Margaret Hamilton was the Apollo Director of Software Engineering. In this picture, she is shown inside a mock-up of the Apollo command module.

Margaret Hamilton in CM simulator

Hamilton is noted for many accomplishments during the Apollo program. She led the team that wrote all of the software for the guidance computers, invented the term "sofware engineering", and insisted on unit and integration tests of all software. Her work was critical to solving computer problems during Apollo 8 and Apollo 11. In 2016, Hamilton was awarded the Presidential Medal of Freedom (the highest U.S. civilian honor) for her contributions to the Apollo program and software engineering.

She also personally wrote many of the routines for the DSKY display and keypad unit, seen above her head in the picture. It is therefore odd that she had posed with her hand on an unrelated control (apparently a toggle switch or a circuit breaker), instead of the DSKY.

Can anyone identify the control that her hand is on in this picture?

  • 26
    $\begingroup$ This sounds like an essay with a one-line question attached at the end. $\endgroup$ Nov 5, 2018 at 6:05
  • 26
    $\begingroup$ @chrylis There’s nothing wrong with that; the “essay” establishes the context clearly, and a straightforward question with clear context is a welcome rarity on this site. $\endgroup$ Nov 5, 2018 at 12:09
  • 14
    $\begingroup$ @chrylis the essay gives needed context, though. It tells me why I should care which control this person is pressing. $\endgroup$
    – jcm
    Nov 5, 2018 at 12:09
  • 24
    $\begingroup$ I just imagine the photographer giving directions: "Great position, Maggie! Stay like that! Now do like you were using the controls." (MH touches a DSKY control.) "Nah, not that one, we can't see your face. Take one of the toggles more to the right." $\endgroup$
    – Dubu
    Nov 5, 2018 at 15:22
  • $\begingroup$ @Dubu has the right answer - that is EXACTLY what would have happened, Although I understand the compositional & aesthetic importance of leading the viewer's eye, the same photographer direction - to change where my finger is pointing at a computer screen from somewhere actually meaningful, just...makes...me....tic!!!! $\endgroup$
    – Esco
    Nov 7, 2018 at 3:56

2 Answers 2


It's the ECS RADIATORS / HEATER / PRIM 1 - PRIM 2 switch.

enter image description here from here

Having given many tours of the cockpit in the shuttle simulators, it's very common to have someone say "Turn around for the camera and put your finger on a switch!"

  • 1
    $\begingroup$ I think this is spot on -- reaching a control above would obstruct her face. $\endgroup$ Nov 5, 2018 at 12:47
  • $\begingroup$ Just out of interest, where do you rest your hand when asked to put it on a switch? $\endgroup$
    – KalleMP
    Nov 5, 2018 at 14:32
  • 3
    $\begingroup$ There are "wickets" (U-shaped pieces of metal) between the switches to help the crew from inadvertently moving two switches at once. You can just put your fingers on the wickets. Also, it took non-trivial force to move a shuttle switch - it wasn't going to move if you brushed it. $\endgroup$ Nov 5, 2018 at 16:11

Organic Marble beat me to it, but in the interest of teaching people to fish:

There's a handy panel locator figure which subdivides the command module control panel into several lettered areas as well as a grid reference. This switch is in area "P", grid J-34 on panel 2.

Looking up that location in the table near the start of the controls and displays section of the Apollo operations handbook identifies it as the "ECS RADIATORS-HEATER - PRIM 1/CENTER/PRIM 2" switch.

This is part of the spacecraft's environmental controls.

Reading further in the (regrettably, not text searchable) handbook, the PRIM 1 position "selects primary heater control no. 1"; CENTER "disables [primary] heater controllers"; PRIM 2 "selects primary heater control no. 2". It's not entirely clear from this document what selecting the heater control actually does; Organic Marble is almost certainly correct that the choice of switch was photo-op arbitrary.

  • $\begingroup$ I sort of remember a question on the site about the heaters on the command module radiators, but I couldn't find it. $\endgroup$ Nov 5, 2018 at 2:56
  • 1
    $\begingroup$ space.stackexchange.com/q/25742/195 ? $\endgroup$ Nov 5, 2018 at 2:57
  • $\begingroup$ That was it indeed, thanks, I was remembering the exchange in the comments on @Uwe's answer. $\endgroup$ Nov 5, 2018 at 2:59
  • 4
    $\begingroup$ The ECS radiator heaters heat the glycol going into the radiators to prevent them from getting too cold and "freezing over". The controller is a thermostat that keeps the radiator temperature above -20°F (handbook There are two controllers for the primary loop, creatively named 1 and 2; the switch selects one (or none) of them to receive power. $\endgroup$
    – hobbs
    Nov 5, 2018 at 8:09
  • 1
    $\begingroup$ (I don't think that the heaters are normally required; normal heat loads should keep the coolant plenty warm to avoid freezing. But since the heaters only come on when needed, I think this switch would normally be set to PRIM 1 to allow automatic control, unless something was broken.) $\endgroup$
    – hobbs
    Nov 5, 2018 at 8:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.