All systems remain "go" for Thursday's launch of the Delta 4 rocket carrying a reconnaissance satellite for U.S. national security, apparently bound for an unusual retrograde orbit around the Earth.

Archived artwork depicts a Delta 4 flying from Space Launch Complex 6.

The official Notices to Airmen, or NOTAMs, released to the public this week to establish clear warning zones around the rocket's flight path indicate the booster is headed southwestward from Vandenberg Air Force Base on its trek downrange.

Typical Vandenberg launches head southward toward polar orbits or slightly southeastward on so-called "coast-hugger" ascents, depending on the payload's desired orbit.

In the upcoming launch, however, the Delta 4 rocket will take a distinct trajectory out over the Pacific Ocean that would result in delivering its classified satellite cargo into a retrograde orbit flying against Earth's natural rotation.

One other recent Vandenberg launch took such a course in September 2010 when an Atlas 5 rocket deployed the first in a new-generation of radar surveillance spacecraft that provide imaging of targets in daylight or darkness, can see through cloud cover and resolve structures beneath the ground's surface.

The National Reconnaissance Office, the customer for this launch and operator of the country's fleet of spy satellites, does not announce the purpose of the spacecraft being sent up on each launch. The flight is known only as the NROL-25 mission.

Liftoff is targeted to occur at 3:30 p.m. local time (6:30 p.m. EDT; 2230 GMT). The usable length of the launch window isn't being disclosed to the public in advance, but officials have said liftoff won't occur after 5:15 p.m. local.

This new type of radar-imaging satellite follows the previous, albeit larger Lacrosse spacecraft launched by the shuttle Atlantis in 1988 and mighty Titan 4 rockets through 2005. The updated design is smaller and less massive, enabling the craft to fly aboard mid-sized Atlas 5 and Delta 4 rockets.

The upper stage of the Delta 4 rocket almost certainly will perform two firings to deliver the payload into a preliminary orbit. The spent stage then executes a deorbit burn to remove itself from space and plunge back into the southern Indian Ocean on its second orbit, also noted in the NOTAMs.

The radar satellite will reside in a retrograde orbit tilted 123 degrees relative to the equator, eventually maneuvering itself to an operational altitude of about 685 miles high to pair up with the constellation's first craft.

The weather outlook for launch forecasts only a 30 percent chance of acceptable conditions for the liftoff time. Gusty ground winds are the prime concern for scuttling the countdown.