Japan has deployed missile batteries in its heaving capital to defend the 30 million people who live in the greater Tokyo area from any North Korean attack.

Here are some key facts about Japan's ability to protect itself.

WHAT IS JAPAN'S MILITARY CAPABILITY?

Japan's Self Defense Forces have a total of approximately 250,000 full time service personnel. As of March 2012, Japan had 143 military vessels and 420 combat aircraft. It spends around $50 billion on its military every year, equivalent to around one percent of Gross Domestic Product.

The military is well-equipped, well-trained and makes use of technology.

Japan is also host to around 47,000 US military service personnel, who have with them a large amount of military hardware.

South Korea has deployed two Aegis destroyers, one off each coast, equipped with advance radar warning systems to track any missile launch.

WHAT HAS JAPAN DONE TO DEFEND AGAINST A NORTH KOREAN MISSILE?

Four Aegis-equipped destroyers routinely ply waters around the archipelago. Presently, two of them are in the Sea of Japan (East Sea).

There are 16 Patriot Advanced Capability (PAC)-3 firing units based in Japan's four different regions. The number of individual PAC-3 launchers is reported to be 28. The US forces in Okinawa are reported to have a further 24.

In anticipation of Pyongyang's launch, four PAC-3 batteries were moved to the Defense Forces headquarters in Tokyo and three other ground force camps surrounding the capital.

However, some commentators warn that North Korea's most recent missile technology allows them to launch from the back of a moving vehicle. This could make any incoming North Korean missile more difficult to intercept because it is harder to calculate a trajectory.

IN WHAT CIRCUMSTANCES WOULD JAPAN TRY TO SHOOT DOWN A NORTH KOREAN MISSILE?

Japan's law allows its military to shoot down a North Korean missile in the air over its territory or over the high seas when it is headed toward its territory and feared to endanger lives or property.

Analysts say Japan is unlikely to be the target of any launch, but that the technological or mechanical failure of a North Korean missile intended to be fired into the Pacific might mean a missile -- or parts of it -- could fall on the country.

It is likely this eventuality that Tokyo is guarding against.

WHAT HAS HAPPENED DURING PREVIOUS NORTH KOREAN LAUNCHES?

North Korea fired a rocket without warning on August 31, 1998, in what it said was an attempt to put a satellite into orbit. The first stage landed in the Sea of Japan and the second crossed Honshu island and fell into the Pacific.

That launch led to the 2003 decision to deploy Aegis destroyers equipped with sea-based SM-3 interceptor missiles and the land-based Patriot surface-to-air missiles.

On March 27, 2009, Japan's defence minister issued a shoot-down order for the first time in response to North Korea's preparations to launch a modified rocket.

The rocket blasted off on April 5, flying harmlessly hundreds of kilometres (miles) above Japan's northeast. Japan's defence forces did not try to intercept it.

On March 30, 2012, another shoot-down order was issued when North Korea said it was launching a satellite, however the long-range rocket disintegrated soon after lift off on April 13.

A third shoot-down order was issued on December 7, 2012, as North Korea readied a rocket that flew south over Okinawa five days later.

The rocket passed over the southern island chain outside the range of Japanese SM-3 and PAC-3 interceptor missiles, which were not launched.

Japan's defence minister issued the latest shoot-down order on April 7.

WHAT IS LIKELY TO HAPPEN THIS TIME?

Toshimitsu Shigemura, a Korean affairs specialist and professor at Tokyo's Waseda University, said North Korea is not aiming the rocket at Japan.

"Japan has the right under international law to shoot down a missile which is passing over if it is launched without warning. North Korea may criticise Japan for such an action but may not do anything further," he said.

Masao Okonogi, an honorary professor at Keio University, said North Korea may be aiming the rocket toward Guam or Hawaii.

"There is a remote possibility that part of the missile will fall over Japan. Such debris will burn up on re-entry so it is very unlikely that anything will actually hit Japan.

"If, however, the rocket is fired toward Japan, it is clearly military action and it is inevitable that Japan will try to shoot it down."