BOSTON — The development schedule for a new U.S.-Israeli missile interceptor system is overly ambitious, and defense authorities likely will have to implement a backup plan if countries like Iran acquire a nuclear-tipped missile before the end of the next decade, according to defense experts.

Advanced sensor and propulsion capabilities envisioned for the Arrow-3 interceptor likely will take significantly longer to develop than the five or six years estimated by Boeing Co., particularly given the program’s funding level, the experts said.

“Look at any system that is developed — it takes 10 years from concept to deployment and there’s not much [funding] going in there,” said Riki Ellison, president of the Missile Defense Advocacy Alliance here. “You usually spend about $1 billion” on research and development in missile defense programs, he said.

Israel and the United States budgeted $20 million for the Arrow-3 program in 2008, and $40 million in 2009, according to U.S. Missile Defense Agency (MDA) officials. While the United States has no plans at this time to use Arrow-3 for its own missile defense needs, it may consider the system at a later date if performance and cost goals are achieved, the officials said.

U.S. Army Lt. Gen. Patrick O’Reilly, director of the MDA, told Congress this year that a land-based variant of the U.S. Navy’s Standard Missile (SM)-3 is one option for defending Israel in case a potential threat materializes before Arrow-3 is ready.

The SM-3, built by Raytheon Missile Systems of Tucson, Ariz., is currently launched from Aegis ships, but the MDA is planning a land-based variant as part of its recently revamped plan for the European missile shield.

Ellison said risk involved in Arrow-3 is very high compared with the SM-3.

The Arrow-3 is being developed jointly by Boeing Integrated Defense Systems of St. Louis and Israel Aerospace Industries of Lod, Israel. Unlike its predecessors, which are designed to destroy missiles with blast fragments inside the atmosphere, the Arrow-3 is equipped with a kill vehicle designed to slam into missile warheads in space.

Bill Dickerhoff, Boeing’s Arrow program manager, said the Arrow-3 would add another layer to existing Israeli missile defenses, giving the country additional chances to fire at incoming warheads. Moreover, he said, the Arrow-3 kill vehicle will feature an advanced propulsion system that can divert it to another missile if its initial target has already been destroyed.

Arrow-3’s hit-to-kill technologies include a very high-resolution sensor on the kill vehicle capable of distinguishing between a missile’s warhead and decoys, Dickerhoff said. This task is far more difficult in the space environment than for the endoatmospheric versions of Arrow, he said.

Boeing and Israel Aerospace Industries also face the challenge of keeping the cost of each Arrow-3 interceptor to $1 million to $2 million so Israel can afford to purchase enough of them to fend off a large number of incoming missiles, Dickerhoff said.

In written testimony to the Senate Armed Services Committee June 16, O’Reilly said the Arrow-3 program has “very high schedule and technical risk,” leading MDA to fund work on a land-based version of the SM-3 as a backup.

In drafting their respective versions of the 2010 defense authorization bill this past summer, both the House and Senate Armed Services committees noted the risks involved in Arrow-3, encouraged development of the land-based SM-3 as a backup option and directed the Pentagon to keep lawmakers apprised of progress on the U.S.-Israeli system.

Dickerhoff acknowledged that  Arrow-3 has risks, but said company reviews of the effort thus far have not uncovered “anything significant standing in its way.”

Boeing is addressing Arrow-3’s technical and cost risks through use of so-called knowledge points, or gates that the program must pass through in specific areas like propulsion, sensing and cost before continuing, Dickerhoff said. The company has used this approach with the Airborne Laser effort as well, he said.

The consequences of missing a knowledge point could include delaying the program for retesting, Dickerhoff said. He declined to speculate on whether this would drive up costs, other than to say that the program is currently meeting its milestones and the unit price goal appears achievable.

Missing a knowledge point also could lead to MDA recommending use of an alternative interceptor to meet Israel’s upper-tier missile defense requirement, Dickerhoff said.

Arrow-3’s specific testing plans and schedule are classified, but the program likely will feature several basic flight tests over the next several years to demonstrate control of the interceptor, followed by multiple shoot-down attempts, Dickerhoff said.

Ellison noted that even if the SM-3 is deployed to defend Israel, the Arrow-3 program likely would continue in order to preserve the country’s missile defense industrial base and technical competencies, he said.

The Arrow-3 program also could help Boeing retain some of its engineering staff as the funding for missile defense programs in its portfolio declines in the years ahead. Boeing is prime contractor on the U.S. Ground-based Midcourse Defense system, whose development and deployment are nearing completion, and on the futuristic Airborne Laser, a program slated to be discontinued after the first flight unit. Boeing also was to supply 10 interceptors as part of a European missile defense plan that the White House recently canceled in favor of an SM-3-based alternative.