Currently, Voyager 1 is floating through the emptiness of space. Does it do anything to indicate its presence, like a light source onboard or broadcasting a signal that makes it more observable to others than just any inert object flying through space?

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    $\begingroup$ If it hits somebody, it is surely observable. If anybody uses infrared sensors, heat from RTGs will be detectable. If radars are used, an echo will come back. Glint from light sources is detectable optically. Can't see the point in asking without specifying precisely what instruments and at what range are used. $\endgroup$ Aug 4, 2013 at 15:54
  • $\begingroup$ @DeerHunter: Please, re-read the question. "Glinting in the sunlight" is not "doing something to indicate its presence". That is a very nice question asking if Voyager actively broadcasts some kind of "ping" (in whatever spectrum or method) to make itself easier to detect, versus just not trying to conceal its own ambience. $\endgroup$
    – SF.
    Aug 4, 2013 at 22:04
  • $\begingroup$ @SF. - No, it doesn't, and you know the reasons - power conservation and the sheer improbability of someone stumbling upon the craft. $\endgroup$ Aug 4, 2013 at 22:35
  • $\begingroup$ @DeerHunter: As for power, a second-long radio broadcast every 24h wouldn't be too "power hungry". As for sheer improbability - it would be far more useful than including the Golden Record. Although "No, it doesn't" is a valid answer. $\endgroup$
    – SF.
    Aug 4, 2013 at 22:41
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    $\begingroup$ @SF. - please note that side- and backlobes from the high-gain comms antenna are detectable. Basically it is shouting mostly in the direction of Earth, but sufficiently close and astute observer will hear the cries from other quarters. $\endgroup$ Aug 4, 2013 at 22:53

1 Answer 1


Any object in space has several sensor signatures:

  • Reflected optical - light reflected from surfaces. If you illuminate Voyager, its glint will be detectable.
  • Emitted optical - light emitted by bulbs aboard. For energy conservation reasons there are none.
  • If there is an insanely sensitive optical sensor exactly on the line between a star and Voyager, occultation detection may be possible.
  • Infrared - due to the work of RTGs and equipment onboard Voyager dumps heat into the environment by radiation. At mission start, the heat loss from the MHW-RTGs was estimated at 200 W.
  • Radar - Voyager was not optimized for stealth, with lots of sharp corners and no radar-absorbents in use, so its radar cross-section must be quite high (NASA should know the numbers, actually).
  • Radiotransmissions - Voyager is still communicating with the DSN back here on Earth. Depending on range and aspect, one can detect the mainlobe, side- and backlobes from this particular source. Other equipment on board may also have detectable side emissions - if you get sufficiently close, you'll spot them. Note: X-band (8.4GHz) transmission comes through two TWT amplifiers of 12.5 and 21.6 W; the antenna dish is 3.65 m in diameter.
  • Radioactivity from the RTGs - only when you come VERY close for inspection (Pu-238 emits alpha-particles, which get caught by the berillium shell and transformed into free neutrons which decay into protons).
  • Kinetic signature - you'll know it when your craft is hit by a Terran probe.

Now let's suppose you've succeeded in detecting something in the vast expanses of space. My bets are on optics and passive radiowave detection, although I don't know who you are and what detectors you can choose from.

How does one tell an artificial object from a natural one using currently available technologies?

  • Rotation. Voyagers are three-axis stabilized, which is sufficiently unusual for rocks to warrant investigation.
  • Active radio transmissions.
  • Infrared signature. Temperature above the background of deep space for such a small object in the middle of nowhere cannot come from tidal heating or other natural sources.
  • ...and, last, when you are close enough, you will see a complex metallic/composite object. No chance for something like that to emerge naturally.


  • Raymond L. Heacock. The Voyager Spacecraft. Proceedings of the Institution of Mechanical Engineers. V.194, no.28. 1980.

Thanks to TildalWave's comment, we have an interesting reminder of human imperfection:

Rosetta probe was misidentified for a near-Earth asteroid "about 20 meters in diameter" and designated 2007 VN84. Its trajectory was calculated to pass Earth closer than 6.000 km, which caused a bit of concern that it will impact Earth and even an alert was issued, until its trajectory was later recognized to match that of the Rosetta spacecraft. Just a short anecdote, I thought it's worth mentioning here. Here's a relevant article from the New Scientist (archived).

  • $\begingroup$ Why do you say one must be VERY close to observe the alpha-particle? I thought alpha emission on Earth has a low range only because of athmosphere? (So yes, you should measure the radiation outside any athmosphere) But of course ther eis the inverse square law for rdioation at work as well - how much radiation are we talking about? $\endgroup$ Aug 5, 2013 at 15:07
  • $\begingroup$ @HagenvonEitzen - berillium shielding of the MHW-RTGs. $\endgroup$ Aug 5, 2013 at 15:19
  • $\begingroup$ @HagenvonEitzen - and even then we're not looking for alpha particles but for the neutrons from the Be+He nuclear reaction (which means we will have to get close enough before the neutrons decay into protons - the latter being almost indistinguishable from the normal radiation environment in these parts). $\endgroup$ Aug 5, 2013 at 15:24
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    $\begingroup$ The same happened with the S-IVB third stage of the Apollo 12 Saturn V rocket, which was misidentified as near-earth asteroid J002E3 en.wikipedia.org/wiki/J002E3 $\endgroup$
    – oefe
    Aug 16, 2013 at 15:57
  • $\begingroup$ The 3 RTG modules produced about 2400 W each of thermal power at the start of the mission, not 200 W. $\endgroup$
    – Hobbes
    Apr 2, 2016 at 19:14

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