WASHINGTON — Spending on most of the nation’s missile defense programs would decline in 2008 under a Missile Defense Agency (MDA) budget request that is $500 million less than the current budget and some $600 million less than what agency officials anticipated at this time last year.

David Altwegg, the MDA’s deputy director for operations, attributed the lower-than-expected request to internal budget pressures at the Department of Defense and said it has forced a number or programmatic adjustments. Among them are the termination of the High Altitude Airship sensor effort and another restructuring of the Kinetic Energy Interceptor (KEI) development program, according to MDA budget documents.

The MDA’s cancellation of the Lockheed Martin-led airship project follows Congress’ decision to trim the agency’s $41 million request for the project in 2007 to $18.5 million.

More dramatic, at least from a budgetary point of view, was the second major scaling back of the KEI program in three years. In submitting its budget request for 2006, a year in which it also took a budget cut, the MDA announced that it would curtail work on the KEI program that was not directly related to a planned 2008 flight test of the booster.

Altwegg said the schedule for the flight test remains unchanged in spite of a KEI request for 2008 that is 36 percent less than the 2007 budget and nearly 50 percent below what the MDA anticipated it would be seeking for 2008 at this time last year. According to MDA officials, the latest restructuring curtails work on a unique KEI kill vehicle, mobile launch platform and fire control equipment.

Altwegg said the MDA could still move ahead with full-scale development and deployment of the KEI should the agency run into intractable problems with its other boost-phase interceptor program, the Airborne Laser. He also said that instead of a unique kill vehicle, the Northrop Grumman-designed KEI could be tipped with the Multiple Kill Vehicle, one of the few MDA projects slated for a budget increase in 2008. The Multiple Kill Vehicle takes a shotgun approach to thwarting missile attacks, destroying both live warheads and decoys, which are difficult to distinguish. Lockheed Martin is the prime contractor on that effort.

MDA officials have previously couched the KEI and Airborne Laser as competing systems, with their fates dependent on how they perform versus one another in upcoming flight tests. But MDA budget documents emphasized the KEI’s role as a possible future replacement for the Ground-based Midcourse Defense system interceptor, the spearhead of the U.S. territorial shield now being installed at Fort Greely, Alaska, and Vandenberg Air Base, Calif.

Meanwhile, Altwegg disclosed that a planned test in which the Airborne Laser — a modified Boeing 747 aircraft equipped with a high-power laser — will attempt to shoot down a target missile has been delayed from 2008 to 2009. He cited unmet programmatic milestones rather than budget cuts as the reason for the delay.

Conversely, the declining budget has forced the MDA to scale back its plans in space. The agency had planned to begin deploying an operational constellation of missile-tracking satellites in 2012 or 201 3, but that schedule has been pushed out to no earlier than 2016 or 2017, according to the 2008 budget-briefing documents. The MDA remains on track to launch a pair of experimental satellites under that effort, known as the Space Tracking and Surveillance System, later this year, Altwegg said.

The MDA also is reining in plans for design work on space-based missile interceptors, Altwegg said. When it submitted its 2007 budget request to Congress last February, the agency anticipated it would be seeking about $45 million for that effort in 2008. The actual request is substantially lower, at $10 million, he said.

The largest program in the MDA’s portfolio remains the Ground-based Midcourse Defense system, led by Boeing Co. Among the changes to that effort is the cancellation of the Boost Vehicle Plus, an alternative interceptor for the system that was being built by Lockheed Martin.

The MDA’s budget request for sensors is significantly higher than this year’s funding level, reflecting agency plans to modify and install a ground-based tracking radar in Europe, the budget documents said.