Reeling In Satellites

By Author Here a cheap way of bringing failed satellites down to Earth is being developed by an American company. The company, Tethers Unlimited, which is based in Clinton, Washington, has booked a launch on a Russian rocket in late 1998 or early 1999 to test its special tether.

In theory, satellites should be able to generate electricity from a long tether. But when NASA launched a satellite to test the idea in 1996, the tether was severed after just six hours, probably by a small piece of space junk. The new tether has a web-like structure to enable it to withstand damage from space junk and the company plans to use it to brake an ailing satellite, causing it to lose height and fall to Earth.

Over the next few years, hundreds of satellites will be injected into low-Earth orbit for mobile phone communications. "Iridium, one of the consortia involved, has already had two failures in the 33 satellites it has launched," says Robert Forward of Tethers Unlimited. "There is an urgent need to remove failed satellites from orbit to make way for replacements."

Using a rocket to take a satellite out of orbit would mean carrying extra fuel into space equivalent to between 5 and 15 per cent of the satellite's mass. The fuel needed to launch this extra mass would add several million dollars to the launch costs.

The tether, which would be up to 10 kilometres long, will generate electric currents of up to 1 amp as it cuts through the Earth's magnetic field. It is the interaction between the current in the tether and the Earth's magnetic field that would brake a satellite. Forward says that a tether would only account for 2 per cent of the satellite's mass and would be able to replace a rocket for about a tenth the price.

The company's Terminator Tether is a multiline cable with a ballast mass of a few kilograms on one end. The tether would be unwound like a bobbin in a cotton factory. It would be pulled taut by the differences in gravity at different altitudes.

According to Forward, once a tether had been unwound it would take a satellite out of a 780-kilometre high orbit in less than a year. It would take a century for the satellite to come down naturally.

The design of the tether, patented by Forward's partner, Robert Hoyt, consists of three parallel lines with crossed wires zig-zagging between. Unlike a single strand, which is vulnerable to being severed by just one projectile, this webbing can resist the bombardment of meteorites and space junk.

Tethers Unlimited, which has a $600 000 contract from NASA to explore the concept of a tether, has hired a Scottish knitwear firm Fleming's Textile of Kilmarnock to develop the design. "We won't have our testing done in time for Iridium," says Forward. However, he says the company will be ready for the next generation of low-orbit satellites.

From New Scientist, 21 February 1998