The US still keeps hundreds of nuclear weapons on hair-trigger alert - here's what it means and why it's a huge risk

• Since the Cold War, the US has kept an estimated 900 nuclear weapons on "hair-trigger alert," meaning the president could decide to launch them in just a few minutes.
• The policy reportedly wasn't discussed during Trump' s meeting with North Korean leader Kim Jong Un on Tuesday.
• Some global security experts and scientists say that hair-trigger alert endangers American cities, because the US could inadvertently start a nuclear war.
• A senior scientist from the Union of Concerned Scientists proposes turning the missiles' hair-trigger alert setting off as a temporary solution.

At a highly anticipated summit in Singapore Tuesday, US President Donald Trump and North Korean leader Kim Jong Un discussed the future of the two nations' nuclear arsenal programs. They ultimately agreed to work toward the complete denuclearization of the Korean Peninsula.

Leading up to the summit, Trump argued that North Korea's highly-secretive nuclear program threatens American security. After the meeting, a joint statement said the two countries would "join their efforts to build a lasting and stable peace regime."

While the meeting seemed optimistic, global security experts say there is another domestic nuclear policy that the Trump administration reportedly did not address - and one just as dangerous as North Korea initiating nuclear war with the US.

Since the end of the Cold War, the US has had 450 land-based missiles and hundreds more missiles undetectable submarines that are all on "hair-trigger alert" - a policy that allows for the launch of nuclear weapons in 10 minutes. Only the president's permission is required to launch these weapons, according to the Department of Defense.

But a growing number of experts believe the US should consider retiring this policy. The Union of Concerned Scientists (UCS), an advocacy group comprised of hundreds of scientists, engineers, and economists, published a report in 2016 on the dangers of hair-trigger alert. The same year, more than 90 prominent American scientists, including 20 Nobel laureates, sent a letter to President Obama that urged him to take the nation's land-based nuclear missiles off hair-trigger alert.

Lisbeth Gronlund, co-director and senior scientist at the UCS Global Security Program, argues that keeping nuclear weapons on hair-trigger alert endangers American cities and towns more than it protects them.

"The president would have 10 minutes to make a decision whether or not to launch the weapons," she told Business Insider. "That's a tremendous amount of time pressure."

The argument for being able to launch 900 nuclear missiles in under 10 minutes

Besides the US, Russia is the only other country known to have a hair-trigger launch status. It's unclear whether Israel, China, and North Korea have their own hair-trigger alert policies, which means that Kim could theoretically have the same nuclear power as Trump.

The US land-based arsenal is siloed in five states, including Nebraska and Colorado, hundreds of miles away from residential communities. It's believed that in total, the US keeps roughly 900 weapons on hair-trigger alert. The locations and exact number of submarines, located in the deep ocean, are confidential.

The US military also keeps around 40 special missiles, called interceptors, on hair-trigger alert as well. It would take three or four of these to ram into and destroy foreign missiles in outer space - in a protocol dubbed "hit to kill." When nuclear missiles are launched into space, they follow a more predictable path, making it easier for interceptors to strike them. (It's still unclear how well these missiles would work in an emergency situation.)

Across the US, the military has installed infrared and satellite sensors that can detect the hot gas a missile expels as it flies through the air. If Russia launched one of these long-range weapons, it would take about 25 minutes to reach the continental US, Gronlund said, and it would take the sensors just a few minutes to identify them.

As the sensors track each missile's movement, an automated system estimates whether or not it's a legitimate attack on the US. The president then has about 10 minutes to decide whether to launch nuclear weapons.

When the US adopted the hair-trigger alert policy during the Cold War, the idea was that if the Soviet Union attacked American military bases with nuclear power, the US could quickly retaliate.

The policy is still in effect today. These days, it's used as an intimidation strategy for other countries, like Russia and North Korea, that have revealed some of their nuclear capabilities, Gronlund said.

Some proponents of hair-trigger alert also say that the US needs those land-based missiles to act as "sponges" for foreign nuclear weaponry. For example, if Russia launched a full attack, it would need to use hundreds of weapons to destroy the US land arsenal, Gronlund said. That means it would have fewer weapons left to strike where people actually live.

Others say that abandoning hair trigger alert would cut jobs at the military bases that monitor the missiles (which could have consequences on local economies).

But USC scientists argue that the risks of false alarms still far outweigh the advantages.

The US has had several close calls with hair-trigger alert weapons

If the US experiences a nuclear attack, it probably won't come from North Korea or Russia, Gronlund said. It will come from our own missteps. (A large-scale nuclear terrorist attack is less likely, but still one of 15 major disaster scenarios planned for by FEMA and other US agencies.)

"The danger is that a system detects an incoming attack, and it's not real, the president makes the wrong decision, and inadvertently starts a nuclear war," she said.

The US has a history of these kinds of close calls, caused by both human error and technical error.

In November 1979, US military computers indicated a large-scale Soviet attack was underway. The military quickly responded by preparing nuclear bombers and intercontinental ballistic missiles (ICBMs) for take-off, and launching at least 10 interceptor planes.

But within 10 minutes, they realized it was a false alarm. Investigators later discovered that a technician accidentally inserted a training tape containing a drill for a nuclear attack into a computer.

In another close call in 2010, a rapid-launch control center at Wyoming's Warren Air Force Base lost contact with all 50 of its ICBMs on hair-trigger alert. For nearly an hour, the crew was unsure whether the missiles - carrying nuclear warheads - would accidentally launch. (The base later regained contact with them.) The cause of the problem was later found to be an electronic circuit card malfunction in one of the computers.

Most recently, in January 2018, a state employee in Hawaii mistakenly triggered an emergency alert warning that a ballistic missile was coming for the island. If the state had not realized that it was a false alarm, other bases containing high-alert missiles could have unleashed them.

"If we launched an attack based on a false warning, you better believe Russia or North Korea would respond," Gronlund said.

A danger to cities across the US

Even though the land-based missiles on high alert are siloed far from major population centers, Gronlund said that keeping such a policy still endangers cities, where over 80% of Americans live today. Simply put, hair-trigger alert increases the likelihood of the president accidentally starting a nuclear war.

"Even if [another country] wasn't directly targeting Boston, they would target things near Boston," causing a widespread nuclear fallout, she said.

Each land-based missile on hair-trigger alert has an on-off switch, which is used when workers perform maintenance on them. Gronlund said that keeping the weapons off could pose a quick, temporary solution that doesn't immediately require negotiations with other nations.

The UCS has become more worried about hair-trigger alert policy since Trump's election, Gronlund said. In the past year alone, the president has boasted about the size of his "Nuclear Button," dropped the nuclear deal with Iran, and threatened to unleash "fire and fury like the world has never seen" on North Korea.

"There are all downsides and no gain," Gronlund said.