Launching a Spacecraft to the Sun: Development and Execution of the PSP Launch Countdown Procedure

NASA's Parker Solar Probe (PSP) mission will revolutionize our understanding of the Sun by traveling through the Sun's atmosphere, closer to the surface than any spacecraft before it, to provide humanity with the closest-ever observations of a star. The PSP spacecraft was designed, built, and operated by the Johns Hopkins Applied Physics Laboratory (APL). A successful launch of the spacecraft occurred on August 12, 2018 from Cape Canaveral, Florida on a Delta IV Heavy launch vehicle using an additional upper stage. This paper describes the development and execution of the launch countdown procedure that was used by the APL spacecraft Integration and Test (I&T) team to prepare the spacecraft for launch. Specific aspects of the complex spacecraft design are introduced where these led to unique driving requirements for the countdown process. The nominal countdown activity timeline is reviewed, and spacecraft contingency procedures are presented that were developed for use in the event of a hold, recovery, recycle, or scrub. Associated test scripts and telemetry display pages specific to the spacecraft countdown are also discussed, as well as pre-launch testing and launch rehearsals. Finally, an overview is provided of spacecraft countdown operations during two launch attempts on back-to-back days. This sequence challenged the spacecraft team to exercise a range of countdown contingency procedures, both to resolve technical issues with the spacecraft ground systems and to support launch vehicle scrubs and recycles. All spacecraft countdown operations were ultimately completed successfully, culminating in the successful launch of PSP.