# First use of an incandescent light in an un-crewed spacecraft?

In early, pre-LED spacecraft indicator lights were incandescent I assume. For larger volumes of crewed areas of spacecraft fluorescent lights were often used.

But what is the first use of an incandescent light on an uncrewed spacecraft?

I don't want to rule out non-intuitive options completely; a radiant heater does illuminate an area with a shower of thermal photons produced by a substantially-hotter-than-ambient surface heated by ohmic (I2R) losses, but I'm only interested in something whose primary function is to generate visible or near infrared light.

An incandescent light that was meant for a crewed situation that just happened to be present in an uncrewed situation won't count. It's primary function has to be to perform as an incandescent source of light whether or not crew are present.

It is the answerer's discretion whether to consider non-human living creatures as "crew".

• Yeah, this could be a hard one to answer. Sep 25 '20 at 10:58
• What do you consider "crew"? Albert II or Laika, for example? Or just humans?
– J...
Sep 25 '20 at 14:15
• There are a few possibilities where an incandescent would ever make sense in unsupervised avionics: a) photointerrupters (vulgo electric eyes) used to control something mechanical, b) optocouplers, c) "disposable" pre-flight diagnostic parts built into assemblies (a small, wired light bulb might be lighter in weight than a usable connector for a test box). Sep 25 '20 at 22:33
• @uhoh: Yeah, the Apollo Experience Reports are quite illuminating. Sep 27 '20 at 2:29
• One could argue that the Mercury chimp flights count - chimps are payload, not crew, and the "instrument panels" were lighted - but that seems at odds with the intent of the question. Sep 29 '20 at 16:08

The Gemini Agena Target Vehicle was an uncrewed docking target utilized during the Gemini program. Although uncrewed itself, it had an externally mounted lighted display panel that could be viewed by the crew in the docked capsule.

Image source How Stuff Works - docking adapter is at the right.

The vehicle also had 'acquisition' and 'approach' lights to aid in the rendezvous and docking.

ACQUISITION LIGHTS

The acquisition lights (Figure Above) are used for visual guidance and tracking of the target vehicle when the vehicles are 20 nautical miles or less apart. Two lights are provided. They are mounted on the docking adapter and are held in the retracted position during the boost and insertion phases of a mission. The docking cone holds the lights in the retracted position until the cone is unrigidizedby the pilots via the Command Link.

Each light consists of a capacitor discharge flashing light system. The lamp flashes at a rate of 65 flashes per minute and has a minimum of 100 candles effective intensity through an included angle of +/- 90 degrees from the lamp longitudinal axis. A reflector increases the intensity so that the lamp is visible from 20 nautical miles with the intensity of a third magnitude star. The pilots turn the lights off and on via the Command Link.

APPROACH LIGHTS

Two approach lights are mounted on the docking adapter (Figure Above). They are positioned so that they shine through the rear opening in the cone to illuminate the notch during final approach of the spacecraft to the target docking vehicle. Some light, however, reaches the entire inner surface of the cone. The pilots turn the lights off and on by using the cone. Electric power is supplied by the target vehicle power system.

Source: http://www.geminiguide.info/Systems/Target.html (annotations mine)

First launch of this vehicle on a mission that achieved a docking was March 16, 1966.

Confirmation by DrSheldon:

1. The acquisition lights were extremely bright, flashing beacons that could be seen up to 20 nautical miles away. Apollo Experience Report: Lighting Considerations, NASA Tech Note D-7290, states Apollo used lights derived from the Gemini Agena Target Vehicle, and that they were xenon lights (and therefore not incandescent):

External lighting (figs. 11 and 12) was used for detection, illumination, and attitude orientation. All of the lighting aids had been proved successful for rendezvous and docking during the Gemini Program. Both the LM and CM were equipped with a flashing xenon light. [...] At 60 nautical miles, the light is equivalent in brightness to a third-magnitude star. It could be detected with optical aids at 160 nautical miles. This light was a modification of the light used on the Gemini Agena Target Vehicle.

2. The approach lights seem to be the Gemini equivalent of the Apollo running lights. D-7290 states "The light fixtures consisted of five grain-of-wheat lamps enclosed within a lensed housing", which would make them incandescent.

3. The display panel is described in Project Gemini: A Technical Summary, NASA CR-1106, pp. 332-333. This document explains the color and meaning of each light and indicator on the Agena Target Vehicle. Unfortunately, it does not mention what type of bulb is used. However, several Gemini astronauts complained that the display was not bright enough to be seen in direct sunlight. This strongly suggests that the lights were incandescent.

So the GATV was uncrewed, and had incandescent lights.

• @ohoh Indeed, flashing bulbs in aviation are (and I think were) typically xenon flashtubes. There's no reason to suspect that the blinking lights on the Agena were any different. I don't think that incandescent bulbs would have been bright enough for this application. Sep 25 '20 at 16:32
• @DrSheldon thanks for the edit. I've been away from home with very limited internet availability or I would have said so earlier. Sep 28 '20 at 19:45

This may be stretching the definition of "incandescent light" quite a bit, but the V-2, which was the first artificial object to cross the Kármán line in June 1944 (and thus technically an uncrewed spacecraft) contained an analog computer as part of its guidance system, which was built using...

These typically contain a small filament known as a "heater" which is responsible for stimulating thermionic emission.

And they glow (typically a dull orange) when they're active.

• I'm sorry, but -1 for stretching the definition a bit too far. It's a great SE answer to an SE question that hasn't yet been asked.
– uhoh
Sep 25 '20 at 8:05
• This answer is no longer relevant to the edited question, as the asker is looking for something whose primary purpose is to emit light; the glow of the heater filament in vacuum tubes is merely a byproduct of their intended purpose. I do believe you answered the initial question as asked, but disagree with uhoh's conclusion that you "stretched the definition a bit too far"; heater elements are indeed incandescent, and do emit light in the visible and IR spectra! Sep 25 '20 at 17:48
• Wien bridge oscillators use incandescent lamps - could there be one of those in the rocket's circuitry? Sep 25 '20 at 18:44
• @AndrewMorton I would assume technology of that era would have used an alternator of some kind instead of a wien bridge if there was a sine wave AC source needed. Sep 25 '20 at 22:28
• Also, doesn't a bright incandescent in a wien bridge mean the oscillator is broken and has gone totally off the rails? Sep 25 '20 at 22:30