Astronauts aboard the space shuttle Discovery will conduct an unprecedented spacewalk this week to try to fix damage to the heat shield that protects the craft from burning up on re-entry. NASA experts have discovered that material used to fill the gaps between Discovery’s heat-resistant tiles has been dislodged in two places and is protruding about two centimetres. The fear is that the material may disturb the flow of air as the shuttle re-enters Earth’s atmosphere, causing a dangerous heat build-up beneath the craft – a problem similar to the one that doomed the Columbia mission. Tomorrow one of the astronauts will venture beneath the shuttle and try to cut away the protruding pieces. Such a repair has never been attempted on a shuttle before, nor has an astronaut ever gone beneath the craft on a spacewalk. But with the Columbia disaster on everyone’s minds, NASA has decided it cannot afford to risk leaving the damage unrepaired.