Apollo XI mission carried a plaque to the Moon featuring a map of the Earth. So do the Pioneers with a plaque with symbolic indications that may help to find its origin.

So, here comes the question: Did the Huygens probe carry any hint that may be helpful to find the Earth? If not, why?

This question may extend to any spacecraft that is expected to survive a landing in other places than Earth, like Mars rovers, or spacecraft whose final location is not a known one, e.g. New Horizons.

  • $\begingroup$ Cassini carries a DVD with signatures of people from around the world, but they of course didn't add a DVD player to it, and I wouldn't expect it to still be readable, so it's all a rather pointless publicity stunt. I'm not aware of any plaques on the Huygens probe, short of a few stickers identifying its parts for the payload integration. It would be helpful if you explained who do you expect to identify these probes' origins? $\endgroup$
    – TildalWave
    May 13, 2014 at 13:53
  • $\begingroup$ @TidalWave I was thinking about ourselves -humans- in the far future, so they could be sure that Huygens came from Earth --just in case the probe's origin gets lost in History. I was thinking about extraterrestrial life also, but I realize that's a very remote possibility. $\endgroup$
    – orique
    May 13, 2014 at 14:01

1 Answer 1


No plaques are needed to identify the origins of any of our deep space probes, landers, rovers and so forth. Any civilisation technologically at least as advanced as we are from the 20th century onward shouldn't have problems identifying that they could only have came from the Earth. In fact, I would dare go as far to claim you'd only really require a small part of one probe, perhaps an electrolytic or a ceramic capacitor, to infer that the only place where it could have been manufactured was on the Earth, and you could likely establish also sufficiently precisely when, by the level of decay of its materials or even carbon dating them.

A whole spacecraft would hold other clues, such as the strength / resolution of its communications subsystem and where its antennae are pointing, limiting possible source within our Solar system or closer, and give you a hint of where to look for other technosignatures. Some of the materials used by it could only have been synthesized and they don't exist naturally anywhere in our neighborhood. Any industry producing them would leave behind markers, such as, say, atmospheric pollutants that could be detected from another celestial in our Solar system, perhaps by astronomical spectroscopy.

So even with a few clues, technologically advanced civilisation could establish when they were manufactured, when they were last used even (exposure to the environment of different parts could give a clue about that), what direction in the skies they were communicating with and a good approximation of at most how far away they were from where their signals were supposed to be picked up. They could then investigate possible candidate celestials and establish that the Earth is the only one that contains traces of industrial pollutants needed for fabrication of some of the probe's materials in its upper atmosphere and that they must be there from around the time when the probe was built. As soon as they'd realize no other celestials in our vicinity match all these requirements, they would've solved the problem of establishing some probe's origins. Simple really.

Plaques and alike would likely be meaningless to anyone not understanding our languages or having required equipment to play records, analogue or digital. If analogy is required, just think of all the data carriers from as little as a few decades ago that we don't even have any readers for anymore. Remember floppy disks? Punched cards? Do you still keep around all the required hardware to read them? Probably not, and even if you did, they might not be working any more, and/or you wouldn't be able to make sense of the data they hold. And even the launching programs don't for some of the spacecraft that are still functional, like e.g. NASA misplacing communications equipment needed to control the ICE/ISEE-3, that is now a crowdfunding effort to redirect it to its original orbit and do science again by means of software-defined radio equipment.

So those plaques, gold records, DVDs (like on the Cassini spacecraft), and whatnot that we're attaching to our probes and landers are really more for ourselves than any yet unknown civilisation (distant future us, or anyone else). And they're a subject to decay, so they won't last or be readable for eternity anyway.

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    $\begingroup$ Given that the large number of satellites we have in geostationary orbit will be there long after we're gone, I think it's a pretty big clue that this little probe probably came from the planet with all the satellite debris orbiting it. $\endgroup$
    – Nickolai
    May 13, 2014 at 15:35
  • $\begingroup$ @Nickolai Well yes, it's another one of the technosignatures, there's many of those we're quite oblivious to yet seem to find it easier to attribute long-term meaning to some tiny commemorative plaques. Some atmospheric pollutants also have a predictable decay rate, and of course, if we'd be still around, we also cause light and radio frequency pollution that could be easily detected from afar. It wouldn't really take a Sherlock Holmes to infer where something came from, if the only place where it could've is the Earth. $\endgroup$
    – TildalWave
    May 13, 2014 at 15:40
  • $\begingroup$ Good points. I just want to add 1) Industrial emissions is such an extremely small fraction of our atmosphere that I bet that it is physically impossible to resolve them from the nearest star regardles of instrument design. Even the largest emission, that of CO2, has increased by only 0.01% of the atmosphere. 2) When I visited Technisches Museum in Munich they were test running a reproduction of the Zuse Z3, by some definitions the first computer (with difficulties because it was surprisingly dependent on a flat stable floor). The historic IT compatibility problem is being addressed nowadays. $\endgroup$
    – LocalFluff
    May 13, 2014 at 15:51
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    $\begingroup$ @LocalFluff Nearest star? What plaques on spacecraft do we have there? And SETI disagrees, that's one of the main selling points of their planned Colossus telescope project. I also didn't necessarily mean CO2 emissions, there's other, clearly non-naturally occurring pollutants with predictable decay rate when exposed to UV radiation, e.g. PCBs come to mind, but there are many others. $\endgroup$
    – TildalWave
    May 13, 2014 at 16:08
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    $\begingroup$ @LocalFluff I think you're forgetting to take into account the context of the question, which is whether or not any alien race that discovers the Huygens probe will be able to determine where it came from. I think we're assuming that if they've discovered Huygens, they're on Titan, not in another solar system. $\endgroup$
    – Nickolai
    May 13, 2014 at 18:50

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