide and long enough to stretch one-fourth of the way to the moon is not something you'll find at your local industrial park. That's the show-stopper for the space elevator. The ribbon.

It bears mentioning that the Milky Way is only one of 150 billion galaxies visible to our telescopes - and each of these will have its own complement of planets.

Sending greeting cards to aliens is hardly a new idea. In 2005, Craigslist solicited messages for broadcast to space by a transmitter in Florida, and in 2008, NASA beamed a Beatles song to the North Star (Polaris), on the assumption that any putative Polarians would appreciate the Fab Four's 1960s-genre compositions.

The principal reason for the universe's poker face is that its constituents are far away. Stars careen through space, and galaxies spin at speeds thousands of times faster than a jet plane. But given their distance, you'd need the patience of Job to notice much change in their appearance or position.

The stars look the same from night to night. Nebulae and galaxies are dully immutable, maintaining the same overall appearance for thousands or millions of years. Indeed, only the sun, moon and planets - together with the occasional comet, asteroid or meteor - seem dynamic.

Each year, thousands of UFOs are sighted and reported, which is an impressive tally of unidentified aerial phenomena. Surveys show that roughly one-third of the populace believes that at least some of this sky show is due to extraterrestrial spacecraft, here to probe our airspace and, when that proves boring, our bodies.

Plate tectonics is not all havoc and destruction. The slow movement of continents and ocean floors recycles carbon dioxide dissolved in the oceans back into the atmosphere. Without this slow speed carbon cycle, Earth's temperatures would cool dozens of degrees below your comfort zone.

If you could drive straight down, into a tunnel bored through the crust of the planet, you'd hit this molten mess in about an hour. It's called the asthenosphere - a sluggish sea, several hundred miles thick, on which floats the Earth's cool epidermis - the so-called tectonic plates.

Like prospecting in the 19th century, reconnaissance of the asteroids would of necessity take place in an arena where trouble is likely and help is distant. Heroic stories of individual triumph and failure, set on landscapes never seen by humankind, are in the cards.