The Sentinel modernization program is not only “absolutely necessary” but “might not be sufficient” for Europe’s defense, a Department of Defense official said last week at a virtual discussion after the NATO summit in Washington.

“We cannot afford not to proceed with Sentinel,” Vipin Narang, Department of Defense acting assistant secretary of defense for space policy, said in a virtual forum on NATO’s nuclear posture hosted by the Washington-based Center for New American Security think tank.

Narang added that since the Sentinel program was sized for a “New START Russia” and for China before it “embarked on its sprint,” he thinks any cost-overrun projections for Sentinel would be incorrect.

The Air Force disclosed in January that the Sentinel program was facing a Nunn-McCurdy breach, which happens when a DOD program’s costs exceed early estimates by 25%. The Sentinel program’s current unit cost is $214 million, compared with its original cost estimate of $118 million and its January cost of $162 million.

Narang said Sentinel did not have a cost overrun but a cost-projection overrun. He also said that while DOD will have to continue with Minuteman III — the current intercontinental ballistic missile (ICBM) used in the nuclear triad — longer than originally planned before transitioning to Sentinel, alternatives “may sound good in theory” but would not work due to either being more expensive or not as prompt.

“We cannot afford not to have an ICBM leg [of the nuclear triad],” Narang said, calling the “totality” of the triad “irreplaceable.”

“The virtue of the ICBM leg the way the U.S. has developed… it’s the most prompt leg of the triad, and that can’t be replicated by other legs of the triad,” Narang added. “And, you know, it complicates adversary decision making.”

Narang also said that as NATO is in the middle of transitioning F-35s, the Dutch and the U.S. would have the first nuclear-certified F-35s and the Dutch have “really led the way in the alliance.” He said that both the 5th generation F-35s and the modern B61-12s nuclear bombs are “a big advance to NATO’s nuclear posture writ large.”

Narang also said that the DOD is proceeding with the nuclear-armed sea-launched cruise missile as prescribed in the NDAA.

“Future NATO capabilities are a discussion for the alliance, and the alliance needs to have those discussions,” Narang said.