By Geoff Brumfiel

For the first time in a decade, a worldwide ban on nuclear testing could be within reach. The combination of a strong commitment from US President Barack Obama, along with new data on nuclear materials and the successful completion of a global nuclear-monitoring network, means that momentum is once again swinging in favour of the Comprehensive Test Ban Treaty (CTBT) that would ban all nuclear explosions for military or civilian purposes.

At the end of this week, US Secretary of State Hillary Clinton will lead a delegation to the United Nations in New York City to discuss the treaty. The delegation will be the first from the United States in a decade, and scientists and other non-proliferation advocates hope that it will reinvigorate the negotiation process. "This will be a different kind of event with the [United States] back at the table," says Oliver Meier, a Berlin-based representative of the Arms Control Association, a non-profit group that provides analysis of arms-control measures. "I think we're as close to entry into force as we've ever been."

Described by former US president Bill Clinton as the "the longest sought, hardest fought prize in the history of arms control", the CTBT is seen as the next major step in reducing the threat from nuclear weapons. Proponents argue that a global ban on nuclear testing will prevent nations from obtaining nuclear capabilities and halt the further development of warheads in those countries that have them. A total of 149 nations have already ratified the treaty, but several countries -- including China, Israel, Iran, North Korea and the United States -- are holding out. Until these and other key states ratify the pact, it is not binding.

Key tests

The United States is seen as a linchpin in the process, according to Meier; if it ratifies the CTBT, then other nations such as China, India and Pakistan may feel more pressure to do so. Clinton signed the treaty in 1996, but three years later, it fell well short of the 67 votes needed to ratify it in the Senate. The failure was in large part political, according to Jenifer Mackby, a fellow at the Center for Strategic and International Studies, a think tank based in Washington DC. Clinton was embroiled in a scandal with Monica Lewinsky, and opponents including Senators Jon Kyl (Republican, Arizona) and Trent Lott (Republican, Mississippi) spread doubt about whether the treaty would work.

Politics may have felled the treaty, but the arguments of opponents were technical. Doubters warned that the United States might need to return to nuclear testing in order to verify its ageing stockpile of weapons. They also claimed that it would be possible for nations to cheat by hiding the seismic signatures of a nuclear blast from a global monitoring network. At the time, scientists had little more than calculations to rebut these criticisms: a voluntary moratorium on US nuclear testing was only seven years old, and a global monitoring network had not yet been built.

"The process of monitoring has notably improved in the past ten years," says Paul G. Richards, a seismologist at Columbia University in New York, who helped carry out a review of the CTBT monitoring network for the US National Academy of Sciences in 2002. Perhaps most importantly, this network successfully detected two small nuclear tests from North Korea that occurred in October 2006 and in May of this year. In both cases, the seismic network picked up evidence of the explosion within minutes.

Ageing bombs

The scientific understanding of the US nuclear stockpile has also improved dramatically. Studies of ageing nuclear materials and computer simulations have shown that the stockpile is reliable and can remain so for decades to come without testing, according to Sidney Drell, a physicist at the Stanford Linear Accelerator Center of the National Accelerator Laboratory in California.

That doesn't mean that technical objections won't be raised this time around. Some US weapons scientists are already objecting to the treaty, warning that it could hurt the country's nuclear readiness in the long term. "Scientists are going to do what scientists do, they're going to raise their own individual concerns, and I welcome that," says Thomas D'Agostino, the administrator of the National Nuclear Security Administration, the independent agency that oversees the US nuclear stockpile.

D'Agostino says that the Obama administration recently asked the National Academy of Sciences to update its largely positive 2002 assessment of the CTBT (see 'Test-ban treaty 'scientifically sound''). That assessment should be ready early next year, when the Senate debate on the treaty is expected to heat up.