Editor's note: This launch was postponed to August 28 by technical issues. As a result, it will be the 301st Delta launch, with the Delta 2/SIRTF mission of August 25 taking the distinction as the 300th.

This weekend's launch from Cape Canaveral will mark a moment in history for the Delta rocket and the finale for a long-running family of U.S. military communications satellites.

The 300th Delta rocket is poised for blastoff Sunday carrying the 65th and last spacecraft for the Defense Satellite Communications System.

The mission logo for this launch of the Delta 4 rocket and DSCS satellite. Credit: Boeing

Launch from pad 37B will occur during a window of 6:58 to 8:21 p.m. EDT (2258-0021 GMT). Forty-two minutes after liftoff, the two-stage, all-cryogenic rocket will complete its mission by deploying the DSCS 3-B6 satellite into geosynchronous transfer orbit.

The Delta era began in May 1960. Although its initial attempt to reach space failed, Delta began racking up successes with its second flight a few months later. Today, the Delta family boasts a remarkable record of 283 successes and 16 failures.

Sunday's mission will be the third for the Delta 4 rocket -- a Boeing-made launcher that became the newest edition to the Delta family last November.

A Delta 4 Medium rocket with no solid rocket boosters will be used to loft the DSCS 3-B6 satellite. Photo: Carleton Bailie/Boeing

Delta 4 was developed in the last decade as part of the Air Force's Evolved Expendable Launch Vehicle program. Dubbed EELV, the government and industry endeavor was aimed at fielding new rockets that would be cheaper to build, easier to operate and more reliable during launch.

But the partnership between the Air Force and Boeing experienced a major bump in the road last week when military investigators determined that Boeing violated federal laws by using propriety documents from rival Lockheed Martin's Atlas 5 EELV program. The penalty levied against Boeing was the loss of seven scheduled Delta 4 launches. Those launches were given to Atlas 5. See our complete story on the Air Force's decision.

Delta 4 still has a dozen Air Force missions on the manifest, including all three heavy-lift launches. Ten flights will occur from Cape Canaveral and two are slated from the rebuilt space shuttle launch complex at Vandenberg Air Force Base in California. A look at the EELV launch schedule is available here.

The Air Force has already placed one of its satellites into orbit using the Delta 4. That successful mission in March lofted the DSCS 3-A3, a sister to the spacecraft launching this weekend.

The Pentagon operates a constellation of DSCS satellites for super high frequency, anti-jam communications around the planet. The system supports troops on the ground, aircraft in the sky, ships at sea, the White House Communications Agency and the State Department.

The system is considered the "backbone" for the U.S. government's global communications network.

The DSCS 3-B6 satellite during pre-flight preparations at the factory. Photo: Russ Underwood/Lockheed Martin Missiles & Space

The first generation of DSCS satellites were just 100 pounds and three feet in diameter. Ford Aerospace built the craft, which were designed to function for three years. Using five Titan 3C rockets, the Air Force launched 35 of the satellites between 1966 and 1968. However, one launch failed and destroyed eight of the craft.

The DSCS 2 program commenced launches in 1971. These 1,200-pound, 9-foot diameter cylinder-shaped satellites built by TRW were more capable than their earlier predecessors and had a five-year life expectancy. Sixteen spacecraft in this series were launched through 1989 using Titan 3C and Titan 34D rockets, however four never reached the proper orbit.

The current generation of Lockheed Martin DSCS 3 satellites began launching in 1982, overlapping the DSCS 2 series. The first 10 DSCS 3 craft had a dry weight of nearly 1,900 pounds; while the final four were 100 pounds heavier with upgrades designed to improve the satellite's capabilities. Their solar arrays span 38 feet tip-to-tip. Titan 34D, the space shuttle, Atlas 2 and 2A and now Delta 4 rockets have been used to successfully launch these birds into space, which were built to operate for at least 10 years.

The DSCS 3-B6 satellite going up Sunday is destined to replace the B7 over the western Atlantic Ocean communications coverage zone. The B7 satellite, which was launched in July 1995 aboard an Atlas 2A rocket, will be shifted into another orbital location in the DSCS constellation to continue its service life.

The $210 million B6 includes the so-called Service Life Enhancement Program upgrades. Overall, the Air Force says B6 represents a 200 percent increase in communications capacity vs. the existing B7 spacecraft.

An artist's concept of the DSCS 3-B6 spacecraft in orbit. Credit: Lockheed Martin

The B6 was originally constructed many years ago for launch aboard a space shuttle in the mid-1980s. However, the Challenger accident grounded the shuttle fleet and DSCS spacecraft were transferred to expendable rockets for the ride to orbit. The satellite has undergone complete overhauls and upgrades in preparation for its upcoming deployment.

After being released into geosynchronous transfer orbit by the Delta 4 rocket, the satellite will use its kick stage to achieve geostationary orbit 22,300 miles above the equator. The Integrated Apogee Boost Subsystem motor puts the craft at an altitude where it matches Earth's rotation and appears parked above a specific spot on the planet.

Although this is the final new addition to the DSCS fleet, the system will remain in use for several more years. But the Air Force is planning for the future by ordering three Wideband Gapfiller Satellites. They will provide increased bandwidth and communications capacity over the DSCS craft.