The Delta II rocket came, it saw, and for a time it conquered

On Saturday morning, the most successful rocket the United States has ever developed flew its final mission. During the pre-dawn hours, United Launch Alliance's Delta II rocket lifted NASA's ice-monitoring mission ICESat-2 into space. It was a bittersweet moment, as the Delta II's retirement marks both a step into the future of US rocketry, while representing a definitive break with the past—and the very origins of US spaceflight.

"Historic day," the chief executive officer of United Launch Alliance, Tory Bruno, said on Twitter. "Retired the shark, Delta II and the mighty Thor." The "shark" was a reference to the shark teeth painted on the payload fairing of GPS launches, an homage to the "Flying Tigers," American volunteer pilots who helped defend China from Japan in 1941 and 1942.

Some history

The Delta II rocket can trace its heritage back to the dawn of serious US rocketry, with the Thor intermediate-range ballistic missile that was developed by the US Air Force in the 1950s and first deployed in Great Britain in 1959. This 20-meter rocket, designed to carry thermonuclear weapons, served as a template for the original Delta rockets.

The first Delta rockets were used by NASA and other government agencies to launch satellites during the 1960s and 1970s. During the 1970s, at the insistence of the White House, the Air Force agreed to work with NASA and its space shuttle program to fulfill the military's launch needs. The shuttle’s first Department of Defense flight launched in June of 1982, but after the space shuttle Challenger accident in 1986, the Reagan administration issued a National Space Launch Strategy that directed the military to develop its own rockets.

This led to an upgraded Delta rocket, with a longer version of the Thor fuel tank, which became known as the Delta II rocket. It first launched in 1989 and has since flown 155 successful missions, with only one total failure. The original goal of the fleet was to launch GPS satellites for government and, later, civilian use. So you can thank the Delta II the next time you check your phone's map for directions.

As useful as the rocket has been for the military, however, it has also been a proven workhorse for NASA. Of the rocket's 155 successful launches, 54 were conducted for the space agency, including eight robotic missions sent to Mars. These missions include the first Martian rover, Pathfinder, in 1996; Spirit and Opportunity in 2003; and the Phoenix lander in 2007.

"We're honored that our customers have trusted us with a lot of critical missions over the years," Scott Messer, the program manager for the Delta II program for United Launch Alliance, told Ars in an interview.

Looking ahead

So why retire the most successful rocket in US history? Because time moves on. Before its retirement, no rocket other than the Russian Soyuz booster had remained active longer than the Delta II rocket. However, in recent decades more-capable, less-expensive options have emerged.

A decade ago, the US Air Force already sought to transition to the more powerful Delta IV and Atlas V rockets, which are also built by United Launch Alliance. Since 2012, the Delta II has averaged fewer than one launch a year, and its cost has escalated due to the need to keep production lines open for so few missions.

Moreover, the Falcon 9 rocket built by SpaceX has also pressured the Delta fleet. At $60 million, it costs significantly less than the Delta II booster, with three to four times the capacity in terms of tonnage to low-Earth orbit.

Listing image by United Launch Alliance