Despite Canada's refusal to take part in the controversial U.S. missile defence shield, the United States will continue with its plans to deploy the controversial system.

Still in its early stages of development, the goal of the U.S. system is to put up a defensive net surrounding the U.S. that would target and destroy incoming intercontinental ballistic missiles (ICBMs).

President George W. Bush is also proposing a multi-national defence system that would cover allies willing to sign up for the program.

The history

Bush's plan is a simplified version of the complex system born in 1983 under former president Ronald Reagan's administration.

The Strategic Defence Initiative -- or Star Wars - under Reagan was designed specifically to protect the U.S. from a Soviet missile attack.

But Star Wars never got off the ground, and succeeding presidents shied away from placing the controversial program high on their agendas.

Bush, however, made missile defence a priority again after the attacks against the U.S. on Sept. 11, 2001.

On June 13, 2002, the U.S. pulled out of the 1972 anti-ballistic missile (ABM) treaty -- an agreement signed by the U.S and the Soviet Union to prohibit the development of a missile defence system.

"We now face new threats from terrorists who seek to destroy our civilization by any means available to rogue states armed with weapons of mass destruction and long-range missiles," Bush said in a statement.

'Son of Star Wars'

Bush called on Congress to fully fund his requested budget for missile defence, in order to meet what he called the "new strategic challenges of the 21st century."

Washington's current plan -- nicknamed "Son of Star Wars" -- would be unable to stop an attack on the scale that a nation such as Russia could mount.

Instead, it's geared more toward defending against 'rogue' states with limited missile capabilities, such as North Korea and Iran.

In May 2002, Bush and Russian President Vladimir Putin signed an agreement to reduce their countries' nuclear arsenals by about 65 per cent by 2012.

In addition, the two nations agreed to cooperate in joint research and development of missile defense systems.

What is it?

The nuts and bolts of the missile defence shield is comprised of:

• an early warning radar system;
• a command centre and missile interceptors (or kill vehicle) -- all based on land; and
• an infrared satellite system in space.

The satellite and radar systems work together to detect an incoming missile as soon as it clears cloud cover.

The satellite provides the missile's trajectory data to the command centre - which controls the entire operation.

The ground radars also track the missile, including any decoy missiles that may have been launched, and transmits the data to the command centre.

When it has enough data on the missile, the command centre relays target information to one or more of the ground-based interceptor (GBI) vehicles.

Each GBI can hold at least 20 interceptor rockets, which are equipped with infrared seeking devices and small on-board rockets to manoeuvre themselves into a collision with the missile.

The rockets are not designed to explode, but instead to destroy the missile with the force of a 120,000 km/h collision.

How effective is it?

Several tests have failed, including one conducted as recently as Feb. 14, 2005. A U.S. interceptor missile did not launch from the Ronald Reagan test site in the Pacific Ocean to stop a mock ICBM fired from Kodiak Island, Alaska.

There's a lot that has to go right in order to successfully intercept a fast-moving target, let alone several of them. It requires multiple shots, and the effectiveness of the rockets depends on pinpoint accuracy of the early warning systems and on-board seeking devices.

The Missile Defense Agency says the cause of the recent failure is still under investigation.

A spokesman for the agency, Rick Lehner, told CTV.ca that early indications point to a malfunction with the ground support equipment, specifically the missile silo, at the test range on Kwajalein Island, and not with the interceptor missile itself.

The previous test, on Dec. 15, 2004 also failed. Lehner said on that occasion there was a problem in the rocket itself.

Previously, the program had gone five-for-eight in attempts to intercept a target.

No date for the next test has been announced. It's unclear how continued test failures would affect two experimental interceptor bases in Alaska and California.

The critics

According to some, the failures of these high-profile, $85 million test launches are significant setbacks.

John Pike, director of military intelligence thinktank Global Security, said the missile defence program is too costly and ill-conceived.

"I don't think the anti-ballistic missile systems will ever work well enough. But the Bush administration is prepared to push ahead …. "

"We're no closer to deployment than during the Reagan administration," Pike said.

He added that the U.S. government has spent more on space-based and land-based anti-missile systems than it has on other more successful defence projects, including the Stealth bomber.

Ted Postol, a professor of science and national security studies at M.I.T., has criticized the missile defense system's ability to distinguish incoming missiles from decoys.

In a letter to the Pentagon, a copy of which he gave to The New York Times, Postol said data he had obtained from the first antimissile test flight in June 1997 showed that the ground-based interceptor was unable to make the distinction, and he even added that the Pentagon and its contractors had tried to hide this failure.

The coverup, he said, was "like rolling a pair of dice and throwing away all outcomes that did not give snake eyes."

What other countries could take part?

Currently, Japan, Australia, South Korea, and Israel are working with the U.S. on the system.

The U.S. has not yet won the support of NATO to pursue the project. Many European countries are worried about the U.S.'s pursuit of a unilateral defence policy, and that this might provoke Russia.

The Chinese have also threatened to boost their own military deployments if the missile defence system is given to South Korea, Japan or Taiwan.