Attached to the Voyager probes are the golden records, which are essentially messages-in-a-bottle sent out into the vast sea of interstellar space, containing, of course, the "message of humanity". Although the primary purposes of these records may have merely been commemorative - and not an actual attempt to come in contact with extraterrestrial life - I think most people have an infinitesimal hope that someday in the far future an extraterrestrial civilization will retrieve these records and "learn of the human species".

Now, suppose another extraterrestrial species has done the same thing, and one of their "Voyager probes" (carrying it's own "golden record") comes travelling straight towards us. For simplicity lets assume that its composition is very similar, if not identical, to our Voyager probes. My question is, if it came near Earth, would we be able to retrieve it? An important qualifying question would be, how close would it need to come for us to even detect it in the first place?

I know this question is quite speculative and loose, but I think it's an interesting thought.

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    $\begingroup$ while being able to detect it is an important pre-requisite, and adding "how close would it need to come for us to even detect it in the first place?" to the question detail is a good change, the question is still about recovery, so changing the title isn't a good edit. $\endgroup$ – JCRM Oct 28 '17 at 20:53
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    $\begingroup$ As an off-topic aside I can imagine a case where our space technology could conceivably overtake Voyager. We could travel to it and back again well before it'll ever reach another system. With that in mind it may well have been better to use Voyager as a time capsule with messages to our future selves ;) $\endgroup$ – Lamar Latrell Oct 29 '17 at 0:32
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    $\begingroup$ We may well be able to detect it if it was heading toward us. In early November 2007 the Rosetta spacecraft was briefly mistaken for a near-Earth asteroid about 20 m (66 ft) in diameter by an astronomer of the Catalina Sky Survey and was given the provisional designation 2007 VN84. Calculations showed that it would pass very close to Earth, which led to speculation that it could impact Earth. However, astronomer Denis Denisenko recognised that the trajectory matched that of Rosetta $\endgroup$ – Dave Gremlin Oct 29 '17 at 17:59
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    $\begingroup$ We had our first interstellar visitor the other day actually, and we still aren't 100% positive what it even was. Most likely an asteroid though. $\endgroup$ – rclev Oct 30 '17 at 17:45
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    $\begingroup$ @ArturodonJuan nasa.gov/feature/jpl/… $\endgroup$ – rclev Oct 30 '17 at 19:18

We should assume that there is no possibility to communicate with the extraterrestrial Voyager. The probe would be out of power, the antenna is not directed to Earth and we don’t know the frequency, modulation and protocol for communication. Therefore the probe may be detected by radar from Earth only.

The asteroid Apophis was detected by radar over a distance of 0.192 AU, see this NASA page. But the probe is much smaller than Apophis and the radar echo would be very weak. I guess radar detection would be possible for a distance of some hundred kilometers distance from Earth.

But to retrieve the probe we would need to build a special spacecraft to fly to the probe, enclose it into a return capsule with heat shield for reentry and to fly back to earth. For the design of the return capsule we should know the size (width, depth and height) and mass of the probe with good precision.

Will the necessary spacecraft be ready before the extraterrestrial probe is to far away? I think it would be too difficult for our available technology.

But if we detect the probe by radar, would we recognise it as an extraterrestrial probe? We would need much more information than only a radar echo. An optical image from a very close distance would be necessary, less than kilometers. A picture with much more resolution than a picture of the ISS made from ground.

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    $\begingroup$ we wouldn't need to build the reentry vehicle for it to capture it - as in ARM we "just" need to get it into orbit nearby where we can study it, and at our leisure decide if we want to bring it back and if so design a vehicle to do that.. I rather suspect the majority would rather redirect it into the sun than land it. $\endgroup$ – JCRM Oct 28 '17 at 19:04
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    $\begingroup$ But to get the probe into an Earth orbit we need to know the size and mass of the probe too to design a docking adapter and the necessary delta-v thrust for orbit insertion. $\endgroup$ – Uwe Oct 28 '17 at 19:19
  • $\begingroup$ ARM proposed a bag, but we would need an estimate of its mass. $\endgroup$ – JCRM Oct 28 '17 at 19:21
  • $\begingroup$ ARM considered up to 500 tonnes - which is enough for a voyager like probe, and the radar signature would be enough to size a bag. Alternately A Study into the Sustainable Disposal of End-of-Life Satellites suggests harpoons and tethers for its "Hunter" craft (from memory). $\endgroup$ – JCRM Oct 28 '17 at 19:30
  • $\begingroup$ If we didn't know the Voyagers were out there and they weren't pointed at us, would an astronomer ever find them? Highly unlikely. +1 $\endgroup$ – Mazura Oct 28 '17 at 20:00

No, we couldn't.

If it was "voyager like" it would just be passing through our system, we probably wouldn't even see it.

If we did, and we worked out it's significance we wouldn't be able to build and launch something to rendezvous with it, capture it and return with it before it has already left the system.

If such an object happened to end up close by with a low relative velocity then the (canned) Asteroid Redirect Mission would have tested many of the technologies needed for doing this.

EDIT: Following the recent discovery of the much larger (180 × 30 × 30 meters) 1I/’Oumuamua Project Lyra is looking at missions to investigate it. It asserts "These values highly exceed the current chemical and electric propulsion system capabilities for deceleration and orbital insertion" and suggests a couple of near-future options, including an impactor mission launched by the in-space refuelled SpaceX BFR striking in 2039; and laser-pushed light-sails carrying 10 gram probes for a flyby in just 7 years. (should the infrastructure have been in place we could have had a flyby in just a year)

The paper further agrees with a gut feeling of mine that intercepting it outside of the system would be difficult "The small size of the object and its low albedo will make it difficult to observe it once it has entered deep space again. This means the navigation problem of getting a sufficiently accurate fix on 1I/‘Oumuamua to get close enough to the object to send back useful data is considerable" while a voyager-like probe is likely to be shinier than an asteroid, it is much smaller, so would be harder to track.

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    $\begingroup$ So, a Voyager probe - no. An Iacarus probe - yes, but only after everyone shat themselves seeing it decelerating into the system! $\endgroup$ – JCRM Oct 28 '17 at 18:58
  • $\begingroup$ Couldn't we detect it if it was still transmitting, assuming it had a battery life long enough to do so? $\endgroup$ – vsz Oct 29 '17 at 18:05
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    $\begingroup$ @vsz Assuming it is similar to Voyager probes would imply it would have run out of power long before it reached another star system. So unless voyager actually has solar panels and will come back to life once it reaches another star, then it wouldn't be transmitting. Now assuming it had solar panels and once it came close enough to power up again would change its attitude to transmit signals towards the star, then the chances of detecting it would be significantly improved. $\endgroup$ – kasperd Oct 29 '17 at 18:24
  • $\begingroup$ to reduce the power requirements, most long distance probe transmissions are highly directional, so even if it was still transmitting we wouldn't be able to hear it. $\endgroup$ – JCRM Oct 29 '17 at 20:57
  • $\begingroup$ Wouldn't that "voyager" be travelling pretty fast by the time it gets here, making it hard for us to catch up with it before it gets out of reach again? $\endgroup$ – JimmyB Oct 30 '17 at 12:11

I think the other answers have made an unnecessary assumption. Namely, that upon detection we have to retrieve it before it starts moving away from us.

I don't think that's necessary (albeit obviously preferable). All we'd have to do is plot its course with sufficient accuracy to be able to predict its future course. Assuming it passes close enough for detection which this answer addresses, then it should be feasible to predict its future motion given that we can do this for asteroids.

Having done that, we can then work on the technology to overtake and retrieve it at our leisure.

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    $\begingroup$ Overtaking and retrieving it means we need to reverse course and come back to us which I'd have to assume would be way too much of a delta velocity to achieve, at least in this lifetime. $\endgroup$ – Scott Oct 29 '17 at 15:03
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    $\begingroup$ we definitely couldn't retrieve it if it had left the solar system $\endgroup$ – JCRM Oct 29 '17 at 17:30
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    $\begingroup$ Retrieving it after it passed by would be at least 60 times more difficult then launching it: what-if.xkcd.com/38 $\endgroup$ – vsz Oct 29 '17 at 18:09
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    $\begingroup$ @vsz And if concrete proof of an extraterrestrial civilization just flew by our system, we would absolutely be willing to spend those resources to bring it back. $\endgroup$ – Ray Oct 30 '17 at 12:47
  • $\begingroup$ @scott The question wasn't, "would it be extremely difficult to retrieve it?", it was "if it came near Earth, would be able to retrieve it?". Since the other answers essentially said "no", on the basis that it's a one-shot deal, I think my point is an important and valid one. $\endgroup$ – JBentley Oct 30 '17 at 15:20

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