“In addition to helping the crew organize its time, the second HP-41 computer was kept ready for flight-critical, deorbit-burn calculations. Once during each orbit around the Earth, the shuttle has an opportunity to land at one of six contingency locations. During a routine flight, Mission Control supplies the shuttle crew with deorbit-burn information. Should the shuttle encounter an emergency, however, the astronauts would rely on the HP-41 for these calculations.”
―”HP-41’s Again Aboard Columbia.” HP Key Notes, March-May 1982.
To prepare an orbiting Space Shuttle for re-entry through the Earth’s atmosphere it is critical that the spacecraft be “balanced” by taking into account the mass of the fuel left in the tanks at the end of a mission. An astronaut would use a handheld computer or programmable calculator to determine how many minutes and seconds of fuel to burn to get the center of gravity correct for a smooth descent and landing. The “personal computing system” used for this was made by Hewlett Packard in the 1980s. NASA donated one of these, a model HP-41CV, to Ladd Observatory after the retirement of the Shuttle program.