BERTHOLD, N.D. (AP) — As a nuclear missileer with his finger on the trigger of the world’s most powerful weapon, Air Force 1st Lt. Andy Parthum faces pressures few others know. He spends his workday awaiting an order he hopes never arrives: to launch nuclear-tipped missiles capable of killing millions and changing the course of history.

Parthum is one of 90 young airmen who carry out their mission not in the air but in a hole in the ground.

Across the northern tier of the U.S., pairs of missileers sit at consoles inside bomb-proof capsules 60 feet underground and linked to groups of Minuteman 3 missiles, a nuclear-armed weapon President John F. Kennedy dubbed an “Ace in the Hole.”

The missileers’ mission was born in the early years of the Cold War, when the threat of nuclear Armageddon was ever-present, yet it lives on despite the emergence of new threats like global terrorism and cyberattack and the shrinking of defense budgets.

The missileers have never engaged in combat, although the Air Force calls them combat crew members. Still, no one can exclude the possibility, remote as it may be, that one day a president will deliver the gut-wrenching order that would compel a missileer to unleash a nuclear hell that would alter world history.

“Absolutely, it weighs on your mind,” Parthum, 25, said on a recent afternoon at Juliet-01, a Minuteman 3 missile launch site on a small patch of prairie 9 miles from the village of Berthold and about 25 miles west of Minot Air Force Base, whose 91st Missile Wing controls 150 of the nation’s 450 Minuteman missiles.

It may come as a surprise to some that the Air Force still operates intercontinental ballistic missiles, or ICBMs. And therein lies part of the problem for missileers, who feel underappreciated in a military that has long since shifted its main focus.

Parthum, however, says he takes pride in his role and accepts its importance.

“It’s something that’s a little abstract, so that could be hard for people, I can see that,” he said. “But once you understand that you’re providing the backbone, the bedrock for United States nuclear deterrence then . it’s a lot easier to appreciate, I think.”

Parthum, a native of Centreville, Virginia, and his crewmate, 23-year-old 2nd Lt. Oliver Parsons of Shawnee, Kansas, showed visitors around the small launch control center where they were several hours into a 24-hour watch over a group of 10 missiles.

It’s a sometimes tedious duty the Air Force calls “standing alert.” Some say their biggest challenge is staying alert.

Missileers, typically 22- to 27-year-old lieutenants and captains, work in pairs, with a relief crew arriving every 24 hours. A missileer generally does two “alerts” a week. It was Parthum’s 118th. (He keeps track.)

It’s not hard to see why some missileers find it hard to adjust to life under the prairie. An 8-ton blast door seals their launch control center from a potential incoming nuclear detonation. Twice last year launch officers were disciplined after admitting they left the blast door open while a crewmate was asleep — a security violation. That and other lapses in discipline, training and leadership were documented by The Associated Press over the past year, prompting Defense Secretary Chuck Hagel to declare that “something is wrong.”

The ICBM launch control center is actually two separate structures. An outer protective shell is made of reinforced concrete lined with a steel plate. A smaller, box-like enclosure where the missileers work, eat and sleep is suspended inside the protective shell by pneumatic cylinders called “shock isolators” attached to the shell’s ceiling by heavy chains; the isolators are designed to keep the space stable in the event of a nuclear blast.

These underground command posts have changed relatively little since they were built in the early 1960s, although the Air Force recently committed to refurbishing them to make a missileer’s life a bit easier. Juliet-01, the command post an AP reporting team was permitted to visit, had just been repainted and spruced up to remove corrosion caused by water intrusion, giving it what one officer called “that new car smell.”

The launch center is accessible only from an above-ground building that resembles a small ranch-style home. An access shaft descends from a vestibule inside the building, which is controlled by a security team and surrounded by alarms and a chain-link fence.

Nuclear weapons duty is a deadly serious business, but it’s not without room for a pinch of missileer humor. A patch on the green leather seat from which Parthum monitors a computer console linked electronically to each of his 10 Minuteman 3 missiles offers these pithy phrases: “This Round’s On The House,” and “Party Til You Nuke.”

In fact, the U.S. has never fired an ICBM, other than for flight testing. Their stated purpose is to help deter nuclear war by convincing a potential attacker that it would have more to lose than to gain.

Upward of two-thirds of missileers were “volunteered” for the job after gaining their officer commission. Once they complete basic ICBM training at Vandenberg Air Force Base in California, they are sent on four-year tours to one of three missile bases: Minot, Malmstrom Air Force Base in Montana, or F.E. Warren Air Force Base in Wyoming.

The responsibility is enormous, the cost of mistakes potentially colossal, ranging from environmental damage to inadvertently triggering a nuclear war.

That is why the Air Force has long-established rules, procedures and backup safety systems to minimize the possibility of a major error. Over time, with the passing of the Cold War, the Air Force lost focus on its nuclear mission.

It also lost a good deal of what remained of the allure of serving as a missileer.

“Even during the Cold War while facing down the Soviets, it could be difficult to convince bright young airmen that what they were doing was worthwhile,” Robert W. Stanley II wrote in a research paper in 2011 before becoming vice commander of the 341st Missile Wing at Malmstrom. Last year he was promoted to commander there but resigned in March 2014 amid a scandal over exam cheating among his missileers.

In his paper, “Reviving a Culture of Disciplined Compliance in Air Force Nuclear Operations,” Stanley called for missileer incentive pay.

“In trying to demonstrate that nuclear duty is not a dying career field, and one worthy of top personnel,” he wrote, “no message could be more tangible than monetary reward.”

The Air Force is heeding that advice. Starting in October, it will offer entry bonuses to newly trained missileers, as well as “duty pay” for security forces, missileers and others who operate in the missile fields. A nuclear weapons service medal will also be offered as part of an intensified effort to make the career field more attractive.

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