The Trump Administration is expected to release a long-awaited assessment of the U.S. defense industrial base next week. Early indications are that it will contain disheartening details about how America is losing ground across a wide range of high-tech industries. One such sector is space, the industry that builds satellites and provides launch services to get them into orbit.Superficially, the U.S. appears to retain its title as the world's leading space faring nation. It certainly spends more on military, civil and commercial space than any other country. But as with other high-tech industries, the devil is in the details when it comes to grasping where the sector is headed and how U.S. interests are likely to fare. The ranks of domestic suppliers are withering and off-shore dependencies are growing even as the Pentagon talks about creating a more "robust, resilient" space posture.A case in point is in-space propulsion, the systems installed on satellites to help them move around once they are in orbit. In-space propulsion enables satellites to maintain the right position and attitude so that they can do their assigned tasks. It enables satellites destined for "geostationary" orbits to transfer from their initial elliptical arcs to circular orbits so they can remain above specific locations on the Earth's surface. And increasingly, it will be used to maneuver satellites being threatened by countries such as Russia and China.But even as the Pentagon warns about growing threats to U.S. space dominance, America's satellite producers are turning increasingly to foreign sources for the in-space propulsion essential to sustaining the functioning of key constellations. This is particularly true of commercial companies producing geostationary satellites for various communications purposes. But it is also true of companies building satellites for NASA's civil space program, and for the military space program.This might not be a major security concern if they were turning just to allies such as Japan, which is a global leader in chemical thrusters used to position geo satellites. But the industry is gradually migrating to so-called electric propulsion technology, and there the key offshore provider is Russia. Even as Congress has been pressing the military to end its reliance on Russian launch vehicles, America's satellites are becoming more dependent on a type of in-space propulsion where Russia has emerged as the global leader.Electric propulsion is not a new idea. Robert Goddard wrote about it over a century ago. In the most commonly used systems, an electric field is applied to accelerate the pace at which propellant is expelled, enabling more efficient use of fuel. If less on-board fuel needs to be carried, then other features of the payload can be increased and/or costs can be reduced. Either way, it's an attractive proposition for companies producing spacecraft to exacting specifications.Russia came to dominate in-space electric propulsion because it has spent more time and money on the technology than other countries. It launched its first satellite equipped with electric propulsion in 1971. Other countries such as China and India have now crowded into the space, subsidizing their producers of both chemical and electric thrusters. The U.S., which once dominated the business, has lagged behind.I know all this because Aerojet Rocketdyne, a contributor to my think tank, is the leading U.S. source on in-space thrusters for satellites. The challenge it faces, like other companies in the domestic supply chain for space systems, is that satellite makers at home and abroad will tend to favor the lowest-cost supplier of in-space propulsion, and -- other things being equal -- that will be a subsidized offshore source.Reliability is an issue on chemical thrusters, but near as I can tell no electric propulsion system has ever failed once placed into orbit. So its no surprise that less than 20% of U.S.-made satellites carrying the most common form of electric propulsion obtained it from U.S. sources. The offshore (subsidized) sources are usually cheaper, and their technology often is cutting-edge. As I said, America used to dominate the market for in-space propulsion, but it doesn't anymore thanks to under-investment by government and industry.Which brings me back to the White House industrial-base assessment to be released next week. The president has repeatedly argued that if the United States fails to lead in critical technology areas such as space, it will have bad consequences for national security. U.S. satellite makers are becoming increasingly dependent on other countries, especially Russia, for a technology essential to the functionality of their products. The more dependent we are on offshore sources of space technology, the more vulnerable we are.If Congress is going to address this emerging vulnerability successfully, it needs to do more than mandate U.S. content on government-funded satellites. It needs to match the investments other countries are making, and create incentives for commercial satellite companies to utilize domestic sources. With the military planning to rely more on commercial satcoms in the future, there needs to be some assurance that the commercial supply chain is secure.Otherwise, the Pentagon's quest for greater "resilience" in space could end up trading one form of vulnerability for another.