Excerpt from article “HP 41 in Orbit“:

By the time of the Space Shuttle’s first flight in 1981, microelectronics technology had advanced rapidly, driven by the economic forces of the consumer market. (To give a sense of this rapid pace, only ten years earlier, Apollo astronauts carried 5-inch slide rules with them on missions to the Moon.) One could thus purchase a pocket calculator, at a very low cost, which gave its user more computing power than a custom-made device produced specifically for a given space mission. For the Shuttle, NASA engineers did just that: they purchased an HP-41C programmable pocket calculator and loaded it with a variety of software for use by the crew. Only minor modifications were made: adding Velcro strips to the case, and removing a few parts that might “outgas,” or give off gases that might contaminate the cabin’s air.

Because of their low cost, NASA was able to buy several calculators and provide them to members of the crew for their personal use. The HP-41C in the National Air and Space Museum’s collections was used by Astronaut Sally Ride and several other astronauts on a total of nine Shuttle missions. The photograph of Ms. Ride shows her on the Challenger flight of June 1983. The calculator later donated to the Smithsonian is the one on her left. NASA eventually replaced these calculators with more sophisticated personal computing devices, including laptop computers, for Shuttle crews.

The Shuttle’s primary functions were still controlled by an on-board, custom-designed system, built by IBM and subject to a rigorous program of testing and validation. But the HP-41C took a lot of the computing load off that machine for mundane but necessary calculations, such as calculating when a given ground station was available to send data to or receive a communication from. The HP-41C also contained a clock, which allowed astronauts to set alarms and schedule experiments. (The clock was not available to consumers at the time of the first Shuttle launches, but NASA obtained pre-production modules that plugged into the calculators. Hewlett-Packard later offered these as an option.) It also served as an emergency backup for re-entry calculations in event of a main computer failure, although this has never happened. One program that was especially useful identified and calculated distance and direction to the nearest emergency runway, if the Shuttle developed a problem and had to de-orbit quickly. Fortunately, this program also was never needed.

Although the calculators themselves were identical to what anyone could purchase at the time, the software developed for these missions was mission-specific and far beyond what most calculator users might encounter. NASA wrote custom software for each mission and loaded it onto read-only-memory (ROM) modules that plugged into the machines. The crew carried spare modules, so that if there were a hardware failure, one could run the programs on one of the other calculators. The calculators weighed about 200 g (seven ounces), measured 14cm x 7.5cm x 3cm, and of course operated independently of the Shuttle’s main power supply.

The Air and Space Museum sought to obtain this artifact for two reasons. The first was its status as a object carried into space and used as personal equipment by a well-known astronaut, Sally Ride. The second was to illustrate how the pace of innovation in microelectronics-a technology whose origins were in the space programs of an earlier day-have given consumers access to computing power that matches what NASA provides its crew.

Challenger shuttle launch, with Sally Ride (and HP-41) onboard

Challenger shuttle launch, with Sally Ride (and HP-41) onboard

Customized HP-41

Customized HP-41

Customized HP-41, closer look

Customized HP-41, closer look

Sally Ride with several HP-41 aboard the Space Shuttle

Sally Ride with several HP-41 aboard the Space Shuttle

HP-41CV

HP-41CV

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