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Years before North Korea fired its first intercontinental ballistic missile, the Pentagon and intelligence experts had sounded a warning: The North was making progress quickly and spy satellite coverage was poor, meaning that the United States might not see a missile being prepared for launch. That set off an urgent search for ways to improve America’s early-warning and early missile striking capability.

The most intriguing solutions have come from Silicon Valley, where the Obama administration began investing in tiny, inexpensive civilian satellites developed to count cars in parking lots and monitor the growth of crops. Some in the Pentagon accustomed to relying on highly classified, multibillion-dollar satellites, which take years to develop, resisted the move. But as North Korea’s missile program progressed, American officials laid out an ambitious schedule for the first of the small satellites to go up at the end of this year, or the beginning of next.

According to the New York Times, the satellites would provide crucial coverage necessary to execute a new military plan called “Kill Chain.” It is the first step in a new strategy to use satellite imagery to identify North Korean launch sites, nuclear facilities and manufacturing capability and destroy them pre-emptively if a conflict seems imminent.

“Kim Jong-un is racing to deploy a missile capability,” Robert Cardillo, the director of the National Geospatial-Intelligence Agency, which coordinates satellite-based mapping for the government, said in an interview “His acceleration has caused us to accelerate.”

The timeline for getting the satellites in orbit, reflects the urgency of the problem. The recent missile launch by North Korea was operated from a new site and according to Capt. Jeff Davis, a Pentagon spokesman, the missile was of unfamiliar design. That mobility is the problem that the new satellites, with wide coverage using radar sensors that work at night and during storms, are designed to address.

Kill Chain was also mentioned in a joint statement issued by the United States and South Korea, a notable shift for the South’s new president, Moon Jae-in. He has rejected public discussion of pre-emptive military action, arguing it plays into the North Korean paranoia that the United States and its allies are plotting to end the Kim government.

Mr. Moon has spoken of reviving direct talks, which he advocated as chief of staff to an earlier South Korean president.

But Trump has tried to build pressure, using warships, sanctions and missile defenses. “The threat is much more immediate,” H.R. McMaster, Mr. Trump’s national security adviser, said recently. “So it’s clear that we can’t repeat the same failed approach of the past.”

Raj Shah, the director of the Defense Innovation Unit Experimental, or DIUx, is already investing in companies that exploit tiny civilian radar satellites, able to operate through darkness or storms, in hopes that the Pentagon can use them by the end of the year, or early in 2018. “It’s a very challenging target,” said Mr. Shah, a former F-16 pilot in Iraq whose extensive experience in Silicon Valley appealed to Defense Secretary Ashton B. Carter, who set up the unit during Obama’s second term and recruited Mr. Shah. “The key is using technologies that are already available, and making the modifications we need for a specific military purpose,” Mr. Shah said. His unit made an investment to jump-start the development efforts of Capella Space, a Silicon Valley start-up. It plans to loft its first radar satellite late this year. The company says its radar fleet, if successfully deployed, will be able to monitor important targets hourly.

“The entire spacecraft is the size of a backpack,” said Payam Banazadeh, founder of the company. Once in orbit, the payload, he added, would unfurl its antenna and solar panels.

“Everything is getting smaller,” Mr. Banazadeh said of the craft’s parts. “Even the next version of the satellite is getting smaller.”

The threat of not noticing missiles grew worse last year as North Korea began using solid fuels after decades of relying on liquid propellants to power its big rockets and missiles. While liquid-fueled missiles can take hours or even days of preparation, solid-fueled missiles can be fired with little or no warning. Kim has made the effort a personal project, posing next to a large solid-fueled motor after a successful experiment test last year. The North followed that firing with four successful flight tests, twice last year and twice this year.

The key to detecting launch preparations is the near-constant presence of satellites that can detect the movement of military gear, including missiles. That requires space-based radars, which over the years have been highly expensive, with their big antennas and tendency to use large amounts of power. Like any radar, they fire radio waves at targets and gather faint echoes.

In 2007, the Congressional Budget Office estimated that a constellation of 21 radar satellites would cost the nation up to $94 billion — or more than $4 billion each. A report, published shortly after the North’s first nuclear detonation, zeroed in on whether the satellites could track Korean missiles on mobile launchers. It called the goal “highly challenging,” and said 35 to 50 spacecrafts would be needed to make such detections rapidly. The new generation of tiny, cheap satellites has made that outcome more achievable.

In addition to Capella, private companies rushing to make and exploit new generations of small radar satellites include Ursa Space Systems in Ithaca, N.Y.; UrtheCast in Vancouver, Canada; and Iceye in Espoo, Finland. Like many new companies seeking to make small satellites, most have strong ties to Silicon Valley.