Imagine a handheld calculator small enough to fit in an astronaut’s pocket, carried on a spacecraft in case the main flight computer ever failed. That is pretty much what happened in July 1975. The Smithsonian’s National Air and Space Museum notes that NASA used a Hewlett-Packard HP-65 pocket calculator for the Apollo-Soyuz Test Project, programmed with rendezvous and guidance equations as a backup in case the main Apollo Guidance Computer failed. Today, a pocket calculator on a spacecraft seems almost quaint, but at the time it was really state-of-the-art technology, and its assigned job was anything but small.
A calculator made for the job
The HP-65 was not an ordinary calculator. According to the Smithsonian's museum record, it was introduced by Hewlett-Packard in 1974 as the first magnetic card programmable handheld scientific calculator in the world. It came with a set of thin magnetic cards that fit into a reader under the display, allowing the user to load or save entire programs. It's that programmability that made NASA choose it for Apollo-Soyuz. The same record explains that, before launch, the engineers pre-programmed the calculator with guidance and rendezvous equations, making it a real backup navigation system instead of just a device for quick sums.
