The United States soon must begin replacing its aging satellites or risk losing critical space-based intelligence capabilities such as spotting an enemy missile strike on America, an analyst said.

A prudent response to this situation should include moving forward with the Space Based Infrared System (SBIRS) constellation of satellites, according to Loren B. Thompson, chief operating officer of the Lexington Institute, a think tank near the Pentagon that focuses on defense and other issues.

It is vital that the United States retains the ability to spot any missile attack shortly after the weapon launches, Thompson said in an issue brief.

But, he noted, the U.S. military could be left with a gap in its missile warning capabilities if SBIRS is delayed in production and deployment.

The last of the old Defense Support Program missile-warning satellites was launched last month, an aging platform design that has been operating since the 1970s.

That old DSP constellation of satellites is vital, he added, "because it is the only system that can reliably warn of a ballistic-missile attack against America no matter where that attack originates on the face of the" planet.

This DSP system is vital not only for a possible war, but as well for peace, he continued.

"Every strategy for averting nuclear war begins with being able to know whether the nation is under attack," Thompson noted. "If the military can’t be sure of detecting an attack, then neither the threat of retaliation nor the possibility of active defense is credible.

"So the fact that there are no more Defense Support Program satellites left to launch has provoked concern among policymakers, because the effort to develop a more capable successor [SBIRS] that was begun in 1995 is running years behind schedule."

And that is fueling fears of a gap in U.S. missile defense and situational awareness abilities.

"After all, satellites don’t last forever, and some of the existing satellites have been in orbit for many years," he observed. "What if they stopped working before the next- generation system became operational? Could the nation be vulnerable to a surprise attack?"

There are no realistic alternatives to the current DSP constellation, he continued, so the Pentagon needs to move, rapidly, to develop SBIRS, Thompson said.

SBIRS detects heat generated by hostile missiles long before they reach their destination. "The baseline constellation of missile-warning satellites today and in the future will consist of at least four spacecraft carrying infrared telescopes in geosynchronous orbits plus two additional sensors piggybacked on electronic eavesdropping satellites in elliptical orbits," Thompson observed.

Each satellite flies at enormous speeds, thousands of miles an hour at an altitude of 22,300 miles above the ground near the equator, so that it remains in a fixed geosynchronous position relative to the slower-turning Earth. Sensors on each satellite thus are able to see, continuously, a large fixed portion of the planet, including the poles.

"The sensors in elliptical orbit cover polar regions not easily visible to satellites above the equator — regions from which enemy submarines might launch missiles," Thompson noted. "Because more than one satellite can see the areas of greatest concern (like Russia and China), it is possible to get multiple perspectives on the same event. But if you lose even one sensor from the baseline constellation, awareness begins to diminish. Lose two, and you could be blind to some threats."

And, he continued, some of the DSP satellites have been in orbit not merely for years, but for decades.

"The government won’t say how many missile-warning satellites are currently operational," he wrote. "However, it appears that every Defense Support Program spacecraft launched over the last 20 years has greatly exceeded its projected service life. In some cases, the satellites have remained operational four times longer than expected."

That is welcome news, but at the same time it is unnerving, Thompson continued.

"The implication is that with the launch of the most recent satellite, the military actually has much more missile-warning capability than is required to satisfy basic mission requirements," but how long will that pleasant reality continue? he asked.

"That situation won’t last indefinitely, so it really is important to push ahead with construction of the SBIRS constellation."

This hopefully isn’t a situation where the United States will lose an immense, and immensely critical, capability soon, he concluded. At the same time, however, prudence dictates moving smartly to replace the old birds.

The Pentagon "still has the time to bring SBIRS to fruition with all of its originally planned performance features intact," he stated.