Meteors are ephemeral. They will usually vanish before you have a chance to point them out

to somebody else. This makes them suitable for starry-eyed lovers to wish upon, but modern technology can put shooting stars to more profitable use. Next time you see one, bear in mind that a dispatcher may be using it to help him marshal a fleet of long-distance lorries.

To human eyes, a meteor is beautiful. To a radio wave, it is just another thing to bounce off, and bouncing radio waves off the sky is not new. Left to themselves radio waves travel in straight lines, which limits their range. To get them round corners, and over the horizon, they need something to bounce off. In the ionosphere—the uppermost level of the atmosphere—the sun's rays break down molecules into positively charged ions and free electrons. These can reflect (and refract) radiation. The ionosphere let Marconi and his contemporaries send radio messages over long distances.

When a pebble falls from space into the atmosphere, moving at tens of kilometers a second, it gets rid of a lot of energy. Like the energy from the sun's rays, this ionizes the molecules of the atmosphere. The meteor's 10—20km path is densely packed with ions. By the 1930s, radio waves bounced off meteor trails had been used by scientists to determine the speed, height and direction of meteors.

The obvious disadvantage of meteors—the fact that they are so transient—might suggest that bouncing radio waves off their trails would remain the preserve of scientists. In overall quantity, though, meteors bid fair to make up what they lack in constancy. On an average day there are a million reasonable-sized ones (one gram), 400 million smaller ones (one-hundredth of a gram), and 160 billion even tinier ones (one ten-thousandth of a gram).

Meteors also have advantage. The greater density of ions in a meteor trail makes it less susceptible to the many things which perturb the ionosphere, and hence the quality of radio signals that bounce off it—such as time of day, weather conditions, sun spots or indeed intrusive meteors. This immunity from "noise" matters to people who want to send digital data. Radio hams may enjoy the tribulations of chit-chat through adversity and static, but such a noisy medium is not good for transmitting error-free sequences of 0s and 1s. That is why meteor-burst communication (MBC) comes into its own when small amounts of data need to be gathered from many places fairly quickly.

A system under construction to monitor the flow of the Nile provides an example. A master transmitter sends a radio "probe" into the sky in roughly the direction of the target. When a conveniently aligned meteor materializes, the probe bounces off it and reaches the receiver. When the receiver hears its master's voice it responds along the same path, spurting out data about the river's recent behavior. The master station acknowledges receipt, gives any further instructions and signs off.

It then directs its probe towards the next of the 250 outstations. Depending on the system's sensitivity, the wait between suitably aligned meteors varies between four seconds and ten minutes. The bursts of communication between master and out-stations may take as little as tenth of a second. It must be completed in the second it takes for the meteor's trail to dissipate.

In America, Meteor Communications of Kent, Washington, is the biggest and oldest of the MBC companies. It has provided meteor-burst equipment for 14 years. Its devices have been planted along the Chinese-Russian border to send short encoded messages back to Beijing. Other systems in Argentina, Australia, Canada, Indonesia, South Africa and Europe have been set up to monitor a variety of things, solar radiation, tides, water supplies, motorway fog, snow conditions and the like.

The military applications are clear, remote unmanned stations could sense approaching enemy ships, a

A．mysterious

B．transient

C．unique

D．wired