On Saturday morning, the most successful missile the United States has ever developed flew its final mission. During the early morning hours, the United Launch Alliance’s Delta II rocket lifted NASA’s ice-sensing mission ICESat-2 into space. It was a bittersweet moment, as the retirement of the Delta II marks both a step into the future of US rocketry, and a definitive break from the past — and the origin of American spaceflight.
“Historic Day”, United Launch Alliance CEO, 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 line of GPS launches, a tribute to the “Flying Tigers”, American volunteer pilots who helped defend China against Japan in 1941 and 1942.
The Delta II missile can trace its heritage back to the dawn of serious American missiles, with the Thor medium-range ballistic missile developed by the US Air Force in the 1950s and first deployed to Britain in 1959. This 20-meter-long missile, designed to carry thermonuclear weapons, served as a template for the original Delta missiles.
The first Delta rockets were used by NASA and other government agencies to launch satellites in the 1960s and 1970s. In the 1970s, at the urging of the White House, the Air Force agreed to partner with NASA and its space shuttle program to meet the military’s launch needs. The Department of Defense’s first flight was launched in June 1982, but after the space shuttle Challenger accident in 1986, the Reagan administration issued a National Space Launch Strategy that instructed the military to develop its own rockets.
This led to an improved Delta rocket, featuring a longer version of the Thor fuel tank, which became known as the Delta II rocket. It was first launched in 1989 and has since flown 155 successful missions, with only one total failure. The fleet’s original purpose was to launch GPS satellites for government use 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 to the military, it is also a proven workhorse for NASA. Of the rocket’s 155 successful launches, 54 were performed for the space agency, including eight robotic missions sent to Mars. These missions include the first Mars rover, Pathfinder, in 1996; Spirit and Opportunity in 2003; and the Phoenix lander in 2007.
“We are honored that our customers have entrusted us with many critical missions over the years,” Scott Messer, program manager for the Delta II program for United Launch Alliance, told Ars in an interview.
So why put the most successful missile in US history out of action? Because time goes on. Before his retirement, no missile other than the Russian Soyuz booster was active longer than the Delta II missile. In recent decades, however, more capable, cheaper options have emerged.
Ten years ago, the US Air Force tried to switch to the more powerful Delta IV and Atlas V missiles, which are also built by United Launch Alliance. Since 2012, the Delta II has been launched on average less than once a year and costs have increased due to the need to keep production lines open for so few missions.
In addition, the SpaceX-built Falcon 9 rocket has also put pressure on the Delta fleet. At $60 million, it costs significantly less than the Delta II booster, at three to four times the capacity in terms of tonnage to low Earth orbit.
List image by United Launch Alliance