David Whitehouse: What other mysterious craft are silently circling above us?

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The Independent Online

We don't know very much about the US Air Force's X-37B, and its sudden appearance has surprised many space watchers. It is certainly unusual for a spacecraft that is supposed to be on a "technology development mission" to materialise in an orbit that is normally reserved for spy satellites. And while the amateur space watchers who spotted it did so with relative ease, the layman could be forgiven for wandering exactly what else might be up there that we don't know about.

Whatever other celestial secrets are out there, they are well-hidden among a remarkable array of other orbiting craft, governmental and commercial alike. And just over fifty years since the first artificial satellite was launched, our world has come to rely on them. Gone is the excitement that heralded the launch of Telstar, the first direct relay communications satellite, in 1962.

Today we take space communications for granted. We have satellite TV and acknowledge its provenance in its name, but we are usually unaware if our telephone, text messages, email or internet connections use satellite links.

The way space has been woven into everyday life is remarkable. In peace or war, space is the commanding high ground.

Not so long ago detailed images of the earth's surface taken by satellites were among the best kept secrets in the world. The United States in particular developed a series of classified spy satellites. Hollywood suggests they could read newspaper headlines from orbit. That is an exaggeration, but only just: they could certainly see if you were reading a tabloid or a broadsheet.

Every minute of every day pictures of sensitive areas were beamed to an anonymous building 30 miles west of Washington, DC for analysis at the US National Reconnaissance Office, top secret until 1994. Radio-listening spysats sift the electromagnetic spectrum when overflying enemy territory, and send signals to computers that extract military and civilian conversations and data transfers.

These days the military has competition. Although not as detailed as secret spy satellites, some commercial organisations have built craft that take remarkable images from space for clients that range from land use managers to building regulators. Some of the images have found their way on to Google Earth. Your house is almost certainly there.

Satellites have revolutionised science, too. We can monitor the ocean surface to a few millimetres. A pair of satellites can detect the minute change in a giant ice sheet's pull of gravity as it flexes and changes shape. And lest anyone think the skies are too full for any further extraordinary additions, consider the satellite that Japan launched last week – one of the strangest yet. Ikaros will pass by Venus in a few months and then voyage to the far side of the sun.

Along the way it will unfurl a series of ultra-thin metallic sails that will billow slightly as sunlight strikes them with a tiny but unremitting pressure. Scientists hope that this effect will one day provide a small probe with enough of a boost to leave our solar system altogether and head for the nearest star.

By then, the X-37B will be long out of service. But there seems little doubt that equally remarkable satellites will be following in its footsteps.

Dr David Whitehouse is a space scientist and author whose latest book is: One Small Step: The Inside Story of Space Exploration