In public statements immediately following the Dec. 21 demonstration flight of the heavy-lift version of Boeing Co.’s Delta 4 launcher, officials with the company and the U.S. Air Force downplayed considerably the problem that caused a premature shutdown of the rocket’s core-stage engines. That should concern Congress and the Pentagon because the next Delta 4 Heavy payload is vital to U.S. national security. Congress should insist that every step possible is taken to ensure that next Delta 4 Heavy launch is a successful one.

The Air Force and Boeing trumpeted elements that went as scripted, but they glossed over the fact that the entire payload — a sensor-laden dummy satellite and two experimental spacecraft for the Air Force Research Laboratory’s University Nanosat Program — was lost.

The Air Force also dismissed any notion that a second demonstration flight, or a serious look at alternatives for launching the two payloads on the Delta 4 Heavy’s 2005 manifest, might be warranted.

Yet the Air Force and Boeing acknowledged that the problem may have been design-related — which in the rocket business can easily mean a redesign that sidelines a system for months.

Investigators believe a disruption in the flow of liquid oxygen in the rocket’s three booster cores basically tricked onboard sensors into shutting the engines down early. In a statement, the Air Force said investigators would “assess whether the design of the liquid oxygen feed system could be creating a cavitation under the flow conditions unique to the Heavy demonstration mission.”

In an interview, Dan Collins, vice president of Boeing Expendable Launch Systems, also stressed that the phenomenon was unique to the demonstration flight profile and that the problem likely would be relatively simple to correct. The Air Force and Boeing both said the Delta 4 Heavy would be ready to launch a Defense Support Program missile warning satellite in September and a classified National Reconnaissance Office payload in December, as scheduled. That the Air Force would select a unique flight profile for the demonstration mission — as opposed to one representative of an operational mission — is puzzling, but the service presumably had its reasons. That said, the Air Force should adopt a more flexible stance on the schedule for launching the payloads on the Delta 4 Heavy’s 2005 manifest.

The DSP 23 satellite is one the nation cannot afford to lose. It is the last of a line of satellites that for more than 30 years have served with distinction as — to borrow the title of Jeffrey T. Richelson’s book on the subject — “America’s Space Sentinals.” These satellites perform a vital function by watching out for missile launches from the former Soviet Union and elsewhere around the globe.

DSP has performed so well over the years — most famously in providing advance notice of Iraqi Scud missile launches during the 1991 Persian Gulf War — that it is tempting to take missile warning for granted.

The DSP constellation, whose exact size and health are classified, also has proven remarkably durable. The United States should be very thankful for DSP’s longevity, because its successor, the Space Based Infrared System (SBIRS) High, is not slated to start launching until around 2008 — six years behind its original schedule.

It remains to be seen whether the first dedicated SBIRS satellite actually will launch at that time , and given the program’s history it is fair to be skeptical. A recent budget planning document that says the Defense Department is considering scaling back the requirements for SBIRS High is troubling because it suggests that the Air Force is still wrestling with problems on the program.

There appears to be little danger at this point that the DSP constellation will degrade significantly before SBIRS takes over. But Congress’ request in the 2005 Defense Appropriations Act for an analysis of the nation’s missile-warning capability during the transition was wise, and it suggests that at least some lawmakers are growing uneasy with the situation.

Surprises in the form of launch mishaps and sudden on-orbit failures of seemingly healthy satellites are part and parcel of space activity. A string of mishaps involving DSP or SBIRS satellites could leave the United States with a missile warning blind spot, which is why it is so important to get DSP 23 safely launched.

This is not to suggest that the Air Force would take any foolish risks with the $400 million satellite. But the service needs to keep its options open until all data from the Delta 4 Heavy demonstration are analyzed thoroughly.

That means holding off on any DSP-Delta 4 integration work at Cape Canaveral Air Force Station, Fla., until the planned two-month post-flight review is completed and the results are checked and double checked. It also might mean paying Lockheed Martin to be ready to provide a flight-tested version of its Atlas 5 rocket for the DSP launch should there be any nagging questions about the Delta 4 Heavy.

Keeping options open might cost the Air Force more money or result in launching the DSP a few or even several months later than now planned. But preserving the integrity of the nation’s missile warning capability as the DSP constellation is replaced by SBIRS is well worth whatever it might cost.