[ix] For consistency and to avoid confusion, the numerical designations of spacecraft within the text conform to the Arabic numeral system. Until 1969, NASA chose roman numerals to designate successful flight missions, although there were notable exceptions. Italics indicate spacecraft that have attained orbit, space probes that have achieved an altitude above 64 000 kilometers, and all manned suborbital flights. Spacecraft launch failures retain their preflight letter designations.
No single system of numbering spacecraft and launch vehicles has been followed by NASA through the years, and often two or more designations have existed for one spacecraft. Usually, however, spacecraft in a series are given letter designations in alphabetical order before launch and successful launches within a series are numbered consecutively with Arabic numerals.
Many satellites and space probes have followed this pattern and most launch vehicles have been, numbered separately. There were exceptions: launches that failed sometimes upset the numbering sequence (for example, Pioneer 3 followed Pioneer 1) and designations for spacecraft within a series at times did not appear to follow a given sequence (ATS-B was launched before ATS-A; OAO 2 was known as OAO-A2 before launch). Launch vehicle development flights were numbered consecutively, including suborbital flights (Atlas-Centaur 2; Atlas-Centaur 3, suborbital; Atlas-Centaur 4).
The numbering systems for the first three manned spaceflight programs-as the new space agency developed approaches in an evolving field-were not consistent. In addition to the overall flight number, each manned flight had a separate designation, named and numbered in sequence, for the launch vehicle combination that was employed. Except for flights in the Mercury program and unmanned flights in the Gemini program, these secondary designations were not official names, but were used by NASA for reference (in the Gemini program) or launch vehicle designation (Apollo). For example Freedom 7 was also known as Mercury-Redstone 3 and Friendship 7 as Mercury-Atlas 6.
In the Mercury program, the choice of the number "7" by the original seven Mercury astronauts precluded the use of roman numerals for the [x] spacecraft. Project Gemini was the only manned program to use roman numerals, and even its early unmanned flights were named by the Mercury system (Gemini- Titan 1, Gemini- Titan 2). In the Apollo program, each mission was assigned an overall number and each command and service module and lunar module was given a separate number designating a specific flight unit (for example, "CSM-108" and "LM-6" designated the specific Apollo 12 modules). Each Apollo launch vehicle was assigned a flight number that indicated both the vehicle model and the specific vehicle used on that mission, such as "AS-201" and "AS-507." The "200" and "500" series referred to Saturn IB and Saturn V launch vehicles; "AS" to "Apollo Saturn." The 10 Saturn I development flights, on the other hand, were designated "Saturn. Apollo."
In addition, each spacecraft or piece that separately entered earth orbit was given a number and letter designation, according to the international designation system (see Appendix B). Spacecraft that separated while in orbit or after leaving earth orbit also were given designations (for example, the Apollo 15 Subsatellite, ejected into lunar orbit from the Apollo command module, and the Soviet softlanding capsules that descended to the Martian surface from the Mars 2 and 3 space probes).