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Q. How much junk is there in space?

A. As debris grows, there are collisions, creating more debris. Space debris is often associated with dead satellites and spent rocket stages. There are about 7,000 large objects in orbit, mainly between 300 and 500 miles high. About 2,000 pieces are payloads, of which only 5 per cent are operational. There are also 40,000 smaller bits and pieces, mainly the debris of exploded rockets. Then there are more than 3 million particles such as flakes of paint, specks of insulation and dust from solid propellant exhaust. The Mir space station and the Space Shuttle have been hit by flakes of paint which have - at a speed of more than 18,000mph - pitted the windows.

Q. How does a fax travel?

A. When you feed a piece of paper into a fax machine, the machine scans it in a series of very thin lines. Each of these lines is converted into a series of ones and zeros. The machine then transmits this code down the phone line with audible high and low notes. At the other end, another fax machine simply reverses the process and prints the thin lines on to a sheet of paper.

Q. What signals do TV detector vans detect?

A. The frequency of broadcast television signals is between 500 and 800 megahertz. Television sets must tune to these frequencies to pick up a picture. To do this accurately is difficult. Instead, a local oscillator that scans between 100 and 200 megahertz is used and then the television set adds 500 megahertz to that signal. TV detector vans detect the local oscillator signal. The only way to stop them picking it up is to cover your TV set completely in tin foil, making it impossible to see the screen.

Q. Why can't some children's toys and smoke alarms use rechargeable batteries?

A. Sensitive battery-powered equipment has been designed around a very specific set of battery specifications. In other words, they expect a particular voltage/current/resistance from a battery. Rechargeable batteries, by their very nature, vary in specification according to age/length or recharging, etc.

Q. When you see shots of the Saturn V rockets taking off, there is always white stuff falling from the sides. What is it?

A. The white stuff is ice. The fuel used by the rockets is liquid hydrogen and oxygen. To keep it liquid, the fuel must be very cold. Water vapour from the air condenses and freezes onto the outside of the rockets and is shaken loose as the rocket takes off.

Q. Why do clouds float?

A. Clouds are made up of tiny water droplets. These droplets fall to the Earth with different speeds, depending on their size. The small droplets are so small that turbulence in the air holds them up. When droplets in the clouds get too big to be held up, they fall to the ground - as rain.

These questions and answers are provided by Science Line. You can use its Dial-A-Scientist service on 0345 600444.