Large section of the shuttle's wing found

Investigations raise new concern over radioactive risk from planned Mars missions

Nasa officials were hoping yesterday that the recovery of a "significant" wing portion of the Columbia shuttle will help them understand what caused a possible rupture in the spacecraft's exterior and led to its disintegration in the Texas skies last weekend.

The piece, found near Forth Worth at the western end of the debris field, includes 18in of wing structure and 26-27in of insulation from the wing's outer edge, the area most exposed to the searing 3,000 degree temperatures of atmospheric re-entry.

The new discoveries follow on from the unearthing of Columbia's nose from a wood in east Texas earlier last week. It remains to be seen whether the pieces are from the crucial left wing, where all available evidence suggests the break-up began.

Nasa has said that all manned spaceflights will be suspended until it can get to the bottom of the Columbia disaster and correct the fatal problem. Concerns about the safety of travel through the earth's atmosphere do not, however, appear to have derailed Nasa plans to launch two unmanned rockets loaded with plutonium in the next few months.

The Independent on Sunday has learned that Nasa has rejected pleas to suspend missions involving radioactive material, even though its own assessments reveal that the rockets are three times more likely to have an accident than the shuttle.

The two Mars explorer rockets, scheduled for launch from Cape Canaveral on 30 May and 25 June, will each contain small amounts of plutonium-238, which in the event of a launch failure could put tens of thousands of people at risk from radioactive fallout. Each probe will carry Radioisotope Heater Units (RHUs) containing 2.7 grams of the deadly isotope.

Nasa's own environmental impact statement acknowledges that there is a one-in-30 chance of a launch accident and a one-in-230 chance of an accidental release of radioactive material for each rocket. The accident rate for the shuttle was calculated at one per 100 launches.

The proposed use of nuclear materials has not generated significant debate in the US because media attention has been focused squarely on the Columbia investigation. For the past two days, television stations have shown and reshown military surveillance photographs from a base in New Mexico indicating a slight bulge in Columbia's left wing and a plume emanating from behind it. The head of the US shuttle program, Ron Dittemore, told reporters the photographs were certainly of interest but added: "All by itself I don't think it's very revealing."

The nuclear issue is nevertheless at the forefront of Nasa's thinking and endorsed at the highest levels of government. The Mars explorer rockets are to be followed by two further launches of deep space probes containing plutonium-238: the Pluto New Horizons project in 2008 and Mars Smart Lander the next year. Each are being designed to carry Radioisotope Thermoelectric Generators which use heat from plutonium to generate electricity.

President Bush, meanwhile, has revived plans for nuclear-powered rockets which were abandoned in the 1960s amid warnings that they could cause a global environmental catastrophe. Nasa proposals for a $2bn research and development grant for the nuclear spacecraft were confirmed by the White House last Monday, just two days after the shuttle disaster, although there has yet to be a formal announcement..

The $3bn programme, codenamed Project Prometheus – after the figure who stole fire from Zeus – is designed to be a 75,000mph rocket, which could put men on Mars. While the projects' initial objectives are ostensibly for scientific exploration, much of the impetus for funding comes from the Pentagon, which wants to launch weapons into space as part of its revived Star Wars initiative.

The push for nuclear power in space follows President Bush's appointment of Sean O'Keefe, an ardently pro-nuclear former defence chief, as head of Nasa. He has likened conventional rockets, which can reach 18,000 mph, to "exploring the old west in covered wagons." Shortly before the Columbia crash he said: "We're talking about doing something on a very aggressive schedule to not only develop the capabilities for nuclear propulsion and power generation but to have a mission using the new technology within this decade."

The last major accident involving radioactive material in space was in 1967, when a US satellite fitted with a nuclear-powered electrical generator failed to achieve orbit and burned up in the atmosphere, spreading radioactive fallout widely over Earth.