America’s oldest rocket just made its penultimate flight

The Delta II rocket first launched in 1989, making it the oldest US orbital launch system still flying today. However, the heritage of this launch system is much older still. The Delta II rocket's first stage is derivative of the Thor intermediate range ballistic missile—the first operational ballistic missile used by the United States Air Force in the 1950s. Thus, the Delta II rocket can trace its roots to the beginning of US rocketry.

But now the end is near for the Delta II rocket. For United Launch Alliance, it is costly to keep supply lines open for the medium-lift rocket (three to six tons to low-Earth orbit) that has flown just three times since 2012. On Saturday morning, the Delta II made its penultimate launch by carrying the first Joint Polar Satellite System spacecraft into orbit for NASA and NOAA. The flight occurred from Vandenberg Air Force Base in California.

Six decades of heritage

The rocket's maker, United Launch Alliance, has enough components of the Delta II rocket left for just one more flight. This mission, scheduled for September 2018, will carry NASA's ICESat-2 into space. After this, United Launch Alliance will cease flying the Delta II rocket, ending a launch vehicle line with a 60-year heritage.

The Air Force began the Thor program in 1954 with a single, significant goal. It wanted the capacity to launch a nuclear weapon from Great Britain into the heartland of the Soviet Union, including Moscow. Industry responded, and by 1958 the first of five dozen missiles was delivered to Britain.

Even as the Thor missiles were deployed across the ocean, the new national space agency, NASA, looked to the Thor vehicle for space launches. It selected the fourth iteration of the Thor launcher, Thor-Delta, for its purposes. By May 1960, NASA had launched its first Thor-Delta from Cape Canaveral Air Force Station in Florida. It failed, but a second launch in August was a success. The payload was the first US communications satellite, Echo 1A.

Delta II origins

Before the space shuttle Challenger accident in 1986, the US government had intended for the shuttle program to carry out all of its national security launches as well as serve NASA's human spaceflight needs. However, after the accident, the US military determined that it would develop its own means of accessing space. This became the origin of the modern line of Delta and Atlas rockets. The Delta II's first stage was derived from the Thor missile.

The first Delta II rockets launched GPS satellites for the government, and soon NASA, too, was purchasing rides on the Delta II rockets for its own Earth-observing payloads. During the 1990s and 2000s, the rocket launched some of the country's most significant space science missions, including eight missions to Mars, such as the Spirit and Opportunity rovers. The Dawn and Genesis spacecraft also flew on a Delta II rocket, as did the exoplanet-hunter Kepler spacecraft.

Should next year's launch be successful, the Delta II rocket would go out on a high note. The venerable rocket has now flown 99 consecutive successful missions. The 2018 mission would mark its 100th flight with mission success in a row, not a bad record for an American-made rocket that traces its roots to the dawn of the industry, and the depths of the Cold War.

Listing image by ULA