SAN FRANCISCO — When U.S. President Ronald Reagan called for an ambitious national effort to develop technology to intercept and destroy strategic ballistic missiles, he warned of the challenges ahead.

“This will be a formidable technical task, one that may not be accomplished before the end of the century,” Reagan said in speech televised nationally on March 23, 1983.

Thirty years later, the U.S. Missile Defense Agency continues to wrestle with technical challenges, but also to demonstrate significant accomplishments, particularly in halting short- and medium-range ballistic missiles.

“It’s gratifying to see the progress that has been made since President Reagan started the Strategic Defense Initiative,” said Riki Ellison, founder of the Missile Defense Advocacy Alliance in Washington.

Still, the current missile defense architecture bears little resemblance to the initial plan proposed by Reagan and his administration to deploy thousands of ballistic missile sensors and interceptors on land, ships and aircraft and in space to counter a full-scale Soviet attack.

Much of that change is due to the changing nature of the threat. “Clearly, Reagan could not have envisioned the extent to which lesser powers would proceed down the nuclear path,” said Baker Spring, a defense policy analyst at the Heritage Foundation in Washington.

Instead of defending against the Soviet Union’s massive arsenal of long-range ballistic missiles, U.S. leaders have become increasingly concerned about the threat posed by nations such as North Korea and Iran that are developing nuclear weapons and the missile technology needed to launch warheads toward the United States or its allies.

The changing nature of the program also stems from a greater understanding of how difficult it would be to establish a comprehensive missile shield, said Victoria Samson, author of the book “American Missile Defense: A Guide to the Issues.”

“During the Reagan administration, U.S. missile defense was going to include space-based interceptors and directed energy weapons to form an impenetrable bubble,” Samson said. “Each of the following administrations dialed it back a notch as they realized it was extremely technically challenging and extremely expensive.”

During the administration of President George H.W. Bush, the Strategic Defense Initiative Organization (SDIO) began to focus on the task of shielding the United States, U.S. forces overseas and U.S. allies from the intended, unauthorized or accidental launch of a limited number of ballistic missiles coming from any source, including missiles scattered in the former Soviet states. That effort was known as Global Protection Against Limited Strikes.

When President Bill Clinton took office he established the Ballistic Missile Defense Organization, which took over SDIO’s role but concentrated on developing and fielding ground-based theater missile defense systems to counter existing threats while continuing to develop technology to fend off long-range ballistic missiles.

President George W. Bush’s campaign to expand strategic missile defenses led to U.S. withdrawal in 2002 from the Anti-Ballistic Missile Treaty and the first deployment in 2004 of Ground-based Midcourse Defense interceptors in Alaska.

Under a plan announced March 14 by U.S. Defense Secretary Chuck Hagel in the wake of North Korea’s third nuclear test, conducted in February, and successful satellite launch in December, Alaska would gain 14 additional interceptors. Hagel also announced plans to identify an interceptor site in the eastern United States and to send a mobile AN/TPY-2 missile-detection radar to Japan.

The current Ballistic Missile Defense System architecture features: Aegis sea-based Ballistic Missile Defense systems deployed on 26 U.S. Navy cruisers and destroyers; 26 Ground-based Midcourse Defense interceptors in Fort Greely, Alaska, plus four interceptors at Vandenberg Air Force Base, Calif.; two Terminal High Altitude Area Defense batteries fielded at Fort Bliss, Texas; and the Command, Control, Battle Management and Communications program to link missile defense sensors and interceptors. In addition, the U.S. Army leads the Defense Department’s Patriot Advanced Capability-3 land-based missile program aimed at countering tactical ballistic missiles.

Noticeably missing from the current architecture are many of the space-based systems that were central to Reagan’s missile defense plan. Those efforts, which continued to attract funding during George H.W. Bush’s administration, were canceled by Clinton, “who curtailed all of the technology programs associated with the Brilliant Pebbles activity,” Henry Cooper, who served as SDIO director from 1990 to 1993, said March 19 during a Heritage Foundation event marking the 30th anniversary of Reagan’s Strategic Defense Initiative speech.

Supporters of Clinton’s decision to halt work on Brilliant Pebbles, a program aimed at putting thousands of non-nuclear missile interceptors in low Earth orbit, said it was based on a careful analysis of the exorbitant cost of launching the satellites, while opponents viewed it as a political move designed to appease members of Congress and international arms control organizations who vehemently opposed space-based weapons.

While serving as director of the U.S. Missile Defense Agency under George W. Bush from 2004 to 2008, retired U.S. Air Force Lt. Gen. Henry Obering strenuously lobbied for funding to conduct space-based interceptor experiments. “I was asking for the modest amount of money: $10 million out of an $8 billion budget,” Obering said March 19 at the Heritage Foundation event. “I was turned down every time.”

In spite of congressional opposition to space-based interceptors, satellite-based sensors have been used to guide interceptors to their targets. The U.S. Navy announced in February that two Space Tracking and Surveillance System-Demonstrator satellites launched in 2009 helped a sea-based Standard Missile launched from an Aegis-equipped guided missile cruiser track down a medium-range ballistic missile over the Pacific Ocean. The Precision Tracking Space System proposed by the Missile Defense Agency would provide a similar operational capability for U.S. forces, Obering said. However, that remains “on the budgetary chopping block,” he said.

Debra Werner is a correspondent for SpaceNews based in San Francisco. Debra earned a bachelor’s degree in communications from the University of California, Berkeley, and a master’s degree in Journalism from Northwestern University. She...